BIO GAS DIGESTER The village Byse, one of the 101 adopted villages of Amrita University as part of their SeRVe

program, is being developed into a self-reliant village. Currently their main income comes from

agriculture, but some of the households have found additional income in selling areca plates,

made from the areca palm sheath. Their areca palm plate selling, which is done individually by

each household will be united into one business venture. Villagers will join hands and work

together in this process. For this venture machines will be improved and an alternate fuel

source will be made available. This energy source will be made with the help of organic waste

and a biogas digester.

In this paper the principles of a biogas digester will be elaborated and discussed. Hereby

different considerations will be looked into and various designs are analyzed. Then several

options are researched in improving the raw materials or defining alternate ones.

The basics of Biogas

The degradation of organic materials is done in several stages with the help of micro-

organisms. The last stage is when the decomposed material is returned to the environment.

This process is done by the methanogens (methane producing bacteria). Through the

degradation of organic material by methanogens under anaerobic conditions, methane gas

is formed. This is the so-called biogas, a renewable source of energy. When looking at the

biogeochemical carbon cycle, the natural production of biogas takes in an important part.

The development of biogas plants for the rural households in India started in the 1950s. With

the strong support by the government there was a massive increase of biogas installations.

Currently there are more than one million biogas plants in India.

Yearly 590 to 880 million tons of methane is produced by micro-organisms worldwide.

Approximately 90% is derived from the decomposition of biomass. The remainder results

from fossils (e.g. petrochemical processes).

The composition of biogas consists of different gasses:

● Methane ( CH4) 40-70 vol %

● Carbon dioxide (CO2) 30-60 vol %

● Other gases 0- 5 vol %

○ Hydrogen (H2) 0- 1 vol %

○ Hydrogen sulfide (H2S) 0- 3 vol %

The characteristic properties of biogas are like any pure gas dependent on pressure and

temperature. Also moisture content affects the properties. The important factors are:

● volume change as function of temperature and pressure,

● calorific value change as function of temperature, pressure and water-vapor content,

and

● change in water-vapor content as a function of temperature and pressure.

The calorific value, which is the amount of heat released by a unit volume or weight of the

substance during complete combustion, is 6 kWh/m3 of biogas. This can be compared to

approximately half a liter of diesel oil. The net calorific value depends on the efficiency of

burners or appliances.

For simple biogas plants not all organic material is suitable, therefore only homogenous and

liquid substrates will be considered. These are faeces and urine from cattle, poultry, pigs and

wastewater from toilets. The excrements need to be diluted with almost the same amount of

liquid, this can be done with urine.

Normally, the biogas that is produced will be able to be used immediately, but also further

treatment can be needed, for example when the hydrogen-sulfide content is high. As a result

from mixing this biogas with air at a ratio of 1:20, a highly explosive gas will form.

The gas demand can be defined on the basis of energy consumed previously. For example, 1

kg firewood then corresponds to 200 l biogas, 1 kg dried cow dung corresponds to 100 l

biogas and 1 kg charcoal corresponds to 500 l biogas.

Benefits of a biogas digester

● The production of energy for heating or electricity. Biogas can replace traditional

energy sources as firewood or fossil fuels thus contributing to combat deforestation

and the emission of greenhouse gases.

● The transformation of organic waste into fertilizer

● Reduce in firewood collecting, thus workload

● Economical benefits through energy and fertilizer substitution and additional income

sources

Principles of a biogas digester

Animal dung and agricultural or kitchen waste along with water are put in the bio gas plant

this will digest into gas and a slurry which can be used as a fertilizer.

The three steps of biogas production

The process for producing biogas can be divided into three steps; hydrolysis, acidification

and methane formation. There are three types of bacteria engaged in the process.

Figure 1: the principles of bio gas diagram

Hydrolysis

The first step is the enzymolyzation of organic matter by extracellular enzymes (cellulase,

amylase, protease and lipase) of micro-organism. The bacteria will break the long chains of

complex carbohydrates, proteins and lipids into shorter parts.

Acidification

In the second step, the acid producing bacteria converts the products from the fermentative

bacteria into acetic acid (CH3COOH), hydrogen (H2) and carbon dioxide (CO2). For the

production of acetic acid, the acetogenic bacteria needs oxygen. This will be taken from the

solution it is in. In this way this bacteria creates anaerobic conditions, which is essential for

the methanogenic (methane producing) bacteria. From a chemical perspective the reactions

that the acid producing bacteria make possible are partially endergonic (i.e. only possible

with energy input).

Methane formation

The methanogenic bacteria, involved in the third step, process the compounds with low

molecular weight (e.g. hydrogen, carbon dioxide and acetic acid) into methane and carbon

dioxide.

