January 1996 • NRELfTP-427-20558

Deploying Anaerobic Digesters:

Current Status and Future

Possibilities

P. Lusk

International Energy Agency

P. Wheeler

ETSU (United Kingdom)

C. Rivard

National Renewable Energy Laboratory

..&..r· .I!S=I ...~J~.'" 1JllE:__ ~.,.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401-3393

A national laboratory of the U.S. Department of Energy

Managed by Midwest Research Institute

for the U.S. Department of Energy

under Contract No. DE-AC36-83CHI0093
NRELrrP-427-20558· DC Category 1503· DE96000508

Deploying Anaerobic Digesters:

Current Status and Future

Possibilities

P. Lusk

International Energy Agency

P. Wheeler

ETSU (United Kingdom)

C. Rivard

National Renewable Energy Laboratory

O'I~5E!..

•

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401-3393

A national laboratory of the U.S. Department of Energy

Managed by Midwest Research Institute

under Contract No. DE-AC36-83CHI0093

Prepared under Task No. WM5l3231

January 1996
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government.

Neither the United States government nor any agency thereof, nor any of their employees, makes any

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness,

or usefUlness of any infonnation, apparatus, product, or process disclosed, or represents that its use would

not infringe privately owned rights. Reference herein to any specific commercial product, process, or service

by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its

endorsement, recommendation, orfavoring by the United States govemment or any agency thereof. The views

and opinions of authors expressed herein do not necessarily state or reflect those of the United States

government or any agency thereof.

Available to DOE and DOE contractors from:

Office of Scientific and Technicallnfonnation (OSTI)

P.O. Box 62

Oak Ridge, TN 37831

Prices available by calling (615) 576-8401

Available to the public from:

National Technical Infonnation Service (NTIS)

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

(703) 487-4650

.#... Printed on paper containing at least 50% wastepaper, including 10% postconsumer waste '1'••
TABLE OF CONTENTS

Introduction .

The Anaerobic Digestion Process, Christopher J. Rivard and David R. Boone .

Animal and Industrial Waste Anaerobic Digestion: USA Status Report, Philip D. Lusk

Animal Manure Digestion Systems in Central Europe, Erwin Koberle .

Anaerobic Digestion ofMunicipal Solid Waste: Technical Developments,

Christopher J. Rivard .

Biogas Utilization, Mark A. Moser .

Environmental Impacts ofAnaerobic Digestion and the Use ofAnaerobic Residues as a Soil

Amendment, Frank E. Mosey .

Economic Viability ofAnaerobic Digestion, Arthur Wellinger .

iii

Page

v

1

7

17

28

36

47

57
IV
INTRODUCTION

Unmanaged pollutants from putrescible farm, industrial, and municipal wastes degrade in

the environment, and methane emitted from their decomposition may contribute to global

climate change. Under modern environmental regulations, these wastes are becoming

difficult to dispose of using traditional means. One waste management system, anaerobic

digestion or AD~ not only provides pollution prevention but can also convert a disposal

problem into a new profit center. This report is drawn from a special session ofthe Second

Biomass Conference ofthe Americas.

The AD Process

AD is a biological process, where synergistic actions between bacteria are occurring at four

different levels. First, hydrolysis converts a wide range of solid organic materials into

sugars and amino acids. Fermenting these materials produces volatile fatty acids (VFAs).

Acidogenesis forms hydrogen, carbon dioxide (C02)~ and acetate from VFAs. Finally,

methanogenesis produces biogas, a mixture of methane, C02~ and numerous trace

elements. The key biological issues are determining the most favorable conditions for each

process stage and how nonoptimal circumstances affect the process as a whole, and the

governing role of hydrogen generation and consumption.

Technology Application

Within the world of AD technology, farm-based AD facilities are perhaps the most

common. A recent survey offarm manure facilities in the United States concluded that

AD is a technology with considerable potential, and that 3000 to 5000 farm digesters could

be economically deployed in the next decade. Farmers in the United States who now

operate a digester often cite odor control and containing excess nutrient runoff as the

drivers influencing their investment decision.

In Europe, AD facilities have had a good record in treating the spectrum ofsuitable farm,

industrial, and municipal wastes. Some ofthese facilities have been in operation for more

than 10 years. The key factor found in successful facilities is simplicity of design. Other

factors influencing success have been local environmental regulations and other policies

governing land use and waste disposal.

Most experience with large digestion facilities has been in Denmark. Recently Denmark's

commitment to AD increased with an energy initiative that will double biogas production

by the year 2000~ and then triple it by the year 2005. One ofthe key policy tools used to

encourage technology deployment is "green pricing," allowing manufacturers ofbiogasgenerated electricity to sell their product at a premium. Interestingly, sales of co-generated

hot water to specially built district heating systems is becoming an important source of

revenue.

v
One new application is the digestion of municipal solid waste (MSW). Several systems

have been developed; each has its own special benefits. Biological treatment processes

such as AD offer the only route for recycling organic matter and nutrients from the

putrescible fraction ofMSW. However, AD is the only biological treatment process that

produces energy.

MSW digestion poses many technical problems. High-solid digestion (HSD) systems

operating with concentrations greater than 30% have been developed. These systems have

the potential to improve the economic merit by reducing digester volume and the parasitic

energy required for the AD process.

Energy and Environment

Biogas is suitable for use in engines to generate electricity, boilers to produce hot water and

steam, or in gas-fired absorption chillers used for refrigeration. When biogas is used to

generate electricity, there is the added potential for harvesting thermal energy. Fuel cells

are a new end-use application, and projects using biogas are currently under way in Asia,

Europe, and North America.

Using the solid and liquid residues from the AD process can also have very beneficial

environmental impacts. AD enriches alternative agricultural practices when the residues

are applied to crops or dedicated feedstocks for alternative fuels such as biodiesel or

ethanol. Treating commodities to produce another energy yield while recycling nutrients

creates a virtuous cycle ofsustainability.

AD mitigates a number of other environmental concerns. The process reduces the

potential for odors, destroys pathogens, and displaces fossil fuels. Further, increased

atmospheric methane concentrations may have important impacts on global climate

change. Methane is considered to be a potent greenhouse gas (GHG) because each

molecule is estimated to have 22 times the heat-trapping impact of a CO2 molecule.

Economic Merit

As noted, AD is a technically feasible option. However, like most renewable energy

options, its economic merit relies on a number ofsite-specific factors. The importance of

selling the energy products produced from biogas at a satisfactory price cannot be

overestimated. Generally, this means that AD facilities must negotiate suitable rate

structures with an electricity utility, and maximize the use ofthe process heat.

There is a major barrier in negotiating utility power purchase contracts that will provide

sufficient revenue. The problem is the "avoided cost" rates offered by utilities to

independent power producers. In many nations, rates are typically low, in part because of

VI
current low prices for fossil fuels and> in some cases> due to the availability of excess

generating capacity.

Individual national conditions such as energy prices> pollution taxes> and the cost of

regulatory compliance strongly influence the economic merit of an AD system. Therefore>

comparing individual facilities and countries is extremely difficult.

Summary

Successful deployment of AD facilities is site-specific. The factor most strongly

influencing the economic merit of an AD facility is maximizing the sales of all usable

coproducts, especially electricity. Legislation that helps internalize social cost is also

important> but in practice> externalities are difficult to quantify. However, formal

consideration ofthe true cost ofthe various energy options can only improve the economic

merit ofAD facilities; thus> evaluating total life-cycle cost is a major task in the near-term.

Understanding total life-cycle cost by politicians and policy makers is important if

renewable energy technology are to become truly competitive.

vii
Acknowledgments

Many thanks to the eight authors who prepared the original papers from which this report

is drawn, and especially to M R Hardee for editorial services.

About the lEA AD Activity

The AD activity aims to provide reliable information on the cost effectiveness ofAD,

markets for biogas, and other co-products, more effective use of biogas, environmental

benefits, and institutional barriers. Copies ofthe special session on AD are available from

the National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado

80401-3393, USA. Document Distribution Code TP-427-20558. Tel: + 1 3032754363;

Fax: +1 303275 3619.

VllJ
THE ANAEROBIC DIGESTION PROCESS

Christopher J. Rivard, Ph.D., Senior Microbiologist

National Renewable Energy Laboratory, Golden, Colorado 80401 U.S.A.

David R. Boone, Ph.D. Professor of Environmental Microbiology

Oregon Graduate Institute, Portland, Oregon 97291-1000 U.S.A.

Abstract

The microbial process of converting organic matter into methane and carbon dioxide is

so complex that anaerobic digesters have long been treated as "black boxes." Research

into this process during the past few decades has gradually unraveled this complexity, but

many questions remain. The major biochemical reactions for forming methane by

methanogens are largely understood, and evolutionary studies indicate that these microbes

are as different from bacteria as they are from plants and animals. In anaerobic digesters,

methanogens are at the terminus of a metabolic web, in which the reactions of myriads

of other microbes produce a very limited range of compounds--mainly acetate, hydrogen,

and formate--on which the methanogens grow and from which they form methane.

"Interspecies hydrogen-transfer" and "interspecies formate-transfer" are major mechanisms

by which methanogens obtain their substrates and by which volatile fatty acids are

degraded. Our understanding of these reactions and other complex interactions among

the bacteria involved in anaerobic digestion is only now to the point where anaerobic

digesters need no longer be treated as black boxes.

1
The Anaerobic Bioconversion Process

Metabolic life in the absence of oxygen has to be frugal. Anaerobic microbes have had I

to adapt to life with less available energy. An example of this is the conservation of the

original energy content of organic substrates in the methane end-product (greater than

90%). As such, the anaerobic process has several distinct advantages such as the

production of low levels of microbial biomass, which thereby reduces the need for

essential nutrients. However, this low level of microbial catalyst production often results

in process instability because of microbial washout or enhanced sensitivity to process

perturbations.

The anaerobic digestion process takes place through the synergistic action of four types

of microorganisms: hydrolytic, fermentative, acidogenic, and methanogenic bacteria.

Numerous reviews describe the level of understanding of the anaerobic digestion process

and the terminal steps of methane production'r'", The hydrolytic bacteria in these

consortia produce a variety of enzymes to depolymerize such constituents as cellulose,

proteins, pectins, hemicellulose, and starches to monomeric subunits such as sugars or

amino acids. The fermentative bacteria convert these monomers to organic acids,

primarily propionic and acetic acid. The acidogenic bacteria convert these acids to

hydrogen, carbon dioxide, and acetate, which the methanogens utilize via two major

pathways to produce methane and carbon dioxide.

Because of the relatively low level of free energy available in most anaerobic

fermentations, microbial communities evolve as symbiotic relationships to help pull

metabolic reactions toward more favorable conditions. Often microbial communities must

be developed to achieve overall biological decomposition. These communities at the

same time protect the more sensitive participants within the community, thereby

increasing the stability of the process to perturbations.

Historical Perspective

Early awareness of methane generated during anaerobic processes was recorded as

mysterious flickering flames in swamps and marshlands known as "will-o-wisp'"." Van

Helmont in 1630 first determined that flammable gases could evolve from decaying

organic matter. In 1776, Volta concluded that there was a direct correlation between the

amount of decaying organic matter and the amount of flammable gas emitted. He also

determined that certain proportions of this flammable gas were explosive in air. In 1808,

Davy determined that methane was present in the gases produced during the anaerobic

digestion of cattle manure. In 1868, Bechamp, a student of Pasteur', attempted to isolate

the microorganism responsible for the anaerobic bioconversion of ethanol to methane.

In reality, Bechamp's attempts resulted in a co-culture of microorganisms'",

The first practical application of anaerobic digestion.for energy production took place in

England in 1896 when biogas from sewage sludge digestion was used to fuel street lamps.

Since that time, the process has received considerable interest to harness its energyproducing capabilities. Applied research began with Buswell's work in the 1920s in

2
which he established fermentation stoichiometries, the fate of nitrogen, and developed

farm-scale digestion systems'Y'". Since that time, numerous studies and demonstration

projects have been attempted with the widest application of anaerobic bioconversion for

the disposing municipal sewage.

Carbon flow

In general, the anaerobic biological process yields consistent end-products, including

methane, carbon dioxide, microbial biomass, and recalcitrant organics. The effective

conversion of feedstock carbon requires the concerted effort of a variety of

microorganisms. The anaerobic process appears to proceed uniformly toward methane

production, but even transient inhibition of one participating consortium group may create

an imbalance that causes intermediary products (primarily organic acids) to accumulate,

thereby reducing sludge pH and ultimately inhibiting more sensitive microbes in the

consortium and throwing the process into a "sour" condition. Process monitoring is thus

important to ensure stable bioconversion. Monitoring biogas production, biogas methane

content, sludge volatile solids destruction, sludge volatile organic acid levels, and pH is

important to predict the general health of the digester process.

Microbiology

Substantial understanding of the microorganisms that participate in the anaerobic

consortium exists, however, this information is far from complete. In general, microbial

populations within the digester consortium are fluctuate continuously, depending on such

factors as changes in feedstock composition, organic loading rate, nutrient availability,

presence _of inhibitors or stimulants, environmental parameters such as pH and volatile

fatty acid levels, and end-product concentrations. In addition, thermophilic and

mesophilic microbes may be present at varying populations in a mesophilically operated

digester", This allows such digestion systems to adapt to thermophilic conditions without

being inoculated by thermophilic microorganisms. A rather slow adaptation period will

allow the thermophilic microorganisms to predominate under the new conditions",

Native hydrolytic enzyme systems

The methanogenic and acetigenic bacteria have slower maximal growth rates than do the

hydrolytic and fermentative bacteria, but when conditions in digestors are maintained to

avoid "sour digestor," their metabolism does not limit the rate and extent of digestion".

The rate-limiting step in the anaerobic bioconversion of polymeric feedstocks such as

agricultural residues or MSW is polymer hydrolysis. Hydrolytic enzymes are required to

depolymerize feedstocks such as hemicellulose, cellulose, starch, pectin, proteins, fats,

oils, and greases. In their monomeric form these molecules may be taken up by the

fermentative microbes and converted to smaller molecular weight organic acids. In

general, hydrolytic enzyme productivity by anaerobes is quite low compared to aerobic

hydrolytic microbes, partly because of the low-level energy yield from anaerobic

metabolism. With lower levels of hydrolytic enzymes present, the rate of polymer

degradation in anaerobic systems is anticipated to be slower than aerobic system for

polymeric feedstocks. Engineering digester systems to "cater" to the hydrolytic microbes

is one way to improve the rate of feedstock polymer hydrolysis. Whereas methanogenic

bacteria prefer a pH in slightly greater than neutral, hydrolytic microbes and their enzyme

3
systems operate best at pH levels of 6 to 6.8. In addition, the optimum temperature for

hydrolytic enzyme activity is often closer to 40°C as opposed to the optimum for most

mesophilic microorganisms of 35°C to 37°C. Designing reactors to enhance the

hydrolysis phase of anaerobic digestion takes advantage of these optima by separating the

hydrolysis from methanogenic stages of the digestion process (two-stage digestion) and

operating each phase under optimum conditions of temperature and pH.

Another approach to enhancing the rate of polymer hydrolysis is to increase the contact

time for hydrolytic enzymes when treating polymeric feedstocks. This may be

accomplished using systems that allow for separate control of hydraulic and solids

retention times or through high-solids digestion systems that contain very little free water.

An additional benefit of high-solids systems is derived from the low rate of mixing in

these systems, in which shear forces which may disrupt the enzyme-substrate binding are

minimized. .

Interspecies hydrogen transfer

As described above, the overall conversion of organic feedstocks to methane and carbon

dioxide is a result of the synergistic action of a consortium of microorganisms. The

ability to enhance the metabolic energy derived from anaerobic fermentations is a feature

of this relationship. When organic substrates are anaerobically fermented, electrons are

"dumped" in the form of dihydrogen. Unless these electrons are actively removed

subsequent fermentation reactions become unfavorable. This is especially true for

reactions with minimal free energy potential. The ability to maintain e.xtremely low

partial pressure of dihydrogen is imperative to pull the anaerobic conversion process to

completion. Dihydrogen utilization is a competitive process in which methanogens and

sulfate reducers are the key participants. Table 1 describes the level of free energy that

may be expected from oxidizing of dihydrogen at the expense of nitrate, sulfate, and

dissolved carbon dioxide.

Table 1. Comparison of Free Energy for Dihydrogen.Oxidation

by Various Electron Acceptors

Electron Acceptor Free Energy (Go)

Nitrate - 143.3

Sulfate - 36.3

Carbonate - 32.4

Feedstock effects

Various feedstock parameters directly affect the yield and quality of the biogas product.

In 1933, Symons and Buswell established an equation to determine the theoretical

methane yield that may be expected from the known composition of the feedstock. Later,

4
Owen et ale (1979) established a more useful relationship for theoretical methane yield

for a "given feedstock based on the chemical oxygen demand value'", In general this

relationship states that for 100% conversion efficiency that one gram of substrate carbon

oxygen demand will result in 350 mL of methane at standard temperature and pressure.

One aspect of the Symons and Buswell conversion equation is that feedstocks with

increased carbon to oxygen (C:O) ratios will result in a higher percentage of methane in

the product biogas. As the value of the biogas product is often based on the Btu content

of the biogas (a direct reflection of the methane content), converting more reduced

feedstocks such as those that contain fat, oil, and grease will ultimately result in a higher

value fuel gas product.

