4.4 Biogas – a way to solve sanitation
problems
Anaerobic
fermentation
is a natural
and
unavoidable
process
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
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
A new look at the cow – and bull
The Biogas Plant
Outlet
Inlet
Biogas digester
Pedro Kraemer, BORDA, India
A biogas plant operates though
anaerobic digestion of organic material
The Biogas Plant
Biogas
Inlet
Outlet
Biogas digester
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
India (in 2004)
12,000,000
3,600,000
Nepal (in 2007)
Vietnam, Thailand, Tanzania,
Bangladesh, Burundi, Brazil
Kenya, Mexico, Cuba, Guyana
Morocco, Ghana, Zimbabwe,
Nicaragua, Jamaica, Bolivia
200,000
x,000
x00
x0
x0
?
?
3,400 (2006)
in Germany
?
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
Excreta viewed
as waste:
Faeces
Urine
250,000 tons/day
1,000,000 m3/day
Dry org. matter (DS) 90,000 t/day
… or
Nitrogen (N) 15,000 t/day
as a
Phosphorus (P2O5)
5,000 t/day
resource
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
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 O
E = -3,475 kJ/mol
Pedro Kraemer, BORDA, India
Biogas appliances
Pedro Kraemer, BORDA, India
Biochemical process of anaerobic
fermentation/digestion
Step 1:
Hydrolysis + Acidogenesis
Step 2:
Acetogenesis
Step 3:
Methanogenesis
Organic waste
Carbohydrates
Fats
Protein
Water
Bacterial
mass
Fermentative
bacteria
Bacterial
mass
H2 , CO2,
acetic acid
Propionic acid
Butyric acid
Alcohols,
Other
components
Bacterial
mass
Methan
+ CO2
H2 , CO2
acetic acid
Acetogenic
bacteria
Methanogenic
bacteria
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
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
Biogas production with continuous feeding
Litres of
biogas
per litre
of slurry
30
20
10
50
100
150
Hydraulic retention time in days
Pedro Kraemer, BORDA, India
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
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
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
Efficiency =
=
0.162
= 0.62
Potential kWh
0.262
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 ?
Yes
Add water and take pH after
one hour
Yes
Add urine or ash (kg/m3) and
wait 1 day
No
Is pH < 6.8 ?
Temperature fallen?
Yes
Try to insulate digester,
less feed, heat substrate.
Wait one day
Yes
Add lime (acute action)
and wait one day
No
Too much feed or of
skewed composition?
Drangert & Ejlertsson, Linkoping university, Sweden
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
Fixed-dome biogas digester
2
1
3
4
Bird´s
eye view
4
1
2
slurry
3
Pedro Kraemer, BORDA, India
Floating-drum unit with water-jacket
Pedro Kraemer, BORDA, India
Anaerobic filter (off-plot system)
gas
manhole
inflow
scum
outflow
filter mass
grill
sludge
sedimentation tank
filter tanks
Pedro Kraemer, BORDA, India
Off-plot system
Anaerobic Baffled Reactor
Anaerobic baffled reactor
gas
manholes
w
um
outflow
dge
sedimentation
inoculation of fresh wastewater with active sludge
final settler
Pedro Kraemer, BORDA, India
Public toilet with hidden treatment unit
wastewater
anaerobic baffled
reactor
toilet section
shower
section
Pedro Kraemer, BORDA, India
A public toilet with a biogas digester
Jan-Olof Drangert, Linköping University, Sweden
Material flows in the toilet complex
Faeces
Urine
Rainwater
Organic waste
System border
Groundwater
recharge
Liquid
urine
Toilet units
& showers
Bio-digester
biogas
Faeces
washwater
Flush
Ablution
water
Liquid urine
Slurry
Slurry
Faeces
compost
Soil conditioner
Urine
drying-bed
Urine powder
Aerobic
pond
Liquid fertilizer
Jan-Olof Drangert, Linköping University, Sweden