An evaluation of central sewage plant gas use and decentralized gas production and use by anaerobic digestion of industrial wastes in the Denver metropolitan area

Material Information

An evaluation of central sewage plant gas use and decentralized gas production and use by anaerobic digestion of industrial wastes in the Denver metropolitan area
Schellenbach, Susan
Publication Date:
Physical Description:
122 unnumbered leaves : illustrations, maps ; 28 cm


Subjects / Keywords:
Waste products as fuel ( lcsh )
Biomass energy -- Colorado -- Denver Metropolitan Area ( lcsh )
Biomass energy ( fast )
Waste products as fuel ( fast )
Colorado -- Denver Metropolitan Area ( fast )
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )


Includes bibliographical references (leaves 121-122).
General Note:
Submitted in partial fulfillment of the requirements for a Master's degree in Planning and Community Development, College of Design and Planning.
Statement of Responsibility:
Susan Schellenbach.

Record Information

Source Institution:
University of Colorado Denver
Holding Location:
Auraria Library
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
08643438 ( OCLC )
LD1190.A72 1981 .S3835 ( lcc )

Full Text
204 00255 4286
Susan S§hellenbach November, 1981
In Partial Fulfillment of the Requirements for the Master's Degree Program in Planning and Community Development
College of Environmental Design University of Colorado at Denver

I wish to acknowledge the help and thank the many people who contributed to this project: Alice Reynolds who ingeniously revised the manuscript into something which could be read;
David Hill, Jim Westkott, Peter Brown, Ray McNeill, Stephen Pearlman, and Judy Zimmerman who patiently read the first draft; Bob Seely and Mary Veatch who supplied technical expertise in the laboratory; and Betty Trampe and all the people at businesses and sewage plants who provided me with the information without which I could not have written a word.
Susan Schellenbach November 23, 1981 Arvada, Colorado

Page No.
Introduction 1
Chapter I: History of Biogas Production and Use
A. The Process of Gas Production by Anaerobic
Digestion 1-2
B. The Economic Value of Biogas 1-5
C. The Government Role in Gas Production
Technology I-10
D. Gas Use at Wastewater Treatment Plants 1-12
E. Gas Utilization in Industry 1-17
Chapter II: Current Sewage Treatment Practices and Biogas Potential in the Denver Metropolitan Area
A. Background: The Formation of Sewage Districts
in the Denver Metropolitan Area 11-2
B. Process Flow of the MDSDD#1 Facility I1-9
C. Population and Wasteload Projections 11-9
D. Gas Potential from Anaerobic Digestion of Industrial Wastewater in the Denver Metropolitan
Area 11-17
Chapter III: The Use of Sewage Gas from Denver's Centralized Waste Treatment Facility
A. Present Operation of the Anaerobic Digestion
System and Gas Use 111-1
B. Centralized Gas Utilization Alternatives 111-3
C. Evaluation of the Four Alternatives 111-12
Chapter IV: A Description of a Decentralized Model for Production of Industrial Waste Sewage Gas
A. Technical Description and Facility Design IV-3
B. Capital Costs and Operating Expenses IV-5
C. Value Projection of Energy and Waste Reduction for Selected Industries in the Denver Metropolitan
Area IV-7
D. Return on Investment IV-16
E. Impact of Industrial Treatment of Wastewater in
the Denver Metropolitan Area IV-22

Page No.
Chapter V: Evaluation and Discussion of the Centralized and
Decentralized Sewage Gas Use Models
A. Criteria V-l
B. Centralized Model V-2
C. Decentralized Gas Use Plan V-6
D. Comparison of Models V-9
E. Discussion of the Difficulties of Predicting Economic Trends and Quantifying Social and
Environmental Impacts V-12
Chapter VI: Conclusions and Recommendations VI-1
Glossary G-l
Bib!iography B-l

by Susan Schellenbach
The population of Denver's thirteen front range counties is predicted to grow 50% by the year 2000, necessitating an increased demand for resources. Among the impacts will be a tripling of electrical demand and a shortage of water. Colorado's Governor Dick Lamm points out the complexity of the proposed solutions to these problems in a paper prepared for the Aspen Institute, stating, "The import of water as a solution to the water shortage runs into the energy crisis. The energy crisis runs into the investment capital crisis." He quotes No Limits To Learning, "innovative societal learning entails .... preparation of the ground for major decisions and an increase in the number of experiments and ideas for new energy options." He continues to discuss the need for vision, planning and institutional change to confront issues before they become crises rather than to discuss the technological answers needed to cope with resource scarcity.
One place to look to expand our limited sources of energy and water is the sewage treatment field. While it is apparent that good wastewater treatment replenishes our water supplies, the method of producing energy from sewage is not widely known. Yet each day Metropolitan Denver Sewage Disposal District #1 (MDSDD#1), the largest facility in the Denver region, flares over 1,500,000 cubic feet of sewage gas containing over 900 thousand cubic feet (MCF) of natural gas^ worth approximately $2,700 ($3 per MCF). This is enough gas to heat approximately 2250 homes. Smaller, local outlying sewage plants flare 6 MCF of raw gas per day. While this is not a large amount of gas, valued at only $18 per day, the waste is notable when considering natural gas prices are predicted to rise by 10% to 20% plus the rate of inflation over the next five years.^

Sewage gas results from one step in the treatment process called anaerobic digestion. Sludges from primary and secondary treatment are placed in an airless tank where bacteria act upon the organic material in the absence of oxygen to produce a gas which is 60% methane (the major component in natural gas). The gas has previously been flared because of the low cost of energy and the expense of cleaning and transporting. Now, with energy prices rising and resources dwindling, is the time to become innovative, do some planning, and even explore the possibility of institutional change if needed, to explore the energy sources around us.
Recently, a planning team composed of the MDSDD#1 staff and a private consulting engineering firm completed a study of gas use alternatives. The most practical course of action appears to be to sell the gas from the central sewage facility to a neighboring oil company. This eliminates flaring the gas and generates some revenue for the district.
While this overlooked source of energy will eventually be used, Denver may have even more sources to draw upon. The indsutries which now put their wastes into the sewer for the central plant to treat might be able to process their wastes to supplement their gas supplies. An additional benefit might be the smaller quantity of wastes which must be treated by the publicly owned facility and economic advantages for the industry and area. This decentralized plan for sewage treatment and gas production should be able to coexist with the central facilities while providing economic, social and environmental benefits. This can be shown by evaluating both against the same criteria of technical feasibility, economic value, social/environmental benefits and ease of implementation. The decentralized approach must have major benefits and rate as well or better than the already existing central plant to deserve further investigation as an energy alternative.
To evaluate the two systesm, the planning team's report of the central plant can be used, but a decentralized plant will be proposed and applied to several different industries in order to compare actual costs and returns. It is probable

that the decentralized plan will not be feasible in every situation but will produce major benefits in the right applications. Four typical industries which have organic wastestreams will be studied to find instances where the decentralized technique could be adapted. It is impractical at this time to catalog all businesses
in Denver. The location of any feasible cases can provide the impetus for further
study of each one and broader investigation of Denver area industries.
First, methods of gas use in other treatment facilities will be examined.
Second, the centralized facilities of MDSDD#1 and their relationship with the
contributing sewage districts will be described. Third, the proposed plan for
gas use and its value and costs will be outlined. Fourth, an alternative plan
for decentralized waste treatment facilities and its costs will be proposed,
with a discussion of its impact on the Denver metropolitan area. Fifth, the
two models will be evaluated in order to examine the impact on the Denver metropolitan
area. Finally, conclusions and recommendations will be made.
1. Energy Conservation and Gas Utilization, Central Plant Facility Plan,
Vol. I-111, Metropolitan Denver Sewage Disposal District, No. 1, prepared by Black and Veatch, Denver, Colorado, Jan., 1981.
2. Martin Robbins, Colorado Energy Research Institute, address to the Colorado Front Range Project, August, 28, 1981.

History of Biogas Production and Use INTRODUCTION
Although sewer gas or biogas was burned in Exeter, England in 1895 to illuminate street lights,'*' its limited utilization today in the United States is the result of a combination of historical circumstances. The two most important of these are the technical development of the sewage industry and the government's role supplying funding to cities to upgrade wastewater plants in the interest of public health. This chapter will discuss the background of biogas production and use in terms of biological processes, history and economic value. Case studies and calculation methods will be cited as examples.
These concepts, facts, methods, and examples are necessary to describe the role that technical development has played in the sewage treatment industry and the large impact that government financing has produced in directing that development.
More importantly, the information is necessary to evaluate the data in later chapters. Furthermore, the case studies illustrate the nonexperimental nature of the technology in order to lay a groundwork for credibility for the practice of increased utilization of the anaerobic digestion process.
First, the biological process which creates the biogas is presented. Second, the methods and purposes of the sewage treatment industry use of the process is described. Third, two methods of calculating net gas production are illustrated and one method for calculating gas value as a replacement fuel is given. Fourth, the history of the government regulation of water pollution control is outlined to show the increasing Federal interest and the large amount of financial subsidy.
The history of Colorado government in enforcement of Federal regulations is also outlined. This established impetus to apply for Federal grants. Fifth, the reasons why anaerobic technology is underutilized are presented, and case studies of gas used in the sewage treatment field are described to show how some municipalities

are using the technology now. Finally, the present situation for use of anaerobic technology in private industry is described, including both the dis-incentives that exist because of the governmental action in the field and the case studies of the experimental nature of the work in progress.
A. The Process of Gas Production by Anaerobic Digestion
1. The Biological Process. There are two basic types of bacteria which decompose organic matter, aerobic (with oxygen) and anaerobic (without oxygen). In each case, the bacteria break down the higher organic matter, leaving various end products of dissolved solids and gases. Aerobic bacteria are the more common and are responsible for the decay of vegetation and animal matter on the floor of the forest, in fields of crop residues and inside compost piles. The gaseous by-products are ammonia, carbon dioxide and traces of other gases. Anaerobic bacteria, on the other hand, are only active in places where air is absent. These include the bottom of lakes or swamps and inside animal intestines where digestion occurs. The resulting gas is named "biogas" and is 60% methane, a combustible gas which is the main component in natural gas. The remainder is carbon dioxide and traces of other gases, such as ammonia and hydrogen sulfide. This gas is also called sewer gas, because it is found in sewage systems where organic matter is decaying, and swamp gas, where upon ignition it is titled a "will-o-the-wisp". Not only is it explosive when mixed with air at biogas concentrations of 8-23%, but the trace of hydrogen sulfide is highly poisonous and has been responsible for many deaths when people enter old septic vaults or manure tanks.
Gas production is the result of the interrelated activities of several strains of bacteria that live in an anaerobic atmosphere. They can be divided into two basic groups. The first group is called acid formers. They hydrolyze the wastes and make simple organic acids, mainly acetic, and much carbon dioxide. These bacteria can tolerate a wide range of acidic, atmospheric,

temperature and nutrient conditions. Furthermore, they multiply very rapidly,
reproducing in 6-10 hours. The second group, called methane formers, metabolize
the acid to make methane as in the following formula:
ch3cooh-----teria------> ch4 + co2
Methane is also formed by the reaction of hydrogen, which can be another by-product of bacterial activity, and carbon dioxide:
4H2 +
These produce the majority of the resulting mixture of gases, referred to as biogas. Traces of other gases are supplied through different metabolic pathways. The methane formers are very sensitive and are unable to function if the environment becomes acidic, if the temperature fluctuates or stays below their optimum operation point or if large quantities of air are introduced.
The acidic conditions created by the first group of bacteria is counterbalanced by 1) the activity of the methane formers as they take up the acids and 2) the neutralizing of acids when buffering compounds are released through other chemical reactions which are occurring. However, to maintain high gas production, these two groups of bacteria must be in a dynamic equilibrium, which is complicated by the lengthy 3 to 10 day reproduction cycle of the methane formers.^ Consequently, to control the bacterial process and proceed at the maximum rate for gas production, the feeding and heating of the entire system must be carefully managed.
2. The Sewage Treatment Industry Use of the Process. The sewage treatment industry is the major industry which puts the process to work. However, it has been concerned with developing the techniques to separate solids from water rather than to optimize production of biogas. Treatment is

achieved in a series of steps. Primary or preliminary treatment uses physical methods of solids separation, such a weirs, flotation chambers, grinders, screens and sedimentation basins. Secondary treatment methods include biological means of breaking up solids, and combinations of aerobic and anerobic bacterial processes. For instance, in a trickling filter, primary treated raw sewage will be sprayed over a bed of rocks, covered with aerobic microorganisms, much like a stream bed. At the same time, thick sludge from settling will be sent to a digestion vessel where air is excluded and heat is applied to make the conditions favorable for anaerobic bacteria to flourish. The liquid from the trickling filter may be given tertiary or advanced treatment or the addition of chlorine and then discharged to the river, while the anaerobic sludge may be centrifuged and the solids buried or sun-dried for use as fertilizer. A wastewater treatment plant will be constructed to conduct several different types of treatment, depending on the nature of the wastes it will receive, such as industrial or residential. However, the list of techniques is immense, only one of which is anaerobic digestion.
For instance, the USEPA publication, Innovative and Alternative Technology Assessment Manual, which describes existing technology so that applications
for innovative ideas can be submitted, catalogs one hundred and twenty-four
fact sheets describing separate existing treatment processes.
The purpose of anaerobic digestion in a sewage treatment plant is to reduce the volume of solids and the highly putrescible nature of the sludge.^
The effect is to liquify some solids and stabilize the solids remaining in -the sludge. The bacteria break the bonds between highly complex organic molecules, releasing water and creating less volatile substances, such as carbon dioxide. The digestion vessel is heated from 90F to 100F, and the heat eliminates approximately 70% to 90% of the disease carrying organisms.7 The digester effluent is black and watery and has a musty odor, although the extent of these characteristics depends on the length of time the digestion

takes place usually 10 to 30 days and original composition of the waste.
The final product retains essentially all the nitrogen, potassium and phosphorus, the primary nutrients for plants, because these elements are not contained in the gases which have emanated from the process. Consequently, the effluent can be conveniently disposed of on fields. Although the area of application should be well drained to disperse the dissolved salts in the effluent, the addition of microbes and trace minerals to the soil can have a beneficial environmental effect.
B. The Economic Value of Biogas
1. Estimation of Production. A portion of the biogas produced is commonly used to heat the process to the approximately 95F required to maintain the environment for the bacteria. In a well insulated vessel, the heat loss through the skin is not as large a heat demand as is bringing the temperature of the influent up to the process level. Figure 1-1 shows how this can vary with the percent of solids in the influent and the temperature difference.
Higher solids and less temperature discrepancy will lessen the need for fuel.
The economic value of the biogas produced must be calculated from net production, taking into account that from 5% to 30% of the gross production, depending on the season, will typically be consumed in the process.
Gross biogas production is dependent on the kind of organics introduced into the system as well as the system management. The bacteria can adapt to a variety of feedstocks, including substances which can be toxic, like
phenols and acetone. Assuming optimum system design and management, the gas yield can be expressed relative to the organics in two ways: 1) as a function of amount of organics fed to the bacteria daily or 2) as a function of the amount of organics destroyed by the bacteria, comparing influent with effluent measurements. Researchers use either or sometimes both methods.
The organics may be expressed as "volatile solids" (VS), "volatile suspended solids" (VSS), BOD or chemical oxygen demand (COD). All expressions can be

Influent Heat Required, Million BTU/Ton Solids
IX Solids Feed
Source: Innovative and Alternative Technology Assessment Manual, USEPA, Office of Water Program Operations, Washington D.C., Office of Research and Development, Cincinnati, Ohio, MCD-53, Feb., 1980, Appendex D, p. D-31.