Feedstock size reduction also affects the hydrolysis rate for polymeric feedstocks.

Smaller particle sizes increase the surface area for microbial attack by hydrolytic

enzymes.

Engineering the ItBlack Box"

With our increased knowledge of the microbial participants and metabolic

interdependence of the anaerobic digestion process, we can now more fully exploit the

process to its maximum potential. The anaerobic digestion of lignocellulosic materials, ·

such as municipal solid waste and biomass, is limited by the rate of polymer

hydrolysisv'", The primary biodegradable polymer in biomass, cellulose, is shielded by

lignin, a_ relatively inert, polyphenylpropane, three-dimensional polymer!", and by

hemicellulose'". This complex structure dictates that under natural environmental

conditions this biodegradation occurs over months or years, rather than hours or days.

Anaerobic digestion of such materials in completely mixed digestors requires a long

contact time between microbes and substrate to obtain satisfactory yields. This results

in very long retention times, reactor volumes, and thus capital costs in large-scale

application. We now have devised systems such as sequencing batch reactors, high-solids

anaerobic digesters, or multiple-phase digester systems that increase the solids retention

time and hydrolytic microbial contact, or optimize the environmental conditions for the

hydrolytic process.

Readily solubilized and fermentable substrates impose a different limitation, that of

organic acid conversion. Improved process understanding has led to the designed systems

that use immobilization to increase the relative concentration of slow-growing acetogenic

and methanogenic microbes. Fixed film or granulation reactors retain these

microorganisms in the digester system and protect these more sensitive microbes by

producing microbial films".

In general, our knowledge of process design, monitoring, and control allow current

anaerobic digestion systems to be operated more closely to their theoretical optimum, and

often with well-defined biological conversion efficiencies.

5
References

1. Buswell. A.M.• and Hatfield. W.O. (1936). Anaerobic Fermentations. State of

Illinois Department of Registration and Education Bulletin. Chicago. 32.

2. MoCarty, P.L. (1964). Public Works 95. 107.

3. Wolfe, R.S. (1971). Microbial Formation of Methane. Adv. Microbial Physiol. 6.

107-145.

4. Mah, R.A.; Ward, D.M.; Baresi, L.; Glass. T.L., (1977), Biogenesis of Methane.

Ann. Rev. Microbiol. 31, 309-341.

5. Zeikus, J.G., (1977), The Biology of Methanogenic Bacteria. Bacteriol, Rev. 41,

514-541.

6. Balch, W.E., Fox. G.E., Magrum, L.J., Woese, C.R., and Wolfe, R.S. (1979)

Microbiol. Rev. 43, 260.

7. Bryant, M.P. (1979), Microbial Methane Production: Theoretical Aspects. J.

Animal Sci. 48, 193-201.

8. Clausen, E.C.; Sitton, O.C.; Gaddy, J.L. (1979). Biological Production of Methane

from Energy Crops. Biotech. Bioeng. 21, 1209-1219.

9. Boone, D.R. (1982), Terminal Reactions in the Anaerobic Digestion of Animal

Waste. Appl. Environ. Microbiol. 43, 57-64.

10. Daniels, L.; Sparling, R.; Sprott, G.D. (1984), The Bioenergetics of

Methanogenesis. Biochem. et Biophys. Acta 768, 113-163.

11. McCarty P.L. (1982), One Hundred Years of Anaerobic Treatment. In Anaerobic

Digestion 1981, Proceedings of the 2nd Symp. on Anaerobic Digestion,

Travemunde, Germany, Sept. 6-11, pp 3-22.

12. Zehnder, A.J.B.; Ingvorsen, K.; and Marti. T. (1982), Microbiology of Methane

Bacteria. In Anaerobic Digestion 1981, Proceedings of the 2nd Symp. on

Anaerobic Digestion, Tavemunde, Germany. Sept. 6-11, pp 45-68.

13. Buswell, A.M. and Neave, S.L. (1930), Laboratory Studies of Sludge Digestion.

State of Illinois, Dept. of Registration and Education, Div. of the State Water

Survey, Urbana, lllinois, Bulletin No. 30, pp 1-84.

14. Metcalf & Eddy, Inc. (1979). Wastewater Engineering: Treatment, Disposal,

Reuse, New York: McGraw-Hill

15. Zhao, Y., H. Zhang et al 1986, Appl. Env. Micro.

16. Zinder, S.H.; Anguish, T.; and Cardwell, S.C. 1984.

17. Owen, W.E: Stuckey, D.C.; Healy, J.B.; Young, L.Y.; and McCarty, P.L. (1979),

Bioassay for Monitoring Biochemical Methane Potential and Anaerobic Toxicity.

Water Res. 13, 485-492.

18. Noike, T.; Endo, G.; Chang, J-E.; Yaguchi, J-I.; Matsumoto, J-I. (1985),

Characteristics of Carbohydrate Degradation and the Rate-Limiting Step in

Anaerobic Digestion. Biotechnol. Bioeng. 27, 1482-1489.

19. Sarkanen, K.V. and Ludwig. C.H. (1971). Lignins: Occurrence, Formation,

Structure and Reactions. Wiley-Interscience, New York, NY.

20. Grohmann, K., Torget, R., Himmel, M.E. (1985) Biotech. Bioeng. Symp. 15:59-80.

21. Young, J.A. and McCarty, P.L. (1965), The Anaerobic Filter for Waste Treatment.

J. Water Pollut. Control Fed. 41, RI60-RI71.

6
ANIMAL AND INDUSTRIAL WASTE ANAEROBIC

DIGESTION: USA STATUS REPORT

Philip D. Lusk, Principal

Resource Development Associates, Washington, DC 20002 USA

Abstract

Pollutants from unmanaged animal and bio-based industrial wastes can degrade the

environment, and methane emitted from decomposing wastes may contribute to global

climate change. One waste management system prevents pollution and converts a

disposal problem into a new profit center. Case studies of operating systems indicate that

the anaerobic digestion of animal and industrial wastes is a commercially available

bioconversion technology with considerable potential for providing profitable coproducts,

including a cost-effective renewable fuel. Growth and concentration of the livestock

industry create opportunities to properly dispose of the large quantities of manures

generated at dairy, swine, and poultry farms. Beyond the farm, extension of the anaerobic

digestion process to recover methane has considerable potential for certain classified

industries-with a waste stream characterization similar to livestock manures. More than

35 example industries have been identified, and include processors of chemicals, fiber,

food, meat, milk, and pharmaceuticals. Some of these industries already recover methane

for energy. This status report examines some current opportunities for recovering

methane from the anaerobic digestion of animal and industrial wastes in the U.S. Case

studies of operating digesters, including project and maintenance histories, and the

operator's "Iessons learned," are included as a reality check. Factors necessary for

successful projects, as well as a list of reasons explaining why some anaerobic digestion

projects fail, are provided. The role of management is key; not only must digesters be

well engineered and built with high-quality components, they must also be sited at

facilities willing to incorporate the uncertainties of a new technology. Anaerobic

digestion can provide monetary benefits and mitigate possible pollution problems, thereby

·sustaining development while maintaining environmental quality. Moreover, economic

development will benefit from the implicit multiplier effect resulting from jobs created

by implementing digester systems. Promising future waste-to-profit activities may add to

the economic performance of anaerobic digestion. New end-use applications that provide

added value to coproducts are discussed.

7
Introduction

Growth and concentration of livestock production and certain classified industries in the

U.S. create new opportunities to properly dispose of waste materials generated at these

facilities. Principal pollutants from decomposing organic wastes are methane emissions,

ammonia, and excess nutrients and pathogens, along with biochemical oxygen demand.

The major pollution problems associated with these wastes are surface and ground water

contamination, and surface air pollution caused by odors, dust, volatile organic acids, and

ammonia. There is also concern about the contribution of methane emissions to global

climate change. Consequently, waste management systems that enable pollution

prevention and energy production are becoming increasingly attractive.

The anaerobic digestion of animal and bio-based industrial wastes is a commercially

available conversion technology with considerable potential for providing profitable

coproducts, including a cost-effective renewable fuel. Anaerobic digesters are commonly

designed to operate in either the mesophilic temperature range (20°C to 45°C) or the

thermophilic temperature range (45°C to 60°C). There are usually two reasons why these

temperature ranges are preferred. First, a higher loading rate of organic materials can be

processed, and because shorter retention times are associated with higher temperatures,

increased outputs for a given digester capacity result. Second, a higher temperature

increases the destruction of pathogens present in raw manure. Anaerobic digestion also

occurs in the psychrophilic temperature range (less than 20°C), but this region has not

been as extensively evaluated by the research community.

Some effort has focused on the anaerobic digestion of poultry manures, but the manures

from dairy and swine operations have been used more extensively for farm-based energy

conversion. Dairy and swine manure management systems are often liquid- or slurrybased, which simplifies the necessary manure movement. Also, poultry manures contain

a higher concentration of fine solids that can quickly fallout of suspension if not

continuously agitated. Built-up solids can reduce reactor and biogas production volumes.

The three main types of anaerobic systems used on farm in the U.S. include the complete

mix, plug flow, and low-temperature covered anaerobic lagoon digesters.

Anaerobic digestion is ideally suited for many of the concentrated wastewaters typical of

many industrial processes today. More than 35 example industries have been identified

with wastewaters amenable for anaerobic treatment, including processors of chemicals,

fiber, food, meat, milk, and pharmaceuticals. The types of anaerobic digesters commonly

used in industrial applications in the U.S. include low-temperature covered lagoons,

complete mix, packed reactors, upflow sludge blankets, and sequencing batch reactors.

The advantages of these technologies compared to other biologically-based processes

include low sludge production, high removal efficiencies, low nutrient requirements, low

maintenance, and, of course, biogas production.

Biogas produced by the anaerobic digestion process is quite similar to «natural" gas, as

it is extracted from the wellhead and is suitable for use in engine/generators to produce

electricity. When biogas is used to produce electricity, there is the added potential for

harvesting thermal energy from the engine's exhaust and cooling systems'. Biogas can

also be burned in boilers to produce hot water and steam used for sanitary washing, or

8
in gas-fired absorption chillers used for refrigeratiorr', Some digesters successfully

compress the biogas to operate light-duty vehicles as well.

Promising future waste-to-profit activities may enhance the economic performance of the

overall waste management system. One new end-use application that can provide added

value to coproducts is the fuel cell for generating electricity and process heat. Fuel cells

appear' to offer a particularly clean and effective method of converting biogas into

electricity and process heat. Instead of the 20%-25% electrical conversion efficiency from

today's engine/generator technologies, fuel cells could produce electricity from biogas at

a 40%-50% conversion efficiency, a two fold increase in productivity. When combined

with process heat recovery, overall fuel cell efficiency is greater than 80%. Because of

their high efficiency, fuel cells emit less carbon dioxide per kilowatt hour of electricity

than do current conversion technologies. The process also produces a minimal amount

of carbon monoxide and nitrogen oxides, an issue of extreme importance to areas with

substandard classifications under the Clean Air Act Amendments of 1990. With no

moving parts, fuel cells also operate very quietly. Around 60 units that operate on natural

gas have been sold world-wide. Two fuel cell projects using biogas are currently under

way: one will demonstrate biogas recovery from a landfill in California, and the other

will recover biogas from a wastewater treatment facility. The second project will also

reclaim waste heat for use by the facility's anaerobic digester.

The economic performance of the overall farm manure management system may be

enhanced by adding value to coproducts and maximizing nutrient utilization in

greenhouses, and production of algae, plant, and fish aquaculture. Following the digestion

process, diluted wastewater effluent can be used as a nutrient source for hydroponic plant

culture in-attached greenhouses. Additionally, attached greenhouses can provide enhanced

plant growth rates if the available carbon dioxide is captured. Discharged wastewater

effluent can also be discharged into ponds and used as a growth culture for algae or

aquatic plants. Algae are up to 50% protein and can be used for many purposes.

Currently, algae are produced for animal feed and as a soil amendment. One alga that can

be grown is Spirulina, a super-nutrient that contains large amounts of beta-carotene. It

is a lipid-rich alga that can be converted into a liquid diesel fuel substitute. Other algae

can be used as natural colorants or dyes. Some species of duckweed, an aquatic plant that

contains 35%-45% protein and has properties similar to algae, have phenomenal growth

rates when grown in waste effluents. Fresh duckweed has been used as fish food with

good results, and dried duckweed meal has been an excellent substitute for soy and fish

meals in poultry rations",

Farm-Based Anaerobic Digestion"

The energy requirements of larger-sized American livestock operations led to the design

and installation of several digesters using model municipal sewage treatment plant

technology as one response to the energy crises of the 1970s. These demonstration

projects represented a transfer of state-of-the-art sewage treatment plant technology and

were the lust generation of complete mix digesters installed for agricultural applications.

Although complete mix digesters can operate in the thermophilic temperature range, the

demonstration projects at facilities such as the Washington State Dairy Farm in Monroe/

operated only in the mesophilic temperature range. At the Monroe project, the digester

9
was sized for the manure volume produced by a milking herd of 180 to 200 Holstein

cows. Although these early complete mix digesters generally produced biogas at the target

design rate, they suffered from high capital costs and from significant operation and

maintenance requirements. In practical application on the farm, the issues of solids

settling, scum formation, and grit removal often presented major problems.

Today's complete mix digesters typically handle manure with a low solids content and

generally can handle substantial manure volumes. The reactor is a large, vertical, poured

concrete or steel circular container. The manure is collected in a mixing pit by either a

gravity-flow or pump system. The total solids percentage can be diluted, and the manure

can be pre-heated before it is introduced to the digester reactor. The manure is

deliberately mixed in the digester reactor. The mixing process creates a homogeneous

substrate that prevents the formation of a surface crust and keeps solids in suspension.

Mixing and heating often improve digester efficiency. This "medium-rate" digestion

technology usually has a hydraulic retention time (HRT), the total time that manure

spends inside the digester, from 10 to 20 days.

A fixed cover is placed over the complete mix digester reactor to maintain anaerobic

conditions and to trap the methane that is produced. The methane is removed from the

digester, processed, and transported to the site of end-use application. The most common

application for methane produced by the digestion process is electricity generation using

a modified internal combustion engine. Both the digester. reactor and the mixing pit are

heated with waste heat from the engine cooling system. As already mentioned, complete

mix digesters operate at either the mesophilic or thermophilic temperatures ranges. Lower

temperatures reduce the rate of methane production, and consequently, a digester operated

in the mesophilic range requires a longer average manure retention time and a larger tank.

Complete mix digester volumes range considerably from about 3500 cubic feet to 14,000

cubic feet. This represents daily capacities of about 25,000 gallons to 100,000 gallons

of manure per digester. Larger volumes are usually handled by multiple digesters.

By the late-1970s researchers at Cornell University" were able to reduce the capital costs

and the operational complexities associated with the early complete mix digesters by

using a simple extension of Asian anaerobic digestion technology. These "plug flow"

digesters were adopted with some success in the cooler climate of the Northeast, where

farms primarily use scraping systems to remove manure. The 1979 project at the Mason

Dixon Dairy Farms in Gettysburg, Pennsylvania was the first plug flow digester operated

on a commercial farm. At the Mason Dixon project, the plug flow digester was originally

sized for a manure volume produced by a milking herd of 250 Holstein cows.

The basic plug flow digester design is a long linear trough, often built below ground

level, with an airtight expandable cover. The manure is collected daily and added at one

end of the trough. Each day a new "plug" of manure is added, slowly pushing the other

manure down the trough. The size of the plug flow system is determined by the size of

the daily "plug." As the manure progresses through the trough, it decomposes and

produces methane that is trapped in the expandable cover. To protect the flexible cover

and maintain optimal temperatures, some plug flow digesters are enclosed in simple

greenhouses or insulated with a fiberglass blanket. The HRT is from 20 to 30 days

depending on the digester temperature. An often vital component of a plug flow digester

10
is the mixing pit, which allows the percent total solids of the manure to be adjusted by

dilution with water. Many systems use a mixing pit with a capacity Toughly equal to one

day's manure output to store manure before adding it to the digester.

Plug flow digesters operate at either the mesophilic or thermophilic temperature ranges.

The amount of methane produced depends on the quantity of manure and the average

retention time in the trough. Lower temperatures will slow the rate of digestion, which

will require a longer retention time, and consequently, a larger, more expensive trough.

Higher temperatures will increase the digestion rate which allows a shorter retention time

and a smaller, less expensive trough. Energy for heating the digester is available in the

waste heat from the exhaust and cooling system of an internal combustion

engine/generator powered by the biogas produced in the digester.

The complete mix and plug flow digestion technologies are not suited for use on farms

that use hydraulic flushing systems to remove manure. Instead an anaerobic lagoon is an

increasingly popular method used to store and treat manure. A properly designed and

operated lagoon system, where the manure retention time exceeds 60 days, will produce

significant quantities of methane. In the early 1980s, the concept of using a floating

cover that collects biogas as it escapes from the surface of an anaerobic lagoon emerged.

The first floating cover that recovered biogas from an anaerobic lagoon operating in the

psychrophilic range was at the Royal Farm operation in Tulare, California? The Royal

Farm's digester used the manure from a 1,600-sow farrow-to-finish farm.

