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4.4 Biogas – a way to solve sanitation problems
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
Anaerobic fermentation is a natural and unavoidable process
Jam-Olof Drangert, Linköping university, Sweden
Spying on Nature – What can we learn from cows?
Inlet OutletBiogas digester
Cows convert biodegradable plants and water to milk, cow dung and urine – and gases
Pedro Kraemer, BORDA, India
A new look at the cow – and bull
Inlet
Outlet
Biogas digester
The Biogas Plant
Pedro Kraemer, BORDA, India
A biogas plant operates though anaerobic digestion of organic material
The Biogas Plant
Inlet Outlet
Biogas digester
Biogas
Pedro Kraemer, BORDA, India
Integrating biogas in agriculture
Pedro Kraemer, BORDA, India
Some examples of biogas plants
Pedro Kraemer, BORDA, India
Where is biogas technology applied?
Approximate numbers of biogas units in selected countries:
Country No of units Volume >100 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
Available human excreta in India compared to the need of fertiliser
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
… or as a resource
Excreta viewed as waste:
Faeces 250,000 tons/day
Urine 1,000,000 m3/day
N-P-K:
X
Y
Z
R
Pedro Kraemer, BORDA, India
Slurry application in agriculture
Pedro Kraemer, BORDA, India
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 OE = -3,475 kJ/mol
Pedro Kraemer, BORDA, India
Biogas appliances
Pedro Kraemer, BORDA, India
Biochemical process of anaerobic fermentation/digestion
Acetogenic bacteria
Fermentativebacteria
Methanogenic bacteria
Organic waste Carbohydrates Fats Protein Water
Bacterial mass
H2 , CO2
acetic acid
Bacterial mass
Methan + CO2
Bacterial mass
Propionic acidButyric acidAlcohols, Other components
H2 , CO2, acetic acid
Step 1: Hydrolysis + Acidogenesis Step 2:
Acetogenesis Step 3: Methanogenesis
Pedro Kraemer, BORDA, India
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
Substrate temperature in the digester
Common temperature ranges for bacteria:• Psychrophillic bacteria below 20oC• Mesophillic bacteria 20 – 40oC• Thermophillic bacteria above 40oC
Anaerobic fermentation can work in an ambient temperature between 3oC and 70oC and, if colder, the reactor has to be insulated and/or heated.
Methane production is very sensitive to changes in temperature
Pedro Kraemer, BORDA, India
Biogas production with continuous feeding
Hydraulic retention time in days
Litres of biogas per litre of slurry
Pedro Kraemer, BORDA, India
10
20
30
50 100 150
pH –value is crucial for a good result
• Optimal production when pH 7.0 – 7.2• Inhibition (due to acids) if pH < 6.2• Inhibition (due to ammonia) if pH > 7.6
pH is a central parameter for controlling the anaerobic process
Deviation from the optimum range results in:
• Lower gas yield
• Inferior gas quality Pedro Kraemer, BORDA, India
C/N ratio is important
Microorganisms need N (nitrogen) and C (carbon) for their metabolism
Methanogenic organisms prefer a
C/N ratio of between 10:1 and 20:1
Recommendation:
Mix different substrates
N must not be too low, or else
shortage of nutrient
Pedro Kraemer, BORDA, India
Nitrogen inhibition
If N concentration is too high (>1,700 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
Changes in dry matter (DM) concentration inside the digester
Pedro Kraemer, BORDA, India
Behaviour of the substrate inside the digester
Pedro Kraemer, BORDA, India
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
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 0.027*6 = 0.162 kWh
Actual kWh
Potential kWhEfficiency =
0.162
0.262= = 0.62
62% efficiency and the other 38% energy remains in the slurry
Pedro Kraemer, BORDA, India
Check-list if gas production is lower than expected
Check Response
Is pH >7.5 ? YesAdd water and take pH after one hour
No
Too much feed or of skewed composition?
Temperature fallen? Yes
Yes
Try to insulate digester, less feed, heat substrate. Wait one day
Add lime (acute action) and wait one day
No
Is pH < 6.8 ?Add urine or ash (kg/m3) and wait 1 day
Yes
Drangert & Ejlertsson, Linkoping university, Sweden
Principles for design and construction
Gas collector:
- fixed dome, or
- floating dome
Continuous feeding or batch feeding
Further treatment or direct use
Pedro Kraemer, BORDA, India
Fixed-dome biogas digester
Pedro Kraemer, BORDA, India
21
4
slurry3
Bird´s eye view
12
34
Floating-drum unit with water-jacket
Pedro Kraemer, BORDA, India
Anaerobic filter (off-plot system)
gas manhole
inflow
scumoutflow
filter mass
grillsludge
sedimentation tank filter tanks
Pedro Kraemer, BORDA, India
Anaerobic baffled reactor Off-plot system
gasmanholes
inflow
scum outflow
sludge
sedimentation inoculation of fresh wastewater with active sludge final settler
Anaerobic Baffled Reactor
Pedro Kraemer, BORDA, India
toilet sectionshowersection
anaerobic baffledreactor
waste-water
Pedro Kraemer, BORDA, India
Public toilet with hidden treatment unit
A public toilet with a biogas digester
Jan-Olof Drangert, Linköping University, Sweden
Material flows in the toilet complex
Toilet units & showers
Faeces
Urine
Bio-digester
Urine drying-bed
Liquid urine
Faeces
washwater
Ablution water
Flush
Liquid fertilizer
compost
Liquid urine Faeces
Organic waste Rainwater
Groundwater recharge
biogas
Soil conditioner Urine powder
Slurry
Aerobic pond
Slurry
System border
Jan-Olof Drangert, Linköping University, Sweden