calculated from laboratory testing of the digester influent and effluent for
VS, VSS, BOD OR COD. Three variations will be used here, because they are
commonly found in the literature: 1) the yield from pounds of VSS destroyed
by the bacteria, 2) the yield from pounds of VS added to the digester daily,
and 3) the yield from pounds of BOD destroyed by the bacteria. Table 1-1
lists some examples from published sources and recent unpublished private g
laboratory work.
For example in the first method (second variation), 400 feeder hogs at a weight of 150 pounds each will give .72 pounds of organic waste per head per day. Each pound of organics, under proper anaerobic conditions, can yield 5.5 cubic feet of methane each day. The equation is:
400 hogs x .72 pounds VS x 5.5 cubic feet methane/lb VS added =
1584 gross cubic feet of methane per day
If the waste material is not allowed to cool off to outside winter ambient
air temperatures, process heat will be on the order of 5%. The net methane
yield is:

1584 cubic feet (5% x 1584 cubic feet) = 1505 cubic feet
methane per day
In the second method (third variation), 10,000 gallons of wastewater containing dissolved sugar could have 96.6 pounds of BOD calculated from laboratory tests. The equation is:
96.6 lbs BOD x 80% destruction x 8 cubic feet methane/lb BOD destroyed = 618 gross cubic feet of methane per day
As in the example above, if the sugar water is not allowed to cool from the

Methane Production Rates from Various Wastestreams
Waste Material Amount lbs VS per day lbs BOD per day % BOD Destroyed Ft Methane per lb VS added per day 3 Ft Methane per lb BOD Destroyed per day
hog 150 lb hog 0.72 5.5
dairy cow 1400 lb cow 12.0 3.5
beef cattle 750 lb cow 4.4 4.2
poultry layers 4 lbs hen 0.037 3.1
packing house wastes 1000 lb cow 17 1.70 75 4.2 5.72
alcohol stillage 1000 lbs dry wt. 98 8.6
cheese whey 100 lbs dry wt. 91 5.6
cellulose fines 100 lbs dry wt. 91 4.4
sugar 10,000 gallons 96.6 80 8

heat of the manufacturing process, the digester will require an amount of gas on the order of 5%. The net methane yield is:
618 cubic feet (5% x 618 cubic feet) = 587 cubic feet of methane per day
The first variation, used in Chapter II and III to express the yield at MDSDD#1, follows this format.
2. Estimation of Value. To translate net gas production into dollars, a fuel type must be selected. Table 1-2 lists value of several fuels based on one million BTU's for each fuel type.
Comparative Cost of Fuels
February, 1981
Fuel Current Retail Price BTU/Uni t Cost/Million BTU
Natural Gas $ 4.00/MCF 850/cu-ft $ 4.71
Methane (CH^) $ 4.00/MCF 1,000/cu-ft $ 4.00
Propane (LPG) $ .68/Gallon 88,806/Gallon $ 7.66
#2 Fuel Oil $ 1.05/Gallon 130,000/Gal1 on $ 8.08
Electricity $ .06/Kwh 3,413 BTU/Kwh $17.57
Anhydrous Ethanol $ 1.78/Gallon 80,000 BTU/GalIon $22.25
The waste gas generated, may be used in locations where natural gas is not available, such as farms or sewage plants on the outskirts of a city. In these cases, the biogas would take on the value of the fuel it replaces.
Propane is a typical fuel used in these situations because it is easily transported by truck. The net hog waste generated gas, then, would be worth $6.02 as natural gas and $11.53 as propane per day or $2,197 and $4,208 annually. The equations are:
1.505 MCF x $4.00 = $6.02 and 1.505 MCF x $7.66 = $11.53

In certain instances, use of the anaerobic process to treat wastes and generate gas has additional benefit. The sludge, which has become stabilized, has a fertilizer value superior to that of the raw waste. Furthermore, both air and water pollution are reduced by implementing the process. Finally, the availability and reliability of controlling the fuel source provides self-sufficiency or may even allow a farmer or company to remain in business in areas where the fuel supply can be interrupted. A dairy is an example of such a business. These extra credits are difficult to assign dollar values and will not be considered any further at this time.
C. The Government Role in Gas Production Technology
1. Federal. Until recently, anaerobic digestion has been investigated
mainly as a technique for waste treatment. Consequently, the history of
government activity in the field parallels its regulation of the sewage
industry. The first Federal law concerning water quality was the Rivers
and Harbors Act of 1899, which gave control of discharge into navigable
waterways to the Army Corps of Engineers.^ It was not until 1938 that the
then Secretary of Interior, Harold Ickes, surveyed municipal facilities.
He concluded that in a few years all accessible population would be reached
by sewers. This did not include industrial wastes, still largely ungoverned
by the Corps. Jobs were more important than the threat of pollutors.
The first Federal pollution law passed in 1948, was the Water Pollution
Control Act. The Health Service provided technical information to the
states for research and Federal funds were provided for sewers and some
enforcement. Appropriations declined through 1955. However, the Act
established the principle of Federal responsibility for financing and
enforcement in the area of pollution.
In 1956, the Federal Water Pollution Control Act, the basis of all current regulation, authorized $500 million over ten years in matching grants to

facilitate the local construction of sewage projects. The Water Pollution Control Advisory Board was established (later known as the Federal Water Quality Administration or FWQA), and research was supported.^ Enforcement was to be provided by the US Dept, of Health, which would call a conference of officials to work out solutions. If no solution was possible within a certain time frame, the Attorney General could eventually bring suit.
This approach evolved until 1972, when Federal regulation reached interstate waters and became based on standards for effluent discharge to a river rather than the waterway itself. Permits for discharge were issued to municipal users by the Environmental Protection Agency or the state, if its capabilities were approved by EPA. This proved more enforceable, and
ammendments were added in 1977 and the name changed to the Clean Water Act.
The goal for 1983 is to make the nation's rivers and streams supportive of
aquatic life and safe for primary contact recreation by the "best available
technology economically achievable", and zero pollution by 1985.^
The real impact of these laws is financial, granting 75% of the costs
of wastewater treatment plants to municipalities. The Clean Water Act
increased this incentive to 85% for innovative alternative technologies.17
This stimulation has clearly improved the quality of the nation's waters
over the last 20 years. For instance, in Colorado, every community has
secondary treatment facilities and all waterways are "holding their own or 1Q
improving". According to the FWQA, about 85% of the communities charge a
fee for use of the sewers, but only about half levy an additional charge on
industries for handling its wastes. For instance, San Francisco has no
charge and New York bases the charge on the volume and strength. Consequently, because the Federal government is building most of the wastewater treatment plants, private industry has not needed to develop the technology.
2. Colorado. Before 1966, water pollution control was vested in the State Health Department and considered mainly a local problem. In 1966, the

General Assembly of the Colorado Legislature enacted the Colorado Water
Pollution Control Act to establish standards and the Colorado Water Pollution
Control Commission to administer the law. It consists of eleven people
from various agencies and constituencies around the state, and its policies
are still to be enforced by the Health Department. The regulations passed
by the Commission have allowed Colorado to participate in the National
Pollutant Discharge Elimination System (NPDES) since 1974. The standards
recognize some 60 pollutant limitations defined by EPA but monitored by the
Enforcement Division of the Health Department. In most cases when an
offender is identified and a "cease and desist" order issued, the Division
and offender agree on a course of action before making a presentation to
the Commission. If a community resists taking action, a moritorium can be
issued on building permits, which is usually effective. This level of enforcement encourages communities to apply for the generous federal funding.
D. Gas Use at Wastewater Treatment Plants.
Although many cases will be cited, they are the exception rather than rule.
The gas is always burned to maintain the process itself, but the remainder is
rarely used. Other gas may be purchased from local utility companies, and an
unmetered gas flare may burn day and night on the back part of the premises
where the employees don't even notice if it should blow out. There are several reasons for this.
1) Natural gas is generally the cheapest available source of energy, as
shown in Table 1-2. The total value of the 17 million cubic feet of gas flared in
the U.S. each day in 1978 was $27,500, but the estimated value of the 18 million cubic
feet expected to be flared in 1985 would be $73,000. Deregulation may change this situation.
2) The trace amount of hydrogen sulfide in the gas makes it corrosive.
Additional equipment would have to be purchased to clean the gas. With cheap

gas, it would take a long time for this equipment to pay for itself.
3) Gas production may vary from day to day, necessitating gas storage equipment and auxiliary burners for back-up fuels for full utilization. The unreliability occurs because the bacteria are sensitive to many pollutants, such as industrial chemicals and caustic cleaning solvents. However, it is the sewage
plant's function to treat these wastes, and of the 280,000 manufacturing businesses
. 25
in the United States, only 25,000 treat their own wastes. Sewage plant
engineers deal with the delicate nature of the bacteria by adding neutralizing
chemicals to the process as insurance against unknown substances coming down the
sewers and by operating in a very dilute concentration. It is not uncommon for
plants to have to shutdown to clean out the digester and grow a new batch of
bacteria because of an uncontrolled contaminant.
Gas utilization is a function of plant size. Small plants may use only
enough gas to heat the digesters while large ones have substantial excesses for other
uses. Unpublished EPA data show that the mean size of all plants with anaerobic
digesters is 2.6 mgd while the mean size of plants which are heating buildings is
3.4 mgd, the mean size of plants running engines is 7.8 mgd and the mean size of
plants doing both is 13.6 mgd.
It is technically feasible to use digester gas on-site for space heat,
mechanical energy and electrical generation. Figure 1-2 diagrams each of these 27
process flows. Off-site gas use requires cleaning and compression equipment as well as the gas use equipment.
Economic feasibility is a site-specific concern, depending on the plant size, type, energy requirements, and local rates for utilities. Costs versus plant size for each of the three types of gas applications is graphed in Figure 1-3. The most cost effective means for gas use is for mechanical energy.
Electrical rates are the one parameter to which the graph is the most sensitive.

Schematic of Heat Generation Scheme
Schematic of Mechanical Energy Generation Scheme
DIGESTER GAS ------------*
Schematic of Electrical Generation Scheme FIGURE 1-2: Biogas Use at Municipal Wastewater Treatment Plants
T 1/1

Energy Costs for Hypothetical Wastewater Treatment Plants in Central U.S. Locations

Described below are cases of digester gas made from wastewater being utilized as an energy source. The examples range from a large plant where the practice has been in effect for many years to laboratory experiments. All examples are publicly funded, either by the sewage construction grant program or government research programs. While they all represent technical feasibility, some are not meant to be economically feasible but to generate data on which a cost-effective plant might be built.
1. Hyperion Project, Los Angeles, California. This plant has been in
operation since the early 1950s. Incoming sewage is dewatered and fed
to anaerobic digesters. The gas is used to fuel 10 diesel gas engines
of 1968 hp each. Six percent of the fuel is diesel, to provide spark
for the sewage gas. Five engines drive 1.2 megawatt electric
generators and the other five power large centrifugal blowers
supplying compressed air to aerate incoming raw sewage. Heat is
recovered from the generators in the form of steam to heat the
digesters. Operations consume 2.4 mi 11 on cubic feet per day, and approximately
1 million cubic feet of excess is sold to a nearby steam-electric power
plant to heat boilers. Originally, the plant was energy self-sufficient,
but the installation of large pumps to discharge sludge further out at
sea has made some purchase of power necessary. This is partially offset
by gas sales. Gas savings in 1975 for the amount consumed by the plant
was $600,000.
2. Winston-Salem, North Carolina. This 23 mgd wastewater plant
generates all its own power through a combination of digester gas, fuel
oil and natural gas. By optimizing gas production, the digesters are
able to provide 56% of the energy. On-site power generation is cost-
effective when compared with other alternative energies, when considering the
additional costs necessary for waste gas disposal, back-up power sources and
payment for building heat and electrical surcharges.

3. Reno-Sparks Wastewater Treatment Plant, Sparks, Nevada. The capacity
of this plant is 20 mgd. Digester gas production has increased since
the plant was inaugurated. Currently, the gas is used to fire boilers
that provide steam for digester and all building heat on the facility.
Excess gas has been flared, but plans are underway to use the gas in an
engine to drive a turbine-producing air compressor for the activated
sludge process and post-aeration tank.
4. Yonkers Joint Treatment Plant, Yonkers. New York. At this 92 mgd plant, sludge gas is used to drive an engine for the aeration process. Digesters provide 95% of the needed energy, utilizing 5% diesel oil as a pilot fuel. The gas is also used as building heat in the winter. The
annual savings by utilizing the gas is $1,944,000 or 58% of the energy
. 32 cost.
These examples were selected because they are in operation. Numerous
plants report plans for electrical generation which will come on-line in
33 34
1981, such as Knoxville, Tennessee and Laguna Niguel, California-
Furthermore, laboratory experiments with high temperature thermophilic digestion
35 36
in packed beds, ambient temperature upflow filters* rotating discs
for bacteria growth, and mixtures of municipal solid wastes and sewage
sludge have been reported. Some of these are in pilot stages and all indicate the intense interest in waste gas recovery as energy prices rise.
Energy costs in the average sewage plant represent 14-27% of the annual
operating dollar, second only to the cost of labor.
E. Gas Utilization in Industry.
There is little encouragement for industries to treat their own wastes. Public financing from 75-85% of the cost of wastewater treatment at the municipal level is an incentive for industry to rely on the government.
The comparable tax write-off to the industry that develops its own waste

facilities is only a combination of 10% pollution credit and 10% investment
credit. Further dis-incentives are more subtle. If a business developed a
new technique for waste treatment and effluent standards were not met when
completed, it would be fined for the discharges. This is a total contrast to
how the government treats a municipality in the same situation. P.L. 95-217,
Section 202 (a) (3) states that "EPA replaces, bearing 100% of the cost, the
failing municipal innovative or alternative technologyHowever, if the
process proved successful, no matter how limited, the technique would be
labelled "Best Available Technology" and all sites, whether appropriate or not,
would be obliged to adapt it (see page 11, paragraph l).41
A survey conducted by Argonne National Laboratory investigated industries'
attitudes towards these problems. Twenty-five questions about energy usage
in industrial wastewater treatment, industrial innovation, and industrial
resource recovery were asked of 80 large firms. It was assumed that
large companies would have more potential than smaller firms for wastes
and conservation as well as having an administrator responsible for a program.
Twenty-eight responded about whether they agreed, disagreed or were unsure of
the statements. Sixty-two percent felt that environmental regulations
discourage industrial innovation in wastewater treatment. Specifically, they
were concerned about 1) what the final removal requirements would be,
2) tight timetables, 3) fines, 4) the requirement of "Best Available
Technology," and 5) the high risk low return investment of waste treatment
which uses up limited venture capital.
Regardless, the survey solicited names of firms who actively pursue
waste treatment activities, and it compiled the following list:
Air Products & Chemicals, Inc. B.F. Goodrich
Sun Oil Amoco
British Petroleum Chevron
DuPont Monsanto

Campbel11s Soup Union Carbide
U.S. Steel
National Steel FMC
These names have been known during the last decade for their detrimental effect on the environment. Although their interest may be for public image or to market new products and services, and in spite of federal discouragement cited above, these industries are developing a reputation for trying to clean up the environment. Conservation, prevention and resource recovery can actually be profitable. The 3M Company began a program they called "Pollution Prevention Pays" in 1975 and claim "environmental gains, plus a worldwide savings of some $20 million. 1,44
Examples of recovery and use of methane by industries are in a developmental stage. The following describe the kinds of activities that are taking place.
1. Celanese Chemical Corporation. This multi-billion dollar corporation
recently announced that after nine years of research, they have developed
a commercial, anaerobic digester for treatment of industrial wastes and
methane generation. The bacteria grow on a "fixed film" inside the
reactor and consequently are more stable than the bacteria in conventional
sewage treatment processes. The bacteria reduce the organics by 80% or
more. The company is demonstrating the system at its own installations 45
in Texas.
2. Pacific Gas and Electric and Southern California Gas Company. In a constant effort to find new gas sources, these two utilities, serving San Francisco and Los Angeles respectively, are looking for methane in
two places. The first is in old solid waste dumps where garbage has fermented over the years, protected by the cover of the ground. At the PG & E installation at Mountain View, California, they are inventing the techniques for extraction and cleaning of this resource.46 Second, in Brawley, California.

they have constructed two 20,000 gallon pilot plants to digest cattle feedlot manure anaerobically. Previously, the manure had been a pollution problem in an intensely farmed valley where chemical rather than organic fertilizers are preferred. In this on-site application, they have been able to show that each pound of organic material fed to the digesters can produce 3-4 cubic feet of methane.^
3. New York Dairy Industry. Researchers at Clarkson College of Technology
and the New York State Research and Development Authority have completed
a feasibility study to produce methane from the cheese whey wastes generated
by New York dairy processers. In 1977, 2.3 billion pounds of whey were
produced. The BOD of each 1000 gallons is equal to that generated by a
population of 1800 people. Their survey showed the most common method of
disposal was to dry the whey to make an animal or human food supplement.
This is an energy intensive process which small producers cannot afford.
The next most common method was to use the local sewers. The researchers
fed whey to an attached film expanded bed anaerobic reactor. This is
similar to the fluidized bed reactors used for dilute chemical wastes in
other industrial processes. The more concentrated whey, however, produced
five to six times the reactor volume of gas and removed 87% of the COD.
They point out that the results are preliminary, but the process is
potentially feasible and could supply 46% of the gas and oil needs at
cheese plants in New York.
The production of methane from organic materials is a researched and documented phenomenon. The biological mechanisms are sensitive but controlable. The sewage treatment industry has found this technology useful for the purposes of sludge stabilization. The economic value of the gas can be ascertained estimating total gas production, subtracting the anaerobic process consumption

of gas, and multiplying the net production by the replacement fuel value.
The Federal and State governments have a long history of stimulating municiple activity in sewage treatment by strong enforcement of pollution laws and generous funding to public entities.
In spite of the many reasons why sewage treatment plants might elect not to use anaerobic digestion or the gas from the process, some do utilize the gas and claim benefits result. Private industries have more negative incentives to develop the technique for their own benefit than public entities, yet some are expanding their activities. Presumably, they are expecting economic benefits from their investigation of the field.