The North Carolina Energy Division and North Carolina State University constructed the

first full-scale covered anaerobic lagoon digester on the east coast at the Randleigh Dairy

in 19888:- The digester processed the wastes from 150 dairy cows. The cooperative

project used funds provided by the U.S. Department of Energys Southeastern Regional

Biomass Energy Program, the North Carolina Agricultural Research Service, and the

North Carolina Dairy Foundation. The project objective was to educate dairy producers

through practical demonstration and outreach about the merits of a low-cost and easily

maintained digester suitable for use on farms using hydraulic flush manure management

systems. The project provided information about the amount of biogas that can be

recovered, along with cost information from which the economic merit of the technology

can be evaluated.

The methane produced in an anaerobic lagoon is captured by placing a floating,

impermeable cover over the lagoon. The cover is constructed of an industrial fabric such

as hypalon that rests on solid floats laid on the surface of the lagoon. The cover can be

placed over the entire lagoon or over the portion of the lagoon that produces the most

methane. Once the cover is installed, the methane produced under the covered area of

the lagoon is trapped. The biogas is harvested using a collection manifold, such as a long

perforated pipe, that is placed under the cover along the sealed edge of the lagoon.

Methane is removed by the pull of a slight vacuum on the collection manifold (e.g., by

connecting a suction blower to the end of the pipe) that draws the collected biogas from

under the cover and on to the end-use application. The cover is held in position with

ropes and anchored by a concrete footing along the edge of the lagoon. Where the cover

attaches to the edge of the lagoon, an airtight seal is constructed by placing a sheet of the

cover material over the lagoon bank and down several feet into the lagoon, and clamping

11
the cover (with the footing) onto the sealed bank. Seals are formed on the remaining

edges by using a weighted curtain of material that hangs vertically from the edge of the

floating cover into the lagoon. The HRT of a "low-loaded" covered lagoon digester is

measured in weeks or months, depending on the specific application and digester design.

At Randleigh Dairy, the HRTwas 67 days.

The covered lagoon digester has several merits. First, it has good potential for widespread

adoption in the U.S., especially in the southeastern and southwestern regions, because

most dairy and swine facilities use hydraulic flushing to collect manure and anaerobic

lagoons to treat waste. Second, constructing and managing this type of reactor are simple

and straightforward compared to complete mix and plug flow digesters. Third, the capital

costs for this type of digester are considerably less than those required for the complete

mix and plug flow types of conventional digesters.

Covering an anaerobic lagoon and harvesting the biogas is a simple technology; however,

the approach raises at least two significant concerns. A key issue is that digestion rate

depends on temperature; therefore, biogas production varies seasonally ifthe lagoon is not

externally heated. This means that methane production is highest during the warm,

summer months and lowest during the cooler, winter months. At the Randleigh Dairy,

daily biogas production during the summer averaged 35% greater than daily production

during the winter. This may make end-use applications more problematic than with

conventional digesters that have fewer significant seasonal variations in methane

production. Moreover, any anaerobic lagoon (covered or not) is impractical in areas with

a high water table because of the potential for groundwater contamination. Lagoons built

into highly permeable soils must be adequately lined to prevent groundwater

contamination.

Mason Dixon Farms has since grown to a total herd of 2,000 milking cows and has built

two additional digesters to accommodate the increased manure volume. Mason Dixon

Farms has now abandoned the linear plug flow approach. The greatly simplified slurrybased "loop" digester now employed further minimizes digester construction and

operating costs. This loop design enables greater convective currents in the digester,

thereby avoiding the solids crusting problem commonly associated with the plug flow

design. This improvement may offer a better opportunity for deploying loop digesters at

caged-layer poultry operations.

A number of other methods for on-farm anaerobic digestion have been proposed,

including variations of covered anaerobic lagoons generally referenced as Advanced

Integrated Pond Systems (AIPS)9. AlPS use a submerged canopy covering a facultative

pond, where the organic wastes are completely converted into methane, nitrogen, carbon

dioxide, and stable residues. The submerged canopy is potentially more cost-effective

than conventional covered lagoons because it is not exposed to weather and other

elements. One intriguing aspect of AlPS is that digester effluent is discharged into

secondary pools and is expressly used as a growth culture for algae.

Surveyed farmers who continue to operate one of the 25 remaining anaerobic digesters

installed on dairy, swine, and poultry farms are generally satisfied with their investment

decision. Some chose to install a digester for noneconomic reasons, primarily to control

12
odor or contain excess nutrient runoff. Those farmers have found that the returns

provided from electricity and other coproduct sales from the digester, however limited,

are preferred to the sunk-cost of conventional disposal which provides no return on

investment. Moreover, without the environmental benefits provided by anaerobic

technology, some might have been forced out of livestock production. Anaerobic

digestion is sometimes the key technology that allows growth in the livestock production

business. Turning a waste liability into a profit center that generates annual revenues can

moderate the impacts of declining commodity prices and diversify farm income.

None of the farmers surveyed to date with an operating anaerobic digester said that they

regret their basic decision. Most would have preferred to spend less money on its design

and installation, but they are unsure of exactly how costs could have been cut. Many seek

new ways to increase profitability by the selling of coproducts, primarily the digested

solids. They would like additional assistance in determining how best to optimize the

added value of coproducts. Beating the odds, a few have met the challenge of making

their systems work despite bad design or equipment. The hard knock of practical

experience makes them the true superstars of farm-based anaerobic digestion technology

in the U.S.

Of the conversion systems evaluated to date, slurry-based designs may offer the most

benefits beyond their low cost to install and simplified operation. When combined with

a mechanical scraping system to collect manure, there is little added water compared to

hydraulic flushing. Because the organic acids are not volatilized, the methane and odor

associated with manure decomposition can be minimized to the greatest extent possible.

Emission rates for manure that decomposes in water can be as high as 90% of the total

methane potential!", Covered. lagoon digesters appear to have great economic merit for the

large number of swine and dairy operations in the Southeast and West that incorporate

hydraulic flushing for manure collection and conventional anaerobic lagoons for waste

treatment. Plug flow digestion is economically sensitive to coproduct utilization and other

offsets from current manure management practices. It is less expensive and technically

easier to operate and maintain than a comparable complete mix digester, but the real

world performance of plug flow digesters should make any potential investor wary of this

approach. Complete mix digesters have higher capital costs and operating and

maintenance requirements than covered lagoon and plug flow digesters. This will

generally limit complete mix digester applications to very large farms and centralized

facilities that have waste streams with total solid concentrations too low for slurry or plug

flow digestion and to locations where the climate is too cold to economically justify

covering a lagoon.

Classified Industry Anaerobic Dlgestlen!'

Beyond the farm, extending the anaerobic digestion process to recover methane has

considerable potential for certain classified industries with a waste stream characterization

similar to livestock manures. More than 35 example bio-based industries have been

identified, and include processors of chemicals, fiber, food, meat, milk, and

pharmaceuticals. Many of these industries use anaerobic digestion as a pretreatment step

to lower sludge disposal costs, control odors, and reduce the costs of final treatment at

a municipal wastewater treatment facility. From the perspective of the municipal facility,

13
pretreatment effectively expands existing treatment capacity. Although it is a common

practice to flare any recovered biogas beyond that required for digester heating, some of

these industries already recover methane for displacing on-site energy requirements.

Some of the types of industries that use anaerobic digestion technology to treat

wastewater include alcohol, baker's yeast, bakery, brewery, candy, canneries, chemical,

chocolate, citric acid, coffee, dairy and cheese, distillery, fermentation, fruit juice, fructose

production, pulp and paper, pharmaceutical, potato processing, rubber production, sludge

liquor, slaughterhouse, soft drinks, starch (barley, com, wheat), sugar processing,

vegetable and fruit, and yeast. Because all the technologies examined were initially

commercialized for food processing applications, it is not surprising that this type of

industry is the leader in using anaerobic treatment. The pulp and paper industry, including

recycle mills, is a distant second as an industry category, and all others comprise only

10% or less of the totals. However, a great potential exists for the future anaerobic

treatment of wastewater in the chemical and pharmaceutical industries.

The use of anaerobic technology offers several advantages over aerobic waste digestion.

U sing aerobic digestion, an appreciable fraction of the waste is converted to new biomass

solids, leading to a potential coproduct disposal problem. In contrast, anaerobic digestion

converts tenfold fewer solids, and the difference is converted to biogas with energy value.

There also are favorable operation and maintenance benefits associated with anaerobic

digestion technology that normally include lower energy requirements for operation.

Certain types of anaerobic digestion processes can be extremely space efficient because

they achieve higher loading rates than is usually possible with "conventional" aerobic

processes. On the other hand, the anaerobic digestion of organic matter is normally not

as complete as can be obtained through aerobic digestion, and, at least in the treatment

of industrial wastewater, it is sometimes advantageous to combine anaerobic

"pretreatment" with an aerobic polishing step.

The oldest and simplest form of anaerobic digestion technology employed by classified

industries is the uncovered, unmixed lagoon. To make practical use of the lagoon

technology and shorten the digestion time requirements, mixing methods were devised to

improve the contact between the organic matter and the bacterial biomass, and a cover

to collect biogas was added. This mixed and covered lagoon is now widely used to treat

industrial wastes. As noted earlier, the HRT is measured in weeks or months, depending

on the specific application and digester design.

Concurrently with efforts to improve the design of the anaerobic lagoon, work progressed

on developing a completely mixed (stirred) tank reactor. This "CSTR" or "contact"

process became the standard of the industry during the 1970s, possibly because of its

similarity to the conventional activated sludge aerobic processes widely used to treat

municipal (domestic) wastewater. The CSTR reactor contains a mixer to maintain good

contact between biomass and the organic material to be digested, and a post-clarification

step with biomass return to ensure a steady quantity of mixed liquor suspended solids

(MLSS) within the reactor. This "medium-rate" digestion technology usually has an HRT

measured in days, so it typically offers greater space efficiency than lagoon technology.

14
The anaerobic filter was commercialized in the late 1970s and, as the name implies, relies

on a media substrate to retain the biomass in the reactor vessel. Different types of

substrate materials have been used for this purpose, and, in efforts to reduce the

possibility of plugging, different "flush-out" methods have been developed. These filters

are operated either in an upflow mode or in the less conunon downflow mode, depending

on the manufacturer. The HRT is typically a day or two, generally making these types of

system somewhat more space efficient than the contact system.

The Upflow Anaerobic Sludge Blanket (UASB) process which basically combined the

well-mixed attributes of the contact system with an internal biogas separation and

clarification mechanism to eliminate the need for an external biomass return loop, was

also a development of the mid and late 1970s. The mixing within the reactor results from

the gassing that occurs as the organic components are distributed in the biomass bed at

the bottom. A convection circulation is established that consists of an upward flow of gasborne biomass and a return downward flow of degassed biomass. The reactor contains no

mechanical components, but does have a topworks baffle arrangement that separates the

gas, liquid, and solid phases. The HRT in the UASB reactor is typically measured in less

than a day, making it more space efficient than any system discussed previously. The very

fast HRT, however, works against efficient digestion of organic material by forming

suspended solids.

The fluidized bed anaerobic technology was also developed during the late 1970s in an

effort to improve on loading rates and reduce reactor size even further. In this type of

technology, biomass is attached to heavier particulate matter such as sand so the biomass

can remain in the reactor vessel even against very strong hydraulic flowthrough rates.

HRT in ibis type of reactor is measured in hours, but the need for "carrier material" can

be an operational drawback. A new generation of "ultra high-rate" reactors which

eliminates the need for carrier material but still retain a fluidized bed which facilitates

good biomass contact, has been developed and commercialized during the past 5 years.

The use of anaerobic digestion technology to treat wastewater from industry has grown

tremendously during the past decade to the point where more than 600 vendor-supplied

systems are operating or under construction throughout the world. European plants

comprise about 44% of the installed base, and only 14% of the systems are located in

North America. A considerable number of systems are located in India and South

America (primarily Brazil, where they are used to treat the vinasse coproduct from sugar

cane-based ethanol production). As with farm-based anaerobic digesters, the economic

advantage of recovering and using the methane-rich biogas becomes more attractive with

larger systems. Given the considerable capital cost of anaerobic digesters and biogas

utilization equipment designed to treat industrial wastewater, "large" systems represent

a considerable percentage of the total installed base. The 230+ large systems (those

containing at least 20,000 kg/d COD) represent more than 35% of the installed base.

Acknowledgments

This paper was. prepared for the National Renewable Energy Laboratory under

Subcontract CAE-3-13383-01 and was sponsored by the Regional Biomass Energy

Program of the U.S. Department of Energy, Office of National Programs. Many thanks

15
to the numerous persons contributing to the case studies, and especially to M.R. Hardee

for editorial services.

References

1. Walsh, et al. (1988) Handbook on Biogas Utilization. Southeastern Regional

Biomass Energy Program, Tennessee Valley Authority, Muscle Shoals, AL.

2. Wiltsee (1994) Heat-Activated Cooling Devices: A Guidebook for General

Audiences. Southeastern Regional Biomass Energy Program, Tennessee Valley

Authority, Muscle Shoals, AL.

3. Skillicorn, et al. (1993) Duckweed aquaculture: A new aquatic farming system for

developing countries. The World Bank, Washington. DC.

4. Drawn from Lusk. (1994) Methane Recovery from Animal Manures: A Current

Opportunities Casebook. RELffP-421-7577, National Renewable Energy

Laboratory, Golden, CO.

5. Coppinger, et at. (1980) Economics and operational experience of a full-scale

anaerobic dairy manure digester. In Biogas and Alcohol Fuels Production, ed. J.

Goldstein, The JG Press, Emmaus, PA.

6. Jewell et al. (1979) "Low cost methane generation on small farms," Proceedings.

Third Annual Symposium on Biomass Energy Systems. Solar Energy Research

Institute, Golden, CO.

7. Chandler, Hermes, and Smith (1983) "A low-cost 75-kW covered lagoon biogas

system," Proceedings. Energy from Biomass and Wastes VII. Lake Buena Vista,

FL.

8. Safley and Lusk (1990) Low Temperature Anaerobic Digester. North Carolina

Energy Division, Raleigh, NC.

9. Oswald (1993) "Ponds in the twenty-first century," Proceedings, 2nd International

Association of Water Quality Conference on Waste Stabilization Ponds and the

Reuse of Pond Effluents. Oakland, CA.

10. Center for Rural Affairs. Manure Management in the European Pork Industry.

August 1994 Newsletter. Walthill, NE.

11. Drawn from Sax and Lusk. (1995) Anaerobic Digestion of Municipal, Industrial,

and Livestock Wastes for Energy Recovery and Disposal, Proceedings. Second

Biomass Conference ofthe Americas. D.L. Klass, ed., National Renewable Energy

Laboratory, Golden, CO.

16
ANIMAL MANURE DIGESTION SYSTEMS

IN CENTRAL EUROPE

Erwin Koberle, Dipl.-Ing.

Fachverband Biogas e. V., 89611 Obermarchtat, Germany

Abstract

This work provides an overview of existing plants in Europe and describes the substrates

being used. It focuses on the individual farm-scale and community plants, as these are

the two main types now being built. It also describes plants currently under construction,

especially in Germany and Denmark, where the major efforts are focused. A description

of how the technique has developed over the past few years, its current state of

development, the motivation and economic balance, and the substrate characteristics, is

presented.

17
Introduction

There are currently about 470 biogas plants in Europe for digesting animal manure (see

Table 1). An exact number is not available, as the last inquiry was made 7 years ago.

Table 1. Existing Biogas Plants in Europe

Country Approximate Number of Plants

Germany 200 ,

Denmark 28 ,

Switzerland 90

Austria 50 I

Italy 20

France 15 ,

Great Britain 30

Sweden 5

Other 30

Total 470

About 150 new plants (120 of them in Germany) have been built in Europe during the

past 7 years. As many as 20 plants may have been closed during this same period.

Most European plants are small- or medium-sized farm-scale plants that use 1-20 m"

substrate per day. Nine large farm-scale plants in Germany use more than 20 m" per day.

There are also several plants of this size in concentrated livestock areas of northern Italy,

the Netherlands, and Denmark.

In this paper I will focus on the plants in Germany and Denmark, as biogas plant

development in these countries is the most dynamic and represents the direction of the

development.

Current Development

Three main types of biogas plants are currently being developed in Europe:

• Small- and medium-sized individual farm plants

• Large-scale individual farm plants (high-technology, industrially built)

18
• Conununity plants that collect manure from individual farms (high-technology,

industrially built)

Small- and Medium-Scale Farm Plants

The small- and medium-sized farm plants represent about 70% of the existing plants (and

about 80% of the actual annual growth). They have been in use for about 60 years, and

the oldest plant (located in southern Germany) has been operating for 36 years.

Most plants in Switzerland, Austria, France, and Great Britain are small fann plants.

About half of the existing 28 plants in Denmark are small- or medium-sized plants.

About 190 of the plants in Germany are small- or medium-sized. Germany has seen the

strongest growth of such plants during the past 2 years, with 25 new plants built during

this period.

Technical Achievements

More than 100 plant designs can be distinguished in Europe. Many solutions (only a few

of which have been used for series production) have been invented and constructed.

Plant building boomed in southern Germany (especially Bavaria), Switzerland, and

Denmark, between 1973 and 1984. About 60 plants were built in Bavaria, 80 in

Switzerland, and 10 in Denmark. Many designs (concrete or steel, vertical or horizontal)

were offered by companies that emerged as biogas plants. In Germany more than 10

manufacturers, most of which had built tanks or slurry equipment, suddenly offered

biogas plants. Only a few built more than one plant, and only one built more than 10.

