The document discusses the microbiology of anaerobic digestion, which is a four phase process utilized by a consortium of microbes to break down organic matter into biogas. The four phases are: 1) hydrolysis, where large organic compounds are broken down into simpler molecules; 2) acidogenesis, where the products of hydrolysis are fermented; 3) acetogenesis, where fatty acids are converted to acetate; and 4) methanogenesis, where methane gas is produced from acetate or hydrogen and carbon dioxide. Each phase provides feedstock for the next and the microbial populations evolve to maintain the optimal sequential process.

1. Microbiology

2. Microbiology of biogas
 Anaerobic digestion utilizes a
consortium of microbes in four distinct
phases
 Products of one phase are feedstocks
for next phase (serial process)
 For optimal performance, each rate
keeps up with previous
 Microbial populations are ever-evolving

3. What is anaerobic digestion?
(Biochemistry perspective)
 Aerobic organisms use oxygen as
their electron acceptor
 Anaerobic organisms utilize
alternative electron acceptors
 Facultative organisms are aerobic
or anaerobic

4. What is anaerobic digestion?
(Biochemistry perspective)
 Anaerobic electron acceptors (oxidizers)
 NO3-, Fe3+, Mn3+, SO4
3+, CO2
 Significantly different energy output
 30 ATP (aerobic) vs. 2 ATP (anaerobic)
 Anaerobic metabolism occurs
throughout nature (hydric soils,
ruminants, etc.)

5. Phases of digestion
 Hydrolysis
 Acidogenesis
 Acetogenesis
 Methanogenesis

6. Digestion Process
Hydrolysis
Acidogenesis
Acetogenesis
Methanogenesis
Carbohydrates, lipids, proteins
Sugars, fatty acids, amino acids
Acetate
H2, CO2
Methane
(Biogas)
Long chain fatty acids, intermediates

7. Hydrolysis
 Large organic compounds are broken
down into simpler compounds
 Performed by many organisms
 Bacteria, fungi, protists
 Facilitated by extracellular enzymes
 H2 and CO2 also produced
Carbohydrates, lipids, proteins
Sugars, fatty acids, amino acids
Vibrio Hartmanella

8. Acidogenesis
 Products of hydrolysis are fermented into
volatile fatty acids
 Also termed fermentative bacteria
 Trace oxygen consumed by facultative
bacteria
 Also produces acetate, H2, and CO2
Sugars, fatty acids, amino acids
Long chain fatty acids, intermediates
Lactobacillus
Propionibacterium

9. Acetogenesis
 Long-chain volatile fatty acids are converted to
acetate, H2, and CO2
 Metabolizes intermediates (proprionate,
butyrate)
 Two groups of acetogens
 Obligate hydrogen-producing acetogens (OHPA) –
more dominant
 Homoacetogens – less dominant
Acetate, CO2, H2
Long chain fatty acids, intermediates
Syntrophomonas

10. Acetogenesis (OHPA)
 Produce acetate, CO2 and H2 from fatty acids
 Metabolize in low-H2 environments
 Mutualistic relationship with methanogens
(Syntropy)
• Methanogens consume H2, OHPA produce
acetate and consume fatty acids
 Positive feedback cycle if balance disturbed
 Homoacetogens produce acetate from CO2
and H2
• Also assist in reducing H2

11. Methanogenesis
 Final metabolic phase
 Methanogens produce methane gas
from acetate, H2 and CO2
 2/3 of methane produced is derived from
acetate
 Often the rate limiting step in anaerobic
digestion
 Can lead to accumulation
of acids
Acetate
H2, CO2
Methane
(Biogas)

12. Methanogens
 All are Archaebacteria
 Obligate anaerobes
 Optimum performance at neutral pH (7)
 Many genera convert H2 and CO2 to
methane (Hydrogenotrophic)
Methanococcus
Methanospirillum

13. Methanogens
 Two known genera convert acetate to
methane (Acetoclastic)
 Methanosarcina – Favor high concentration
 Methanosaeta – Favor low concentration
Methanosarcina Methanosaeta

14. Questions?