1. Metcalf & Eddy, Wastewater Engineering, McGraw-Hill, N.Y., 1972, p. 592.
2. Ibid, p. 416.
3. McCarty, Perry, "Anaerobic Waste Treatment Fundamentals," Public Works,
Sept., 1964, p. 110.
4. Metcalf & Eddy, p. 417.
5. Innovative and Alternative Technology Assessment Manual, USEPA, Office of Water Program Operations, Washington D.C., Office of Research and Development, Cincinnati, Ohio, MCD-53, Feb., 1980, Appendix A.
6. Manual of Instruction for Sewage Treatment Plant Operations, Water Pollution Control Board, Health Education Service, Albany, N.Y.
7. Stafford, D.A.; Hawkes, Dennis; Horton, Rex; Methane Production From Waste Organic Matter, CRC Press, Boca Raton, Florida, 1980.
8. Speece, R.E., "Fundamentals of the Anaerobic Digestion of Municipal Sludges and Industrial Wastes," Duncan, Lagnese and Associates, Inc., Pittsburg,
Pa., Nov., 1981.
9. American Society of Agricultural Engineers' Yearbook, ASAE D384, Dec., 1976; also Bio-Gas of Colorado, Arvada, Colorado, unpublished laboratory reports; also "Elimination of Water Pollution by Packinghouse Animal Paunch and Blood", by Beefland International, Inc., Council Bluffs, Iowa, EPA Project #12060 FDS, Nov., 1971; also Fordyce, I.V., L.C. Benedict, W.D. Ransdell, H. Sointer,
"The Anamet Waste Water Treatment Plant, Principles of Operation, Design Parameters, Operational Performance, Moorehead and East Grand Forks Plants," presented at the ASSBT conference, 1981.
10. Young, R.A., and Radosevich, G.E.; Economic and Institutional Analysis of Colorado Water Quality Management, Completion Report Series No. 61, OWRR Project No. B-042-Colorado, submitted to Office of Water Research and Technology, US Dept, of Interior, Environmental Resources Center, CSU,
Ft. Collins, March, 1975, p. 13.

12. Zwick, David and Benstock, Marcy; Water Wasteland, Grossman, New York, 1971, p.55.
13. Ridgeway, p.48
14. Ibid, p.51
15. Energy Conservation and Air/Water Planning Processes, Energy Consciousness Planning Project, Office of Energy Conservation, State of Colorado, Colorado Office of Energy Conservation, July, 1980, p.3.
16. Ibid, p.14.
17. Manual, p. 1-1,
18. Environmental Quarterly, 10th Annual Report of the Council on Environmental Quarterly, Dec. 1979, p 75.
19. Matter, Fred; Personnal Communication, Colorado Department of Health, Enforcement Division Director, July 17, 1981.
20. Ridgeway, James; The Politics of Ecology.
21. Energy Conservation and Air/Water Planning Processes, Energy Consciousness Planning Project, Office of Energy Conservation, State of Colorado,
Colorado Office of Energy Conservation, July, 1980, p 15.
22. Personal Communication, Fred Matter, Director of Enforcements Division, Water Quality Office, Colorado Department of Health.
23. Personal Communication, supervisor, Marshall Street Wastewater Treatment Plant, Arvada, Colorado.
24. Rodgers, Eric A., & Wong, A.L.; "Economic Assessment of Digester Gas Utilization", Proceedings of the USDOE Energy Optimization of
Water and Wastewater Management for Municipal and Industrial Applications Conference, Vol. 1, Dec., 1979, Argonne National Laboratory, Aug., 1980, p. 139.
25. Ridgeway, p.73,

26. Rodgers, p. 141.
27. Ibid, p.144.
28. Ibid, p. 148.
29. Bio-Energy Directory, Bio-Energy Council, Washintgon, D.C. 1979, p. 246.
30. Proceedings of the USDOE, Vol I, p.515.
31. Gonzales, John G.M.; Auckly, Chester W.; and Davis, Eugene; Energy Considerations in Sludge Management At The Reno-Sparks Wastewater Treatment Plant, Vol. II, p.257.
32. Srinivasaraghavan, R.; Reh, C.W.; Sullivan, T.J.; Wall, S.E.;
Energy Recovery and Reuse at Yonkers Wastewater Treatment Plant,
Vol. II, p.291.
33. Brower, George R,, Sc.D., P.E., and Priddy, Charles F., Jr., P.E., Envirodyne Engineers, Inc., and Dice, B. Eugene, Mechanical Engineer; Anaerobic Digester Gas Utilization At Knoxville's New Advanced Wastewater Treatment Facility., Vol II, p. 323.
34. Faisst, William K., and Smith, James G., Private-Public Cooperation for Effective Energy Conservation, Vol. II, p.486.
35. Schwartz, Leander J., Kelner, Thomas A., and DeBaere, Luc A., University of Wisconsin; Anaerobic Digestion of a Wastewater Treatment Plant Sidestream; Vol. I, p. 235.
36. Koon, John H., and Davis, Gary M., Associated Water and Air Resources Engineers, Inc., and Genung, Richard K., and Pitt, W. Wilson, Oak Ridge National Laboratory; The Feasibility of An Anaerobic Upflow Fixed-Film Process for Treating Small Sewage Flows; Vol. I, p. 193.
37. Firedman, A.A., Syracuse University and Tait, S.J., Sr., International Paper Company; Wastewater Treatment and Energy Recovery with Anaerobic Rotating Biological Contractors, Vol I, p. 179.

38. Perron, C.H., "Refuse Conversion to Methane", Energy from Biomass and Wastes, Symposium Papers, January, 1980, Institute of Gas Technology, Chicago, Illinois, May, 1980.
39. Biggers, Mark A., and Uhler, Robert B. and Reithmayr, Kurt 0.,
Energy Conservation At An Advanced Secondary Treatment Plant,
Vol. II, p.299.
40. Shite, Peregoine, Jr., "A Survey of Industries Concerns on Energy
and Wastewater Treatment", Proceedings of the USDOE Energy Optimization of Water and Wastewater Management for Municipal and Industrial Applications Conference, Vol I, p.397.
41. Ibid, p. 398.
42. Ibid, p. 402.
43. Ibid, p. 403.
44. Royston, Michael G. , Pollution Prevention Pays, Pergamon Press, 1979,
p. xiv.
45. Feder, Barnaby, J., "War on Waste with Microbes", The New York Times,
August 20, 1981.
46. "Mountain View Landfill Gas Recovery Project", Pacific Gas and Electric
Co., EPA Grant No. S-803390, March, 1981.
47. Schellenbach, SuSan; "Imperial Valley Biogas Project", Energy from Biomass and Wastes IV, Symposium Papers, January, 1980, Institute of Gas Technology, Chicago, Illinois, May, 1980.
48. Switzenbaum, M.S., Danskin, S.C., and Nadas, D., "Methane Generation from Whey for Energy Production and Pollution Control", Proceedings of the USDOE, Vol I, p. 417.

Current Sewage Treatment Practices and Biogas Potential in the Denver Metropolitan Area
MDSDD#1 is the largest sewage treatment facility in the Denver metropolitan area because it processes the discharges of 21 different small area districts. Consequently, it has the highest volume of organic solids and the greatest single gas production potential of any existing local plant. For the purposes of this investigation, it represents a centralized model of sewage gas or energy production. Various industries in the Denver area have organic wastes. They represent examples where decentralized sewage treatment facilities could be constructed for gas production. The two models must coexist because the decentralized model treats only a portion of the area sewage. Both will be described to predict the impact of supplementing the centralized model with instances of decentralized waste treatment. This chapter will develop gas production projections for the centralized model based on published data of population and wasteload increases and for the decentralized model from records of industrial sewage strength for four sample industries and laboratory studies of gas yields. Gas production will be calculated according to the three methods presented in the previous chapter.
First, the history of MDSDD#1 will be outlined to show how the sewage in the metropolitan area is collected centrally. This provides the background information for applying the different types of sewage district charges to the examples of waste reduction in the economic discussions in later chapters. Second, the treatment process at MDSDD#1 is described to verify the use of anaerobic digesters, the kind of waste treated in them and the gas yield. Third, populations and wasteloads for Denver to the year 2004 are documented. Gas projections are reported. Fourth, wasteloads from Denver's industries are discussed, and examples of four companies are used to project daily methane productions.

A. Background: The Formation of Sewage Districts in the Denver Metropolitan Area The South Platte River is 459 miles long from its point of origin to its confluence with the North Platte River in Nebraska. The river and its tributaries, the major ones listed in Table 11-1,1 drain the Denver, Colorado metropolitan area and is known as the South Platte River Basin.
Major Tributaries to the South Platte River in the Denver Metropolitan Area
Tributary Name River Mile Annual Discharge in acre feet
Bear Creek 320.9 24,470
Cherry Creek 312.3 11,800
Sand Creek 306.8 5,000
Clear Creek 305.5 64,200
The area is characterized geologically by sand and gravel and a shallow aquifer. Ground water is located from 1 to 65 feet below the land surface.
The ground water flows into the creeks and river and, conversely, some of the creek and river water recharges the aquifer. The interrelationship of the area water drainage patterns increases the importance of developing a sound sewage management program.
In 1960, the Colorado State Legislature passed an act, sponsored by the Joint Sanitation Commission, which was composed of a number of interested districts, municipalities and the Inter-County Regional Planning Commission.

The legislation permitted the formation of a Metropolitan Sewage Disposal District
in any metropolitan area in Colorado. This provided for the organization of the
Metropolitan Denver Sewage Disposal District #1 (MDSDD#1). An engineering study
was performed by a group of selected consultants, Denver Metro Engineers, with
representatives from three different consulting firms. They recommended and
subsequently designed a plant to provide primary and secondary treatment, and sludge
disposal. The then existing Denver Northside plant (DNS), which treated most of the
metropolitan Denver area, would supply only primary treatment and anaerobic digestion
of primary sludge and send the wastes to the new plant for secondary treatment and
sludge disposal. The new facility would provide secondary treatment for plants
which provided only primary treatment and would completely replace some district
plants at capacity. The initial capacity was designed to treat 27 mgd for primary
and 117 mgd for secondary treatment for 46 different municipalities and sanitary
districts. The plant was completed and received sewage in December, 1966. The location of MDSDD#1 is indicated in Figure II-1.
Further investigation into the sources of pollution and control of water quality of the South Platte was instigated by the Governor of Colorado. On October 29, 1963 the first session of a Conference under the provisions of the Federal Water Control Act convened and established a study to be undertaken by the Public Health Department. The goals and objectives of the Project were to inventory legitimate water users and sources of pollution in order to make recommendations for pollution abatement. The study identified 100 separate entities directly involved in the collection and treatment of sewage in the front range area. Two-thirds of the sewage was found to pass through the Denver Northside plant and the remaining third through 25 other facilities. The treatment plants reduced the BOD by 50% for 95% of the area population. The wastes of the remaining 5% of the population were treated by individual septic tanks. Table 11-2 lists the facilities,

T T_/l

and Figure 11-1 shows the locations of the plants. The study was completed in December, 1965.
From 1974 to 1976, the MDSDD#1 was expanded and anaerobic digesters were added, designed by CH^M Hill, a private engineering and consulting firm. From 1977 to 1979, Black and Veatch, consulting engineers, completed Phase I of a two phase study, which is still in progress, examining the alternatives for increased water quality in Denver and the South Platte river.^ They utilized three previous CH^M Hill studies of adjacent planning areas. The earlier studies compared use of a centralized plant with construction of satellite facilities. The former was found to be more
economically feasible than the latter. The four planning areas are:
1) Lower South Platte
2) Sand Creek
3) Clear Creek
4) Central Denver
Figure 11-2 shows the boundaries of the areas and the locations of the MDSDD#1 and DNS facilities. Phase II, not yet completed, will incorporate public comment and describe the final plans for the selected alternatives. The overall planning area includes five Section 201 plans, from P.L. 92-500 or the Clean Water Act, as well as the Central Denver area. Since the regulations stipulate a 20 year planning period and the projected operational date is 1984, the timeframe reaches to 2004. Table 11-3 lists the 21 current member districts and municipalities participating in the central MDSDD#1 facility in 1981.
Each member district charges different rates, a situation resulting from the diverse histories of the districts. Usually, a base rate is set for residential hookups and industrial users are charged for volume of flow and surcharged for the amount of wastes carried above residential as measured by the BOD and suspended solids (SS). The BOD is a measure of how much material is present which will react with biological organisms in a five day test under controlled conditions.