Technical difficulties and falling oil prices forced most to withdraw from the market.

Today only one company is offering medium-scale plants.

Fortunately, before and during the boom years many farmers built their own plants. With

help from scientists at the agricultural school at Weihenstephan in Bavaria and from

ecological groups, the fanners expertise could be gathered and disseminated.

Several engineering offices, which support fanners in building their own plants, were

formed from the ecological groups. Most plants built in Germany since 1985 are based

on this self-construction design, which succeeded because the technique was simplified,

standardized, and combined with individual plant planning. The planning principle

consists of the following parts:

• A specialized engineer performs individual planning

• The prefabricated parts are delivered as a building kit

• Local craftsmen and workers are hired

• The possibility also exists to "do-it-yourself"

Two main plant types, were developed:

19
• Horizontal flowthrough steel digester, using a standard steel tank, often previously

used as a gasoline tank

• Vertical steel storage digester, using a standard slurry storage tank as the digester

The horizontal steel digesters generally range from 50 to 100 rrr', and occasionally to 150

rrr', A stirring axle and arms reach each square foot, making this digester usable for all

substrate types. Because of its limited volume and time-consuming construction, it is

currently used mostly for problematic substrates such as chicken and other solid manures

(see Figure 1).

......, " .--..

11\ "i\ J: .

UeAlnn~

L._'file DI~M Plnnt

Figure 1. The biogas plant with horizontal steel digester.

20
The vertical concrete digester is based on a standard concrete slurry tank as used in

southern Germany. These tanks are series products that provide low-cost volume. They

can be insulated and made gastight, and are often built underground, thereby reducing

space demand. The volumes range from 250 to 600 rrr', but some are 800 to 1200 m" and

range in depth from 3 to 6 m and measure 8 to 16 m in diameter (see Figure 2).

Figure 2. Vertical storage type digester

Depending on the substrate, some stirring systems also handle substrates such as solid

manure with a high fiber content. Special gastight stirrers have been developed based on

a standard slurry stirrer. These can be handled from the outside and adjusted to the

optimum working level (see Figure 3).

Figure 3. Small farm-scale plant, P. Schuler Wennedach

21
Until 1985, most individual farm plants used gas for heating purposes only. Today most

plants use it for cogeneration, with electricity as the main product and heat as a byproduct; however, it is also used to heat homes and water.

The gas is stored in balloons and protected in containers, silos, or shelter huts. The

storage capacity is 60-100 m", This biogas is stored at night, when the biogas engine is

not running.

Substrate Characteristics

The main substrate is liquid slurry from cattle, pigs, and chickens. One-third of the

German plants also use solid manure that includes straw and is liquefied in special mixing

devices with slurry or water.

During the past 2 years, many biogas producers began adding organic matter from the

food processing industry to their manure. Most types of organic waste from food

processing can be used in biogas plants including wastes from slaughterhouses, oil and

fatfrom frying, spoiled food, etc. In areas that have many biogas plants, there is already

a shortage of industrial waste material.

Economic Aspects

There have been only two company-made turnkey plants built in Germany, and two in

Austria, during the past 2 years. However, more than 25 self-constructed plants have

been built during this same period. In Switzerland there are only company-built plants.

In recent years only one or two of these have been built each year. The same applies to

Denmark.

In Germany, electricity from biogas can be sold to the grid for $0.10 (U.S.) per kWh.

The gas can also be cogenerated to provide heat for the farmhouse for $2,000 (U.S.) per

year. During the summer a great deal of excess heat is lost. The cost of a selfconstructed plant for 100 cows (a typical size for southern Germany) is

$100,000-$120,000 (U.S.). Subsidies are granted in most German states for biogas plants

at a rate of 20%-25%. A reasonable return on investment can be achieved under these

conditions. Few company-built plants are built because they cost 50%-100% more than

self-constructed ones.

Large-Scale Farm Plants

These plants are built by industrial contractors at large farms that have a high

concentration of livestock. They use high-technology to treat and reduce the volume of

slurry. During the past 5 years, five such plants have been built in Germany, and a few

have been built in the Netherlands, Great Britain, and Denmark. A demonstration project

was initiated to cover 50% of their costs, however, the overall costs were proven to be

too high. The former Deutsche Demokratische Republik (East Germany) had seven such

plants in operation, the largest of which is in Nordhausen, which produced more than

20,000 m" of biogas per day. This plant now produces half that volume, as the volume

of livestock has decreased by half. Today five plants, which depend on the cogeneration

of industrial waste, still operate.

22
Community Plants

Community plants use the manure from many fanners in a particular area. The first such

plant began to operate 11 years ago. Today 14 plants, which use up to 80 manure

deliverers and up to 440 tonnes per day of substrate, are in operation.

Conununity plants are especially popular in Denmark for the following reasons:

• Individual farm plants had minimal success in Denmark

• The Danish culture stresses cooperation and community involvement

• Most villages have heat distribution grids with central boilers that can

make use of the waste heat produced from biogas cogeneration systems.

The lust three plants were built in Jutland (in the far north) during 1986 and 1987. At

that time, numerous technical and economic problems interfered with their success.

However, the Danish Energy Agency started an action program for centralized biogas

plants that repaired or rebuilt existing plants and constructed new ones. Under the

Agency's supervision, the builders and engineers applied lessons learned from previous

constructions and tested new technologies without damaging the plants.

Technical Achievements

The manure is collected from individual farms by lorry tankers. Up to now, mesophilic

digestion has been used over thermophilic digestion by a 10:5 ratio. However, four of

the next five plants scheduled to be built will use thermophilic digestion.

The development of effective and cleanable heat exchangers for the slurry allows high

process temperatures to be used hygienically while the plant's process heat demand

remains below 20%. A major task of the plant is to sanitize the substrate (eliminate

pathogens), as slurry from many farms is collected and redistributed. Therefore, each

plant either operates at thermophilic temperatures or includes a thermophilic step

performed at 55°C or higher for several hours. When sewage sludge or house waste is

codigested with manure, a sanitizing step performed at 70°C for at least 1 hour must be

included.

Each of the 14 plants has a different digester design and stirrmg system. Some use

horizontal steel cylinders or vertical steel tanks assembled by welding or bolting the

concrete plates. Some plants use large, central, slow-moving stirrers; some use fastturning propellers. Some new plants use vertical steel constructions and slow-turning

central stirrers. Gas may be stored in high- or medium-pressure tanks, in bell-type

gasometers, in balloons, or in pressureless or double membrane systems.

The gas in most plants is cogenerated, and the heat is delivered to a municipal heating

grid. The gas line can reach consumers up to 8 km away. Low-pressure systems have

proven to be the most economical for storing and transporting gas.

Substrate Source

The substrate is the decisive element for all corrununity plants. The lust plants were

constructed to use only livestock manure, but their survival depended on adding industrial

23
and household waste. Today all plants depend heavily on municipal and industrial waste.

Slaughterhouse waste, fish oil, frying oil, bentonite filter mass, household waste, and other

food processing wastes are added in quantities up to 37% of the total input. Gas

production can thus be doubled or tripled, and the process can be stabilized.

The slurry is collected from farms within a 10-km radius of the plants by lorry tankers,

which allows the area's manure to be well managed. The farms with surplus manure can

send the product to farms that need more fertilizer. One of the plant staff's central tasks

is to carry out the logistics of slurry transportation. The manure is stored, sanitized,

homogenized, evaluated for nutrient content, and distributed to farms or fields. This

enables the improved fertilizing qualities of the digested manure to be used where needed.

See Table 2.

Table 2. First 10 Centralized Biogas Plants, 1994*

Plant Input Manure Organic Digestion HRT Gas Gas

(m31 (%) Waste Temperature (days) Production Use

day) (%) CC) (m3/day)

V. Hjennitslev 44 63 37 37 34 4400 CHP**

Vegger 58 73 27 56 15 4500 ClIP

Skovsgard 53 70 30 35 29 3100 ClIP

Davinde 27 86 14 37 28 900 Boiler

Sinding 132 70 30 52 16 7100 CHP

Fangel 152 77 23 37 21 7100 ClIP

Revninge 37 75 25 44 15 1200 Networ

k

Ribe 401 84 16 53 12 11800 ClIP

Lintrup 385 67 33 37 20 11400 CHP

Lemvig 453 79 21 52 17 14800 CHP

*

**

Progress report on the economics of centralized biogas plants, February 1995,

Danish Energy Agency.

Combined heat and power

Economic Aspects

Biogas energy in Denmark costs about $0.28 (U.S.) per m3 when used to cogenrate

combined heat and power (CHP). This stipulates a methane content of 65%. Once the

biogas is converted into heat and electricity, its value increases to $0.42 (U.S.) per m",

Transporting manure and industrial waste feedstocks to centralized biogas plants

24
constitutes 35%-50% of the plant's total operating costs. All plants own the vehicles;

outside transporters are employed for most industrial wastes.

The first conununity plants needed a substantial subsidy (30%-40%), but new plants

require less than 20% (0% in some cases) to achieve economic balance. During the past

8 years construction has improved and prices have decreased; however, the long-term

challenge for centralized biogas plants will be economic viability without industrial wastes

or investment grants. Organic wastes comprise almost 90% of the substrate; already,

organic wastes are being imported from Germany. Biogas plant costs must be reduced

by 15%-20% if they are to be able to operate economically on manure alone.

Individual Farm-Scale Plant in Germany (owned by Paul Schuler, Wennedacb)

Mr. Schuler's farm in Baden-Wiirttenberg is quite typical for southern Germany. He

owns 70 cows and 60 other animals, so his farm is actually somewhat larger than average

at present; however, the average is expected to increase to at least his level over the next

10 years. His main income is from milk. and cereals. His family (three people) are the

farm's only employees. The biogas plant was built during the summer of 1994 and began

operating in December (see Table 3).

Table 3. Basic Figures for the Farm-Scale Biogas Plant

Daily Feedstock 6 m3/day

Digester Size 475 m"

Digesting Temperature 37°C

Effective Digesting Volume 425 rrr'

Stirrer Horizontally and vertically movable,

submerged propeller, 7.5 kW, gastight

Gas Yield 150 m3/day (60% methane)

Process Heat 25% to heat the digester

Gas Storage 70-m3 balloon in 40' container, pressureless

Gas Cleaning Biocatalytic reaction in the digester by

adding a small quantity of air to the gas

zone

Cleaning Rate < 200 ppm hydrogen sulfide

Gas Use, Cogeneration 30 kW, dual-fuel diesel engine

Electricity Production 90,000 kWh/year

Electricity Used on the Farm 20,000 kWh/year

25
Heat Use 50,000 kWh/year to heat the living house

(220 rrr') and provide hot water

Investment 150,000 Deutsche Mark*

Subsidy 35,000 Deutsche Mark

Annual Cost for Investment, Interest, and 14,000 Deutsche Mark

Repair

Annual Income for Electricity Sale, Oil 19,000 Deutsche Mark

Savings

* 1.5 Deutsche Mark = $1 U.S.

Community Plant in Denmark: Ribe

The Ribe community biogas plant started in 1990 and is the first large-scale plant with

thermophilic operation. It is owned by shareholders (the farmers that deliver the manure,

a slaughterhouse, the local electricity company, and investment houses). The gas is

delivered to the electricity company, which runs a cogneration set and delivers the heat

to the town of Ribe (see Table 4).

Table 4. Basic Figures for the Community Biogas Plant

Daily Input, Manure 300 ton/day

Daily Input, Industrial Waste 40 ton/day

Digester Size, Total 4650 rrr', three tanks of 1550 rn"

Digesting Temperature 53°C

Effect, Digesting Volume 4400 m3

Stirrer Top-mounted, continously slowly rotating

Gas Yield 11,800 m3/day (65% methane)

Process Heat < 20% to heat the digester

Gas Storage 1000 rrr' balloon with double-air membrane,

use of iron cIorin

Investment $7.5 million (U.S.)

Subsidy $3.0 million (U.S.)

Sales per m3 Input $8.20 (U.S.)

Costs per rrr' Input $5.20 (U.S.)

Income per m3 Input $3.20 (U.S.)

26
References

Progress Report on the Economy of Centralized Biogas Plants, February 1995, Danish

Energy Agency.

Biogas-Nutzung und Potential in Europa. M. Kottner, 74592 Kirchberg, Fachverband

Biogas e.V.

Wellinger, Arthur. Infoenergie, Tanikon.

27
ANAEROBIC DIGESTION OF MUNICIPAL SOLID WASTE:

TECHNICAL DEVELOPMENTS

Christopher J. Rivard, Ph.D., Senior Microbiologist

National Renewable Energy Laboratory, Golden, CO 80401 U.S.A.

Abstract

The anaerobic biogasification of organic wastes generates two useful products: a mediumBtu fuel gas and a compost-quality organic residue. Although commercial-scale digestion

systems are used to treat municipal sewage wastes, the disposal of solid organic wastes,

including municipal solid wastes (MSW), requires a more cost-efficient process. Modem

biogasification systems employ high-rate, high-solids fermentation methods to improve

process efficiency and reduce capital costs. The design criteria and development stages

are discussed. These systems are also compared with conventional low-solids

fermentation technology.

28
Introduction

The anaerobic digestion process takes place through the synergistic action of four types

of microorganisms: hydrolytic, fermentative, acidogenic, and methanogenic bacteria.

Numerous reviews that detail the level of understanding of the anaerobic digestion process

and the terminal steps of methane production have been published (1-10). The hydrolytic

bacteria in these consortia use cellulase enzymes to depolymerize cellulose to simple

sugars. Proteins, pectins, hemicellulose, and starches (if present in the feedstock) are also

degraded enzymatically. The fermentative bacteria convert these monomers to organic

acids, primarily propionic and acetic acid. The acidogenic bacteria convert these acids

tp hydrogen, carbon dioxide, and acetate, which the methanogens utilize via two major

pathways to produce methane and carbon dioxide.

Historical Commercialization

Anaerobic digestion has been used successfully for centuries to dispose of municipal

sewage (11-14). When used to treat sewage wastes, its primary purpose is to reduce the

organic content, volume, odor potential, and pathogenic microorganism concentration of

the sludge (11-16). Because this organic waste stream is relatively dilute, the overall

effect of sewage treatment is to convert a water pollution problem into a solid waste

disposal problem. However, because of the high level of water in the waste, anaerobic

digestion is carried out in conventional low-solids, stirred tank reactor systems. These

systems are large and require substantial energy inputs for heating and mixing. However,

in treating municipal sewage, the perception of system reliability and utility often

outweighs the process economics.

Solid Waste Conversion

The anaerobic digestion of lignocellulosic materials, such as biomass, agricultural

residues, food processing wastes, and MSW is limited by the rate of polymer hydrolysis

(9,15). The primary biodegradable polymer in most biomass--cellulose-is shielded by

lignin, a relatively inert, polyphenylpropane, three-dimensional polymer (16), and by

hemicellulose (17). This complex structure dictates that natural biodegradation occurs

over a period of months or years, rather than hours or days. Such slow rates of polymer

degradation require long retention times, and large reactor volumes, which result in

greater capital costs in large-scale application.

Economics of Biogas Production

Unfortunately for biogas, not all energy is created equal as described in Table 1. In

general, transporation fuels of common use garner the highest value per energy unit

This information indicates that processing costs for some fuel sources must be minimal

in order to maintain profit margin. As natural gas prices are approximately one-half of

petroleum and almost one-third of gasoline, the process must be maintained as low cost

as possible.

29
Table 1. Current Energy Value per million Btu

I Fuel I $ per Million Btu I

Electricity 20.72

Gasoline 9.33

Petroleum 7.04

Natural gas (methane) 3.79

Coal 1.47

Because the methane produced is relatively low value, the anaerobic process must be

rather simple in design, require little energy to operate, and produce gas at high rates.

The conversion process must also result in near complete digestion to maximize

energy production and residue value.

Economic Criteria for Solid Waste Process Development

When considering the anaerobic bioconversion process for solid organic wastes, we

must be mindful of the economics of competing technologies for solid waste disposal.

Incineration and landfilling will continue to be used (at least in the near future), with

changes being made to meet more stringent environmental mandates. Similar

requirements may be dictated for any disposal process, including those involving

biological treatment. Biological treatments, although desirable in their natural sense of

carbon cycling and co-product production (compost), are often time-consuming.

Without "SUfficient contact time for microbial biodegradation, (especially for cellulose),

incomplete digestion results, which reduces the net energy production and compost

product quality. Conventional low-solids anaerobic fermentation systems have been

used successfully for digesting biomass, animal manures, and MSW (8,22,32).

However, because of relatively slow conversion rates and long contact times, these

reactor systems are large and capital intensive.

In preliminary economic evaluations of anaerobic digestion processes for producing

fuel gas from solid wastes. reactor capital costs have been identified as an important

cost factor. If the reactor volume could be reduced significantly and power use

maintained or decreased. anaerobic digestion process economics would improve

greatly. Increasing the solids concentration within the reactor would be particularly

beneficial, because a decreased reactor volume is possible while the same solids

loading rate and retention time are maintained. However, high-solids slurries are very

viscous and resemble solid materials more closely than typical fluids. Therefore,

conventional mixers such as those employed in continuous stirred tank reactor systems

do not ensure homogeneity within the reactor; problems can develop in providing

adequate dispersion of substrate. intermediates, and microorganisms while minimizing

power requirements.

