This presentation provides an overview of anaerobic digestion and biogas production. It discusses the basic anaerobic sludge digestion process, including descriptions of different process types and key operational parameters like temperature and retention time. It also examines the biological activities involved, including the microbial composition, survival of pathogens, and biochemistry of the digestion process. The goal is to provide a foundation for exploring the feasibility of biogas-to-energy facilities in local communities.

1. Biogas Basics
presented by
Tom Marshall, Ph.D., P.E.
Operator Training Committee of Ohio
CLASS III & IV
WORKSHOP
THURSDAY, AUGUST 13, 2009
RAMADA PLAZA HOTEL & CONFERENCE
CENTER
4900 Sinclair Road
Columbus, OH 43229

2. Bringing Biogas to Your Community Could
Bring Energy Stimulus Money, Jobs and
Energy Independence
• This presentation is intended to provide a
basic process overview of anaerobic digestion
and biogas production
• A solid technical background provides a
platform upon which further investigation into
the feasibility of biogas-to-energy facilities in
your community can be explored
Dr. Thomas Marshall
(marshall.15@osu.edu)

3. Process Overview
• Anaerobic Sludge Digestion
• Biological Activities in Anaerobic Digestion
• Practical Applications
Dr. Thomas Marshall
(marshall.15@osu.edu)

4. Anaerobic Sludge Digestion
– Process Description
– Operational Parameters
Dr. Thomas Marshall
(marshall.15@osu.edu)

5. Anaerobic Sludge Digestion
Process Description
• Involves a complex mixed culture of anaerobic
organisms to hydrolyze, ferment and convert
to methane the fats, proteins and
polysaccharides in organic sludge solids
• Organic solids are typically reduced by 50%
Dr. Thomas Marshall
(marshall.15@osu.edu)

6. Anaerobic Sludge Digestion
Process Description
• Batch Process
– Supernatant Withdrawal
• High-Rate Process
– Continuous flow
– No Supernatant Withdrawal
• Semi-Batch Process
– Intermittent flow
• Single stage or two stage
Dr. Thomas Marshall
(marshall.15@osu.edu)

7. Anaerobic Sludge Digestion
Operational Parameters
• Temperature
– Psychrophilic Range (less than 68 degrees F)
• Typically Unheated
– Mesophilic Range (between 77 and 104 degrees F)
• Requires Heat
• Most Common Process
– Thermophilic Range (between 122 and 158
degrees F)
• Unusual Set of Microorganisms
Dr. Thomas Marshall
(marshall.15@osu.edu)

8. Anaerobic Sludge Digestion
Operational Parameters
• Retention Times
– Mesophilic and Thermophilic Range
• Typically 25-35 days
• Can be as low as 12-15 days
– Psychrophilic Range
• In excess of 100 days
• Requires large storage volume
Dr. Thomas Marshall
(marshall.15@osu.edu)

9. Anaerobic Sludge Digestion
Operational Parameters
• Substrate Loading
– Normally expressed as lbs/cu ft - day of volatile
solids
– Typical loading rates are .1 - .2 lbs/cu ft - day
• Assumes Primary or Blended Sludge
Dr. Thomas Marshall
(marshall.15@osu.edu)

10. Biological Activities in Anaerobic
Digestion
– Microbial Composition of Digesting Sludge
– Survival of Pathogens
– Biochemistry of Anaerobic Digestion
– Microbial Growth Parameters
Dr. Thomas Marshall
(marshall.15@osu.edu)

11. Biological Activities in Anaerobic Digestion
Microbial Composition of Digesting Sludge
• Process depends exclusively on bacterial activity
(fungi and protozoa contribute little to the
process)
• Bacteria Types are classified as
– Hydrolytic
– Fermentative
– Methanogenic
• The most significant of the strict anaerobes are of
the genus Bacteroides
• Found at densities of 1010 per gram in feces
Dr. Thomas Marshall
(marshall.15@osu.edu)

12. Biological Activities in Anaerobic Digestion
Microbial Composition of Digesting Sludge
• Some fermentative organisms are facultative
in anaerobic digestion but the predominate
population is that of obligate anaerobes such
as Clostridium
• Reducing conditions release hydrogen sulfide
– Sulfate is reduced by Desulfovibrio desulfuricans
• The methanogenic population is also strictly
anaerobic
Dr. Thomas Marshall
(marshall.15@osu.edu)

13. Biological Activities in Anaerobic Digestion
Survival of Pathogens
• Salmonella is often used as indicator organism
for pathogen survival
– Survival rates are low with infrequent detection
• Vibrio cholerae and Giardia are largely
destroyed
• Viruses can survive
• Survival depends on temperature, detention
time and process
Dr. Thomas Marshall
(marshall.15@osu.edu)

14. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• While anaerobic digestion is typically
described in terms of acid formers and
methane producers, there are actually four
distinct phases:
– Hydrolysis
– Fermentation
– Acetogenisis
– Methanogenisis
Dr. Thomas Marshall
(marshall.15@osu.edu)