TABLE 11-2
Map Ident No. Name of Facility River Mileage Type Treatment
1. City of Brighton 289.5 Secondary
2. S. Adams Water & San District 301.2 Secondary
3. City of Thornton 303.5 Secondary
4. N. Washington Water & San District 305.5 Secondary
5. Denver Northside 308.8 Primary
6. S. Lakewood San District 314.1/2.1 W Secondary
7. City of Englewood 319.7 Secondary
8. City of Littleton 323.5 Secondary
9. Baker Water & San District 305.5/3.0 Secondary
10. City of Westminster 305.5/3.6/1.6 Primary
11. City of Arvada 305.5/6.2/0.3 Secondary
12. Clear Creek Valley San District 305.5/7.0 Secondary
13. City of Wheatridge 305.5/7.5 Secondary
14. Fruitdale San District 305.5/10.0 Primary
15. N. W. Lakewood San District 305.5/10.2 Primary
16. City of Golden 305.5/15.5 Secondary
17. Denver Eastside 306.8/4.7 Primary
18. City of Aurora (Westerly) 306.8/5.5/1.1 S Secondary
19. City of Aurora (Sand Creek) 306.8/6.8 Secondary
20. Fitzsimons Hospital 306.8/6.9/0.9 Secondary
21. Buckley Air Station 306.8/11.9 Secondary
22. City of Glendale (a) Secondary
23. Colo. State Industrial School for Girls (a) Secondary
24. Federal Correctional Institution 320.9/5.5 Secondary
25. City of Evergreen 320.9/19.3 Secondary
26. Rocky Mountain Arsenal (b) Secondary
(a) Plant discharge to oxidation pond , no effluent.
(b) Discharges to First Creek, thence to Burlington Canal Does
(c) not enter South Platte River. No secondary settling facilities available.

niiMiMii min siiici iismu
mum m i

T T-7

TABLE 11-3
21 Current Member Districts and Municipalities of MDSDD#1
Alameda Water and Sanitation District Applewood Sanitation District City of Arvada City of Aurora
Bancroft/Clover Water and Sanitation District Berkeley Water and Sanitation District Crestview Water and Sanitation District City and County of Denver East Lakewood Sanitation District Fruitdale Sanitation District City of Golden City of Lakewood
North Pecos Water and Sanitation District North Table Mountain Water and Sanitation District North Washington Street Water and Sanitation District Northwest Lakewood Sanitation District .
Pleasant View Water and Sanitation District
City of Thornton
City of Westminster
Westridge Sanitation District
Wheat Ridge Sanitation District

The SS is the amount of solids which remain on a filter of a specified mesh when a sample is drawn through the filtering apparatus. It includes organic and inorganic solids. Both units are common terminology for indicating the extent of foreign, polluting or nutrient (for agricultural irrigation water) material in wastewater. For example, during 1981, the North Washington Sanitation District charged 40<£ per 1000 gallons for flow, and surcharged 8.15 In contrast to this method of billing, MDSDD#1 bills each member district quarterly,
based on total tons of BOD and SS received.
B. Process Flow of the MDSDD#1 Facility
Wastewater enters the MDSDD#1 complex through various interceptors and is metered in Parshall flumes. The water can then be diverted to eithdr the north or the south complex. Each is composed of several processes, listed in Table 11-4 and Table I1-5 and shown in Figure II-3. Both provide preliminary, primary, and secondary treatment, as well as disinfection to the water discharged through the outfalls to the South Platte or to the Burlington Ditch. All treatment in both complexes is either physical or biological. The remaining sludge is pumped to the anaerobic digesters, which are described in more detail in Chapter III.
After digestion, the digester effluent is filtered to thicken it and trucked to a land disposal site where it is tilled into the soil. Future plans include dewatering with a centrifuge and composting to demonstrate use as a fertilizer and
soil amendment.
C. Population and Wasteload Projections
Potential gas production is directly proportional to the amount of organic material in the wastewater stream that is loaded into the digester. Table I1-6 shows the population projections that will generate the waste stream for the area sewage facilities. These are taken from DRCOG's (Denver Regional Council of Governments) "1977 Population and Household Estimates" as reported by Black and

Bar Screens Secondary Clarifiers
Type Mechanically Cleaned Type Center Fe
Number 3 Number 12
Dimensions 4' wide, 1" openings Dimensions 130' diameter b
Hydraulic Capacity, mgd 65
Return Sludqe Pumps
Grit Basins Type Variable S
Type Aerated Basins Number 16
Number 2 Capacity, gpm each 4,000 at 25
Dimensions 30' x 51' x 10' SWD HP, each 30
Hydraulic Capacity, mgd 65 tat 5 minutes detention time)
Waste Sludqe Pumps
Grit Cl ass i f1 cation Type Centrifug
Type Cyclonic Grit Separators Number 8
Number 2 Capacity, gpm each 1,000 at 40
Pump Capacity, gpm each 280 at 90' TDH HP, each 20
Primary Clarifiers . Chlorine Contact Basins
Type Center Feed Number t
Number 4 Dimensions 30' x 250' x
Dimensions 106' diameter by 8.75 SWD Volume, mg 2
Sludqe Pumps Chlor inators
Type Duplex Plunger-type Number i
Number 6 Capacity, Ibs/day each 8,00C
Capacity, gpm total 580
Capacity, Ibs/day each 8,00(
Aeration Basins
Number 12, 3 passes each
Dimensions, each pass 210' x 30' x 15' SWD "Current modifications will allow only 2,000 lbs per
Type Single-stage Centrifugal
Number 4
Capacity, cfm each 35,000

TABLE 11-5
Bar Screens Type Number Dimensions
Hydraulic Capacity, mgd
Grit Basins Type Number Dimensions
Mechanically Cleaned Oxvqen Generation Equipment Type Cryogen i c
3 Number 2
6' wide, 3/4" openings Capacity, tons/day each 55
Gravity Cdetritorl Return Sludqe Pumps Type Number Mixed-flow Centrifugal 6
2 Capacity, gpm each 6,300 at 46' TDH
40' x 40' x 3.25' SWD HP, each 100
Grit Classification
Grit Pump Number Pump Capacity, gpm each
Primary Clari f iers
CycIonic 2 2
200 at 43.3' TDH
Center Feed 4
150' diameter by 10' SWD
Waste Sludge Pumps
Capacity, gpm each HP, each
Secondary Clarifiers
Vertical Nonclog 2
830 at 58' TDH 20
Rlm-feed, Rim-effluent 10
140' diameter by 14' SWD
Primary Sludge Pumps
Capacity, gpm each
Simplex Piston 6
100 at 100' TDH
Chlorine Contact
Type Final Effluent Channel
Dimensions 10' x 10' x 1,700'
Volume, mg 1.3
Primary Effluent Pumps
Each, gpm TotaI, mgd
Aeration Basins Type of Aerators Number Dimensions Aerator Horsepower Fi rst Stage
Second and Third Stage, pe
Vertical Mixed-flow 6
27,500 at 18.9' TDH 238 at 18.9' TDH
Mechanical Surface 8 with 3 stages each 46' x 138' x 17.5' SWD
stage 60
Chlori nators Number
Capacity, Ibs/day each
Capacity, Ibs/day each

Aeration Air Blower Bldg. Administration Bldg.
Control Bldg.
Chlorine Contact Basin Sludge Digester Electrical Substation Grit Basin Laboratory Maintenance Bldg.
Cryogenic Oxygen Plant Outfall North Complex Outfall South Complex Primary Clarifier Personnel Bldg.
Primary Effluent Pumping Sta Bar Screens Secondary Clarifier Sludge Flotation Bldg.
Sludge Holding Tanks Primary Sludge Pumping Sta. Scum pumping Station Sludge Filtration Bldg. Effluent Pump Sta.
Property Limit Sludge Flow
Raw & Treated Sewage Flow

TABLE 11-6
Population Projections for Denver Metropolitan Planning Area Basins
Estimated Population
Basin 1977 1980 1990 2000 2004
Clear Creek 224,500 238,000 276,000 321,000 339,000
Sand Creek 138,700 170,000 253,000 359,000 400,000
Lower South Platte 60,000 61,000 68,000 82,000 88,000
Central Denver 613,800 655,000 712,000 766,000 791,000
Totals: 1,037,000 1,124,000 1,309,000 1,528,000 1,618,000

Table 11-7 lists the wastewater flows projected from 1977 records of DNS and MDSDD#1. They include Infiltration/Inflow, which is ground water that enters the system and is subsequently treated by the plants. The facilities which treated 116 mgd in 1977 will need to process 185 mgd in 2004.^
Primary treatment at DNS removes 35% of the BOD and 55% of the SS before the waste is sent to MDSDD#1 for secondary treatment. Table 11-8 gives the wasteload projections in the wastewater flows from each basin to the central facility. They are based on 1977 data which take into account both populations and land use. TheSe were applied to the population projections and wastewater flows reported above, and land use projections on the basis of DRCOG data and information available from local governmental planning agencies.^
Projected Wasteload to the Central Plant Wasteloads, tons/aay (dry)
Planning Area 1980___________1990__________2000__________2004______
Clear Creek 25.1 27.9 31.1 32.5
Sand Creek* 15.2 19.3 24.6 26.8
Lower South Platte 6.8 7.7 8.8 9.1
Central Denver (DNS) 45.1 49.8 55.2 57.1
Total 92.2 104.7 119.7 155.5
Clear Creek 26.3 29.4 33.0 34.1
Sand Creek* 15.2 19.4 24.7 26.8
Lower South Platte 7.2 8.1 9.3 9.7
Central Denver (DNS) 35.3 39.0 43.2 44.7
Total 84.0 95.9 110.2 115.3
* Includes wastes from Packaging Corporation of America, not a municipal facility but an entity contributing to MDSDD#1.
TT- 14

TABLE 11-7
Wastewater Facility Design Flows (mgd)
_________Tributary Basin__________
Clear Sand South Central
Year Wastewater Type Creek* Creek* Platte* Denver** *** Total
1977 Total (actual) 18 15 9 74 116
1980 Sanitary 22 16 6 68 112
Infiltration/Inflow _3 _2 1 20 26
Total 25 18 J'k'k'k 88 }38
1990 Sanitary 25 21 7 75 128
Infiltration/Inflow _4 _3 I 20 28
Total 29 24 8 95 156
2000 Sanitary 28 29 8 83 148
Infiltration/Inflow _4 _4 I 20 29
Total 2Z 22 1 .171
2004 Sanitary 30 31 8 86 155
Infi ltration/Inflow __4 _5 I 20 30
Total M 22 1 106 185
*Flows taken from other facility plans and revised for constant unit flow rates in time and average I/I.
**Based on maximum I/I.
***Northglenn excluded.
II- 15

These loadings to the plant represent the total raw material the plant receives and the maximum potential for gas production. The primary and secondary processes concentrate the solids into a 5% total solid sludge before feeding to the anaerobic digesters. Sludge is measured by VSS, or the organic portion of the SS. In MDSDD#1 sludge, nearly the entire suspended portion of digester feed is volatile, and the VSS is approximately equivalent to the VS. Inside the anaerobic digesters, the bacteria destroy or reduce the volatiles by making gas.
The consultants and planning engineers have debated the actual gas production rate at length, because of the difficulty of projecting annual production on the basis of a few measurements. Table 11-9 lists the unpublished numbers which are currently accepted.^ They are based on VSS of 75.4% and 49% of the VSS destroyed to produce the gas. This results in a yield of 20.76 cubic feet of biogas per pound of VSS destroyed. Each cubic foot is assumed to have 600 BTU's. This number can also be expressed as 6.10 cubic feet of methane per pound of VSS added, or 6.09 cubic feet in terms of "total VS added" as described in Chapter I, Section B (based on a representative average of 4.19% TS and 75.6% VS).9
TABLE 11-9
Projected Digester Loadings and Gas Production at MDSDD#1
Year Tons VS Loaded per Day Tons Sludge Loaded per Day Million Standard Cubic Feet Biogas per Year Standard Cubic Feet per Hour
1980 74.3 98.7 552 63,000
1985 79.4 105.5 591 67,400
1994 88.9 118.1 661 75,500
2004 100.2 133.1 745 85,100
T T 1 C

This yield has increased from the 1978-1979 data, which was reported at 16.6 scf of biogas per pound of VSS destroyed and is higher than the engineers expect according to industry standards. Volume III of the Central Plant Facility Plan uses the old yield and predicts 47,100 scfh in 1984 and not until 2004 is 63,700 scfh of biogas achieved. However, it should be noted that actual gas production measurement is difficult. The reported rate is a calculation based on metering only the fixed roof digesters. Assumptions have been made that the rates in the other two digesters are equal, although it is apparent that biological activity varies from watching the foaming rates. Furthermore, the meters themselves may be unreliable, and boiler consumption meters have been documented as reading only 63% of the measured flow. Consequently, this unexpectedly high yield is understood to be the best number at this time and will need future confirmation. There is little precedent in the literature with which to compare the yield because most plants digest only primary sludge, and report far lower yields. Only a few plants, such as in Los Angeles and Chicago, like MDSDD#1, digest a mixture of primary and waste activated sludge.
The high yield at MDSDD#1 would indicate that the process is working well biologically.
D. Gas Potential from Anaerobic Digestion of Industrial Wastewater in the
Denver Metropolitan Area.
While officials at MDSDD#1 have done a study on the sources of chemical or potentially toxic industrial contributors to the plant, there is no existing list of Denver industries with strictly organic wastewater streams. However,
Denver is not a highly industrialized city. The 1965 study notes only four
plants which have a significant amount of industrial wastes: Coors (which
treats Golden's waste and will leave the MDSDD#1 system in 1981), North Washington, Clear
Creek Valley and Denver Northside. Table 11-10 lists the extent and nature
of the wastes they treat.^

TABLE 11-10
Estimated Denver Industrial Wastes, 1965 (based on 5 day week)
Number Daily
Plant Industry Type of Large Industries % Industrial BOD Loadinq % Industrial SS Loadinq % BOD Removal % SS Removal Total Flow (mg)
Coors (7 day wk) Brewery 1 91 63 96 89 2.70
Clear Creek Valley Meat Pkg. 1 78 59 67 84 1.49
Denver Northside Misc. 1411 47 47 42 75 72.53
North Washington Meat Pkg. 4 84 55 62 72 1.50

Table 11-10 shows that the largest amount of industrial waste is treated
by the Northside plant, where the industrial 47% of its load is far larger
than all the others combined. However, the single, most common type of
industry in the Denver metropolitan area is meat packing.
Since the number and variety of industries is small, four representative
industries which produce primarily organic and non-toxic wastes will be examined
to explore the potential value of the gas which could be made from the anaerobic
process. The gas production information is summarized in Table 11-11. The BOD
and SS measurements are from sewage district records; the percent TS and VS are
from Bio-Gas of Colorado, Inc. testing laboratory. The methane yields are taken
from literature or Bio-Gas of Colorado studies and are referenced in the footnotes.
The overall economic feasibility will be discussed in Chapter IV.
1. Packaging Manufacturer. This example was selected because it is the only
private customer at MDSDD#1. All others are municipalities or sewage districts
which bill their customers according to their own rates and methods. The
central plant is not aware of the identity of the different industries. However,
the packaging company is a nearby neighbor whose interceptor connects directly
with MDSDD#1 and is individually metered. Furthermore, the technical
feasibility of generating methane from wastes from the pulp and paper plants
has been demonstrated.
In 1980, the annual flow of wastes from the packaging company was 156.83 million gallons, or an average of 429,000 gallons per day. The SS was 1063.13 tons or 5825 pounds per day and the BOD was 558.62 tons or 3,060 pounds per day. From a sample taken in October, 1981, the average total solids was .36% and the average total VS was 50.09% (dry basison a reported daily flow of 350,000 gallons per day. Assuming the wastes are comparable to the cellulose fines listed in Table I-1, each pound of volatile solids added to a digester per day will produce 4.4 cubic feet of methane per day. The gross daily methane production available is 23,448 cubic feet. Because the wastes