30
Discussion

High-Solids Digestion Systems

High-solids anaerobic fermentations are conunonly found in the landfill disposal of MSW.

However, landfill high-solids anaerobic bioconversion is very slow because process

control is lacking (18). High-solids anaerobic bioreactor technology development has

employed a variety of designs, including non-mixed (i.e., packed bed) batch-operated

reactors (generally with liquid recycle, [19-26]), and mixed or partially mixed reactor

designs (27-30). In general, mixing has been viewed as a requirement for effective

anaerobic fermentation for increasing the accessibility of the consortium to the substrate.

In conventional low-solids systems, various agitation systems have been used successfully

to anaerobically treat municipal sewage, including impeller agitators, recirculating pumps,

and gas injection. However, substantial modifications of low-solids mixing systems are

required to obtain mixing at high-solids levels.

The use of packed bed systems that are operated in a batch mode are enhanced through

a passive mixing system that involves free liquid. These systems may approximate semicontinuous operation by using staged operation as in the SEBA~ process (31). The

passive mixing of solids with a pumping action is demonstrated by the bio-funnel, in

which material must flow over an expanding bed (30). However, the bio-funnel requires

that lower-solids materials be used to ensure a minimum level of plasticity to effectively

move materials within the reactor system (Le., > 21 %). The VALORGA system uses gas

mixing to develop zones of mixing similar to low-solids "air-lift" fermentation systems.

However.' mixing high-solids materials with gas injection requires substantial pressures

(i.e., 4-8 bar overpressure). Materials flow through the system based on a central baffle

and sludge removal is enhanced by operating the reactor at slight overpressure.

Mechanically mixed systems may be accomplished either through internal mixers to the

reactor or alternatively by external mixing. The DRANCO system employs external

mixing of the digester sludge with a smaller amount of fresh feed and reintroduces it to

the unmixed reactor. In contrast, the NREL high-solids system uses a" conventional

internal mixer similar to a pug-mill to mix in a horizontally positioned reactor. All these

designs affect in some way the fermentation efficiency, capital costs, operating and

maintenance costs, and complexity of the overall process. Additionally, with at least one

mixing system, (the bio-funnel), the level of solids is limited so that materials in the

reactor can be effectively moved.

The mixing systems described here are representative designs for nuxmg high-solids

materials; the list is incomplete. However, representative data from these systems are

listed in Table 2 for comparison with conventional low-solids systems. This review of

fermentation data has been limited to systems operated on the food/yard waste fraction

of MSW. Here the MSW, obtained primarily from residential sources, has been subjected

to some level of mechanical/manual separation process to remove inert (and recyclable)

materials and size reduced before anaerobic digestion.

31
Conclusion

The data indicate that when using a mixture of primarily food/yard wastes derived from

residential MSW. organic loading rates of 15-19 kg volatile solids (VS)/m3.d may be

achieved while operating the anaerobic fermentation at sludge total solids levels exceeding

30%. The data for the high-solids systems - DRANCO. VALORGA. and NREL - are

comparable with the exception of the methane yield. Potentially; by using a completely

mixed system, the overall fermentation performance is enhanced in the NREL system.

Additionally. differences in MSW feedstock processing may account for differences in the

substrate biodegradability.

Effects of High-Solids Fermentation on Reactor Sizing

Several conclusions may be drawn about the advantages that high-solids operation affords

the anaerobic process. When comparing low-solids systems to a representative high-solids

system, significant advantages in relative reactor size result from reducing the level of

process water. thus increasing the sludge total solids. Additionally. the high-solids system

is capable of significantly higher organic loading rates, and maintains overall anaerobic

bioconversion yields, that further reduce the required size of the reactor system.

Combining the two effects of reducing the process water content in the reactor and

increasing the organic loading rate dramatically reduces the reactor size over the

conventional low-solids reactor system to treat the same volume of waste.

References Cited

1. Buswell, A. M., and Hatfield, W.O.• (1936), Anaerobic Fermentations, State of

lliinois Department of Registration and Education Bulletin, Chicago, 32.

2. McCarty, P.L., (1964), Public Works 95, 107.

3. Wolfe, R. S., (1971). Microbial Formation of Methane. Adv. Microbial Physiol. 6,

107-145.

4. Mah, R. A.; Ward, D. M.; Baresi, L.; Glass, T. L., (1977), Biogenesis of Methane.

Ann. Rev. Microbiol. 31, 309-341.

5. Zeikus, J. G., (1977), The Biology of Methanogenic Bacteria. Bacteriol. Rev. 41,

514-541.

6. Balch, W. E.• Fox, G. E., Magrum, L. J., Weese, C. R., and Wolfe, R. S.

(Microbiol. Rev. 1979, 43. 260.

7. Bryant. M.P., (1979), Microbial Methane Production: Theoretical Aspects. J. Animal

Sci. 48, 193-201.

8. Clausen, E. C.; Sitton, O. C.; Gaddy. J. L., (1979), Biological Production of

Methane from Energy Crops. Biotech. Bioeng. 21, 1209-1219.

9. Boone, D. R., (1982), Terminal Reactions in the Anaerobic Digestion of Animal

Waste. Appl. Environ. Microbiol. .43, 57-64.

10. Daniels, L.; Sparling, R.; Sprott, G. D., (1984), The Bioenergetics of

Methanogenesis. Biochim. et Biophys. Acta 768, 113-163.

11. Metcalf & Eddy, Inc.• (1979). Wastewater Engineering: Treatment, Disposal, Reuse,

New York: McGraw-Hill

32
12. U.S. Environmental Protection Agency, (1979)~ Process Design Manual for Sludge

Treatment and Disposal, EPA 625/1-79-011, Cincinnati, OH: Environmental

Research Information,

13. Arora, M.L. (1980)~ Water and Sewage Works~ 127, 24.

14. Federal Register, (1989), Standards for the Disposal of Sewage Sludge, Proposed

Rule.

15. Noike, T.; Endo, G.; Chang, J-E.; Yaguchi, J-I.; Matsumoto, J-I.~ (1985),

Characteristics of Carbohydrate Degradation and the Rate-Limiting Step in

Anaerobic Digestion. Biotechnol. Bioeng. 27, 1482-1489.

16. Sarkanen, K.V.~ and Ludwig, C.H., (1971), Lignins: Occurrence, Formation,

Structure, and Reactions, Wiley-Interscience, New York, NY.

17. Grohmann, K., Torget, R., Himmel, M.E., (1985) Biotech. Bioeng. Symp, 15: 59-80.

18. Rivard, C.J. "Municipal Solid Waste: The Mounting Problem". J. Environ. Health

52, 99-100 (1989).

19. Ghosh, S. "Leach-bed Two-phase Digestion- The LanFilgas Concept," in Proc.

Ninth Annual Symp, on Energy From Biomass and Wastes, 743-761 (1985) January

28 - February 1.

20. Goebel, R.P. High Solids Anaerobic Digester for Rural Use, #DE-FG03-81 SF

11613, Washington, DC: U.S. Department of Energy, 1983.

21. Hall, S.J., Hawkes, D.L., Hawkes~ F.R.~ and Thomas, A. "Mesophilic Anaerobic

Digestion of High Solids Cattle Waste in a Packed Bed Digester," J. Agric. Eng.

Res. 32, 153-162~ (1985).

22. Jewell, W.J. Anaerobic Fermentation of Agricultural Residue, Potential for

Improvement and Implementation, US DOE Project #DE-AC02-76ETI0051,

Washington, DC, 1980.

23. Lin, Y.-M. "Coupled High Solids Fermentation and Anaerobic. Filtration of

Cellulosic Residues," Ph.D. Thesis, Michigan State University, 1983.

24. Molnar, L.~ and Bartha, I. "High Solids Anaerobic Fermentation for Biogas and

Compost Production," Biomass 16, 173-182 (1988).

25. Snell Environmental Group Rapid Methane Generation from Solid Waste, #DEFG02-81R510329, Washington, DC: u.S. Department of Energy, (1983).

26. Wujcik, W.J.~ and Jewell, W.J. "Dry Anaerobic Fermentation," in Proc. from the

Second Symp. on Biotechnology in Energy Production and Conservation, 43-65

(1979) October 3-5.

27. Begouen, 0., Thiebaut, E.~ Pavia, A .., and Peillex, J.P. "Thermophilic Anaerobic

Digestion of Municipal Solid Waste by the Valorga Process," In Proc. of the Fifth

Int. Symp, on Anaerobic Digestion, 789-792 (1988) May 22-26.

28. de Baere, L.~ and Verstraete, W. "High Rate Anaerobic Composting with Biogas

Recovery," Biocycle 25., 30-1 (1984).

29. Gaddy, J.L., and Clausen, E.C. SERf Anaerobic Digestion Program 1984 Annual

Report, SERI/PR-231-2691, Golden, CO: Solar Energy Research Institute, 1985.

30. Goldberg, T., Bjergbaek, B., Djernaes, E., and Mouritzen., A. "An Anaerobic

Digester for Continual High Solid Loading: The Bio-Funnel (A Radially Expanding,

Overflow Digester)," in Proc. of the Int. Gas Res. Conf. 734-741 (1981).

31. O'Keefe, D.M., Chynoweth, D.P., Barkdoll, A.W., Nordstedt, R.A.~ Owens, J.M.,

and Sifontes, J., "Sequential Batch Anaerobic Composting of Municipal Solid Waste

(MSW) and Yard Waste,lI Water Sci. recn; 27:77-86 (1993).

33
32. Isaacson, R., Pfeffer, J., Mooij, P., and Geselbracht, J., "RefCoM-Technical Status,

Economics, and Market," in Klass, D., ed. Energy from Biomass and Wastes, XI,

Institiute of Gas Technology, 1987.

33. Ghosh, S., "Solid-Phase Methane Fermentation of Solid Wastes," Paper presented

at the 11th ASME National Waste Processing Conference, Orlando, FL, 1984.

34. De Baere, L., Van Meenan, P., Deboosere, S., and Verstraete, W., "Anaerobic

Fermentation of Refuse," Resources and Conservation, 14:296-308, 1987.

35. Bonhomme, M., "The VALORGA Process for the Recycling of Urban Waste by

Methanization," in Klass, D., ed., Energy from Biomass and Wastes, XII, Institute

of Gas Technology, 1987.

36. Rivard, C., "Anaerobic Bioconverion of Municipal Solid Wastes Using a Novel

High-Solids Reactor Design: Maximum Organic Loading Rate and Comparison with

Low-Solids Reactor Systems," Appl. Biochem. Biotech., 39/40:71-82, 1993.

34
Table 2. Analysis ofHigh-Solids Reactor System Fermentation Performance

with a Conventional Low-Solids System

System Solids Temp. RT Scale OLR CH4 Yield Reference

(% sludge) ro (d) kg VS/m3.d SCM/kg VS added

CSTR 2.7-6.3 60 6-27 2xl274 m' 3-9.6 0.13-0.30 32

Packed Bed 30-40 35 90 0.5 m' 1.12 0.21 33

DRANCO 30-35 50 20 56.6 m' 16 0.28 34

VALORGA 35 37 15 400 m' 15 0.2 35

NREL 28-35 35 14-20 0.02 m' 18.5 0.33 36
BIOGAS UTILIZATION

Mark A. Moser, MSAE

Resource Conservation Management, Inc.

P.O. Box 4715

Berkeley, CA 94704 U.S.A.

Abstract

Options for successfully using biogas depend on project scale. Almost all biogas from

anaerobic digesters must rust go through a gas handling system that pressurizes, meters,

and filters the biogas. Additional treatment, including hydrogen sulfide-mercaptan

scrubbing, gas drying, and carbon dioxide removal may be necessary for specialized uses,

but these are complex and expensive processes. Thus, they can be justified only for

large-scale projects that require high-quality biogas. Small-scale projects (less than 65

cfm) generally use biogas (as produced) as a boiler fuel or for fueling internal combustion

engine-generators to produce electricity. If engines or boilers are selected properly, there

should be no need to remove hydrogen sulfide. Small-scale combustion turbines, steam

turbines, and fuel cells are not used because of their technical complexity and high capital

cost. Biogas cleanup to pipeline or transportation fuel specifications is very costly, and .

energy economics preclude this level of treatment.

36
Introduction

This paper is applicable to biogas utilization up to 90,000 fe per day (62.5 cfrn), the

average amount of biogas prodoced by 100% conversion of the manure from 1,500 dairy

cows or 22,500 pigs. Biogas is produced by a digester at a steady daily rate. It is

assumed that other papers have covered biogas production. This paper covers in-vessel,

low-pressure «0.5 psi) biogas storage, transmission, handling, and use.

Limitations

Because of technical complexity and high cost, medium- (up to 200 psi) and high-pressure

(>2000 psi) biogas storage will not be considered. Pressurized storage requires gas

treatment to clean and dry biogas and high-pressure compressors, piping, and storage

tanks. To date there is no profitable method for storing biogas at these pressures.

This paper does not cover steam generation for process steam or steam turbines, direct

combustion gas turbines, Sterling engines, or fuel cells. These technologies are not

currently in use at any farm digesters.

Published resources and manufacturers were consulted to verify the reasons these

technologies are not in use. The excluded technologies require a greater degree of

engineering, installation, and operating skills than normally found in the farm community.

Small-scale equipment is custom built and supplied by a very limited group of suppliers.

Service for these units is generally unavailable.

A more detailed discussion of equipment can be found in Biogas Utilization Handbook

published by the U.S. Department of Energy, Southeastern Regional Biomass Energy

Program, Tennessee Valley Authority, Muscle Shoals, Alabama 35660. Individual

equipment suppliers should be contracted for equipment performance and specifications.

Biogas Utilization Considerations

Location

Energy utilization equipment should be protected from animals and machinery in an area

of the farm frequented by farm personnel. All buildings and equipment should be

installed with adequate room for servicing and in accordance with all building codes.

Short distances for transmitting biogas and hot water through buried pipes are preferable.

Safety Considerations for Biogas

The major components of biogas are methane (CH4 ) and carbon dioxide (COz). Another

component of concern is hydrogen sulfide (H2S). Biogas such as "manure gas" can be

toxic if inhaled directly, corrosive to equipment, and potentially explosive in confined

space when mixed with air. When properly managed, biogas is as safe as any other fuel

such as propane used on the farm. If improperly managed, these elements can be very

hazardous, as shown by the number of "manure gas" incidents in which fanners have

been injured or killed (see Appendix).

37
~ystem Safety

Buried pipes should be placed adjacent to. but not in. roadways to avoid crushing.

~boveground plastic pipe should be placed where it cannot be frozen or damaged.

~encing or impact barrier posts may be needed. The very low pressure «1 psi) of biogas

n the piping system limits risks associated with damaged piping. Because of the potential

'or accumulation of gases. biogas equipment should be installed in properly ventilated

.ooms with adequate air inlet and exhaust. Exhaust stacks, flares, or emergency gas

pressure relief valves should be located away from buildings and ventilation intakes. and

should be placed high enough to avoid danger to people and livestock. "Confined space

entry," "no smoking," and "rife hazard" signs should be posed where necessary. A simple

gas detection monitor and alarm are recommended.

Biogas Production

Daily biogas production, with storage limited to less than 12 hours of production, is the

key factor in sizing all system components.

Biogas Flow Rate

The maximum biogas flow rate (fe/min) with a 1.5 safety factor should be used to size

all components.

Selection of Materials

All materials. such as PVC. ceramic, stainless steel. or aluminum, should be corrosion

resistant. Carbon-steel should be painted with multiple coats of an epoxy paint (or

equivalent) to reduce corrosion.

Value ofEnergy

By far the most important issue in biogas use is the value of the energy replaced by

biogas.

Heat Recovery for Digester

IT heat recovery for digester heating is required, it must be considered throughout a design

process.

Biogas Transmission

Biogas must be conveyed via a transmission system from the digester to the gas handling

system.

Components

Low-Pressure Storage

The digester cover system may be designed to store a small amount of biogas for a short

time. Floating or inflatable covers may be used to store 1 to 12 hours of biogas

production at pressures less than 2 inches water column pressure (.07 psi). Properly

designed fixed covers may be able to store 1-3 hours of gas production at up to 10 inches

water column pressure (.37 psi) depending on the head space available below the cover.

38
Biogas Intake

The biogas intake is a pipe (usually PVC) extending under the biogas collection cover and

into the .gas space to collect biogas and withdraw it to the biogas transmission system.

A minimum 6 inches intake should be used to reduce clogging.

Gas Pressure Relief

A pressure relief value releases pressure buildup when biogas is produced faster than it

is being used, and should release excess biogas in a pipe to a flare. Commercial pressure

relief valves are available, or simple manometer-type pressure reliefs can be built.

Flame An-ester (optional)

A flame arrester prevents flame from burning back down a pipe. This is not expected to

be needed, because there is no oxygen in the gas to allow combustion. Some local

building codes may require a flame arrester. Commercial flame arresters are available,

or simple small arresters can be built.

Condensate Drain

Biogas, as produced, is saturated with water vapor. When biogas exits the digester and

enters the pipeline, water vapor condenses on the cooler surfaces and must be drained

from the pipe. At least one condensate drain is needed. Simple "J" or "U" traps below

T fittings are suitable condensate drains that require little or no maintenance. An access

sump or manhole may be required for observation and service. Additional condensate

drains will be required at all low points in a biogas transmission pipe. Condensate drain

manholes should not be located where surface water or groundwater can enter the

manhole, because water may backflow from the manhole into the biogas transmission pipe

and cut off gas flow.

