This document provides an overview of anaerobic digestion as a renewable energy resource. It discusses: 1. The stages of anaerobic digestion including hydrolysis, acidogenesis, acetogenesis, and methanogenesis in which complex organic matter is broken down by microorganisms into methane and carbon dioxide. 2. The history of anaerobic digestion and some key developments such as the first digestion plant being built in 1859 and regulations supporting renewable energy in Germany in 2000. 3. Illustrations of the anaerobic digestion process showing how biomass is converted into biogas through a digester, and how the biogas can then be used to generate electricity, heat, or fuel. 4. Fact

1. RENEWABLE ENERGY RESOURCES
Lecture # 21
Presented by:
Dr. Bilal Ahmad Zafar Amin
Anaerobic Digestion (AD) of Biomass to Biogas
Solar Energy
Wind Energy
Biomass Energy
Thermal Energy
Renewable
Resources
Oil
Coal
Natural Gas
Nuclear
Non-Renewable
Resources

2. Week # 12, Lecture # 1 Renewable Energy Resources
 Concept of anaerobic digestion of biomass to energy resources
 History of Anaerobic Digestion (AD) of Biomass
 Illustration of anaerobic digestion process
 Stages of anaerobic digestion of biomass
 Illustration of hydrolysis, acidogenesis, acetogenesis & methanogenesis.
 Biochemistry of hydrolysis, acidogenesis, acetogenesis & methanogenesis
 AD of Lignocellulosic biomass to Biogas
 Visual presentation of AD
Lecture Outlines:

3. Biomass Energy Conversion Technologies
Biomass Conversion
Direct
Combustion
Thermochemical
Conversion
Biochemical
Conversion
Gasification Pyrolysis
Anaerobic Digestion (AD) Fermentation
+ O2
- O2
limited O2
- O2
Week # 12, Lecture # 1 Renewable Energy Resources

4. Week # 12, Lecture # 1 Renewable Energy Resources
History of Anaerobic Digestion (AD) of Biomass
Jan Baptita Van Helmont determined that flammable gases could evolve from
decaying organic matter
Principle Discovery
1600
Sir Humphry Davy determined that methane was present in the gases
produced during the AD of cattlemanure
Methane Detected
1808
First digestion plant was built in Mumbai, India
1st Digestion Plant
1859
Buswell and others identified anaerobic bacteria and the conditions that
promote methane production
Anaerobic Bacteria
1930
German Renewable Energy Act passed into law setting a feed-in tariff for
electricity production
German Regulations
2000
First legally-binding framework to cut carbon emission. Sets 80% cut in
greenhouse gas emissions by 2050
UK ClimateAct
2008
AD realizes “greatest environmental benefit” of any treatment option from
inedible food wastes
AD benefits
2011

5. Week # 12, Lecture # 1 Renewable Energy Resources
Bio-Chemical conversion
Biochemical conversion makes use of microorganisms (like fungi and bacteria) and their
enzymes to break down biomass and convert it into fuels. This conversion process includes
anaerobic digestion (AD) and fermentation.
1. Anaerobic digestion involves the microbial digestion of biomass. The process takes
place at low temperature up to 60oC & requires a moisture content of at least 80%, and
generates a gas consisting of CO2 & methane (CH4)
2. Fermentation is the breakdown of complex molecules in organic compound under
influence of a ferment such as Yeast,bacteria, enzymes etc.
Anaerobic digestion (AD) is a biochemical process that breaks down
organic materials (feedstocks) in the absence of oxygen (anaerobic
conditions) into methane (CH4) and carbon dioxide (CO2)
What is AD?
“Organic” means coming from or made of plants or animals. This may
include Principally plant residue, animal manures, food scraps, fats, oils &
greases, industrial organic residuals and sewage sludge (“biosolids”)
What organic
materials?
Anaerobic digestion takes place in closed spaces where there is no air (or
oxygen), hence the name “anaerobic”
Where does it
takes place?
What is the
result of process?
When microorganisms break down or eat the organic materials biogas is
generated. The material that is left after the process is called digestate which
is rich in nutrients and can be used as fertilizers/soil additives

6.  This is a process in which organic material directly converted to a gas which is termed as
biogas.
 It is mixture of methane, carbon dioxide and other gases like hydrohgen, hydrogen sulphide
in small quantities.
 Biomass is converted in anaerobic environment by bacteria, which produces a gas having an
energy of 20-40% of heating value of the feedstock.
 This process is suitable for organic wastes having high moisture about 80-90%.
 This biogas can be directly used in spark ignition gas engines and gas turbines and can be
upgraded to higher quality natural gas by removing carbon dioxide.
 The overall conversion efficiency of this process is 21%.
AD Process
Week # 12, Lecture # 1 Renewable Energy Resources