TABLE 11-11
Daily Waste Characteristics and Gas Potential
Industry Gallons 1981 1980 lbs BOD 1980 lbs SS
Packaging 350,00015 3,060 5,825
Candy Manufacturing 29,00016 302 15
Meat Packing 175,80023 1,970 1,062
Cheese Processing r1 CO o o o n r^ c,23, 32,530 29 2,418
1981 lbs TS 1981 lbs VS % TS or BOD Destruction ft3 ft3CH4 CH4 Production Yield Net
10,622 5,329 (TS) 91%14 4.4a14 22,275 a
(BOD) 80%19 8b'19 l,933b
(BOD) 75%26 5.72b26 8,451b
38,490 35,025 (TS) 93%30 5.6 a30 186,000a
per pound VS added
per pound of BOD destroyed
Assumes BOD is 66% of COD

are warm, only 5% will be used for process heat on an annual average, and 22,275 cubic feet is the net daily methane production.
2. Candy Manufacturing. The production of candy is a year around process with three seasonal peaks near holidays. The wastes consist of dissolved sugars, flavorings, dyes and human wastes.1^ This Denver company is located in a suburban sewage district which joined MDSDD#1 rather than expand its own facilities, and it merely maintains administrative offices for billing
and retaining of a consulting engineering firm to supply technical inspections,
testing and interpretation. MDSDD#1 does all the actual treatment.17 In
1980 on a 6 day work week, the company contributed 14,775 gallons per
day with 302 pounds of BOD and 15 pounds of SS. Because most of the organic material in these wastes is dissolved, the food will be readily available to the bacteria.
Anaerobic digestion of sugar wastes has been employed by the American
Crystal Sugar Company in the Red River Valley of North Dakota and Minnesota
since 1977. Digestion takes place in a 6 to 7 million gallon tank using a
Swedish process called ANAMET. A paper presented in 1981 reports the
bacteria have been able to destroy 80% of the BOD, yielding approximately
8 cubic feet of methane per pound of BOD reduced. Destructions up to
90% have been achieved. Using the more conservative 80% figure and the 302 pounds of BOD reported in 1980, a methane production of 1933 cubic feet per day could be attained on the Denver candy manufacturer's waste stream by application of presently developed technology.
3. Meat Packing. There are three types of meat packing facilities:
slaughter houses which kill and dress the carcasses, meat processing plants
which do no killing but process parts or the waste materials, and meat
packing plants which both kill and process the meat. The wastestream contains many different types of material, including manure, paunch manure (the

contents of the intestinal tract), blood and miscellaneous organic tissue.
Some of the waste has economic value and is recovered, for instance, as cooking
oil or animal feed. Packing houses have various types of recovery equipment
or wastewater treatment facilities. Primary treatment, lagooning, aerobic
and anaerobic processing are practiced. While Denver has all three kinds
of packing plants, most are located in the North Washington Sanitation
District and are in the older, industrial parts of the city where there
is not room for space intensive forms of treatment.
The example analyzed in this study is a by-products plant which has
a waste flow of 175,800 gallons per day averaged over 30 days. The BOD
content is 1345 mg/1 and the SS is 725 mg/1, or 1970 pounds of BOD per
day and 1062 pounds of SS per day. Packing industry wastes have been
24 25
a topic of investigation since the 1950s. In a more recent report for the
federally funded Solar Energy Research Institute in Golden, Colorado, a
research and development corporation reported from literature and laboratory
work that a yield of 5.72 cubic feet of methane could be obtained for each
pound of BOD destroyed. Furthermore, a destruction rate of 75l of the 26
BOD was stated. Using these rates, a daily methane production of 8,450
cubic feet of methane could be produced. Although this volume of gas can
be projected, the capital costs to process 175,800 gallons per day could
be excessive. Consequently, some compromise between recovering all the
gas and the cost of equipment will be necessary. This problem will be
dealt with in Chapter IV when the economic value is assessed.
4. Cheese Producer. Denver does not have a large cheese manufacturing
company but does have the research division for a company which has sizeable
production units in other parts of the country. When milk is coagulated ,
to make cheese, a liquid or whey with 5% to 6% dissolved solids remains.
Cheese whey has been investigated for many kinds of economic return,
27 28 29
including methane production. * * The cheese research division in

Denver does not have a consistent or large enough waste stream to use for
energy production, but tests performed on samples have indicated excellent 30
potential. Because the methane yield numbers are exact for this wastestream,
they will be applied in this example to a large plant owned by the same
company but actually sited in a populous area of California near San Francisco.
This plant produces a similar whey and disposes of fifteen 50,000 pound
truckloads of whey per production day (average six day per week) in a
leach field for a cost of $10 per truckload. This is 77,000 gallons per
day on a seven day basis, and using Table I-1, the 6% solids and 91%
volatile solids could yield 196,000 gross cubic feet of methane. A part of this large volume could be used to produce electricity and offset two kinds of energy consumed by the plant. Future plans are uncertain, except that in an area growing in suburban population, heavy loads of wastes will become difficult to place in land disposal sites. In California where the electrical costs are high, the incentive to produce electricity is even higher than in Denver.
The Denver metropolitan area is composed of a complex membership of sewage districts, each of which may charge different rates to its customers. MDSDD#1 collects the discharges of 21 member districts and uses anaerobic digestion as one method of treatment. Denver's population and volume of sewage is projected to increase significantly by the year 2004. The gas production rate of 63,000 cubic feet per hour at MDSDD#1, already higher than expected, will increase proportionately. This illustrates gas production at a centralized sewage collection facility. Denver does not have a large amount of organic waste from industrial contributors. A 1965 study gives only four sewage districts with significant industrial customers. DNS, the only large district, collects 47% of its flow from industries. However, individual industries

with primarily organic wastestreams can be selected to assess the private or decentralized anaerobic treatment of their current contribution to the sewage districts. Four examples and their projected methane production are a packaging manufacturer, 22.275 MCF per day; a candy manufacturer, 1.933 MCF per day; a meat packer, 8.451 MCF per day; and a cheese processor, 186.0 MCF per day.

1. Municipal Waste Report Metropolitan Denver Area South Platte River Basin, U.S. Dept, of Health Education and Welfare, Public Health Service, Denver, Colorado, Dec., 1965.
2. Ibid, p. 4-5.
3. Ibid, p. 2-4
4. Central Plant Facility Plan* Phase I Report, Black & Veatch, Denver, Colorado,
June, 1979, p. 2-2 and 2-3.
5. Personal Communication, Ray McNeill, Planning Engineer, MDSDD#1,
Oct., 1, 1981.
6. Central Plant, Vol. I,. p. 2-13.
7. Ibid, p. 2-3, p. 2-16, p. 2-17.
8. Personal Communication, McNeill.
9. Calculation: 49% of total VSS added 3 or 1 lb was destroyed to make 20.76 ft :
total VSS added = 2.04 lbs. VSS/20.76 ft^; 20.76 ft3 600 BTU biogas produced
for each 1 lb VSS destroyed or each 2.04 lbs VSS added; 1000 BTU methane
. . 20.76 ft3 bioqas 600 BTU's , ____ 3
yield = 2.0'4nbs_V'SS added' xToOO' BTCPT" or 6*10 ft methane is produced
for each pound of VSS loaded into the anaerobic digesters. Rates from
10. Municipal Waste Report, p. 16.
11. Personal Communication, Moe Tabatabai, Chief Engineer Operations; plants with high BODs.
12. Takeshita, N., Fugimura, E., Minota, N., "Energy Recovery By Methane Fermentation of Pulp Mill Waste Water Sludges.," Pulp and Paper Canada, .
Vol. 82, No. 5, May 1981, p, 99-103.
13. Records, MDSDD#1, Stephen Pearlman, MDSDD#1 Connector Service Chemist.

14. -. Unpublished laboratory report, Bio-Gas of Colorado, Arvada, Colorado,
Oct., 1981, and Robert Seely, "Paper Manufacturing Waste Materials",
Bio-Gas of Colorado, Dec., 1980.
15. Personal Communication, Dan Kozloski, Water Chemist, Packaging Corp. of America, Oct., 1981.
16. Personal Communication, Keith Krickbaum, Jolly Rancher, Nov., 1981.
17. Personal Communication, Sandy Stenson, Fruitdale Sanitation District,
Oct., 1981.
18. Personal Communication, Krickbaum.
19. Fordyce, I.V., L. C. Benedict, W.D. Ransdell, H. Sointer, "The Anamet Waste Water Treatment Plant, Principles of Operation, Design Parameters, Operational Performance, Moorehead and East Grand Forks Plants," presented at the ASSBT conference, 1981.
20. Sell, N.J., Industrial Pollution Control: Issues and Techniques, Environmental Engineering Series, Van Nostrand Reinhold Co., 1981, p. 262.
21. Beefland International, Inc., "Elimination of Water Pollution by Treatment of Packinghouse Animal Paunch and Blood," EPA Project No. 12060 FDS,
Council Bluffs, Iowa, Nov., 1981.
22. Sell, p. 269.
23. Personal Communication, Jim Jamsey, North Washington Sanitation District,
Nov., 1981.
24. Schroepfer, G.J., Fullen, W.J., Johnson, A.S., Ziemke, N.R., and Anderson, J.J., "The Anaerobic Process as Applied to Packinghouse Waste," Sewage and Industrial Wastes, 1955, 27:460-486.
25. Hemens, J. and D.C. Shurbin, "Anaerobic Digestion of Waste Waters from Slaughter Houses," Food Trade Review, 1959, 29:7.
26. Ashare, E., A. P. Leuschner, C.E. West, B. Langton, Assessment of Secondary Residues, Final Report, Contract No. EG-77-C-01-4042, Solar Energy Research Institute, SERI/TR-98175-2, March, 1981, p. 4-8.
T T_9C

27. Switzenbaum, M.S., Danskin, S.C., and Nadas, D., "Methane Generation from Whey for Energy Production and Pollution Control," Proceedings of the USDOE, Vol I, p. 417.
28. Ashare, E., p. 6-1 to 6-26.
29. Modler, H.W., "Wiping Out Our Whey Woes," presented at the American Cultured Dairy Products Institute, San Antonio, Texas, March 23-25, 1981.
30. Seely, Robert, "Biogas Production from the Anaerobic Digestion of Cheese Whey," Oct., 1981.

The Use of Sewage Gas from Denver's Centralized Waste Treatment Facility
Currently, MDSDD#1 is generating approximately 63,000 cubic feet of biogas per hour. Only fifteen percent of this is utilized to heat the anaerobic digesters.
The remainder is flared. The waste gas has been considered an energy source which could be put to use. A planning team has been considering the possibilities. This chapter describes the alternative plans as reported by the consulting engineering firm employed by MDSDD#1 and as updated by the MDSDD#1 staff. This shows how the process of formulating a gas use plan has advanced, how decisions have been made to direct the process, how one particular plan appears feasible, and how some difficulties arise in instituting the plan. In a later chapter, the problems described here will help to specify criteria for evaluating both the centralized gas use plan and the decentralized plan.
First, the equipment and operation is described to give a clear idea how the gas is produced. Second, the criteria selected by the planning team is listed and the four alternative plans considered are presented. Finally, the planning team's analysis and decision in selecting the best alternative is stated and discussed.
A. Present Operation of the Anaerobic Digestion System and Gas Use
1. Equipment and Process Flow. From 1976 to 1978, MDSDD#1 constructed eight anaerobic digestion tanks, each 100 feet in diameter and 31 feet tall.
Six are constructed with concrete fixed roofs and two with steel roofs. As gas is generated, the movable steel roofs rise, allowing some gas to be stored under some pressure. The capacity of each digester is 243,500 cubic feet or 1.8 million gallons. Each digester is stirred by a 25 hp

gas compressor, with an output of 200 scfm at 18 psi, which recirculates the digester gas. Heating is provided by hot water heat exchangers and mixing the incoming cold sludge with hot digester sludge.*
The sludge which goes to the digesters is a combination of all primary and secondary sludges generated at the central plant. Seventy percent is waste activated sludge. The sludges have undergone extensive aerobic treatment,
reducing the BOD and SS by 35% to 50%. The sludge fed to the digesters is
. 2 approximately 5% total solids and 72% volatile solids. The anaerobic process
destroys 49% of the volatile SS fed, producing 20.76 cubic feet of biogas with
600 BTU's per cubic foot for each pound VSS destroyed.
2. Operation. The digesters have been difficult to operate at their design point because excessive foaming interferes with the gas removal line.
The plant staff is not able to define why the foaming takes place since no patterns are apparent and it occurs in different digesters at unpredictable times. The mixture of organic compounds causes interactions that are not understood, but the problem is controlled in several ways. The operation level of each digester has been reduced 3 feet and the loading rate hasf
been lowered to .05 pounds of volatile solids per cubic foot of digester.
Foam is removed from the top of the digesters when necessary. A thermophilic temperature range (140F) was attempted, but the situation did not improve. Operation has been returned to mesophilic temperature (95F). Phase II plans include modifications of the digesters to correct these problems. The flares and gas lines will be rebuilt, and digester covers will be insulated to improve heating and conserve energy. The mixing system will be converted from gas recirculation to mechanical agitation. With design and operational
changes, the digesters are expected to handle the projected needs of the plant.
3. Gas Use. The gas produced by the digesters is used to heat the incoming sludge to keep the process at the optimum temperature. From October 1978 to September 1979, 50 million standard cubic feet of gas were consumed, although

only half the sludge in the entire plant was treated in the digesters.
Twice the amount of gas would have been used if the digesters had operated continuously and at full capacity. Fifteen percent of the gas is used and the remainder flared. The cost of gas use is virtually free, because the only equipment required is a 7.5 hp compressor.6
The plant purchased 35 million cubic feet of natural gas in 1979 for space heating and deodorizing gas. This represents only 4.8% of the total energy use at the facility, the majority being electricity. However, in 1980 the deodorizing equipment was changed to operate on digester gas, reducing the natural gas demand by 47%. The price of natural gas has increased several times in the last few years. The current price is $2.80 per million BTU's (Jan., 1981) and it is not expected to decrease.^
B. Centralized Gas Utilization Alternatives
Extensive analysis of different plans for digester gas utilization at
MDSDD#1 and DNS was performed by Black and Veatch. Five criteria were established which must be met by each alternative. Each alternative had an A and B case, where A considers gas from MDSDD#1 only and B from both MDSDD#1 and DNS. Because gas production figures have been revised upwards since the Black and Veatch study, production and benefits in all cases would be higher than reported here.
1. The plan must be designed to utilize or accommodate all gas generated.
2. The plan must provide for the digester heating requirements.
3. The plan must provide direct benefits to the plant operating cost.
4. The plan must have enough flexibility in the utilization rate to permit reasonable matching with the gas generation rate.
5. The plan must be possible to implement in a reasonable time period, with a reasonable amount of risk, and without adversely affecting plant operations during the implementation period.

1. Alternative 1. All the gas would be used to generate electricity,
and waste heat from the engines would be recovered to provide digester process temperature. The electricity would be used on-site and reduce the annual purchased amount. This is shown in a block diagram in Figure 111-1. The major equipment needed is four 1200 kw generators, a heat recovery system for engine heat, gas cleaning and storage equipment, and electrical switching gear. The amount of electricity generated is listed in Table III-l and the costs in Table 111-2.
Average Electrical Generation For Full Generation Alternative
Alternative 1A Alternative IB
Year Annual Annual
kwh kwh
1984 23,700,000 30,500,000
1994 28,500,000 34,800,000
2004 33,300,000 39,600,000
Construction Costs For Full Generation
Alternative 1A Alternative IB
Engine Generators $ 2,593,000 $ 3,242,000
Heat Recovery 368,000 460,000
Gas Cleaning and Storage 1,905,000 1,905,000
Electrical Equipment and Distribution 488,000 540,000
Structures 995,000 1,244,000
Pipeline and Compressor Facility 349,000*
TOTAL $ 6,349,000 $ 7,740,000
*Add $550,000 if a new gas transport line is needed.


2. Alternative 2. All the digester gas would be used as fuel for
engines driving aeration blowers. As in Alternative 1, heat would be recovered for the digestion process. Figure 111-2 is a block diagram of the alternative. The major equipment required is engine driven blowers, a heat recovery system and gas cleaning and storage equipment.
The amount of power available is listed in Table 111-3 and the costs in Table III-4.
TABLE 111-3
Annual Power Available For Engine Driven Blowers
Alternative 2A Horsepower
37.800.000 44/700,000
Alternative 2B Horsepower
TABLE 111-4
Construction Costs For Engine Drives
Alternative 2A Alternative 2B
Engines $ 1,900,000 $ 2,375,000
Blowers 1,134,000 1,417,000
Heat Recovery 443,000 554,000
Gas Cleaning and Storage 1,905,000 1,905,000
Cleaned Gas Transport Pi peline 77,000 77,000
Structures 875,000 1,094,000
Pipeline and Compressor Facility 349,000*
TOTAL $ 6,334,000 $ 7,771,000
*Add $550,000 if a new gas transport line is needed.