15. Biochemistry of Anaerobic Digestion
Dr. Thomas Marshall
(marshall.15@osu.edu)

16. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• Hydrolysis
– The initial phases of the hydrolysis of a substrate
occur extracellularly
– Only a few bacteria are able to produce
degradation enzymes for large polymer substrates
Dr. Thomas Marshall
(marshall.15@osu.edu)

17. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• Hydrolysis
– Lipid hydrolysis is not well understood
• Clostrdia and micrococci appear to produce most of the
extracellular lipases
• Some of these enzymes are very specific while others
will also hydrolyze proteins
Dr. Thomas Marshall
(marshall.15@osu.edu)

18. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• Hydrolysis
– Polysaccharides include cellulose, pectins and
starch
– Polysaccharides are degraded extracellularly by a
variety of different bacterial genre
Dr. Thomas Marshall
(marshall.15@osu.edu)

19. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• Fermentation
– Amino acids and sugars are the most readily
fermentable substrates
– Sugars are fermented to alcohols
– Pyruvate is an important intermediate in the
fermentation of amino acids from which formate
and acetate are formed
Dr. Thomas Marshall
(marshall.15@osu.edu)

20. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• Fermentation
– Acidogenic fermentation results in acetate
– Carbon dioxide and molecular hydrogen are also
fermentative end-products
Dr. Thomas Marshall
(marshall.15@osu.edu)

21. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• Acetogenesis
– For many years is was assumed that methanogens
could convert all of the fermentative end-products
directly to methane. However, there is an
important intermediate step in which acetate and
hydrogen are produced with OHPA bacteria
• Obligatory Hydrogen-Producing Acetogenic bacteria
Dr. Thomas Marshall
(marshall.15@osu.edu)

22. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• Methanogenesis
– Methanosarcina bareri is the only species that can
produce methane directly from acetate
– Twice as much methane is generated from acetate
than from the reduction of carbon dioxide
Dr. Thomas Marshall
(marshall.15@osu.edu)

23. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• Process stability
– Hydrolysis and fermentation processes proceed
slowly but the micro-organisms are robust
– Acetate and hydrogen are present in low
concentrations during stable operations
– Overload of carbohydrates will result in elevated
concentrations of acetate
– Acetate accumulation results in low pH and
process inhibition
Dr. Thomas Marshall
(marshall.15@osu.edu)

24. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• Process stability
– OHPA bacteria are inhibited by their own
metabolic product, hydrogen, which is removed by
the methanogens
– Early detection of elevated volatile fatty acid
concentrations (2000 – 5000 mg/l) is an early
warning to process upset.
– pH adjustments can counter process failure
Dr. Thomas Marshall
(marshall.15@osu.edu)

25. Biological Activities in Anaerobic Digestion
Biochemistry of Anaerobic Digestion
• Process stability
– Methanogenic inhibition is also caused by:
• Heavy metals
• Chloromethane
• Detergents
Dr. Thomas Marshall
(marshall.15@osu.edu)

26. Biochemistry of Anaerobic Digestion
Dr. Thomas Marshall
(marshall.15@osu.edu)

27. Biological Activities in Anaerobic Digestion
Microbial Growth Parameters
• It is commonly thought that the slow growth rate
of the methanogens limits the rate of anaerobic
digestion, however, the OHPA bacteria have a
slower growth rate (1/10 of the rate of
methanogens)
• The detention time must be sufficient to allow
the OHPA bacteria to produce acetate
• The yield coefficients for anaerobic bacteria are
50% of aerobic bacteria due to organic carbon
being converted to gas rather than biomass
Dr. Thomas Marshall
(marshall.15@osu.edu)

28. Practical Applications
• Typical gas production value is approximately
15 cubic feet of gas (65% methane) per pound
of volatile solids
• Typical biogas has 650 BTU/cu ft
• H2S is undesirable and can be removed with
scrubbers
Dr. Thomas Marshall
(marshall.15@osu.edu)

29. Practical Applications
• Gas can be used to directly provide heat, run
internal combustion engines or micro-turbines
• Operating gas pressures are typically 12-15
inches of water head
• Cogeneration is cost effective for communities
with 1 MGD or higher wastewater flows
• If electrical costs double, payback period for
biogas facilities is divided by a factor of 2
Dr. Thomas Marshall
(marshall.15@osu.edu)

30. Conclusion
• This presentation was intended to provide a basic
process overview of anaerobic digestion and
biogas production
• A solid technical platform provides a basis upon
which further investigation into the feasibility of
biogas-to-energy facilities in your community can
be studied
• Bringing Biogas to Your Community Could Bring
Energy Stimulus Money, Jobs and Energy
Independence
Dr. Thomas Marshall
(marshall.15@osu.edu)

31. Biogas Basics
presented by
Tom Marshall, Ph.D, P.E.
Operator Training Committee of Ohio
CLASS III & IV
WORKSHOP
THURSDAY, AUGUST 13, 2009
RAMADA PLAZA HOTEL & CONFERENCE
CENTER
4900 Sinclair Road
Columbus, OH 43229