7. Livestock
Waste
Crops
Waste-
water
Food
Waste
Anaerobic
Digester
Biogas
Digestate
Heat Electricity
Bio-
methane
Fuel Gas Grid
Fertilizers/Soil
Amendments
Livestock
Bedding
Week # 12, Lecture # 1 Renewable Energy Resources
AD Process Illustration

8. Transport use
Electricity
Process heat fordigester
Biomass Biogas
Vehicle fuel
Digester
Gas
boiler
Biogas
Storage
vessel
Compressor
Turbine /
generator
Week # 12, Lecture # 1 Renewable Energy Resources
AD Biomass Conversion to Energy

9. Week # 12, Lecture # 1 Renewable Energy Resources
Principles/Stages of Biogas Production by AD

10. Week # 12, Lecture # 1 Renewable Energy Resources
AD of Lignocellulosic biomass to Biogas

11.  Complex organic matter, Carbohydrates, Fats &
Proteins are broken down into glucose
molecules, fatty acids and aminoacids
 This stage is carried out by hydrolytic eco-
enzymes (such as cellulase, amylase, protease and
lipase) exerted by fermentative microorganisms
 Some products of hydrolysis such as hydrogen
and acetate may be used by methanogens.
However, majority of the molecules produced are
relatively large and must be further converted to
smaller molecules for production of methane
 Relatively slow step - can limit the rate of the
overall process
Carbohydrates,
Proteins
& Fats
Sugar,
Amino
Acids &
Fatty
Acids
Volatile
Fatty Acids
Acetic
Acid
H2 andCO2
Methane
& Carbon
dioxide
1 2 3 4
Hydrolysis
Acidogenesis Acetogenesis Methanogenesis
Week # 12, Lecture # 1 Renewable Energy Resources

12.  Bacteria break down glucose molecules,
fatty acids & amino acids into volatile fatty
acids & alcohols resulting in by products
like hydrogen sulphide, carbon dioxide and
ammonia
 The organic are converted by acid-forming
bacteria to higher organic acids such as
propionic acid, butyric acid and to acetic
acid, hydrogen and carbon dioxide
Carbohydrates,
Proteins
& Fats
Sugar,
Amino
Acids &
Fatty
Acids
Volatile
Fatty
Acids
Acetic Acid
H2 and
CO2
Methane
& Carbon
dioxide
Week # 12, Lecture # 1 Renewable Energy Resources
1 3 4
Hydrolysis
Acidogenesis
Acetogenesis Methanogenesis
2

13.  Volatile fatty acids & alcohols are
converted into hydrogen, carbon dioxide
and ammonia
 Higher organic acids produced during
acidogenesis are subsequently transferred
to acetic acid and hydrogen by acetogenic
bacteria
 It is not always possible to draw distinctions
between acidogenic and acetogenic
reactions. Acetate and hydrogen are
produced during acidification and
acetogenic reactions and both of them are
substrates of methanogenic bacteria
Carbohydrates,
Proteins
& Fats
Sugar,
Amino
Acids &
Fatty
Acids
Volatile
Fatty
Acids
Acetic Acid
H2 and
CO2
Methane
& Carbon
dioxide
Week # 12, Lecture # 1 Renewable Energy Resources
1 4
Hydrolysis Acidogenesis
Acetogenesis
Methanogenesis
2 3

14.  Microbes convert hydrogen and acetic acid into
methane and carbon dioxide
 Methane is produced by methanogenic
bacteria which are capable of metabolizing
formic acid, acetic acid, methanol, carbon
monoxide, and carbon dioxide and hydrogen
into methane
 The methanogenic bacteria are crucial to
anaerobic digestion process since they are slow
growing and extremely sensitive to the changes in
the environment and can assimilate only a narrow
array of relatively simple substrates
 Methanogenesis is a critical step in the entire
anaerobic digestion process, and its biochemical
reactions are the slowest in comparison to those in
other steps. Methane-producing bacteria are strict
anaerobes and are vulnerable to even small
amounts of oxygen
Carbohydrates,
Proteins
& Fats
Sugar,
Amino
Acids &
Fatty
Acids
Volatile
Fatty
Acids
Acetic Acid
H2 and
CO2
Methane
& Carbon
dioxide
Week # 12, Lecture # 1 Renewable Energy Resources
1
Hydrolysis Acidogenesis Acetogenesis
Methanogenesis
2 3 4

15. In hydrolysis reaction, cellulose, starch, and simple sugars can be broken down by water and
enzymes (eq 1).
(1) biomass + H2O → monomers + H2
(Sources: cellulose, starch, sugars, fats, oils) (Products: mono-sugars [glucose, xylose, etc.],
fatty acids)
Week # 12, Lecture # 1 Renewable Energy Resources
Hydrolysis Biochemistry
During acidogenesis, soluble monomers are converted into small organic compounds, such as
short chain (volatile) acids (propionic, formic, lactic, butyric, succinic acids), ketones
(glycerol, acetone), and alcohols (ethanol, methanol) (eq 2 & 3).
(2) C6H12O6 + 2H2 → 2CH3CH2COOH + 2H2O
(3) C6H12O6 → 2CH3CH2OH + 2CO2
Acidogenesis Biochemistry