FIGURE 111-2

3. Alternative 3. Digester gas would be used to heat the anaerobic
process and the remainder would be sold. Three neighbors were contacted to determine the market interest. Between Public Service of Colorado and two refineries, Conoco and Asamera, the former could use the gas to replace coal at 75 TABLE 111-5
Annual Surplus Gas Available For Sale Alternative 3A________ _____________Alternative 3B
1984 151,900,000 91,100,000,000 240,500,000 144,000,000,000
1994 188,600,000 113,200,000,000 292,600,000 175,000,000,000
2004 235,100,000 141,100,000,000 330,800,000 198,000,000,000
Construction Costs For Sale Of Gas
Alternative 3A Alternative 3B
Pipeline $ 226,000 $ 575,000
Compressors, Valves, and Meteri ng 175,000 330,000
Structure 75,000 150,000
TOTAL $ 476,000 $ 1,055,000

FIGURE 111-3
T T T n

4. Alternative 4. Digester gas would be used to heat the anaerobic process and only the excess would be used in generators for electrical production. This alternative is shown in Figure III-4. Major equipment is three 700 kilowatt generators, gas cleaning and storage equipment and electrical switching gear. The amount of electricity generated is listed in Table 111-7 and the costs in Table 111-8.
TABLE 111-7
Average Electrical Generation For Limited Generation Alternative
Alternative 4A Annua I
Alternative 4B Annua I
1984 9,200,000 1994 11,400,000 2004 14,100,000
TABLE 111-8
Construction Costs For Limited Generation
Alternative 4A AI Iternative 4B
Engine Generator $ 1,138,000 $ 1,517,000
Gas Cleaning and Storage 1,587,000 1,587,000
Electrical Equipment and Distribution 192,000 192,000
Structures 450,000 600,000
Pipeline and Compressor Faci1ity TOTAL 349,000*
$ 3,367,000 $ 4,245,000
*Add $550,000 if a new gas transport line is needed.

FIGURE 111-4

C. Evaluation of the Four Alternatives
1. The Analysis. The Black and Veatch study performs an economic analysis
by capital costs and net annual costs. The four alternatives are compared
by present worth cost and rate of return. The rate of return is defined as
"the percent of capital returned on an equal annual payment from benefits
received over the twenty year study period." The "benefits" taken into consideration are reduced electrical costs or gas sale revenue, not environmental or social benefits. The presentation may have taken this form because of the extensive previous analyses which discussed social and environmental impacts.
A second reason is the generally accepted understanding of the inherently positive impact of this project waste gas utilization which conserves natural resources and concerns an industry sewage treatment which enhances the environment for social health and welfare. The report explains why the rate of return is used as a basis for selection by stating:
The annual cost analysis illustrates the flow of funds for each alternative. Because different levels of capital investment are needed for the different alternatives, the benefits resulting from the project must be related in some way to its capital requirement.
The rate of return on investment provides this relationship.
The report does not continue to compare the alternatives through the description or discussion of any other benefits, nor does it state that all the other benefits for the four alternatives are equal.
Table 111-9 lists the rates of return for cases A and B of all four alternatives. They are calculated with and without energy inflation rates after 1984, and they are adjusted for construction contingency, escalation for inflation at 9%, interest during construction, and miscellaneous costs for engineering, overhead and financial and legal services. This gives a realistic view of the extent of the investment and equates each alternative with the others to illustrate the cost/economic benefits of each fairly.

Full Generation Engine Drives Sale of Gas Ltd. Generation
Alternative Alternative Alternative Alternat Ive
Item 1A IB 2A 2B 3A 3B 4A 4B
Present Worth of Benefits S( 16,418,700) $(20,252,900) $(16,418,700) $(20,252,900) $(3,624,100) $(5,497,200) $(6,570,000) $ (9,992,600)
Present Worth of O&M Costs 5,201,200 6,609,100 5,213,000 6,598,400 1,130,200 1,794,200 2,519,100 3,620,000
TOTAL CAPITAL COST 10,543,000 12,853,000 10.518,000 12,906,000 789.400 1,752,000 5,591.000 7.052.000
NET PRESENT WORTH $ (674,500) $ (790,800) $ (687,700) $ (748,500) $(1,704,500) $(1,951,000) $ 1,540,100 $ 679,400
Rate of Return 0.6? 0.6$ 0.6$ 0.5$ 20.1$ 10.4$ -2.5$ -0.8$
Present Worth of Benefits $(16,646,000) $(21,422,100) $(16,646,000) $(21,422,100) $(4,183,800) $(6,607,600) $(6,461,800) $(10,046,300)
Present Worth of O&M Costs 5,235,300 6,673,500 5,247,100 6,662,700 1,138,300 1,830,900 2,531,100 3,645,100
TOTAL CAPITAL COST 10,543,000 12,853,000 10,518,000 12,906,000 789,400 1,752,000 5.591.000 7,052,000
NET PRESENT WORTH $ (867,700) $ (1,895,600) $ (880,900) $ (1,853,400) $(2,256,100) $(3,024,700) $ 1,660,300 $ 650,800
Rate of Return 0.8$ 1.4$ 0.8$ 1.3$ 26.6 $ 16.1$ -2.7$ -0.8$
A Central Plant gas only.
B Central Plant gas and Northside Plant surplus gas

2. The Selection. Alternative 3 for the sale of the gas, which also could
be placed in service earliest, is the clearly best choice on an economic basis. The return on cases 3A and 3B ranged from 10.4% to 26.6%, while the other alternatives all fell below 1.5% or were negative.^ Alternative 3 requires a low capital investment, one-half to $1 million, instead of $3 to 4 million for Alternative 4 or $6 to 8 million for Alternatives 1 and 2. The difference in the costs is that no large equipment, such as generator sets, engines and gas storage, must be purchased. Consequently, the subsequent operation, maintenance and management requires a significantly smaller budget amount.
This is an important consideration. The more expensive alternatives force MDSDD#1 into the utility generation field, an entirely different business than sewage treatment. It is simpler as well as less expensive to transfer the gas to a company which has the staff and equipment to handle it.
A decision has not been made between the 3A and 3B Alternatives.
Alternative A utilizes the gas from MDSDD#1 only at a cost of $476,000 and a return on investment of 26.6%, while Alternative 3B takes the gas from both MDSDD#1 and DNS at a cost of $1,055,000 and a rate of return of 16.1%.
The major portion of increase in the cost is the gas pipeline from DNS to MDSDD#1. Two pipelines are already in place for sludge and primary treated wastes. A plan to use the sludge line for transmitting gas is being evaluated. This would reduce flexibility in the scheduling of the DNS plant operation.
If this plan is acceptable, the cost for Alternative 3B may decrease and
the return on investment increase enough to make 3B competitive with 3A.
Other ideas for gas use at DNS have been discussed informally. Presently, the DNS plant burns 30% of the digester gas generated to heat the process and to incinerate the screenings collected from the first steps of the primary treatment. Electrical generation does not fit the plant since its electrical consumption to perform only primary treatment is not large enough to pay for expensive generators in a reasonable amount of time. Furthermore,

there are no energy consuming neighbors such as MDSDD#1 has at the location of DNS. Consequently, the dhoices now are between continued flaring or transmission to MDSDD#1. A less defined choice is to invite an entrepreneur with appropriate assets, ability and equipment to initiate a small business or generating station. While this has been suggested to the management at DNS, a plan of action has not been formulated at this time.
The decision between alternatives will be made jointly by the Boards of Directors of MDSDD#1 and DNS. This creates a political atmosphere for the decision making process. Discussions are underway, as they have been for years, to consolidate the two plant managements. This slows the decision but if it occurs, may simplify the process. ^
Presently, the details of a contract with either of the oil refineries are being investigated. One difficulty is the requirement of the refineries to have an air discharge permit to burn the uncleaned gas. The District, a publically owned service, is allowed to flare the gas, but the privately owned refineries are subject to government controls which regulate their air discharges. In actuality, the gas is merely being moved from one burner to another and the impact on the air quality would be the same. However, bureaucratic machinery must be employed to either transfer the District permit or alter the refinery permit.
MDSDD#1 has operated eight anaerobic digesters since 1977. Excessive foaming has caused mechanical problems, but modifications are planned to control the foaming and enable the equipment to handle the projected needs of the plant. Fifteen percent of the gas is currently used to maintain process temperature, and the remainder is flared. A study has been performed to develop a proposal to use the excess gas. Four alternative plans were devised which fit a group

of five criteria set by the planning team. The criteria state that the plan must utilize all the gas, provide digester heating requirements, directly benefit the plant operating costs, be flexible enough to match gas use rate with gas generation rate, and be implemented in reasonable time with reasonable risk to plant operations. Alternative 3, sale of the excess gas to a neighbor was selected on the basis of speed of implementation and economic benefit. The rate of return for Alternative 3 was 10.4% to 26.6% while the other plans were below 1.5% or negative. No social or environmental considerations were included in the criteria or in the selection process. A decision has not been made at this time about the use of DNS gas in the same plan. Problems with implementing the
plan are the complication of two governing bodies (DNS and MDSDD#1) having the
responsibility of the decision and the difficulty of the neighbor obtaining an air discharge permit.

1. Phase I: Central Plant Facility Plan, Vol. I, Black & Veatch,
Metropolitan Denver Sewage Disposal District #1, Denver, Colorado, June, 1979, p. 419.
2. Phase I: Central Plant Facility Plan, Energy Conservation and Gas Utilization, Vol. Ill, Black and Veatch, Denver, Colorado, Jan. 1981, p. 4-2.
3. Personal Communication, Ray McNeill, MDSDD#1 Planning Engineer.
4. Phase I: Vol. Ill, p. 4-19.
5. Personal Communication, McNeill.
6. Final Report on the Value Engineering Workshop for MDSDD#1 Central Plant,
Flare Renovation Project, Culp/Westner/Culp, March, 1981.
7. Ibid.
8. Phase I: Vol. Ill, p. 4-5 to 4-27.
9. Ibid., p. 4-16.
10. Ibid., p. 4-26
11. Ibid., p. 4-27.
12. Personal Communication, McNeill
13. Personal Communication, Moe Tabatabai, Chief Engineer Operations, DNS.
14. Personal Communication, McNeill

A Description of a Decentralized Model for Production of Industrial Waste Sewage Gas
Centralized sewage treatment performs an indispensible function preserving public health by restoring all residential, industrial and storm runoff waters to acceptable stream quality. The tendency in Denver is towards more consolidation, with the closing of smaller districts, the formation of MDSDD#1 and the consideration of combining the central plant and DNS. While there are still separate districts in the Denver area, such as Golden, Glendale, Ken Caryl Ranch, Littleton, and Mission Viejo which is still under construction, they are primarily residential. All plants face the prospect of having to treat heavier wasteloads in the future, perhaps causing more small plant closures.
The pressure of the growing population and volume of wastewater on the central Denver facilities could be mitigated if industries treated a portion of their wastes. Some in the Denver area do now, such as meat packing houses removing grease which has economic value when used in other products, or a candy manufacturer recovering sugar to reprocess. Sometimes these practices can increase revenue enough to offset their costs and improve the company's profits.
Anaerobic digestion of industrial wastes in order to recover methane gas is another method of reducing the load on the public sewage facility. Based on the organic wastewater flows described in the foregoing chapters, the potential to make gas from industrial wastes in Denver is available. Effluent from the digestion system would be placed in the existing sewage system for final treatment at the central facility. The BOD and SS levels received at the central plant would be reduced.
The purpose of this chapter is to determine if reducing overhead in energy costs and sewage fees will be cost effective. A model of the equipment is proposed and

the costs to build a digestion system are estimated. The design is then applied to the four industrial wastestreams described in Chapter II to assess their economic feasibility and discuss their impact on the Denver area.
This design is intended to serve as an all purpose system for any wastestream, which is an oversimplification in meeting the needs of the bacteria which will do the digestion. The most recent research in anaerobic digestion indicates that dilute or soluable wastes like cheese whey, sugar and the packaging wastes are handled best in a digester designed with a packed or fluidized bed to which the bacteria attach.^ The waste flows past them and a very compact digestion vessel can be used. Another alternative is allowing the bacteria to flocculate, attaching to each other and forming a blanket or layer in the lower part of the vessel.
In both cases, high rates of throughput, gas production and solids destruction are reported. The reason these rates can be achieved is because the bacteria remain in the vessel rather than discharging each day with the effluent as in an overflowing, stirred vessel.
For the purposes of this examination, the interior details of optimum design for each wastestream will not be taken into account. Rather, the flow rates, gas productions and destructions already documented in literature are utilized. It is assumed that if an advanced vessel design were actually constructed, the capital costs would be equivalent to the design presented here because any additional modifications would be compensated for by savings in a reduced vessel size. It is not recommended that any of these systems be constructed without further laboratory testing to optimize the process. The values used here are being assigned to perform an economic analysis as part of an evaluation of sewage gas production and use. Each of the four systems is presented with slight variations from the basic design to respond to the different characteristics of the waste material.
First, the technical description and facility design of the proposed decentralized sewage treatment plant is given. Second, the capital costs and operating expenses are listed. Third, each of the four examples is discussed to identify the variations

necessary to the basic design for treatment of the individual wastestream. Then, the gas production is translated into an economic value for the fuel it will replace. Also, the sewage treatment value or savings in fees paid to a district facility is computed according to the closest possible approximation of each district's accounting system. Fourth, the returns on investment proforma statements as calculated by the Bio-Gas of Colorado computer program are presented. Fifth, the impacts of adopting decentralized wastewater treatment in the Denver metropolitan area is discussed.
A. Technical Description and Facility Design
The process flow and equipment for an industrial anaerobic digestion system is pictured in Figure IV-1. One of the most important considerations in controlling the biological activity in an anerobic digestion system is furnishing a consistent quantity and quality of waste material or feed to the bacteria. This prevents the bacteria culture from becoming starved, which reduces the population, or overloaded, which causes the solution to become acid and inhibits bacterial activity. The process begins in a feed tank where the daily feed can collect and be metered by an automated pump to the digestion vessel. The tank must be large enough to allow a volume to accumulate for weekend feeding when industrial operation is interrupted. Often, industries have such a collection point where wastes are held until they can be disposed of properly. For heated feeds with high suspended solids, a mixer must be provided to keep them in suspension, and the tank should be insulated to retain temperature.
The digestion vessel is a heated, insulated, gas-tight tank which will hold 3 to 20 days' volume of feed. The volume depends on the relationship of digestible to refractory solids in the feed material. The vessel is equipped with piping to enable the feed pump to mix the contents and deliver new feed to a position underneath an internally mounted heat exchanger. Temperature is sensed by a thermocouple and controlled by a thermostat. Digested liquid can overflow from a discharge pipe at the top as the vessel fills.

