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