Flore

A flare is recommended to burn off excess biogas. There may be occasional excess

biogas production or times when the gas use system is under repair and excess gas must

be released. Flaring will reduce potential dangers associated with unburned biogas.

Commercially available flares or farm-built flares may be used. A windscreen .prevents

flare blowout. A farm-built flare can be constructed from excess construction materials

in about 4 man-hours. Automatic ignitors should be resistant to corrosive gases.

Transmission Pipe

PVC pipe is commonly used to transmit biogas from the methane recovery system to the

utilization system. Underground transmission pipe is sloped to a condensate drain. If

installed aboveground, PVC must be painted and protected from breakage. Some city or

county codes may require exposed biogas piping to be metal.

Biogas Handling

To properly size components, the gas use must be identified before the biogas handling

system is developed. The biogas transmission system is designed to deliver biogas at a

pressure between 0.5 and 8 inches of water column, depending on the collection cover

system.

39
Components

Gas Filter

MI gas streams should have an inline "Y" type strainer-screen filter as a minimum to

catch debris left in the pipeline during construction.

Gas Treatment (optional)

Biogas is saturated with water vapor and will contain H2S.. Table 1 shows ranges of H2S

found in biogas streams. Properly operated engines and boilers will not require gas

treatment. Gas treatment may be needed for gas uses where hydrogen sulfide could be

corrosive to equipment such as combustion turbines, direct fire room heaters, adsorption

chillers, and forced air furnaces.

For small-scale biogas uses, the most cost-effective biogas treatment technique is dry

chemical oxidation to bind sulfur compounds. Iron sponge, activated carbon, and

chemically impregnated filter cartridges can be used to remove in line debris, water vapor,

or H2S. The filter should be sized to have a maximum pressure drop between 0.5 to 1

inch when new and a 6 to 12-month filter material service life.

Table 1. Ranges of Hydrogen Sulfide Found in Biogas from Various Sources

Digester Type

Covered Lagoon

Complete Mix Digester

Plug Flow Digester

Range of H2S in Biogas

500-2500 ppm

1800-6000 ppm

1500-2800 ppm

Gas Pump or Blower

A gas blower or pump is used to increase biogas pressure up to 10 psi. The gas

blower/pump can be either a vane or lobe type and either belt or direct driven. The gas

blower should be made of noncorrosive materials and have a noncorrosive shaft seal.

Gas Meter

An accumulator-type flow-through gas meter with a readable face is used to measure

biogas flow to the gas use. A lobe or vane meter is preferred. A bellows-type natural

gas meter should not be used. Meter components that contact biogas should be stainless

steel. The daily gas quantity is used to monitor and infer the biological health of the

methane recovery system. The ratio of a daily output (kWh, tons of cooling, Btu of heat)

to daily gas quantity is monitored to assess the status of the equipment.

Gas Backpressure Regulator (optional)

A commercial backpressure regulator is used to relieve excess gas pressure back to the

beginning of the gas handling system.

Condensate Drain

Condensate drains are required at all low points.

40
Biogas Use

Biogas that is pressurized and metered can be used as fuel for heating, adsorption cooling,

electrical generation, and cogeneration. Biogas can be substituted for low-pressure natural

gas or propane in the equipment listed in Table 2. Many other types of equipment are

possible. Only those currently in use will be further described.

Table 2. Common Biogas Use Options

Biogas Fueled Engine

Electrical generator-electricity for use or sale, heat recovery optional

Direct Combustion Options

Hot water boiler-for space heat, process and cleanup hot water

Demonstrated but not in use-direct rue heaters, adsorption chillers

System Operation Plan

A system operating plan must be developed to determine how to use the biogas. Tables

3 and 4 list important factors in developing a biogas use option.

Table 3.. Important Factors-Biogas Supply

1) Biogas is produced year-round

2) Biogas storage for longer than 23 h is very expensive

3) Biogas production from heated complete mix and plug flow digesters

should be stable, if manure collection is stable

4) Biogas production from covered lagoon digesters will vary seasonally

5) Digester may require heat

Table 4.. Important Factors-Biogas Use

1) Space heat and cooling requirements vary seasonally

2) Product refrigeration requirements are usually continuous, though total

requirements vary seasonally

3) Electricity use may be widely variable daily and seasonally

4) Electricity use varies between farms and management techniques

5) Adsorption chillers and engine-driven uses can recover hot water

6) Digester may require heat

41
Selecting and Sizing Biogas Use Options

Biogas use must be matched to biogas availability and energy requirements. The most

appropriate use for biogas is to replace purchased energy for heating, cooling, or

electricity. A gas use should be selected to maximize economic return. The price,

quantity, and pattern of electrical and thermal energy consumption must be analyzed to

assess potential project returns. An electricity cost in 1995 of $0.065 per kWh or a

propane value of $0.70/gallon are probably the minimum values at which biogas

substitution can be feasible.

Biogas Requirement of Gas Use Equipment

Table 5 summarizes the estimated Btu requirements of various biogas use options.

Table 5. Equipment Biogas-Btu Use Estimates

(estimation purposes only)

Assumes biogas is 600-750 Btu/ft"

Biogas Fueled Engine at 23.5% efficiency

Engine-driven generator-15,000 biogas Btu/kWh

Direct Combustion Options

Hot water boiler-Natural gas Btu requirement x 110%

Direct rue room heater-Natural gas Btu requirement x 110%

Adsorption cooling-3-30 ton/h standard - 27,500 biogas Btu/h/ton cooling

30+ ton/h double effect - 13,400 biogas Btu/h/ton cooling

Biogas Fueled Internal Combustion (IC) Engines

The most common use of biogas on a farm is to fuel an engine generator to produce

electricity for on-farm use or off-farm sale. Hot water for digester heating and on-farm

uses can be recovered from the engine jacket water and exhaust. Hot air has been

recovered from radiator exhaust to heat buildings.

Natural gas or propane engines are easily converted to bum treated biogas by modifying

carburetion and ignition systems. Natural gas engines are available in virtually any

capacity. The most successful engines are industrial natural gas engines that can bum

untreated biogas. A biogas fueled engine-generator will normally convert 18%-25% of

the biogas Btu to electricity, depending on engine design and load factor. Gas treatment

is not necessary if proper maintenance procedures are followed. Biogas engines less than

200 horsepower (150 kW) generally meet all California air pollution restrictions without

modification if run with a lean fuel mixture.

42
Control systems for engine-generators are well developed. The control system must

operate in the harsh environment of a farm, and excess automation often fails- where

simple manual and mechanical controls usually succeed.

Electricity Generation Options

Isolated versus Parallel Power Production

A farm may choose to use a stand-alone engine-generator to provide all or part of its own

electricity as an "isolated" system or operate connected to and mixing electricity with the

utility "in parallel."

An isolated system must be able to function continuously, without interruption, to meet

fluctuating levels of electricity demand while maintaining a smooth and steady 60-cycle

current. Varying electrical loads or large motor starting loads can lead to drift in the 60-

cycle current. Drift results in wear on motors, speedup or slowdown of clocks and timers,

and operating problems with computers and programmable logic controllers.

Isolated systems require a sophisticated control system and a gas reservoir to meet

changing loads. They are generally oversized to accommodate the highest electrical

demand, but operate less efficiently at average or partial load.

A parallel system is directly connected to the utility and matches the utility phasing,

frequency, and voltage so that farm-produced power blends directly with utility line

power. A utility intertie panel with safety relays is required to operate in parallel and to

disconnect the farm generator if there is a problem with either the utility or the farm

generation.

Parallel operation allows the farm generator to run at a constant output regardless of farm

demand. Constant output allows more efficient use of biogas and less wear on the

engine. The engine-generator can be sized for biogas availability rather than for farm

requirements. The farm buys power when it underproduces and sells power when it

overproduces. The utility is the system backup if engine maintenance is required.

Generator Options

Induction Generators versus Synchronous Generators

A synchronous generator will operate either isolated or in parallel. It can provide

electricity to the farm if the utility is shut down. Synchronous parallel generation requires

a sophisticated intertie to match generator output to utility phase frequency and voltage.

An induction generator will operate in parallel with the utility and cannot stand alone.

Induction generation derives its phase, frequency, and voltage from the utility. Fewer,

less-sophisticated relays are required to protect the utility. Negotiations with a utility for

intertie of a small induction generator are generally much easier.

43
Parallel Operating Schemes

Generation for On-Farm Use and Surplus Sale

The goal is to produce primarily to avoid purchasing electricity from the utility.

Shortages are purchased and excess is sold.

Sale ofElectricity to the Utility

The goal is to produce and sell electricity to the utility. Also known as "buy-sell"

agreement, the utility pays for all electricity produced and the farm pays for all electricity

used. Most utilities currently buy electricity for approximately 25% of the sale price.

Preferred Operation Scheme

An ideal agreement would be a monthly offset of excess kilowatts sold off-farm against

any power purchased on-farm with appropriate payment or credit.

Utility Issues

The key issue in developing a profitable biogas recovery system is the value of the energy

to the owner. A careful review of utility rates and interconnection requirements is

necessary before selecting the operating mode. Rate negotiation is appropriate for farmscale projects, as most rules are set up for very large independent power producers.

Effects of Utility Rate Structures

The farm has to be careful in rate analysis because high "demand" charges can negate half

the value of the electricity produced. "Demand" is usually the highest rate of electricity

consumption for 15 minutes during a month. To offset demand charges, a generator must

achieve 99.95% operation. Some utilities offer a "backup" or "standby" charge, which

is usually lower than a demand charge.

Under Section 210 of the Federal Public Utility Regulatory Policies Act of 1978

(PURPA), utilities must purchase power produced by qualifying cogenerators and small

power producers. Biogas-fueled electricity generation qualified by definition. However,

the utilities seldom offer reasonable purchase prices. H electricity can be sold more

profitably to a neighboring facility, this option can be investigated.

Intertie Requirements

Each utility has intertie requirements for protective relays to disconnect the generator

automatically if the power line near the farm is accidentally broken or if there is a

problem with the generator. These relays are necessary to protect farm and utility

personnel. A professional familiar with intertie equipment should negotiate with the

utility and supply the appropriate gear. Negotiation is necessary because of the potential

cost of the intertie. Solid-state relays and electromechanical relays perform the same

function; however, electromechanical relays may cost 10 times more. A utility may need

high-cost relays for very large power producers, but lower-cost relays operate well for

farm-scale installations.

44
Hot Water Boiler

Thousands of biogas-fired boilers are in use at municipal waste treatment plants in the

U.S., where they provide hot water for building and digester heat. Conversion efficiencies

are typically 75%-85%. Several have been installed on farm digesters.. Farms require hot

water year-round, but there is typically more biogas available than hot water required.

Farrow to wean and farrow to nursery hog farms in cold climates are the only types of

farm where hot water use could consume most or all of the available biogas production

potential.

A cast iron natural gas boiler can be used for most farm applications. The air fuel mix

will require adjustment, and burner jets will have to be enlarged for low Btu gas. Cast

iron boilers are available in a wide range of sizes from 45,000 Btu/h and larger.

Untreated biogas can be burned in these boilers; however, all metal surfaces of the

housing should be painted. Flame tube boilers with heavy gauge flame tubes may be

used if the exhaust temperature is maintained above 3000P to minimize condensation.

High HzS concentration in the gas may clog flame tubes.

Future Alternatives-Forced Air Furnaces, Direct-Eire Room Heaters, Adsorption

Chillers

Forced air furnaces, direct-fire room heaters, and adsorption chillers fueled by treated

biogas have been demonstrated but are not commonly used in the U.S. This equipment

is manufactured from thin metal and is not corrosion resistant; therefore, gas treatment

for water and HzS removal is needed. Also, these units must be rejetted with larger

burner orifices. These technologies are most suited to covered lagoon digester gas

production because they are generally not designed to return hot water necessary for

digester heating. However, most facilities do not need either heating or cooling yearround.

Forced air furnaces could be used on hog farms in place of direct-fire room heaters.

Direct-fire room heaters are commonly used in hog farrowing and nursery rooms. A farm

will typically have multiple units. Biogas-fired units have not been installed in the U.S.

because owners are reluctant to risk losses in animal production. These heaters are

available and in use in Taiwan.

Gas-fired adsorption chillers produce cold water for milk cooling or air conditioning.

Dairies cool milk every day of the year. Chilled water or glycol can be used in milk

precoolers in place of well water. Units are under development that should produce

glycol at temperatures lower than 30°F and allow direct refrigeration. A dairy generally

requires 0.014 tons of cooling per hour per milk cow per day, which is about 15% of the

potential biogas production from the same cow (one ton cooling = 12,000 Btu/h).

Double effect chillers, producing hot and cold water simultaneously, are available for

applications more than 30 tons and could be coupled with a heated digester.

45
Appendix

Safety Concerns of Biogas Components

Methane

Potential hazards: asphyxiation, explosion. When collected from a digester, methane

makes up 60%-80% of the biogas volume. As generated and collected, biogas does not

contain oxygen. As collected, there is no combustion or explosion hazard due to the lack

of oxygen. The piped biogas will not bum until mixed with air. Methane generally does

not collect in pits as it is lighter than air. If released in a confined space and diluted to

approximately 17%-23% concentration in air and given a source of ignition, an explosion

can occur. If it leaks into a sealed or poorly ventilated room and displaces air, the lack

of oxygen can be harmful to animals and humans. Smoking should be prohibited in areas

where biogas is collected and used.

Carbon Dioxide

Potential hazards: asphyxiation. CO2 makes up 20%-40% of the biogas volume, is

heavier than air, and will collect in poorly ventilated pits and confined spaces, while other

gases dissipate. CO2 does not bum or explode. If it leaks into a sealed or poorly

ventilated room and displaces air, the lack of oxygen can be harmful to animals and

humans. Entry into pits or confined spaces filled with COz may result in asphyxiation.

Hydrogen Sulfide

Potential hazards: human toxicity, equipment corrosion. HzS is present in the air of all

confined animal feeding operations. It is lighter than air and dissipates rapidly. Very low

concentrations corrode steel, iron, aluminum, and copper. HzS makes up 0.1%-0.3% (500

ppm to 6000 ppm) of the volume. HzS can be detected by its "rotten egg" smell in

concentrations of 1-100 ppm. Direct inhalation of HzS in concentrations above 10 ppm

is harmful to the respiratory tract. Direct inhalation of HzS above 500 ppm will cause

rapid respiratory failure, and inhalation of concentrations above 1000 ppm can cause

unconsciousness and death.

46
ENVIRONMENTAL IMPACTS OF ANAEROBIC DIGESTION

AND THE USE OF ANAEROBIC RESIDUES AS

SOIL AMENDMENT

Frank E. Mosey BSc, Environmental Consultant,

VFA Services Ltd, 32 Park View, Stevenage,

Herts, SG2 8PS, United Kingdom

Abstract

This paper defines the environmental. role of anaerobic digestion within the overall

objective of recovering energy from renewable biomass resources. Examples and

opportunities for incorporating anaerobic digestion into biomass-to-energy schemes are

discussed, together with environmental aspects of anaerobic digestion plants. These

include visual, public amenity, pathogens and public health, odor control, and gaseous

emissions.. Digestate disposal and the benefits of restrictions on recycling organic wastes

and biomass residues back to the land are discussed, particularly as they relate to

American and European codes of practice and environmental legislation. The paper

concludes that anaerobic digestion, if performed in purpose-designed reactors that

efficiently recover and use biogas, is an environmentally benign process that can enhance

energy recovery and aid the beneficial land use of plant residues in many biomass-toenergy schemes.

47
Introduction

Anaerobic digestion (methanogenesis) is a natural bacterial process during which organic

matter is fermented to produce a biogas, which typically contains about 60% CH4 and

40% CO2, It occurs wherever wet organic matter accumulates in the absence of air, in

the sediments of oceans and lakes, in wetlands and rice paddies, and within the digestive

tracts of many animals, especially termites and ruminants. It is an environmentally

benign process that benefits mankind by removing putrescible waste materials, destroying

odors and pathogens, and recycling plant nutrients and humus solids to improve soil

fertility. The CH4 it produces is somewhat of a mixed blessing. Released directly to

atmosphere it is a powerful greenhouse gas. Captured and burned to CO2 and water, it

provides a useful renewable fuel.

Anaerobic Digestion for Renewable Energy Production

Pure methane has a gross calorific value of 39.7 MJ/m3

, and the biogas produced by the

anaerobic digestion process has a gross calorific value of 23.8 MJ/m3 (640 Btu/ft"), which

makes it a useful renewable fuel for generating electricity or steam. The gas bubbles

directly from the fermentation vessel and can be collected without any sophisticated

extraction procedure. Even after deducting the electricity required to operate plant

machinery, anaerobic digesters are net energy producers and offer an efficient method of

capturing residual energy values from wastes and by-products that are too wet or

otherwise unsuitable for incineration. The major energy requirement of the process is

low-grade heat to warm the fermentation tank: (to 35°C or 55°C, as appropriate).

Biogas, Methane, and Energy Yields

Methanogenic fermentations generate very little metabolic heat and, as shown in Table

1, complete quantitative transfer of calorific values to methane is theoretically possible.

Table 1. Theoretical Biogas, Methane, and Energy Yields from Anaerobic

Digestion of Pure Compounds (per kg Organic Matter Destroyed).