1. Insulated feed tank 8. Process boiler 15. Hydrogen sulfide purifier
2. Feed and mix pump 9. Temperature sensor 16. Compressor and Water Removal
3. Automatic valve 10. Temperature controls 17. Pressure control switches
4. Feed mixer 11. Discharge assembly 18.' Pressure release valve
5. Digester liquid delivery pipe 12. Over-pressure vacuum valve 19. Pressurized gas storage
6. Insulated digestion vessel 13. Gas piping 20. Pressure relief valve
7. Heat exchanger 14. Flare 21. Gas use piping

Six hundred BTU's per cubic feet biogas is emitted by the bacteria as the digestion, a kind of fermentation, proceeds. The gas collects in the area inside the roof of the digestion vessel. As gas pressure builds, the compressor switches on and pulls the gas through a purifier which eliminates the trace amounts of hydrogen sulfide, a poisonous and corrosive gas. A waste gas flare is placed before the purifier as a safety release if gas is being produced and the gas is not being consumed. The compressor also removes water from the gas and places the gas in a high pressure storage tank at approximately 250 psi. Gas relief valves are located on the digestion vessel, the compressor and the gas storage tank as safety features. The gas can be used in any type of application as natural gas or propane, such as burning in boilers and coolers, operating engines and generators or, with higher compression, powering vehicles. Gas use other than for a burning application requires higher capital expense. Most industrial applications need the gas to burn on-site to conduct their manufacturing activities.
B. Capital Costs and Operating Expenses
In order to estimate capital costs, an application and a size that can be generalized to many industries will be assumed. Since industrial organic wastes are often heated and diluted with wash water, a large vessel will be selected; but since they may have high dissolved solids which are readily available to bacteria, a short retention time will be used. Consequently, a wastestream of 248,000 pounds of 30,000 gallons per day will be analyzed. Table IV-1 lists the equipment, installation and start-up costs. This will vary with particular cases, and in many instances add up to a smaller total when equipment on-site can be utilized. The total cost for installation of a 263,000 gallon plant is $265,400. This assumes the plant already has a feed collection tank and needs only 3 to 4 hours of gas storage.
Annual operating expenses are listed in Table IV-2. They include chemical costs for any supplementary nutrients that need to be added to the feed, packing

Capital Costs for an Industrial 30,000 Gallon Per Day Digestion and Gas Recovery Facility
Digestion Vessel $ 81,300
Insulation 4,160
Paint 390
Feed Tank Modifications and Insulation 2,600
Feed and Mix Pump 5,200
Automatic Valve 1,560
Feed and Mix Piping 650
Boiler 6,500
Heat Exchanger 1,300
Heat Recirculation Pump 260
Temperature Control 390
Heat Delivery Piping 260
Liquid Discharge Assembly 1,300
Hand Valves 1,300
Check Valve 520
Flanges 460
Gas Compressor 12,480
Over Pressure Vacuum Relief Valve 650
Gas Piping 1,300
Hydrogen Sulfide Purifier 6,500
Pressure Relief Valves 1,040
Pressure Control Switches 2,600
Waste Gas Flare 2,600
Pressurized Gas Storage 29,900
Controls 1,950
Equipment Shipment 2,600
Site Preparation 2,600
Design Engineering 11,700
Installation Labor 39,000
Installation Supervision 1,300
Electrical Hookup 1,300
Start-up Costs & Operator Training 15,600
Contingency 10% 24,130
TOTAL $ 265,400
Annual Operating Expenses for an Industrial 30,000 Gallon Per Day Anaerobic Digestion and Gas Recovery
Daily Labor $ 1,370
Maintenance Labor 2,740
Maintenance Materials 5,400
Gas Purifier Packing 6,000
Management Overhead 4,110
Laboratory and Consulting Fees 6,000
TOTAL $ 25,620

material for the purifier, operation labor of one-half hour per day to check out the system, maintenance materials and labor, and management overhead.
An important consideration is the continued successful operation of a facility which performs a different function than the company's normal manufacturing activities. All the equipment provided is common to most industries, such as pumps, tanks and compressors, so general maintenance should not be expected to be a problem. However, the biological process inside the digestion vessel is not one with which most managers and operators would have familiarity. Control of the process is accomplished in two ways.
The start-up costs in Table IV-1 include establishment of a viable bacteria culture, preparation of an operation manual and training of the system operators and managers. It includes twelve weeks of laboratory tests so that a baseline sampling program is initiated. Secondly, laboratory and consulting fees are a regular part of the operating expense to maintain a healthy bacteria culture. This will be particularly necessary any time a new waste product is introduced into the waste stream as the manufacturing is improved or expanded. During consistent operation, the bacteria culture will adapt to the feed and become more stable.
C. Value Projection of Energy and Waste Reduction for Selected
Industries in the Denver Metropolitan Area
These projections are made for the four specific industries, packaging,
candy manufacturing, meat packing and cheese processing, described in Chapter II,
Section D. Any variation from the preceding design of a decentralized plant
in process flow, equipment or operation costs will be explained. Process flows
are diagramed in Figure IV-2. The gas potentials are taken from Table 11-11.
The sewage fee reduction potential is based on current rates for each sewage district
which serves the individual industry. Each method is different and complex.
Most district fees are calculated on flow volume and a surcharge for BOD and SS
above a basic residential rate. Flow is metered by a portable unit for major
sewage district customers sometimes as infrequently as once a month. Samples

Process Flow of Waste Treatment Systems at Four Selected Industries
Mix Tank
^ Digestion Vessel



Process Use
Mix Tank

___-- 'i' __
Digestion Vessel
^ Scrubber


Process Use
Mix Tank
Digestion Vessel



Storage Process Use
Wastes? Mix Tank
Digestion Vessel
? Scrubber

Process Use

are tested for levels of BOD and SS also on an occasional basis. The testing is usually done at MDSDD#1 but sometimes at a private laboratory. Projections may be made for the coming year for current billing. Adjustments to the bill may be made according to the previous year's test results. The numbers used for the purposes of this thesis are taken from the rates and formulas for each district as listed in Table IV-3 in a highly simplified version. The results are not verified by the sewage districts and should be accepted only as the closest estimate possible.
BOD and SS Surcharge Rates for Various Denver Sewage Districts*
District Industry $ BOD $ SS Year
MDSDD#1 Packaging $112.0247/ton $96.3765/ton 1980
Fruitdale Candy $.050 (BOD-175 mg/1) $.042 (SS-200 mg/1) 1980
North Washington Meat Packing .0017 (BOD-230 mg/1) .0015 (SS-200 mg/l)1980 (plus flow charge factor)
Denver North Side Cheese $.041 (BOD-250 mg/1) $.037 (SS-300 mg/1) 1981
Denver North Side Cheese $.021 (B0D-250 mg/1) $.028 (SS-300 mg/1) 1980
* All districts include changes for volume of flow. Volume would remain the same after digestion.
1. Packaging Manufacturer. Although the manufacturer expects to reduce the amount of waste from the current level of 350,000 gallons per day, the pounds of solids, which will generate the gas, will remain roughly the same. Some of the clarified water will be removed from the wastestream, leaving 200,000 gallons per day. The waste characteristics are high in SS and still very dilute. Because of the dilute nature of the wastes, the solids will need to be separated and treated with a different loading rate than the liquid. Separation can be done by the addition of a chemical which will flocculate the solids in the mix tank. Two digestion vessels will be necessary. The
IV- 9

flocculated solids will be pumped to one digestion vessel from the bottom of
the mix tank, and the clear liquid will overflow from the top of the mix
tank to the other vessel. The digestion process will need further chemical
additions to supply nutrients missing from the packaging wastes such as nitrogen,
phosphorus, nickel and cobalt. The two digestion vessels will be smaller than but equal in cost to the one quoted in the basic system in Section IV-B, but the equipment needed for flocculant and chemical additions will add $30,000 to the costs and double the maintenance materials. The process flow diagram for this system is illustrated in Figure IV-2.
As reported in Chapter II, the net daily methane is 22,275 cubic feet or 22.275 MCF or 22,275,000 BTU's. The company is presently paying $3.19 for 1000 cubic feet of 840 BTU per cubic foot fuel. This is equivalent to a
rate of $3.80 per million BTU's or per MCF. The daily value of the net gas
produced is $84.65; the annual value calculated on a 300 day year to allow for interruptions in production for maintenance or holidays when the gas will be wasted is $25,395.
Sewage charges at MDSDD#1 are determined by developing a complete budget for the coming year. A fixed percent of the budget is allocated to be collected for flow charges, BOD and SS, totalling 100%. Also, estimations are made for the total flow and BOD and SS loads to the plant as well as from each contributor. The total flow is divided by the percent to find a flow factor which can be multiplied times the expected contribution of each member. During the following year, the actual or measured flows are compared with the estimations. The entire budget amount is accounted, but adjustments are made for the cost per unit or the flow factor. BOD and SS are handled in the same way. Consequently, any year's billing is a composit of the readjustments according to the samples collected from a previous year and the projections of the current year. The projected reduction in the packaging company's sewage costs by pre-treatment for methane recovery is calculated here only on the current MDSDD#1 cost for tons of SS and BOD without adjustments from past sampling. A 365 day year

applies because the daily BOD is calculated on a complete year and the digester must be fed daily. Assuming the total solids destruction rate given in Table 11-11 would reduce both BOD and SS, and using the cost figures in Table IV-3, the savings is as follows:
91% x 3060 lb BOD/day x 365 days t 2000 lbs/ton x $112.0247/ton BOD
= $56,930
91% x 5825 lb SS/day x 365 days t 2000 lbs/ton x $96.3765/ton SS
= $93,233
The total savings potential at 1981 sewage fees and 1980 BOD and SS levels
is $150,163. This is an 82% savings over the fee paid to MDSDD#1 in 1980.
The difference is the 9% undigestible material that will not be destroyed by
the bacteria plus the MDSDD#1 flow charge. Future charges will increase,
as the 1982 budget is 41% higher than the 1980 budget, a 20.5% annual inflation
factor. The increase does not necessarily represent an increased service
area but does reflect higher costs for personnel, electric power and other ... 4
operational activities.
2. Candy Manufacturing. A treatment system for the candy manufacturing company would require 188,500 gallons of digestion volume to take the current waste of 29,000 gallons per day and hold a 6.5 day flow, as reported by the American Crystal Sugar Company. A smaller vessel can be used than in the basic plant described in Section IV-B, reducing the capital costs of the tank by 28%. Further savings are made because of the warm, dissolved characteristics of the waste in the feed pump and heating system, although the piping remains the same as the basic plant. Finally, the gas handling equipment is reduced in size because of the small volume of gas produced. Installation and start-up costs are equivalent to the basic plant. Consequently, the total installed plant cost is $170,750. As also reported by the American Crystal Sugar Company, no chemicals will have to be added to supplement

nutrients, lowering the maintenance materials budget of the operating expenses by $4,400. A smaller amount of gas purifier packing will be needed, further reducing the operating costs. The total annual operating budget is $15,720.
The daily methane production as reported in Chapter II is 1933 cubic feet or 1.933 MCF or 1,933,000 BTU's. At a value of $3.80 per million BTU's, the daily value is $7.35; the annual value calculated on a 300 day year to allow for closings on holidays and Sundays is $2,204.
The sanitary district's method of figuring the entire sewage rate is highly complicated, involving 29 separate steps, and only the surcharge for BOD and SS, which can be altered by the destruction in the digestive process, will be considered here. The savings in charges are calculated from the 80% destruction of the 302 pounds of BOD and 15 pounds of SS in 29,000 gallons of flow, as presented in Table II-11. Destruction of 80% of the SS reduces the total amount of SS to less than a residential sewer connector in the district contributes, or 200 mg/1. This eliminates the entire SS surcharge. The BOD cost can be calculated by taking the remaining 20% of the BOD and subtracting the allowed amount for the residential connector, or 175 mg/1. This concentration in 29,000 gallons allows 42.28 pounds of BOD to be discharged without a surcharge. Twenty percent of 302 is equal to slightly more, 60.4 pounds. The total annual BOD and SS surcharge would be calculated by the figures in Table IV-3 as follows:
$.05 £(20% x 302 lbs BOD) 42.28 lbs B0d£| -F (29,000 gal. x 8.3 Ib/gal)
x 1,000,000 ppm x 365 days = $1,369
The entire surcharge cost in 1980 was $5,161.12, a 73% reduction.
3. Meat Packing. The meat packing industry waste stream is high in nutrients, very dilute and most of the BOD is in the suspended solids.
Modification of the mix tank will be needed to help collect the suspended solids and separate some of the water. The mix tank will be built as a settling device. It will have a vertically mounted draft tube in the center.

Fresh wastes will be placed in the tank at the top of the draft tube so the solids can fall directly to the bottom. The length of the tube will be about 80% of the height of the mix tank, and the liquid will be able to circulate freely around it. Solids will be pumped off the bottom of the tank to feed the digester. Approximately 60% of the solids can be settled so that the 263,000 gallon digester is operated at a 12 day retention time.
The remaining 40% would overflow off the top of the tank to the current sewage treatment plant, or a secondary facility could be constructed in the future if the price of energy and sewage rates increase enough to warrent the expense. The digestion vessel would also be modified by a floor drain and a center mounted nozzle, so that solids settled in the digester can be drained directly off the bottom or can be resuspended by attaching an auxiliary pump. The cost of these additions to the basic system is $15,000, bringing the total capital costs to $280,400. The operating cost of the facility would remain the same as in the basic design.
Because of the necessity of wasting 40% of the solids, only 60% of the projected total energy reported in Chapter II will be generated, or 5,070 cubic feet of methane per day. Annual energy is calculated on a 365 day year because the industry is very energy intensive and the methane produced will be a small part of the required amount.^ It will be consumed to keep boilers up to temperature on holidays and weekends. The annual production is 1,850.55 MCF, valued at $7,032 per year at a rate of $3.80 per MCF.
The value of the savings for sewage treatment costs is higher than the gas value. The method of calculation used by the North Washington Sanitation District is to devise factors for BOD, SS and flow, add them together and multiply times 40<£ per thousand gallons. This is calculated monthly.
Assuming the annual charge in 1980 was 12 times the one month reported,
the 1980 cost for sewage treatment was $93,408. Using a 75% reduction of
BOD from Table 11-11 on 60% of the solids treated by the anaerobic system, the

following formula can be completed for resulting sewage fees.
[[25% x 60% x 1345 mg/1 BOD) + (40% x 1345 mg/1 BOD)] 230 mg/1 BOD
[(25% x 60% x 725 mg/1 SS) + (40% x 725 mg/1 ssf|
- 200 mg/1 SS
flow factor:
Total Factors: .87 + .30 + 1 = 2.17
Formula: 2.17 x $.40 x 63,288 M gallons per year = $54,934
The total savings, then, is $93,408 minus $54,934 or $38,474. This is a 41% savings.
4. Cheese Processing. This application involves the largest volume of wastes and the largest gas production considered in these four examples.
The vblume of wastes will require twice the digestion volume of the basic system described in Section IV-B. Since the whey is almost an entirely dissolved nutrient, it cannot be flocculated or settled before pumping to the digestion vessel (as with the paper wastes), and will need trace nutrients added in the digestion stage. The gas yield is greater than can be consumed by the cheese making process. Consequently, it is assumed that half the gas will be burned for process heat and the remainder will be utilized by generators to put electricity into the grid. The increased capital costs for a larger digestion vessel and gas handling equipment is $95,600, but since the generator will make constant use of the gas, a savings of $29,900 is realized for the gas compression equipment. The increase of equipment capital costs is $65,700. The installed cost of two 200 kilowatt generators is $160,000 at $400 per kilowatt. The operating costs will increase $15,000 per year for maintenance and materials for the chemical feed system. Operation costs for the maintenance of the generators will be accounted for by taking a 1.0<£ per kilowatt discount on the sale of electricity to the local utility company. The process flow is pictured in Figure IV-2. The total capital costs including the generators is $491,100 and the annual operating costs are $40,620.

The return on gas use at the cheese processing plant is calculated on half the produced gas, or 93,000 cubic feet of methane per day or 93.0 MCF
or 93,000,000 BTU's. The rate is $3.80 per million BTU's. Annual value on a 300 day year for gas consumption is $106,020. The return on electrical production is calculated at 4.6<£ per kilowatt or 5.6 This is the projected 1984 rate in Denver and is used in the centralized plant calculations.^ The remaining 93,000 cubic feet of gas will generate 7750 kilowatts per day or 323 per hour continuously at an efficiency of 12 cubic feet of gas to one kilowatt. Annual electrical value on a 365 day year is $130,123; waste heat from the generator can be used in the plant as a bonus.
The total energy value of both gas and electricity is $236,143, or an average of $3.47 per MCF.
The savings to the cheese processing plant in reduced sewage fees is not used in the calculation of the return on investment because the actual plant, located in Califronia, does not use the services of a local sewage plant.
Fifteen truckload per day would still have to be hauled at the same cost to the leaching facility. The volume to be disposed of would not change, although the BOD and SS strength would be reduced 93%. This could have a future benefit of extending the life of the leaching field by reducing its daily load. The possible sewage fee reduction for a plant of this size if located in the DNS sewage district is calculated on a BOD and SS destruction of 93% from Table II-11 and DNS fees in 1981 in Table IV-3 and the total pounds BOD and SS in Table 11-11 The entire cost of the SS is eliminated because the remaining 7% SS is less than the allowed 300 mg/1. The BOD is high because this is the most concentrated of these four examples.
$.041 x f(93% x 50,712 mg/1 BOD) 250 mg/1 B0d] x 365 days
= $702,040
The total potential sewage fee savings is $702,040 when DNS sewage rates are

applied. This high cost suggests why the company is using the leaching field rather than local sewage facilities. The disposal cost of effluent from the digester is:
$.041 x Y[l% x 50,712 mg/1 BOD) 250 mg/1 BOd] x 365 days
= $49,382
This is just slightly more than the $45,000 paid for hauling the gallonage to the disposal field now. The total savings would be $652,658.
D. Return on Investment
Table IV-4 lists the numbers used for calculating the returns on investment and the payback periods for the four industries examined. Tables IV-5 to IV-8 are the pro forma statements. The returns are based on a ten year cash flow. The proqram lists revenue from energy production as "gas" and savings from sewage cost reduction as "cake". Energy values and operation costs are both escalated at 10% per year. The calculations also assume that the investor can take advantage of tax credits for energy production and investment totaling 20% and accelerated depreciation of equipment. The program does not take into account the cost of financing. The rates of return on investment range from 0% to 45.0%.' The candy manufacturer could not show a return because the operating costs were higher than the revenue. This project cannot generate positive returns unless the operation costs are written off to other parts of the business or unless energy escalates dramatically. To have a positive return in ten years, the capital costs of the project would have to be reduced 2.5 times.
This suggests that this scale of project is not feasible without research that will make a significant change in gas production, solids destruction, capital costs or operation. The other three projects all yield positive returns, although the meat packing plant is viable only if the cost of money is lower than the return. The cheese processing and packaging businessess, yielding 34% and 45% respectively, could be productive projects based on cash flow alone. Each site and waste could be studied in more depth to verity the numbers presented.

