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

2. 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

3. A new look at the cow – and bull
The Biogas Plant
Outlet
Biogas digester
Inlet
Pedro Kraemer, BORDA, India

4. A biogas plant operates though anaerobic digestion of organic material
The Biogas Plant
Biogas
Inlet
Outlet
Biogas digester
Pedro Kraemer, BORDA, India

5. Integrating biogas in agriculture
Pedro Kraemer, BORDA, India

6. Some examples of biogas plants
Pedro Kraemer, BORDA, India

7. 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
DK, NL, S, Thailand,
99% of all systems do not use pumps, agitator, and heating
Pedro Kraemer, BORDA, India 7

8. 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)
Potential biogas
50 mil m3 day
N-P-K: X Y Z R
… or
as a
resource
Pedro Kraemer, BORDA, India

9. Slurry application in agriculture
Pedro Kraemer, BORDA, India

10. 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= kJ/mol
Burning of biogas:
2CH4+ 6O2  CO2 + 6 H2 O
E = -3,475 kJ/mol
Pedro Kraemer, BORDA, India

11. Biogas appliances
Pedro Kraemer, BORDA, India

12. Biochemical process of anaerobic fermentation/digestion
Step 1:
Hydrolysis + Acidogenesis
Step 2:
Acetogenesis
Step 3: Methanogenesis
Bacterial
mass
Methan
+ CO2
Bacterial
mass
Propionic acid
Butyric acid
Alcohols,
Other components
H2 , CO2,
acetic acid
Organic waste
Carbohydrates
Fats
Protein
Water
Bacterial
mass
H2 , CO2
acetic acid
Fermentative
bacteria
Acetogenic
bacteria
Methanogenic
bacteria
Pedro Kraemer, BORDA, India

13. 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

14. 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

15. Biogas production with continuous feeding
Litres of biogas per litre of slurry 10 20 30 50
100
150
Hydraulic retention time in days
Pedro Kraemer, BORDA, India

16. 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 < 6.2
Inhibition (due to ammonia) if pH > 7.6
Deviation from the optimum range results in:
Lower gas yield
Inferior gas quality
Pedro Kraemer, BORDA, India

17. Methanogenic organisms prefer a
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
N must not be too low, or else
shortage of nutrient
Recommendation:
Mix different substrates
Pedro Kraemer, BORDA, India

18. 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 mg/l of NH4-N given the pre-requisite that the uncharged ammonia (NH3 controlled by pH) level does not exceed mg/l
Pedro Kraemer, BORDA, India

19. Changes in dry matter (DM) concentration inside the digester
Pedro Kraemer, BORDA, India 19

20. Behaviour of the substrate inside the digester
Pedro Kraemer, BORDA, India

21. 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

22. 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 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 = kWh
Actual kWh
Potential kWh
0.162
0.262
Efficiency =
= = 0.62
62% efficiency and the other 38% energy remains in the slurry
Pedro Kraemer, BORDA, India

23. if gas production is lower than expected
Check-list
if gas production is lower than expected
Check Response
Add water and take pH after one hour
Yes
Is pH >7.5 ? No
Add urine or ash (kg/m3) and wait 1 day
Yes
Is pH < 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

24. 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

25. Fixed-dome biogas digester2 1 3 4
Bird´s
eye view 4 1 2
slurry 3
Pedro Kraemer, BORDA, India 25

27. Floating-drum unit with water-jacket
Pedro Kraemer, BORDA, India 27

28. Anaerobic filter (off-plot system)
Pedro Kraemer, BORDA, India

29. Anaerobic baffled reactor
Off-plot system
Anaerobic baffled reactor
Anaerobic Baffled Reactor
Pedro Kraemer, BORDA, India

30. Public toilet with hidden treatment unit
Pedro Kraemer, BORDA, India

31. A public toilet with a biogas digester
Jan-Olof Drangert, Linköping University, Sweden 31 31

32. Material flows in the toilet complex
Toilet units
& showers
Faeces
Urine
Bio-digester
drying-bed
Liquid urine
washwater
Ablution
water
Flush
Liquid fertilizer
compost
Organic waste
Rainwater
Groundwater
recharge
biogas
Soil conditioner
Urine powder
Slurry
Aerobic
pond
System border
Jan-Olof Drangert, Linköping University, Sweden