Pure Calorific Biogas Yield Methane Calorific Value

Compounds Value (m3/kg) Yield (m3/kg) of Methane*

(MJ/kg)

Fats 38,200** 1.44 1.03 40,900 MJ

Proteins 17,400** 0.96 0.51 20,200 MJ

Carbohydrates 16,000** 0.79 0.39 15,500 MJ

Acetic Acid 14,600 0.75 0.37 14,700 MJ

Ethanol 29,000 0.97 0.73 29,000 MJ

Glycerol 18,000 0.73 0.43 17,100 MJ

* Gross CV per kg of substrate converted to methane

** Varies slightly according to its precise composition

48
In practice, even 100% biodegradable substrates are only 75%-85% converted to biogas.

The remainder appears as new microbial biomass in the final digestate product. Even so,

potential conversion efficiencies remain high and anaerobic digestion is an efficient way

of recovering energy values from biodegradable materials.

Lignin, Cellulose, and the Limits of Biodegradability

Up to 30% lignin may be present in woody plants. Its purpose is to protect the plant from

microbial decay. Lignin is completely nonbiodegradable in anaerobic environments, and

even its aerobic degradation is slow and limited to a few species of soil bacteria and

fungi. Pure cellulose is readily fermented in anaerobic digesters, but its natural forms

are often protected from biodegradation by their lignin content. For example, hay, straw,

alfalfa, and bagasse are 40%-70% degradable in the rumen and in anaerobic digesters,

but sawdust and coconut fiber are virtually inert to anaerobic digestion (Huub et al. 1988).

Processed woody biomass (paper and board) also varies greatly in its anaerobic

biodegradability.

Biogas from Biowastes

The traditional role of anaerobic digestion is to stabilize, deodorize, and disinfect

biodegradable waste materials for disposal. Here the energy is generally regarded as a

by-product of the waste stabilization. Examples include:

Anaerobic Digestion of Sewage Sludges

Anaerobic digestion was first developed as an engineered process to deodorize and

disinfect sewage sludges, and is widely used to allow wastewater treatment utilities to

spread their sludges onto farmland and improve soil fertility without creating a public

nuisance or a human health hazard. The process is traditionally undertaken by fermenting

dilute (3%-6%DS) slurries at 35°C in completely mixed cylindrical concrete tanks. At

larger installations, biogas from the digesters is used to fuel gas engines to generate

electricity. The largest installation in the UK is at Becton STW (serving northeast

London), where the biogas is used to fuel gas turbines that generate 7.8 MW of

electricity. The most sophisticated sludge-to-energy recovery plant in the world is at the

Hyperion Sewage Treatment Plant (serving the city of Los Angeles, California), which

includes a gas turbine combined cycle power train that generates 12.6 MW of electricity

from sludge digester gas (Makansi 1984). Smaller installations generally use sparkignition or dual-fuel (biogas/oil) CHP gas engines to generate electricity, with waste heat

from the engine jacket cooling water being used to heat the digestion tanks.

Anaerobic Digestion of Farm Livestock Wastes

The same technology has also been used to treat animal slurries from intensive livestock

farms. For individual farms, these tend to be "low-tech" construction with the digestion

tank: set in the ground for minimal visual intrusion and improved thermal insulation. U.S.

farm practices tend to produce dilute (1%-2%DS) slurries and to favor covered lagoons.

European farm practices tend to produce smaller volumes of thicker (4%-8%DS) slurries

that can permit thermophilic (55°C) operation. In Denmark, individual farmers form

49
Figtrre 1. Centralized biogas plant for thermophilic anaerobic digestion of animal

manure, at Thorso, Denmark.

Anaerobic Digestion of Municipal Solid Wastes

The development of anaerobic digestion technologies that can ferment high-solids

feedstocks such as municipal solid wastes began in the 1980s. The first generation of

these plants is now coming into service, mostly in Germany, Denmark, Belgium, and the

Netherlands (lEA 1994). Figure 2 shows the plant used to treat source-separated biowaste

from the city of Salzburg, Austria. The ability of these plants to handle high-solids

feedstocks also makes them useful for improving the overall energy and environmental

aspects of emerging renewable biomass technologies.

Biogas from Biomass

Most biogas produced in agriculture comes from intensive livestock production. Here

biogas is collected as a by-product of an environmental protection process. But energy

crops such as silage grass, myscanthus, or sugar beet leaves can also be used as direct

feedstocks for biogas production in a dedicated anaerobic digestion plant. The process

offers a net energy yield of 52.9 GJ/ha (CEC 1994a), but biogas and methane are not

convenient fuels for vehicle propulsion and need to be used on-site or exported (as

electricity) to the local power utility.

50
Figure 2. High-solids anaerobic digestion of 55 tid of source-separated municipal

biowaste from the city of Salzburg, Austria.

Biogas from Biofuel Residues

1\10st biomass-to-energy schemes are targeted to producing the energy as a liquid fuel

(vegetable oil 'or ethanol), which can be sold off-site as vehicle fuel. Here the role of

anaerobic digestion would be to extract residual energy values from surplus by-products

to improve the overall energy efficiency and economics of the scheme.

51
Biogas and Biodiesel

More than 300 plant species can be used to produce vegetable oils, with the highest yields

usually found in tree crops (palm oil, olive oil, etc). Field crops such as rapeseed or

sunflower seeds are easier to harvest. The simplest method of extracting the raw oil is

in a screw press or expeller, followed by solvent extraction of the residual press cake.

The recovered oil then needs to be chemically modified to convert the original

triglycerides into the methyl esters of their component long-chain fatty acids. A typical

conversion of rapeseed oil would be:

rape oil

(RME)

1000 kg

+

KOH

methanol -------->

110 kg

glycerol +

110 kg

rape oil methylester

1000 kg

Typically, 1 ha of rape crop would produce about 3000 kg of rapeseeds, which would

eventually yield about:

1000 kg rape oil methyl ester

1900 kg pressed seed cake

110 kg glycerol

for vehicle fuel

for animal feed

for chemical feedstock

An overall energy flow diagram for producing rape oil methylester (biodiesel) is shown

in Figure 3. The net energy yield of 47.8 GJ/ha is comparable with putting the same field

to silage for biogas production, but the energy has been captured in a much more versatile

liquid fuel. The problems begin when the markets for biodiesel by-products (rapeseed

meal and glycerine) become saturated. But it doesn't have to be so, because you can

have your biodiesel and biogas as well. Anaerobic digestion could convert about half the

28.2 GJ/ha in the surplus rapeseed meal into biogas, conserving its nutrients and

providing a stabilized residue for recycling back to the crop.

Biogas and Bioethanol

A wide range of crops can be grown to provide sugars and starches for fermentation to

ethanol. Their yields and climate ranges vary, but they all produce a high proportion of

wastes and by-products that require marketing, processing, or disposal. Anaerobic

wastewater plants are already used to treat wastewaters from sugar refineries and vinasses

from ethanol distilleries. Separate high-solids digesters could yield even more biogas

from plant residues and pulps.

52
glycerol

2.2CJlha

00 .5 rape oil

.§ partly refined

48.1 CJlha

rape straw

78.2CJlha

00 rape seed meal

.~ 28.2 CJjha

::a rape seed t====-:~=~:JF=:::a..--T"':"';::%::;;'_ u.. 77.8 CJlha ] rape seed oil raw

'0 49.6 CJlha

tcchnical

means

sun

energy

156CJjha

4.6CJlha process enc..:rg::Y_~E::t=:F::;:~~~=====~t==::::5==:;;=~:2~_

methanol factory

(2.7 GJ/ha) (0.3 G]/ha)

deguuuning

(O.SGJjha)

transcsterUication energy losses

(0.4 GJ/ha) (23.1 GJtha)

Figure 3. Energy flow diagram for the production of rape oil methylester (CEC 1994b)

Recycling Biomass Residues Back to the Crop

Plant Nutrients for Field Crops

Field crops tend to be hungry for nutrients, and their fertilizer requirements make a

significant impact on the overall energy balance and economics of biomass-to-energy

schemes. Anaerobic digestion and composting both have a role to play in promoting the

effective recycling of plant nutrients back to the growing crop. Both processes stabilize

the organic residues to improve their storage and distribution characteristics. Anaerobic

digestion is particularly well suited to processing wet pulps and soft, squashy residues that

become more liquid and easier to pump. It also converts a proportion of the bound

nitrogen into soluble ammonium ions for immediate uptake by the plants. Nutrient

requirements of some common field crops are summarized in Table 2.

Table 2.· Typical Nutrient and Pesticide Requirements of Some Common Crops

as a Percentage of Total Yield

(Dry Farming in a North American Climate-CEC 1994d)

Sorghum Soybeans Safflowers Sugar Beet Sugar Cane

(Kansas) (Illinois) (California) (California) (Louisiana)

Nitrogen 20.54 2.64 70.58 3.54 33.71

Phosphates 1.84 9.02 41.76 0.00 20.69

Potassium 0.48 8.52 -- -- 31.79

Insecticides 0.32 -- -- 0.04 0.53

Herbicides 0.54 1.43 -- -- 1.32

Total yield 1840 1994 1700 53644 4687

(kg/ha)

53
Organic Matter and Humus for Marginal Land

For upgrading marginal land and reclaiming derelict land, the prime requirement is for

bulk quantities of well-rotted organic matter (humus) to improve the soil's physical

structure and water retention. Digested or composted biomass residues are equally

suitable, with composting being the most obvious choice for highly structured woody

residues (and vice versa).

Slow-Release Fertilizers for Forestry

For forestry, and especially when planting young trees, the requirement is for slow-release

fertilizers from digestate solids or composted crop residues that can nurture the young

trees through their first years of growth. Early addition of quick-release soluble nitrogen

simply produces a sudden flush of rank grass.

Anaerobic Digestion as an Odor-Abatement Technology

The methanogenic fermentation at the heart of the anaerobic digestion process destroys

the malodorous organic compounds commonly associated with simple anaerobic

putrefaction. High-nitrogen wastes will still have some residual odor from ammonia, and

high-sulphur wastes will tend to generate hydrogen sulphide, but other odors are very

effectively destroyed.

Anaerobic Digestion for Pathogen Destruction

Anaerobic digestion is one of several processes available for disinfecting or sanitizing

waste materials to permit their safe return to the environment, and is very widely used

around the world to deodorize and disinfect high-risk materials such as sewage sludge.

Regulations concerning the precise degree of sanitization required vary from country to

country. UK practice is summarized in Table 3, which lists those processes considered

to be effective barriers to infection in the UK.

Originally developed- to protect public health and farm livestock, these processes and

codes of practice also provide a useful barrier to infection of the growing crop by

diseased plant residues and crop pests from the central biomass processing plant.

Visual and Amenity Aspects of Anaerobic Digestion Plants

Anaerobic digestion plants comprise mostly enclosed tanks with a similar appearance to

the grain stores commonly found on farms in the American Midwest. The anaerobic

digestion tanks themselves are generally odor-free, quiet, and unobtrusive. Any peripheral

problems with odors, gaseous emissions, noise, or road traffic tend to arise from the

loading/unloading or pre-processing of raw feedstocks or digestate products. These areas

of the plant need to be carefully designed to ensure that they too are unobtrusive and

"neighborhood-friendly." Road traffic can sometimes be a problem where the plants are

processing large throughputs of a bulky feedstock, and the plants need to be carefully

located to minimize haulage distance and provide good vehicle access.

54
Table 3. Treatment Processes for Sewage Sludge (as Defined by the UK DOE

Code of Practice for Agricultural Use of Sewage Sludge)

Process Description

Pasteurization Minimum of 30 min at 70°C or minimum of 4 h at 55°C

(or appropriate intermediate conditions), followed in all

cases by primary mesophilic anaerobic digestion.

Mesophilic Anaerobic Mean retention period of at least 12 d primary digestion in

Digestion temperature range 35°C+3°C, or of at least 20 d primary

digestion in temperature range 25°C+3°C, followed in each

case by a secondary stage that provides a mean retention

period of at least 14 d.

Thermophilic Aerobic Mean retention period of at least 7 d digestion. All sludge

Digestion to be subject to a minimum of 55°C for at least 4 h.

Composting in The compost must be maintained at 40°C for at least 5 d

Windrows or Aerated and for 4 h during this period at a minimum of 55°C

Piles within the body of the pile, followed by a period of

maturation adequate to ensure that the compost reaction is

substantially complete.

Lime Stabilization of Addition of lime to raise pH to greater than 12.0 and

Liquid Sludge sufficient to ensure that the pH is not less than 12.0 for at

least 2 h. The sludge can then be used directly.

Liquid Storage Storage of untreated liquid sludge for at least 3 mo.

Dewatering and Conditioning of untreated sludge with lime or other

Storage coagulants followed by dewatering and storage of the cake

for at least 3 mo. If sludge has been subject to primary

mesophilic anaerobic digestion, storage to be for at least

14 d.

Summary and Conclusions

Natural methane fermentations occur widely in the environment and form an integral part

of the life processes of our planet. Methane released into the atmosphere has been

increased in recent years by intensive livestock farming, deposition of large quantities of

putrescible solid wastes in landfills and by the release of geological methane during the

extraction, distribution, and use of fossil fuels. There is an environmental need to capture

this methane and put it to use as fuel to benefit mankind and his habitat. Simply flaring

off surplus biogas is wasteful but it does, at least, decrease the greenhouse effect of the

gas because, molecule for molecule, carbon dioxide is 20 times less powerful than

methane as a greenhouse gas. Using the gas as a renewable fuel is a better environmental

option. It conserves fossil fuel resources and decreases overall emissions of carbon

dioxide to the atmosphere. It also make good economic sense and substantially improves

the energy efficiency and economic cash flows of other biomass-to-energy schemes.

55
References

CEC. (1994a). "Gaseous Biofuels: Biogas and Producer Gas," in Biofuels: Application

ofBiologically Derived Products as Fuels or Additives in Combustion Engines, European

Commission Directorate-General xn, Science, Research and Development, EU 15647 EN,

pp.52-67.

CEC. (1994b). "Plant Oils and Methylesters," in Biofuels: Application ofBiologically

Derived Products as Fuels or Additives in Combustion Engines, European Commission

Directorate-General XII, Science, Research and Development, EU 15647 EN, pp.2-20.

CEC. (1994c). "Alcohols: Ethanol' and Methanol," in Biofuels: Application of

Biologically Derived Products as Fuels or Additives in Combustion Engines, European

Commission Directorate-General XU, Science, Research and Development, EU 15647 EN,

pp.22-50.

CEC. (1994<1). "Biofuels for Motor Vehicles," in Biofuels: Application ofBiologically

Derived Products as Fuels or Additives in Combustion Engines, European Commission

Directorate-General XII, Science, Research and Development, EU 15647 EN, pp.

104-122.

DOE. (1989). "Code of Practice for Agricultural Use of Sewage Sludge," London,

HMSO, ISBN 0 11 752256 2.

Huub, J.M. et al. (1988). "Effect of Lignin on the Microbial Degradation of

(Ligno)cellulosic Wastes by Rumen Microorganisms," Applied Microbiology and

Biotechnology 29, pp. 408-412.

lEA. (1994). "Biogas from Municipal Solid Waste," published by Danish Energy Agency,

Copenhagen, Denmark, July 1994.

Makansi, J. (1984). "Power from Sludge," Power 128, February, pp. SI-816.

56
ECONOMIC VIABILITY OF ANAEROBIC DIGESTION

Arthur Wellinger

Head of INFOENERGIE

8356 Ettenhausen, Switzerland

Abstract

The industrial application of anaerobic digestion is a relatively new, yet proven waste

treatment technology. Anaerobic digestion reduces and upgrades organic waste, and is

a good way to control air pollution as it reduces methane and nitrous gas emissions.

For environmental and energy considerations, anaerobic digestion is a nearly perfect waste

treatment process. However, its economic viability is still in question. A number of

parameters -type of waste (solid or liquid), digester system, facility size, product quality

and end use, environmental requirements, cost of alternative treatments (including labor),

and interest rates - define the investment and operating costs of an anaerobic digestion

facility. Therefore, identical facilities that treat the same amount and type of waste may,

depending on location, legislation, and end product characteristics, reveal radically

differentcosts.

A good approach for evaluating the economics of anaerobic digestion is to compare it to

treatment techniques such as aeration or conventional sewage treatment (for industrial

wastewater), or composting and incineration (for solid organic waste). For example, the

cost (per ton of waste) of in-vessel composting with biofilters is somewhat higher than

that of anaerobic digestion, but the investment costs 1 to 2 times more than either

composting or anaerobic digestion. Two distinct advantages of anaerobic digestion are:

(1) it requires less land than either composting or incinerating, which translates into lower

costs and milder environmental and community impacts (especially in densely populated

areas); and (2) it produces net energy, which can be used to operate the facility or sold

to nearby industries.

57
Introductory Remarks

The industrial application of anaerobic digestion (AD) is a relatively new, yet proven

waste treatment technology. Anaerobic digestion reduces and upgrades organic waste, and

is a good way to control air pollution as it reduces methane and nitrous gas emissions.

The political implication of environmental control makes evaluating the economics of AD

on an international scale extremely difficult. A number of country-specific parameters

define the investment and the operation and maintenance (O&M) costs such as the type

of waste (liquid, solid), digester system, size of installation, quality and end use of the

products, environmental requirements, cost and labor of alternative treatments, interest

rates, etc. As a result, identical installations treating the same amount and type of waste

might lead to different costs in different countries.