16. Week # 12, Lecture # 1 Renewable Energy Resources
The acidogenesis intermediates are attacked by acetogenic bacteria; the products from
acetogenesis include acetic acid, CO2, and H2. The reactions 4-7 shows the reactions that occur
during acetogenesis:
(4) CH3CH2COO- + 3H2O → CH3COO- + H+ + HCO3
- + 3H2
(5) C6H12O6 + 2H2O → 2CH3COOH + 2CO2 + 4H2
(6) CH3CH2OH + 2H2O → CH3COO- + 2H2 + H+
(7) 2HCO3
- + 4H2 + H+ → CH3COO- + 4H2O
Acetogenesis Biochemistry
The last phase of anaerobic digestion is the methanogenesis phase. Several reactions take place
using the intermediate products from the other phases, with the main product being methane.
Reactions 8-13 show the common reactions that take place during methanogenesis:
(8) 2CH3CH2OH + CO2 → 2CH3COOH + CH4
(9) CH3COOH → CH4 + CO2
(10) CH3OH → CH4 + H2O
(11) CO2 + 4H2 → CH4 + 2H2O
(12) CH3COO- + SO4
2- + H+ → 2HCO3 + H2S
(13) CH3COO- + NO- + H2O + H+ → 2HCO3 + NH4
+
Methanogenesis Biochemistry

17. Week # 12, Lecture # 1 Renewable Energy Resources
FactorsAffecting Anaerobic Digestion
Bacteria
Must have enough living organisms and the two different types bacteria types required in balance
Waste
Solid concentration and frequency of feeding can impact the process
Contact
Stabilization cannot occur without actual contact of the bacteria with the food. This contact can take place
in several ways but the most effective is mixing. Mixing can be achieved artificially by mechanical mixers
or by natural means.
Time
Two main factor may be considered in this aspect namely hydraulic retention time (HRT) and the solids
retention time (SRT). SRT depends on the degree of sludge retention achieved and HRT. The SRT/HRT
ratio, therefore, directly implies the efficiency of a treatment system. Higher the ratio, more efficient
and economic the system
Temperature
The rate of food stabilization increases and decreases with temperature within certain limits
pH
The pH of the contents of a digester depends on the relationship between the volatile acid, alkalinity and
percentage of carbon dioxide in the digester gas. Many reporters have indicated that the optimum pH for
the digestion of organic waste is in the range 6.8
Toxics
Inhibitory effects of certain materials on digestion if their concentration become to high
pH

18. Week # 12, Lecture # 1 Renewable Energy Resources
Key Options & Control Parameters
Maintain adequate quantity &
plan for restart
Minimize amount of inorganic entering and
eliminate toxic material
Amount applied to the treatment process should
be according to system size
Bacteria
Food
Loading
Temperature strongly influences many factors
affecting all stages of microbial activity and
especially methane and VFAproduction
Temperature
Methanogens are sensitive to variations in pH
and it is critical to maintain the correct pH
range for efficient operations
pH
Sources of alkalinity like ammonia and
bicarbonate are produced during digestion and
help maintain pH. A well-performing digester
should not require alkalinity supplementation
Alkalinity

19.  Elimination of
malodorous compounds
 Reduction of pathogens
 Deactivation of weed
seeds
 Production of sanitized
compost
 Decrease in GHGs
emission
 Promotion of carbon
sequestration
 Beneficial reuse of
recycled water
 Protection of
ground/surface water
 Net energy producing
process
 Biogas facility generates
high quality renewable
fuel
 Surplus energy as
electricity and heat is
produced
 Reduces reliance on
energy imports
 Biogas is a rich source of
electricity, heat, and
transportation fuel
 Transforms waste
liabilities into new profit
 Adds value to negative
value feedstock
 Income can be obtained
from the processing of
waste (tipping fees), sale
of organic fertilizer,
carbon credits and sale of
power
 Biomass-to-biogass
reduces water
consumption
 Reduces dependence on
energy imports
Environmental Benefits Energy Benefits Economic Benefits
Week # 12, Lecture # 1 Renewable Energy Resources
Advantages of Anaerobic Digestion

20. Week # 12, Lecture # 1 Renewable Energy Resources
Drawbacks of Anaerobic Digestion
Slow start up
Close monitoring required
Sensitive to temperature,
load and toxicity
High BOD and Pin
supernatant
Extreme confined space
hazard
Production of explosive
gas
Cleaning & maintenance difficult with
sealed tanks
Heating, mixing, gas collection
equipment & plumbing adds cost,
complexity

21. Week # 12, Lecture # 1 Renewable Energy Resources

22. Thanks for your Attention