Training on
Technologies for Converting Waste Agricultural Biomass into Energy
Organized by
United Nations Environment Programme (UNEP DTIE IETC)
23-25 September, 2013
San Jose, Costa Rica
Biological Conversion Technologies
Anaerobic Digestion
Surya Prakash Chandak
Senior Programme Officer
International environmental Technology Centre
Division of Technology, Industry and Economics
Osaka, Japan
CONTENT

Introduction

Fermentation

Anaerobic Digestion
 Basic Process
 Factors Influencing Biogas Production
 Types of Biogas Digesters
 Biogas Yield
2
INTRODUCTION
• Biological Conversion
– Conversion of the biomass to fuel by exposing
biomass to certain microorganisms is called
biological conversion.
– The secondary fuels are produced as a result of
metabolic activity of the microorganisms.
– Fermentation and anaerobic digestions are the two
most common biological conversion processes and
products of these processes are ethanol and biogas.
– Today, ethanol is widely used as an alternative
source of liquid fuels for the transport sector in
countries and regions like USA, Brail, EU and China.
– Ethanol as a fuel offers many advantages such as
high octane number (99) than petrol (80–100), low
emission.
INTRODUCTION
• Biogas
– Biogas originates from the bio-degradation of organic
material under anaerobic conditions.
– Today biogas has several applications such as
industrial and household cooking, lighting, radiant
heaters and incubators for agricultural purposes and
absorption refrigerators.
– Biogas system provides a whole range of benefits for
their users, the society and the environment in
general.
INTRODUCTION
• Biogas
– Benefits:
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Production of energy (heat, light, electricity),
Transformation of organic waste into high quality fertilizer,
Improvement of hygienic conditions via reduction of
pathogens,worm eggs and flies,
Increase of productivity, mainly for women, in firewood
collection and cooking,
Environmental advantages through protection of soil, water,
air and woody vegetation,
Micro-economical benefits through energy and fertilizer
substitution,
Additional income sources and increasing yields of animal
husbandry and agriculture,
Macro-economical benefits through decentralized energy
generation, import substitution and environmental
protection.
FERMENTATION
• Overview
– Fermentation is a natural process initiated by
microorganisms, similar to common yeast cultures,
under anaerobic conditions.
– Ethanol can derive from any material which contains
sugar.
– In the Fermentation process, sugar elements such as
glucose, fructose and sucrose are converted into
ethanol and carbon dioxide as metabolic waste
products.
– The net chemical equation for the production of
ethanol from glucose is:
C6 H12 O6  2C2 H5 OH  2CO2
FERMENTATION
• Bio-ethanol
– The raw materials used in the production of ethanol
via fermentation are mainly classified into three types
as sugars, starches, and cellulose materials.
– Sugars (extracted from from sugarcane, sugar beets,
molasses, and fruits) can be converted into ethanol
directly.
– But starches (from corn, cassava, potatoes, and root
crops) and cellulose (from wood, agricultural
residues, waste sulfite liquor from pulp, and paper
mills) are needed to pre-treat prior the fermentation.
– The final product, ethanol (C2H5OH) mixed with small
amounts of methanol or higher alcohols and diluted
in water.
FERMENTATION
• Bio-ethanol
Sugars
(From sugar
cane,
molasses
and fruits)
Starches
(From corn,
cassava,
potatoes, and
root crops
Enzymetic
hydrolysis
Enzymetic
hydrolysis
Cellulose
(from wood,
agricultural
residues)
Fermantable
Sugars
Fermentation
Acid
hydrolysis
Gasification
Synthesis
gas
Flow diagram of bioethanol production
Distillation
Bio
Ethanol
ANAEROBIC DIGESTION
• Basic Process
– A simplified stoichiometry for anaerobic digestion of
biomass is;
C6H10O5  H2O  3CH4  3CO2
– The whole process of biogas production from organic
wastes occurred in main three steps namely
hydrolysis, acidification, and methane formation.
– Three types of bacteria namely fermentative,
acetogenic and methanogenic are involved in
hydrolysis, acidification and methane formation,
respectively.
ANAEROBIC DIGESTION
• Basic Process
– The three stage anerobic fermentaton of biomass
Organic waste
(Cellelose, Starch,
Protein, Lipid)
Fermentative
bacteria
Alcohol, organic acids,
amino acids , hydrogen
sulphide and other
compounds
H2, CO2, Acedic
Acid
Acetogenic
bacteria
Methanogenic
bacteria
Hydrolysis
Acidification
H2, CO2, Acedic Acid
CH4, CO2
Methane
formation
ANAEROBIC DIGESTION
• Hydrolysis
– In hydrolysis aerobic micro-organisms convert
complex organic compounds (carbohydrates,
proteins and lipids) into simple forms which are
soluble and can be consumed by the microorganisms.
– As an example, Polysaccharides are converted into
monosaccharides, lipids to fatty acids, proteins to
amino acids and peptides.
• Acidification
– In the second step, acid-producing bacteria
(acetogenic bacteria), convert the intermediates of
fermenting bacteria into mixture of acetic acid
(CH3COOH), H2, CO2, alcohols, organic acids, amino
acids and hydrogen sulphide.
ANAEROBIC DIGESTION
• Acidification
– The oxygen requirement for producing acetic acid is
fulfilled by the oxygen solved in the solution or
bounded-oxygen.
– The acid-producing bacteria create an anaerobic
condition which is essential for the methane
producing microorganisms.
• Methane formation
– In the third step, methane-producing bacteria utilize
hydrogen, carbon dioxide and acetic acid formed in
acidification process to form methane and carbon
dioxide.
ANAEROBIC DIGESTION
• Factors influencing the biogas production
– Substrate temperature

