4.4 Biogas

1
4.4 Biogas a way to solve sanitation problems
Anaerobic fermentation is a natural and
unavoidable process
How much biogas can be produced from excreta and
biomass? How safe is the process and its sludge??
Learning objectives to know about the
fundamental processes in biogas production, and
get an overview of biogas generation in the world
Jam-Olof Drangert, Linköping university, Sweden
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Spying on Nature What can we learn from cows?
Inlet
Outlet
Biogas digester
Cows convert biodegradable plants and water to
milk, cow dung and urine and gases
Pedro Kraemer, BORDA, India
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A new look at the cow and bull
The Biogas Plant
Outlet
Biogas digester
Inlet
Pedro Kraemer, BORDA, India
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A biogas plant operates though anaerobic
digestion of organic material
The Biogas Plant
Biogas
Inlet
Outlet
Biogas digester
Pedro Kraemer, BORDA, India
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Integrating biogas in agriculture
Pedro Kraemer, BORDA, India
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Some examples of biogas plants
Pedro Kraemer, BORDA, India
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Where is biogas technology applied?
Approximate numbers of biogas units in selected
countries
Country No of units Volume gt100 m3
China 12,000,000 x0
India (in 2004) 3,600,000 ?
Nepal (in 2007) 200,000 ?
Vietnam, Thailand, Tanzania, Bangladesh, Burundi, Brazil x,000 3,400 (2006) in Germany
Kenya, Mexico, Cuba, Guyana x00 ?
Morocco, Ghana, Zimbabwe, Nicaragua, Jamaica, Bolivia x0 DK, NL, S, Thailand,
99 of all systems do not use pumps, agitator,
and heating
Pedro Kraemer, BORDA, India
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Available human excreta in India compared to the
need of fertiliser
Excreta viewed as waste
Faeces 250,000 tons/day
Urine 1,000,000 m3/day
Dry org. matter (DS) 90,000 t/day
Nitrogen (N) 15,000 t/day
Phosphorus (P2O5) 5,000 t/day
Potassium (K2O) 3,000 t/day
Carbon (C) 35,000 t/day
Calcium (CaO) 5,000 t/day
Potential biogas 50 mil m3 day
N-P-K X Y Z R
or as a resource
Pedro Kraemer, BORDA, India
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Slurry application in agriculture
Pedro Kraemer, BORDA, India
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Energy balance for composting and digestion

• Aerobic conversion (composting)
• C6 H12 O6 6O2 ? 6 CO2 6 H2 O
• ?E -3,880 kJ/mol
• Anaerobic conversion (digestion)
• C6 H 12 O6 2H2 O ? 3 CO2 3CH4 2H 2O
• ?E - 405 kJ/mol

Burning of biogas 2CH4 6O2 ? CO2 6 H2
O ?E -3,475 kJ/mol
Pedro Kraemer, BORDA, India
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Biogas appliances
Pedro Kraemer, BORDA, India
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Biochemical process of anaerobic
fermentation/digestion
Step 1 Hydrolysis Acidogenesis
Step 2 Acetogenesis
Step 3 Methanogenesis
Organic waste Carbohydrates Fats Protein
Water
Acetogenic bacteria
Fermentative bacteria
Methanogenic bacteria
Pedro Kraemer, BORDA, India
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What parameters affect anaerobic digestion?

• The most important determinants of good
  living conditions for anaerobic bacteria and
  therefore efficient gas production, are
• Temperature
• Retention Time
• pH-level
• Carbon/Nitrogen ratio (C/N ratio)
• Proportion of dry matter in substrate suitable
  viscosity
• Agitation (mixing) of the substrate
• If any one of these determinants is outside
  acceptable range, the digestion may be inhibited

Pedro Kraemer, BORDA, India
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Substrate temperature in the digester
Anaerobic fermentation can work in an ambient
temperature between 3oC and 70oC and, if
colder, the reactor has to be insulated and/or
heated.

