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