Optimal temperature range for biogas production: 20-28°C.

This can be achieved in a satisfactory level only where mean
annual temperatures are around 20°C or above or where the
average daily temperature is at least 18°C.
If the temperature is below 15°C, gas production will be so
low that the biogas plant is no longer economically feasible.

– Changes in temperature
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The process is very sensitive to changes in temperature.
Most of the biogas plants are builds in underground in order
to overcome this issue.
The temperature fluctuations between day and night are no
great problem for plants built underground, since the
temperature of the earth below a depth of one meter is
practically constant.
ANAEROBIC DIGESTION
• Factors influencing the biogas production
– Available nutrient
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In order to grow, bacteria need organic substances as a
source of carbon and energy.
In addition to carbon, oxygen and hydrogen, the generation
of bio-mass requires an adequate supply of nitrogen, sulfur,
phosphorous, potassium, calcium, magnesium and a
number of trace elements such as iron, manganese,
molybdenum, zinc, cobalt, selenium, tungsten, nickel etc.
"Normal" substrates such as agricultural residues or
municipal sewage usually contain adequate amounts of the
mentioned elements.
Higher concentration of any individual substance usually
has an inhibitory effect, so that analyses are recommended
on a case-to-case basis to determine which amount of which
nutrients, if any, still needs to be added.
ANAEROBIC DIGESTION
• Factors influencing the biogas production
– Retention time
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The effective retention time may vary widely for the
individual substrate constituents depending on the vessel
geometry, the means of mixing, etc.
Selection of a suitable retention time depends on process
temperature as well as on the type of substrate used.
For liquid manure undergoing fermentation, the following
approximate values apply:
- liquid cow manure: 20-30 days
- liquid pig manure: 15-25 days
- liquid chicken manure: 20-40 days
- animal manure mixed with plant material: 50-80 days
If the retention time is not maintained properly and it is too
short, the bacteria in the digester are "washed out" faster
than they can reproduce, and fermentation practically comes
to a standstill. This problem rarely occurs in WAB systems.
ANAEROBIC DIGESTION
• Factors influencing the biogas production
– pH Value
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The best condition for the methane-producing bacteria is
neutral to slightly alkaline conditions.
Once the process of fermentation has stabilized under
anaerobic conditions, the pH will normally take on a value of
between 7 and 8.5.
If the pH value drops below 6.2, the medium will have a toxic
effect on the methanogenic bacteria.
– Nitrogen inhibition
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Nitrogen in the substrate inhibits the process of
fermentation.
Noticeable inhibition occurs at a nitrogen concentration of
roughly 1700 mg ammonium-nitrogen (NH4-N) per liter
substrate.
The main prerequisite is that the ammonia level does not
exceed 200-300 mg NH3-N per liter substrate.
ANAEROBIC DIGESTION
• Factors influencing the biogas production
– C/N ratio
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Microorganisms required both nitrogen and carbon for
assimilation into their cell structures.
Various experiments have shown that the metabolic activity
of methanogenic bacteria can be optimized at a C/N ratio of
approximately 8-20, whereby the optimum point varies
depending on the nature of the substrate.
– Substrate solid content
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Solid content of the substrate impaired the mobility of the
methanogens within the substrate.
Therefore the biogas yield decreases with the increase of
solids content.
No generally valid guidelines can be offered with regard to
specific biogas production for any particular solids
percentage.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– There are various types of biogas plants available in
the world and they are classified mainly based on
feeding method and construction.
– Based on the feed method, three different forms can
be distinguished:
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
Batch plants,
Continuous plants,
Semi-batch plants.
– Based on the construction, two main types of simple
biogas plants can be distinguished:


Fixed-dome plants,
Floating-drum plants.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Batch type digesters.
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In batch type plants, materials fed into the digester at a time
and sealed only allowing the gas to exit and then emptied
completely after a fixed retention time.
Each design and each fermentation material is suitable for
batch filling, but batch plants require high labor input.
The major disadvantage of this type is unsteady gas-output.
3
1
2
1 - Digester.
2 - Gasholder.
3 - Gas pipe.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Continuous type digesters.
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Once the process started, regular quantity of waste are fed
and regular quantity of material discharged, continuously.
They empty automatically through the overflow whenever
new material is filled in.
Therefore, the substrate must be fluid and homogeneous.
This technology is suitable for both medium and large scale
waste treatment and large scale biogas production.
Advantages of this type are constant and higher gas
production
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Semi-batch type digesters.
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If the two materials which have completely different
digestion rates (such as straw and dung) are to be digested
together, a biogas plant can be operated on a semibatch
basis.
The slowly digested straw-type material is fed in about twice
a year as a batch load. The dung is added and removed
regularly.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Fixed Dome type digesters.

A fixed-dome plant comprises of a closed, dome-shaped
digester with an immovable, rigid gas-holder and a
displacement pit.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Fixed Dome type digesters.
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The gas is stored in the upper part of the digester.
When gas production commences, the slurry is displaced
into the displacement tank.
Gas pressure increases with the volume of gas stored, i.e.
with the height difference between the two slurry levels.
If there is little gas in the gasholder, the gas pressure is low.
The digesters of fixed-dome plants are usually masonry
structures, structures of cement and ferro-cement exist.
Main parameters for the choice of material are technical
suitability (stability, gas- and liquid tightness), costeffectiveness, availability in the region and transport costs
and availability of local skills for working with the particular
building material.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Fixed Dome type digesters.

Currently, various types of fixed dome plants are available
such as Chinese fixed dome plant, Janata model,
s the archetype of all fixed dome plants and extensively use in China. The
Deenbandhu and CAMARTEC model.
ists of a cylinder with round bottom and top.
rious types of fixed dome plants are available such as Chinese fixed dome plant,
l, Deenbandhu and CAMARTEC model. Chinese fixed-dome plant, shown in
Chinese fixed dome type biogas digester
Fixed dome biogas digester: CAMARTEC model
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Fixed Dome type digesters.
Gas collector,
fixed dome
Biogas
Waste
1
1.
6
5
Slurry
9
2
4
8
3
10
Automatic
overflow
7
Mixing tank with inlet pipe
and sand trap.
2. Digester.
3. Compensation and removal
tank.
4. Gasholder.
5. Gas pipe.
6. Entry hatch, with gastight seal.
7. Accumulation of thick sludge.
8. Outlet pipe.
9. Reference level.
10. Supernatant scum
Fixed dome biogas digester - Nicarao design
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Floating drum type digesters.