• Common temperature ranges for bacteria
• Psychrophillic bacteria below 20oC
• Mesophillic bacteria 20 40oC
• Thermophillic bacteria above 40oC

Methane production is very sensitive to changes
in temperature
Pedro Kraemer, BORDA, India
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Biogas production with continuous feeding
Litres of biogas per litre of slurry
Hydraulic retention time in days
Pedro Kraemer, BORDA, India
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pH value is crucial for a good result
pH is a central parameter for controlling the
anaerobic process

• Optimal production when pH 7.0 7.2
• Inhibition (due to acids) if pH lt 6.2
• Inhibition (due to ammonia) if pH gt 7.6

• Deviation from the optimum range results in
• Lower gas yield
• Inferior gas quality

Pedro Kraemer, BORDA, India
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C/N ratio is important

• Microorganisms need N (nitrogen) and C (carbon)
  for their metabolism

Methanogenic organisms prefer a C/N ratio
of between 101 and 201
N must not be too low, or else shortage of
nutrient
Recommendation Mix different substrates
Pedro Kraemer, BORDA, India
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Nitrogen inhibition
If N concentration is too high (gt1,700 mg/l
of NH4-N) and pH is high, then
growth of bacteria is inhibited due to
toxicity caused by high levels of (uncharged)
ammonia
Methanogens, however, are able of adapt to
5,000 - 7,000 mg/l of NH4-N given the
pre-requisite that the uncharged ammonia (NH3
controlled by pH) level does not exceed 200-300
mg/l
Pedro Kraemer, BORDA, India
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Changes in dry matter (DM) concentration inside
the digester
Pedro Kraemer, BORDA, India
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Behaviour of the substrate inside the digester
Pedro Kraemer, BORDA, India
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Stirring the substrate

• Stirring improves the efficiency of digestion by

• Removing metabolites (gas removal)
• Bringing fresh material in contact with bacteria
• Reducing scum formation and sedimentation
• Preventing temperature gradients in the digester
• Avoiding the formation of blind spots (short
  cuts)

However, excessive stirring disturbs the
symbiotic relationship between the different
bacteria species
Simple biogas units normally do not have
mechanical stirring devises
Pedro Kraemer, BORDA, India
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Efficiency of a biogas unit
Input 1 kg of dry (95) cattle dung will
produce 2.5 kWh (rule of thumb)
1 kg dry (100) matter can generate 2.5/0.95
2.63 kWh
Slurry contains 10 dry matter, thus 1 litre can
generate 0.263 kWh
1 litre slurry (27oC, 90 days retention) releases
27 litre biogas
1 m3 of biogas can generate 6 kWh (rule of thumb)
So, 1 lit of slurry generates 0.0276 0.162 kWh
Actual kWh Potential kWh
0.162 0.262
0.62
Efficiency
62 efficiency and the other 38 energy remains
in the slurry
Pedro Kraemer, BORDA, India
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Check-list if gas production is lower than
expected
Check Response
Add water and take pH after one hour
Yes
Is pH gt7.5 ?
No
Add urine or ash (kg/m3) and wait 1 day
Yes
Is pH lt 6.8 ?
Try to insulate digester, less feed, heat
substrate. Wait one day
Temperature fallen?
Yes
No
Add lime (acute action) and wait one day
Yes
Too much feed or of skewed composition?
Drangert Ejlertsson, Linkoping university,
Sweden
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Principles for design and construction
Continuous feeding or batch feeding

• Gas collector
• fixed dome, or
• floating dome

Further treatment or direct use
Pedro Kraemer, BORDA, India
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Fixed-dome biogas digester
2

1

3

4

Birds eye view
4
1
2
slurry
3
Pedro Kraemer, BORDA, India
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(No Transcript)
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Floating-drum unit with water-jacket
Pedro Kraemer, BORDA, India
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Anaerobic filter (off-plot system)
Pedro Kraemer, BORDA, India
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Anaerobic baffled reactor
Off-plot system
Anaerobic Baffled Reactor
Pedro Kraemer, BORDA, India
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Public toilet with hidden treatment unit
Pedro Kraemer, BORDA, India
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A public toilet with a biogas digester
Jan-Olof Drangert, Linköping University, Sweden
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Material flows in the toilet complex
Jan-Olof Drangert, Linköping University, Sweden