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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 l.jpg

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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
Integrating biogas in agriculture organic material

Pedro Kraemer, BORDA, India


Some examples of biogas plants l.jpg
Some examples of biogas plants organic material

Pedro Kraemer, BORDA, India


Where is biogas technology applied l.jpg
Where is biogas technology applied? organic material

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 l.jpg
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 l.jpg
Slurry application in agriculture fertiliser

Pedro Kraemer, BORDA, India


Energy balance for composting and digestion l.jpg
Energy balance – for composting and digestion fertiliser

  • 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 l.jpg
Biogas appliances fertiliser

Pedro Kraemer, BORDA, India


Biochemical process of anaerobic fermentation digestion l.jpg

Bacterial fertiliser

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 l.jpg
What parameters affect anaerobic digestion? fertiliser

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 l.jpg
Substrate temperature in the digester fertiliser

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 l.jpg

30 fertiliser

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 l.jpg
pH –value is crucial for a good result fertiliser

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 l.jpg
C/N ratio is important fertiliser

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 l.jpg
Nitrogen inhibition fertiliser

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


Changes in dry matter dm concentration inside the digester l.jpg
Changes in dry matter (DM) concentration fertiliser inside the digester

Pedro Kraemer, BORDA, India


Behaviour of the substrate inside the digester l.jpg
Behaviour of the substrate inside the digester fertiliser

Pedro Kraemer, BORDA, India


Stirring the substrate l.jpg
Stirring the substrate fertiliser

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 l.jpg
Efficiency of a biogas unit fertiliser

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 l.jpg

Check-list fertiliser

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 l.jpg
Principles for design and construction fertiliser

Continuous feeding

orbatch feeding

  • Gas collector:

  • fixed dome, or

  • floating dome

Further treatment or

direct

use

Pedro Kraemer, BORDA, India


Fixed dome biogas digester l.jpg
Fixed-dome biogas digester fertiliser

2

1

3

4

Bird´s

eye view

4

1

2

slurry

3

Pedro Kraemer, BORDA, India


Floating drum unit with water jacket l.jpg
Floating-drum unit with water-jacket fertiliser

Pedro Kraemer, BORDA, India


Anaerobic filter off plot system l.jpg
Anaerobic filter fertiliser (off-plotsystem)

Pedro Kraemer, BORDA, India


Anaerobic baffled reactor l.jpg

Off-plot fertiliser system

Anaerobic Baffled Reactor

Anaerobic baffled reactor

Pedro Kraemer, BORDA, India


Slide30 l.jpg

Public toilet with hidden treatment unit fertiliser

Pedro Kraemer, BORDA, India


A public toilet with a biogas digester l.jpg
A public toilet with a biogas digester fertiliser

Jan-Olof Drangert, Linköping University, Sweden


Material flows in the toilet complex l.jpg

Faeces fertiliser

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


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