Major difference between fixed dome and floating drum type
plant is, a floating-drum plant consists of a floating gasholder, or drum.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Floating drum type digesters.
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This floats either directly in the fermenting slurry or in a
separate water jacket.
The drum in which the biogas collects has an internal and/or
external guide frame that provides stability and keeps the
drum upright. If biogas is produced, the drum moves up, if
gas is consumed, the gasholder sinks back.
Floating-drum plants are used mainly in continuous feed
mode of operation.
They are used most frequently by small- to middle-sized
farms (digester size: 5-15m3) or in institutions and larger
agro-industrial estates (digester size: 20-100m3)
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Advantages of fixed dome type.
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Produce just as much gas as floating-drum plants, if they are
gas-tight.
Low cost operation
Simple design
Long life of the plant (20 years or more)
– Disadvantages of fixed dome type.
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Utilization of the gas is less effective as the gas pressure
fluctuates substantially.
Labor-intensive design
Not easy to build.
Difficult to achieve gas tightness.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Advantages of floating drum type.
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Simple operation
Provide gas at a constant pressure
Stored gas-volume is immediately recognizable by the
position of the
– Disadvantages of floating drum type.
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The steel drum is relatively expensive and maintenanceintensive.
Removing rust and painting has to be carried out regularly.
The life-time of the drum is short (up to 15 years; in tropical
coastal regions about five years).
ANAEROBIC DIGESTION
• Biogas Yield
– Biogas yield of a biomass material depends on the
organic fraction of dry matter in the material and the
waste management system associated with it.
– The dry matter (DM) of the waste is the matter left
after removal of its moisture content. It may be
obtained as the weight loss on heating to a
temperature of 105 C.
– Whereas, Volatile Solids (VS) are defined as the
organic fraction of dry matter in waste.
– Around 50-60% of the initial energy content in the
organic material can be converted to biogas in a
properly operated digester.
ANAEROBIC DIGESTION
• Biogas Yield
–
Biogas production after addition of substrate
ANAEROBIC DIGESTION
• Biogas Yield
– The resultant gas mixture consists of about 50 - 70%,
CH4 and the rest is CO2 with small amounts of water
vapours, H2S, NH3, and some organics that give bad
odour.
– Methodology of Estimation:
• Amount of biogas that can be potentially produced from
recoverable wastes
= Amount of dry matter recoverable (kg DM/year) 
Volatile solids fraction in dry matter (kg VS/kg DM) 
Biogas yield (m3/kg VS)
• Energy potential of the biogas recoverable (MJ /year)
=Amount of biogas recoverable (m3/year)  Heating value
of biogas (MJ /m3)
ANAEROBIC DIGESTION
• Biogas Yield
– The heating value of the biogas depends on its
composition, especially the amount of methane.
– HHV of methane is about 35.8 MJ/m3 and therefore
biogas with 60% methane, HHV could be taken as 20
MJ/m3.
– Selected values for waste characteristics
Animal Type
Cattle
Fraction of Volatile Biogas Yield
Solid (VS/DM)
(m3/kg of VS)
0.8
0.20 – 0.3
0.8
0.35 – 0.60
Pigs
0.7 – 0.8
0.25 – 0.50
Straw
0.8 – 0.9
0.15 – 0.35
Grass
0.9
0.55
Fruit Waste
0.75
0.25 – 0.50
Garden Waste
0.9
0.20 – 0.50
Poultry
ANAEROBIC DIGESTION
• Biogas Yield
– Benchmarks for specific methane yields
The End
35