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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? ?.

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4 4 biogas a way to solve sanitation problems

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

spying on nature what can we learn from cows
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
A new look at the cow – and bull

The Biogas Plant

Outlet

Biogas digester

Inlet

Pedro Kraemer, BORDA, India

a biogas plant operates though anaerobic digestion of organic material
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
Integrating biogas in agriculture

Pedro Kraemer, BORDA, India

some examples of biogas plants
Some examples of biogas plants

Pedro Kraemer, BORDA, India

where is biogas technology applied
Where is biogas technology applied?

Approximate numbers of biogas units in selected countries:

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
Available human excreta in India compared to the need of fertiliser

Excreta viewed

as waste:

N-P-K:

X

Y

Z

R

… or

as a

resource

Pedro Kraemer, BORDA, India

slurry application in agriculture
Slurry application in agriculture

Pedro Kraemer, BORDA, India

energy balance for composting and digestion
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
Biogas appliances

Pedro Kraemer, BORDA, India

biochemical process of anaerobic fermentation digestion

Bacterial

mass

Bacterial

mass

H2 , CO2,

acetic acid

Methan

+ CO2

Bacterial

mass

Propionic acid

Butyric acid

Alcohols,

Other components

H2 , CO2

acetic acid

Biochemical process of anaerobic fermentation/digestion

Step 1:

Hydrolysis + Acidogenesis

Step 2:

Acetogenesis

Step 3:Methanogenesis

Organic waste

Carbohydrates

Fats

Protein

Water

Fermentative

bacteria

Acetogenic

bacteria

Methanogenic

bacteria

Pedro Kraemer, BORDA, India

what parameters affect anaerobic digestion
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
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

30

20

10

50

100

150

Biogas production with continuous feeding

Litres of biogas per litre of slurry

Hydraulic retention time in days

Pedro Kraemer, BORDA, India

ph value is crucial for a good result
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
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
Nitrogen inhibition

If N concentration is too high(>1,700 mg/l of NH4-N) and pH is high, then

growth of bacteria is inhibiteddue 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

stirring the substrate
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
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

ActualkWh

Potential kWh

0.162

0.262

= = 0.62

Efficiency =

62% efficiency and the other 38% energy remains in the slurry

Pedro Kraemer, BORDA, India

slide23

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

principles for design and construction
Principles for design and construction

Continuous feeding

orbatch feeding

  • Gas collector:
  • fixed dome, or
  • floating dome

Further treatment or

direct

use

Pedro Kraemer, BORDA, India

fixed dome biogas digester
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
Floating-drum unit with water-jacket

Pedro Kraemer, BORDA, India

anaerobic filter off plot system
Anaerobic filter(off-plotsystem)

Pedro Kraemer, BORDA, India

anaerobic baffled reactor

Off-plot system

Anaerobic Baffled Reactor

Anaerobic baffled reactor

Pedro Kraemer, BORDA, India

slide30

Public toilet with hidden treatment unit

Pedro Kraemer, BORDA, India

a public toilet with a biogas digester
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

Faeces

biogas

washwater

Flush

Ablution

water

Faeces

Liquid urine

Slurry

Slurry

compost

Urine

drying-bed

Aerobic

pond

Soil conditioner

Urine powder

Liquid fertilizer

Material flows in the toilet complex

Jan-Olof Drangert, Linköping University, Sweden