Returns on Investments Pro Forma Statement Information
Item Packaging Candy Meat Cheese
Price of equipment $295,400 $170,750 $280,400 $491,100
MCF gas per year utilized 6,682.5 579.9 1,805.5 61,845
Tons BOD destroyed 508.19 47.65 162* 5,479
Tons SS destroyed 967.39 0.78 87* 315
Fuel value replaced/MCF $ 3.80 $ 3.80 $ 3.80 $ 3.47
Savings/ton BOD destroyed $112.0247 $100.00 $154.51* 0
Savings/ton SS destroyed $ 96.3765 $ 84.00 $154.51* 0
Annual Daily Labor $ 4,110 $ 4,110 $ 4,110 $ 4,110
Annual Operaging Expenses $22,800 $ 7,500 $ 7,500 $37,800
Management Overhead $ 4,110 $ 4,110 $ 4,110 $ 4,110
Tax Bracket 45% 45% 45% 45%
Return on Investment 45.0% 0.0% 5.7% 34.0%
Payback Period 2.22 years infinite 7.92 years 2.85 years
*Approximation from complicated formula; total savings equals $63,520 per year.

GAS CAKE 25393. 150170. ?7933 30726 165187t 181705* 33799, 199076. 37179. 219064. 40896, 241850, 44986, 266035. 49485, 292638, 54433. 321902. 59877. 354092.
TOTAL 175563* 193120, 212432. 233675, 257042. 202746, 311021. 342123, 376335. 413969.
LABOR 4110. 4521, 4973. 54/0. 6017, 6619, 7281. 8009. 8810. 9691.
CHEM ELEC MAINT 22800. 25080, 27588. 30347. 33381. 36720, 40392, 44431, 43374, 53761.
OVERHEAD 4110. 4521. 4973. 5470. 6017, 6619, 7281, 8009. 8810. 9691.
H, MANAGEMEN1 t-EE < 0. 0. 0. 0. 0. 0, 0. 0. 0. 0.
oo TOTAL 31020. 34122, 37S34, 41288, 45416. 49958, 54954, 60449. 66494. 73144.
OPERATING INCOME 144543. 158998, 174897, 192387. 211626. 232788. 256067. 281674, 309841, 340825.
DEPRECIATION 47937. 47937, 47937, 47937. 47937. 3687, 3637. 3687. 3637. 368/,
LOSS CARRYFORWARD 0. 0. 0. 0. 0. 0. 0. 0. 0, 0.
PRETAX INC 96606. ~lU060r 126960, 144450. 163688. 229101. 252380. 277986. 306154. 337138.
INCOME TAX 43473. 49977, 57132, 65002, 73660. 103095, 113571. 125094, 137769. 151712.
NET INCOME 53133. 61083, 69828. 79447, 90029, 126005, 138809. 152893, 168385. 105426.
NET INCOME 53133. 61003, 69828. 79447. 90029. 126005, 138809. 152893. 168305. 185426.
+ DEPRECIATION 47937. 47937, 47937, 4/937, 47937, 3637. 3687. 3637. 3687. 3687.
+ LOSS CARRYFWB 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.
} INVEST TAX CRED 59000. 0. 0. 0, 0. 0, 0. 0. 0. 0,
+ NET SALVAGE 0. 0. 0. 0. 0. 0. 0. 0. 0. 36875.
NET FLOW 160071. 109021. 117765. 127385. 137966. 129693. 142496, 156580. 172072, 225988.

$Y5TEK SALES PRICE ^ t 170750 ITC = 20% REVENUE GROWTH 1 = 10.%/YEAR
AFTFR 10 YFARS 8Y8TFM 1 SOLD FOR 1 nr (ipyrtmai .uai iif
GAS 2204. 2424. 2666. 2933. 3226. 3549. 3904, 4294. 4724, 5196.
CAKE 3791. 4170. 4587. 5046. 5551 6106. 6716. 7388. 8127. 8939.
TOTAL 5995. 6594. 7254. 7979. 8777. 9655, 10620. 11682. 12850, 14135.
LABOR 4110. 4521. 4973. 5470. 6017. 6619. 7281 . 8009. 8810. 9691.
______ CHE M ELEC MAI NT 7500. 8250. 9075. 9982, 10981. 12079, 13287. 14615, 16077. 17603.
OVERHEAD 4110. 4521. 4973. 5470. 6017, 6619. 7281. 8009. 8810. 9691,
MANAGEMENT FEE 0. 0. 0. 0. 0, 0, 0. 0, 0. 0.
h TOTAL 15720. 17292, 19021. 20923, 23016, 25317, 27849. 30634, 33697. 37067.
3 OPERATING INCOME -9725. -10698. 11768. -12944. -14239. -15663. -17229. -18952. -20847, -22932.
DEPRECIATION 27747. 27747, 27747, 27747, 27747. 2134. 2134, 2134, 2134. 2134.
LOSS CARRYFORWARD 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.

PRETAX INC -37472. -38445. -39514. -40691, -41986. -17797. -19363. -21086. -22981. -25066.
INCOME TAX 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.
NET INCOME- -37472. -38445, -39514. -40691, -41986. -17797, -19363, -21086. -22981, -25066.
_ NET INCOHE -37472. -38445. -39514. -40691, -41986. -17797, -19363. -21086. -22981. -25066.
t DEPRECIATION 27747. 27747. 27747, 27747, 27747, 2134. 2134. 2134. 2134, 2134.
+ LOSS CARRYEWD 0. 0. 0. 0. 0, 0. 0. 0. 0. 0.
+ INVEST TAX CRED 34150. 0. 0. 0. 0. 0. 0. 0. 0. 0.
+ NET SALVAGE 0, 0. 0. 0. 0. 0. 0. 0. 0. 21344.
NET FLOW 24425. -10698. -11768. -12944. -14239. -15663, -17229. -18952. -20847, -1588.
r. t ivit i*iA*rr*
A /

GAS 7032, 7735, 8509, 9360. 10296. 11325. 12458. 13704. 15074, 16581.
CAKE 38473. 42320, 46552. 51208, 56328. 61961. 68157. 74973, 82470. 90717.
TOTAL 45505. 50056. 55061. 60567. 66624. 73286. 80615. 38677, 97544. 107299.
LABOR 4110. 4521. 4973. 5470, 6017. 6619, 7281, 8009. 8810. 9691.
CHEM ELEC MAINT 17400. 19140. 21054. 23159. 25475, 23023, 30825. 33908, 37298. 41028.
OVERHEAD - 4110. 4521. 4973. 5470. 6017, 6619, 7281. 8009. 8810, 9691.
H MANAGEMENT FEE 0. 0. 0. 0. 0. 0. 0. 0, 0. 0,
1 ro TOTAL 25620. 28182, 31000, 34100, 37510. 41261, 45387. 49926, 54919. 60411.
o OPERATING INCOME 19885, 21874. 24061. 26467, 29114. 32025, 35228. 38750, 42625. 46888.
DEPRECIATION 45565. 45565, 45565, 45565, 45565. 3505. 3505. 3505. 3505. 3505.
LOSS CARRYFORWARD 0, 0, 0. 0, 0, 28520, 31723. 35245. 10936. 0.
PRETAX INC -25680. -23691. -21504. -19098. -16451. 0. 0. 0, 28184. 43383.
__ INCOME TAX . 0. 0. 0. 0. 0. 0. 0. 0, 12683. 19522.
NET INCOME -25680. -23691, -21504, -19098. -16451. 0. 0. 0, 15501. 23861.
NET INCOME -25680. -23691. -21504, -19098, -16451. 0. 0. 0. 15501. 23861.
+ DEPRECIATION 45565. 45565, 45565, 45565, 45565. 3505. 3505. 3505. 3505. 3505.
+ LOSS CARRYFWB 0. 0. 0. 0. 0, 28520, 31723. 35245. 1097,6. 0.
f INVEST TAX CRED 56080. 0. 0. 0. 0. 0, 0. 0, 0. 0.
+ NET SALVAGE 0. 0, 0. 0. 0. 0. 0. 0. 0. 35050.

NET FLOW 75965. 21874, 24061. 26467. 2.9114, 32025, 35228. 7,8750. 29943, 62416.

income ---
214602. 236062. 259669. 285635. 314199. 345619. 380181. 418199. 460019, 506021.
0. 0. 0. 0. 0. 0. 0. 0. 0, 0,
214602. 234062, 259669. 285635. 314199. 345619. 380181. 418199. 460019, 506021.
4110. 4521. 4973. 5470. 6017. 6619. 7281. 8009. 8810. 9691.
37800, 41580. 45738, 50312, 55343. 60877, 66965. 73662, 81028. 39130.
4110. 4521. 4973. 5470. 6017. 6619. 7281. 8009. 8810, 9691.
0. 0. 0. 0, 0. 0. 0. 0, 0. 0.
46020. 50622, 55634, 61253, 67378. 74116, 81527, 89680, 98648, 103513.
168582. 185440. 203984. 224383, 246821. 271503. 298654. 328519. 361371. 397508.
79804. 79804, 79804, 79804, 79804. 6139. 6139. 6139, 6139. 6139.
0. 0. 0. 0. 0. 0. 0. 0. 0. 0.
88778. 105637. 124181. 144579. 167017. 265364. 292515. 322380. 355232. 391369.
39950_ 47536, 55381, 65061, 75138. 119414, 131632. 145071, 159854. 176116.
48823. 53100, 68299, 79518, 91860. 145950, 160883. 177309, 195378. 215253.
48828. 58100. 68299. 79518. 91860. 145950. 160803. 177309. 195378. 215253.
79804._ 79804L 79804, 79804, 79804. 6139, 6139. 6139. 6139. 6139
0. 0. 0. 0, 0. 0. 0. 0. 0. 0.
98220. 0. 0, 0. 0. 0. 0. 0. 0, 0.
0. 0. 0. 0, 0. 0. 0. 0. 0. 61387.
NET FLOW 226852. 127704. 148103. 159322. 171663. 152089. 167022. 183448. 201516. 282779.
nTPnnmiT bati*
nt a A /

E. The Impact of Industrial Treatment of Wastewater in the Denver Metropolitan Area
Both the residential and the industrial sectors in Denver are projected to grow as described in Chapter II, Section C. The demand on MDSDD#1 will go up proportionately. As stated, the population in the basin which the sewage district serves is expected to increase 44% from 1980 to 2004; the COD load will increase 69%; the SS, 37%; and the entire wastewater stream, 34%. The resulting gas production increase from anaerobic digestion at the central sewage facility is expected to be 35%. Expansion to meet these needs will require further capital improvements at the plant, such as financed by the bond issue passed by a 5 to 1 margin in October, 1981.
Decentralizing sewage treatment by industries anaerobically digesting their wastes before discharging to the central plant will not have a measurable effect.
Table 11-10 shows that DNS is the only plant with a significant portion of industrial contributors. In 1965, 47% of the load in the 72.53 mgd flow was contributed by industry. This is less than one third of the flow received by the central plant. As has been shown, the economic feasibility for all industries to do their own treatment is not good. Only in the cases of some of the largest industries which have organic wastes will the process be possible. Consequently, the effect on the central facility would be to reduce the total wasteload at a rate which is less than the rate at which the population and new wasteload is increasing.
Not only will the effect of industrial treatment of wastes to the municipal facilities be relatively small and unnoticeable, but it will occur gradually. Industries are beginning to become aware of the possibilities as sewage rates increase. Denver city statutes which set the rates of the DNS plant increased the surcharge for BOD by 95% and the surcharge for SS by 24% from 1980 to 1981 (see Table IV-3). Only two nationally known companies offer digestion equipment to recover gas, Joseph Oates Corporation and Celanese Chemical Company, Inc.
As businesses assess their potential for gas production at different times,

the solids flow to the central sewage facilities will not make any sudden changes The effect will be comparable to conservation-like actions as businessess tighten up their operation to reduce their expenses. This provides the community with economic stimulation.
A further conservation-like effect is energy production. As an industry generates a portion of the natural gas it normally consumes, the total demand for public supplies will decrease. This has the effect of increasing Colorado's gas reserves by not placing a demand on the local gas supply while maintaining the productivity of the industry.
More energy will be available for the Denver area if industries generate gas for process consumption than if the wastes are transported to MDSDD#1 where gas will be made. Solids, which are the bacteria nutrient for gas production, are degraded in transport or are charged by the aerobic treatment at the central plant. The highest yields of gas are achieved from the freshest solids because
the fresh soluable organic material is highly available for bacteria food.
This technology is not labor intensive and because there are few large industries in Denver, there will be few projects built. Generally, the equipment can be operated by the present personnel at the companies which install digesters, Besides some temporary design and construction jobs, some employment will be created for laboratory technicians to test effluent and support the operators and some other employment will be created to train the operators. Maintenance and
operation materials are a small part- of the overall costs and would not provide
a significant number of jobs.
Of all these possible impacts, it is important to ask which has the greatest effect or if any of these impacts are significant. Most of the impacts discussed cannot be quantified, such as increased gas reserves by processing the organics for energy where they are fresh at the industrial site instead of at the treatment plant. The impact of diminishing pressure on the taxpayers to improve existing sewage facilities is a social advantage, but not one which is large enough to draw attention. Even the environmental impact of cleaner air by using purified

gas at the industrial sites has little effect when the sewage plant does not
exceed its discharge permit limitations at this time. On the other hand, the economic advantages are both quantifiable and significantly large in two of four cases. Furthermore, the most valuable between energy revenue and disposal savings is the latter. In all cases except the cheese processing which does not use a treatment plant, the savings from sewage disposal fees is two to five times the revenue from energy. This would be true of the cheese processing plant as well if it used a comparable disposal technique. Consequently, an investigation which began with MDSDD#1 studying gas use plans to avoid wasting energy and continued through this investigation of how more gas could be created if industry treated their own wastes, ends with discovering a greater benefit in savings of disposal fees. Sewage treatment is expensive. If it escalates faster than,energy, the impact of this analysis would be impressive. Unfortunately, the pro forma statement program does not separate escalation of "gas" from "cake". To observe the effect of escalation of both, the meat packing example was analyzed at 20% instead of 10%, as shown in Table IV-9. The effect on the return on investment was to increase it from 5.7% to 15.3%. Although this rate is true for 1981 to 1982 it is unlikely to continue for ten years without other social effects changing the whole infrastructure of the city.
Finally, it is necessary to point out that although application of anaerobic digestion to industrial sites may not have a large impact on the Denver metropolitan area as a whole, the effect on the individual industry could be substantial. Financial strengthening of these individuals will have a beneficial effect on the community at large.
An anaerobic system can be designed to digest industrial wastes. Capital and operating costs can be estimated. Further laboratory studies or piloting should be performed on most industrial wastes before hardening the design. The returns