Table 1 compiles of the most important parameters. Sensitivity analyses are not detailed;

however, preliminary evaluations seem to indicate that legislation and the price of

electricity predominate. Equally important are the external or social costs, i.e.,

environment and health not covered by the actual energy price but paid by the entire

society, in particular by future generations. They are internalized, i.e., added to the

energy price, and they can drastically disfavor some current waste treatment techniques.

In summary, the profitability of waste treatment processes is equally defined by political

decisions and engineering solutions.

Table 1: Influence of Various Parameters on Investment and

Running Costs of Anaerobic Digesters

Parameters Influencing Major Effect on Major Effect on

the Economy Running Cost Investment Cost

Type of Equipment Pretreatment Pretreatment

Type of Digester Type of Digester

Upgrade of Biogas Upgrade of Biogas

Chemical Additives Use of Biogas

Storage of Biogas

Capacity Efficiency of Electricity Economy of Scale

Production

Profitability Interest Rates Subsidies

Taxes

Amortization

Laws Post-Treatment Post Treatment

End Use of Products Impact Studies

Security Requirements

Cost of Living Labor Labor

Insurance Land Prices

Transportation Cost

58
Parameters Influencing Major Effect on Major Effect on

the Economy Running Cost Investment Cost

Type of Waste Biological Oxygen Source Separation

Demand (BOD)

Content Inhibitors

Economical Considerations of Different Process Designs

Like other waste treatment processes, not all kinds of organic materials can be treated

anaerobically in the same type of installation. Three basic designs of digesters can be

distinguished for municipal solid waste (MSW), for liquid (solubilized) waste (industrial

waste water), and for heterorganic waste (animal manure). Economical comparisons can

reasonably be done only within the respective types of waste. The examples given below

were evaluated in project studies for a same object in 1992. The young market of AD

moves fast; hence, the absolute figures no longer reflect the actual situation. However,

the comparative significance between the different procedures is still valuable.

Municipal Solid Waste Digestion

In Europe, source separation of household waste is widespread. Common fractions are:

metals, glass, polyethylene terephthalate (PEn, paper, organics and the remaining

fraction. The goals of separate collection are to recycle and reduce the waste volume to

be landfiHed or incinerated. In particular, removing the humid organic fraction leads to

lower emissions of methane and volatile organic carbons (VOCs) when Iandfilled or to

higher heating values when incinerated. In the European Union, landfilling organic

material will be drastically decreased and ultimately prohibited.

With the increasing requirements for stack gas treatment (CO, ashes, NOx, dioxins) and

the need to solidify the ashes, incineration became a very expensive technique (Figure 1).

Reducing the volume by source separating the organic fraction will therefore reduce costs.

Composting or AD are the two methods of choice to upgrade the organic fraction. Both

techniques yield high-quality humus that can be recycled to replace imported peat.

The main distinctive advantages of AD over composting are that it:

- produces net energy (Table 2)

- reduces greenhouse gas emissions

- requires less land, which translates into lesser effects on the community

- has fewer odor emissions

59
Figure I: Average Prices for Landfilling and Incineration in Selected Countries

250-,------------------------------__

DK aRD CH

Legend

• Lanctftlung

o Incineration

o

200 -+-----------------------------

.s 150 -F=====:===,-------------------~

:s

.fi

Ii.

M"100

::::> -F=======~-------------------­

Table 2: Energy Balances of Aerobic and Anaerobic Processes

(adapted from Edelmann et al. 1994)

System Composting Anaerobic Digestion

Box Indoor Tunnel Thermo- Thermo- Mesophilic philic philic

Producer BIOCON BUHLER BAV KOMPO- DRAN- VALORGAS CO GA

Capacity 50,000 40,000 50.000 10.000 50.000 52,000

(t/a)

Electric- 20 35 25 35 33 75

ity Consumption

(kWh/t)

Electric- -20 -35 -25 75 79 78

ity Production

(kWh/ta)

The application of aerobic composting to MSW is that the wet organic waste tends to

compact and oxygen transfer during composting becomes limited. As a result, the

60
compost starts stinking and eventually may even "sour." MSW therefore must be

composted in closed halls together with the addition parts of dry organic material which

increases air flow through the material. In this way composting tends to become slightly

more expensive that anaerobic digestion (Table 3).

In contrast, a Dutch study (Haskoning 1993) with data from 1990 found that composting

was approximately 20% to 40% cheaper than AD in the range of 120,000 to 25;000 tons

per year. However, this evaluation was not site specific. In addition, AD was not well

established at that time, and offers still had a relatively high security factor.

Both biological treatments are much cheaper than incineration. The example of the

VALORGA plant indicates a distinct economy of scale. Construction and running cost

of AD were reduced during the past two years thanks to experience gained and

increasing competition. But still there is a clear-cut specific cost. The comparable costs

of the large-scale plant are $208 per ton for incineration and $99 per ton for landfilling.

Table 3: Comparison of Investment on Running Cost of MSW Treatment

(adapted from Edelmann et al. 1993)

System Anaerobic Digestion Composting

DRANCO KOMPO- VALOR- VALOR- INDOOR

GAS GA GA WINDROW

1992 1992 1992 1994 1992

Capacity (t/a) 10,000 10,000 10,000 52,000 10,000

Investment US$ US$ US$ US$ US$

Cost 6,286,200 5,666,000 9,861,500 34300,000 8,331,200

Running Cost

- Capital 734,200 608,400 1,087,300 3,118,500 889,800

- Operation & 167,600 135,000 258,500 311,900 207,800

Maintenance

- Personnel 146,000 146,000 146,000 727,700 219,000

- Divers 190,000 179,700 221,700 519,700 206,400

Total Running

Cost 1,237,800 1,069,100 1,713,500 4,667,800 1,523,000

Specific Cost US$/ton US$/ton US$/ton US$/ton US$/ton

With Land- 123 107 172 152

Without Land 121 105 168 104 142

Digestion of Industrial Wastewater

Most often AD is used as a first step in the treatment of high-strength wastewater (more

than 2,000 ppm chemical oxygen demand [COD] to reduce the cost of sewage treatment.)

The digested material overflows in the sewer. Sometimes AD is followed by a

61
denitrification step. In a few cases, particularly in developing countries, AD--eventually

followed by an aeration step--is used as a full treatment of wastewater. Most of the 13

plants in Switzerland were erected because the treatment cost was lower than a necessary

upscale of the sewage treatment plant because of higher industrial production. In one

case, AD was introduced to minimize the odor problems from the sewage treatment plant,

which evoked complaints from residents. The differences of treatment cost between the

various AD procedures are marginal (Table 4). However, AD is always more cost

effective than aeration, which often is not considered alone because of its space

requirement.

Table 4: Comparison of Investment and Running Costs of Industrial

Wastewater Treatment (adapted from Edelmann et al. 1993)

System Contact UASB- Fluidized Contact Aeration

Reactor Reactor Bed Reactor

Performance

Wastewater m3!d 6,000 6,000 6,000 1,600 1,600

m3!a 1500,000 1500,000 1500,000 580,000 580,000

COD-Load kg/d 10,000 10,000 10,000 9,600 9,600

kg/a 2500,000 2500,000 2500,000 2500,000 2500,000

Investment

Cost US$ 5923,000 5961,500 5461,500 1100,000 1200,000

Running Cost US$!

- Capital a 681,000 644,400 627,000

- Operation & 115,000 96,000 105,000 52,000 300,000

Maintenance

- Energy -98,400 -130,000 -80,700 156,000 98,000

- Personnel 69,000 69,000 699,000

- Divers 443,800 444,200 439,200

Total Running US$!

Cost a 1210,400 1123,600 1159,500

Specific Cost US$! 0.80 0.74 0.77

m3

US$! 0.48 0.45 0.46

kg

CSB

Digestion of Agricultural Wastes

AD of industrial or municipal wastes has to compete with alternative procedures with

relatively high treatment cost. It is therefore easy to reach economic compatibility. With

agricultural wastes, economic viability has to be reached by biogas production alone.

Advantages such as improving fertilizer quality or reducing the environmental impact of

62
manure are hardly taken into account. Hence, the potential for constructing economically

viable digesters is rather limited.

The few solutions discussed below deal with cattle only. However, all systems apply to

pig manure. In fact, chances of economical feasibility with swine waste are far better

because it has higher specific gas potential.

In Europe farm sizes are very small. Typical sizes for biogas farms vary between 30 and

50 cattle units (CU) (1 CU is equivalent to 500 kg live weight), To be profitable

investments costs must generally be lower than $750 per CU. Essentially this means

either partial self-construction or a joint central biogas plant.

An alternative to low construction cost is the addition of industrial waste material from

slaughterhouses, fish processing, tanneries, milk and vegetable processing, etc., which

yield high gas production and bring an additional gain through treatment fees.

Iii a number of European countries biogas installations are subsidized either by direct

support or indirectly through high market prices for the electricity fed to the grid (Figure

2).

Figure 2: Market Prices of Electricity Produced from Biogas

14

12

10

!

r

6

4

2

0

D UK Ir BRD N.. B F E at

A number of small-scale on-farm biogas installations (accumulation systems) in Germany

and in Switzerland have payback periods of less than 13 years, which is the estimated

average lifetime of a biogas installation. Profitabili-ty was reached by self-construction,

subsidies, and in part by adding other waste materials. On the basis of the best run

biogas plants in Denmark, joint biogas plants can be operated profitably (Danish Energy

Agency 1991), with a few preconditions:

63
- Supplement at least 15% industrial waste

- Use the biogas produced steadily, with high subsidized market prices for the electricity

- Abundant manure must be available within a reasonable radius

- Plant management must keep O&M cost low

In the U.S. the income is less favorable. Both electricity and heat; i.e. fuel prices, are far

lower than in Europe. On the other hand, specific investment cost can be kept low

because of the large herd sizes. The economy of scale allows a favorable economy with

the same simple digester designs as in Europe. A recent study on operating U.S. biogas

plants (Lusk 1994) shows that covered lagoons and mixed systems were built at

reasonably low prices from $250 (750 head) to $400 (300 head) per head of cattle. A

more detailed economical analysis was not possible, because data of the amount of

manure actually fed into the digester could not be determined. Instead, the author has

developed an economical model for herd sizes from 250 to 1000 head. He assumed

technically and volumetrically optimized digesters with cost from $96 to $374 per head

of cattle as a function of herd size (250 or 500 head), designed (covered lagoon,

completely stirred tank) and collected amount of manure (14% or 55%). Within this

framework a positive cash flow was calculated with simple payback periods of less than

13 years. This strongly indicates that biogas production from agricultural waste is

economically feasible in the U.S. .

Energy and Environmental Legislation

Legislation has the strongest potential impact on the economy of AD. The higher the

environmental requirements, the higher the cost of conventional processes; i.e., the better

applicability of AD, which is a proven clean technology. Swiss standards on emission

of stack gases and requirements for landfilling ashes and products of advanced flue gas

treatment (denox) render incineration extremely expensive (Figure 1). AD of the organic

fraction is therefore a valuable, low-cost alternative.

The requirement for very clean composts after AD increases the cost of waste separation

and control before and after collection. The concentrations of heavy metals and other

toxic substances must meet national guidelines. The product also must be visually

attractive; i.e., free of plastics, glass, and stones to ensure its marketability.

Increasingly stringent emissions standards require the reduction of NOx or CO

concentrations from co-generation, often requiring the removal of hydrogen sulfide from

the biogas. Legislation usually defines the market price of the electricity produced. High

prices stimulate the construction of biogas installations (Figure 2). In some European

countries, such as Austria and Denmark, the distribution of heat from co-generators is

subsidized.

Finally, CO2 and/or energy taxes on fossil fuels influence the profitability of biogas use.

However, its impact remains marginal when compared to the politically increased market

prices. A realistic economical calculation is possible only when the social costs are

internalized.

64
Social Cost

The social or external costs include all liabilities not covered by the purchase price of the

energy; i.e., transport risks, impacts on the environment, buildings, human health,

accidents, reduction of plant yields, etc. The highest input on the greenhouse effect, for

example (50%) is caused by CO2 emissions from energy production (including traffic)

with fossil fuels. The second most important source is the halogenated hydrocarbons

(CFC) with approximately 20% (Enquete-Commission 1990).

Biomass also adds to the greenhouse effect. Methane and N20 produced by livestock,

landfills, rice fields, etc. contribute about 15%. Particularly large surfaces with organic

material are major sources for the production of methane, CO2 , and VQCs. (Lusk 1994)

estimated that U.S. livestock manures emit about 3 million metric tons of methane

annually, accounting for approximately 10% of total U.S. methane emissions. Of that,

liquid slurry adds about 30% and the remaining 60% comes from solid waste. The U.S.

Environmental Protection Agency has estimated that about one-third of the emissions

might be profitably reduced at the farms. .

A recent Austrian study (Steinlechner et al. 1994) demonstrated that 32% of a total of 411

tons a year of methane emitted derive from MSW disposal, 7% from the storage of

animal manure, and 2.8% from industrial wastewater. All three types of waste can be

easily treated anaerobically under controlled conditions. Hence, AD could prevent up to

40% of the methane emissions in Austria.

If governments would include external costs in energy prices and support technologies

which represent environmental betterment through added income, all renewable energies

would immediately have breakthrough.

In Switzerland, these social costs were evaluated (Ott et al. 1994). However, they are

voluntarily applied for public construction only. The added costs are conservative

estimates (Table 5) and do not necessary reflect the real cost. However, it is more

important that they are approximately right than precisely wrong. The only thing that

counts is their acceptance and application by the concerned representatives.

Table 5: Social Cost of Energy in Switzerland

Energy Source Actual Price (cents/kWh) Social Cost (cents/kWh)

Electricity 3.8 - 17 4.2

Oil (extra light) 2.0 - 3.5 6.0

Oil (heavy) 1.5 - 3.5 7.0

Natural Gas 3.0 - 5.0 4.0

Wood Chips 3.0 - 5.0 1.0

65
Literature

Danish Energy Agency; 1991, Centralized Biogas Plants in Denmark. Conclusions and

recommendations from the main report, p 9.

Edelmann, W. et al.; 1993. Vergarung von hauslichen Abfallen und Industrieabwassem.

Bundesamt fur Konjunkturfragen, p 68.

Edelmann, W. et al.; 1994. Biogas aus festen Abfallen und Industrieabwassem. Eckdaten

fur Planer. Bundesamt fur Konjunkturfragen, p 83.

Enquete-Kommission des deutschen Bundestages; 1990. Schutz der Erde, Teilband I.

Verlag C.P. Muller, p 686.

Haskoning; 1993. Conversion Techniques for VOF waste. Developments of 1992. p 121.

Lusk, Ph.; 1994. Methane Recovery from Animal Manures: A Current Opportunities

Casebook, p 90.

Ott, W. et al.; 1994. Exteme Kosten und kalkulatorische Energiepreiszuschlage fur den

Stromund Warmebereich, Bundesamt fur Konjunkturfragen, p 169.

Steinlechner, E. et al.; 1994. Moglichkeiten der Vemeidung und Nutzung anthropogener

Methanemissionen. Inst. fur Umweltgeologie und Okosystemforschung, p 342.

66
REPORT DOCUMENTATION PAGE Form Approved

OMB NO. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewin~ instructions, searching existing data sources, gathering and maintaining the data needed. and completing and reviewing the collection of information. Send comments regardinrc t is burden estimate or any other aspect of this

g~~r;t~~~~~0E::~i~~'o'~~~r1i~3t~~~~i~~8i~33t,u;~~t~ht~:~~~' ~~:%~~~i~~~~t ~~~d~~3~it~p;~=k ~i~~~~~~e~~~i~«r'8l0P~)~~~~~i~~to~~8crt2bJgj~ ! Jefferson

1. AGENCY USE ONLY (Leave blank) I2. REPORT DATE I3. REPORT TYPE AND DATES COVERED

January 1996 NREL Report

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Deploying Anaerobic Digesters: Current Status and Future Possibilities WM513231

6. AUTHOR(S)

P. Lusk, P. Wheeler, C. Rivard

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

REPORT NUMBER

National Renewable Energy Laboratory

1617 Cole Boulevard OE96000508

Golden, CO 80401

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

National Renewable Energy Laboratory

1617 Cole Boulevard NRELlTP-427-20558

Golden, CO 80401-3393

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

National Technical Information Service UC-1503

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

13. ABSTRACT (Maximum 200 words)

Anaerobic digestion is a biological process, where synergistic actions between bacteria are occurring at four different levels. First, hydrolysis converts a wide range of solid

organic materials into sugars and amino acids. Fermenting these materials produces volatile fatty acids (VFAs). Acidogenesis forms hydrogen, carbon dioxide (CO:J, and

acetate from VFAs. Finally, methanogenesis produces biogas, a mixture of methane, CO2 , and numerous trace elements. The key biological issues are determining the

most favorable conditions for each process stage and how nonoptimal circumstances affect the process as a whole. and the governing role of hydrogen generation and

consumption.

14. SUBJECT TERMS 15. NUMBER OF PAGES

66

anaerobic digestion, biogas, municipal solid waste, green pricing 16. PRICE CODE

17. SECURITY CLASSIFiCATiON I

18. SECU RITY CLASSIFICATION I

19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

OF REPORT OFTHIS PAGE OF ABSTRACT

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std Z39-H

298-10~