Cattle dung and manure

To determine the biomass supply, the livestock inventory is used. Therefore data concerning

the amount of manure produced by different species and per live weight of the livestock unit

is needed.

Dung yield = live weight × number of animals × specific quantity of excrements [ kg/d ]

The use of cattle dung is for methane production is most preferable, as methanogenic

bacteria are already present in the stomach of the ruminants. However due to pre

fermentation the proportion of produced methane is approximately 65%. The homogeneous

consistency makes it easy to use in continuous plants if it’s mixed with equal amounts of

water.

The fresh cattle dung is mixed with water. Straw and fodder left in the dung is removed. This

will prevent clogging and reduce the scum formation. The urine from the cows can improve

the methane production, however its collection on most farms in developing countries is to

difficult..

Chicken droppings

The use of chicken dropping is only possible if they roast above a collecting area. Otherwise

sand or sawdust fraction will affect the methane production negatively. The use of pure

chicken droppings can result into a to high ammonia concentration. The use of it with cow

dung is possible. The dropping are hard and dry, for its use it has to be pulverized and mixed

with water before using it as input for the digester. The methane proportion in biogas

resulting from chicken excrement is up to 60%.

The problem of scum

Stirring of the substrate can prevent scum. It’s filthy and though material and after short time

it will become solid. To remove it with fermentation it has to be kept wet. This can be done

by watering it from the top or by pushing it down in the liquid. This is a costly procedure,

which include expensive apparatus. For simple biogas plants stirring are not viable and the

only solution is selecting suitable feed material and mixing the dung with liquid thoroughly

before input.

In the case of heave gas releasement from the inlet, when there is not enough gas, it’s

because of the scum layer. By continuously gas release from the inlet pressure building in the

digester is prevented.

The pipe gas can be blocked when scum rises due to daily feeding without daily discharge.

The scum has to be removed, this can be done by hand. On the surface, straw, grass, stalks

or dried dung floats. In the case of proper mixing of the substrate and not to high water

contents, separation is prevented due to friction within the mixture. Solids and mineral

material tends to sink to the bottom and can block the outlet pipe or reduce the active

digester volume.

In the use of pure and fresh cattle dung, scum will not be a problem. Floating layers will be a

problem when indigestible husks are in the fodder.

Design of the biogas digester

The material used in the design of the biogas digester need to be inexpensive, widely

accessible materials and technology. The digester can be quite simple, it consist of a tank in

which the material is digested combined with a system that collects and stores the biogas

along with another outlet for the produced slurry.

Currently there are three designs for biogas digesters, the balloon plat, fixed dome plant and

a floating drum plant.

Balloon type

A balloon plant consist of a kind of balloon or bag in which the gas is stored. The inlet and

outlet are both attached to this. Gas pressure is achieved through the elasticity of the

balloon as this can change. The main advantage of this type is the low costs and materials

needed to build it.

Fixed dome type

In a fixed-dome plant the gas holder is fixed on top of the digester. The produced gas floats

to the top, the volume of gas decides the gas pressure and the level and height difference of

the slurry in the digester and compensation tank. The advantage of a fixed dome type

digester is the long life span.

Floating drum type

The floating drum plant consist of an underground digester and a gas holder on the ground.

The gasholder floats either directly on the fermentation slurry or in a water jacket of its own.

The gas is collected in the gas drum, which rises or moves down, according to the amount of

gas stored. The gas drum is prevented from tilting by a guiding frame. If the drum floats in a

water jacket, it cannot get stuck, even in substrate with high solid content.

Types of plants

The type of plant that is favorable with the rural environment is the continuous plants or the

semi-batch plants.

Continuous plants are fed and emptied continuously. When new material is fed overflow will

cause the plant to empty automatically. Therefore it’s necessary that the substrate is fluid

and homogeneous. The advantages are a constant gas production, with minimum labor

input.

Semi-batch plants include the use of example straw and dung. Straw-type material will digest

slowly and is to be put in twice a year as a batch load. The dung is added and removed

regularly.

Considerations

For the design of the biogas digester there are several considerations that need to be kept in

mind to make a working design favorable in the area of Byse. These considerations are listed

below.

• climatic and soil conditions; Biogas technology is feasible in most climatic zones under all

climatic conditions, where temperature or precipitation are not too low. Biogas production

works best between specific temperatures. High precipitation can lead to high groundwater

levels, causing problems in construction and operation of biogas plants. It will be an indirect

impact on anaerobic fermentation.

• the capital available; around 600 rupees

• the availability of skills for operation, maintenance and repair.

The design

The design used for the biogas digester is a floating drum type, this is most used and proven

to work type used in India. However the considerations are to make units which are low-cost

and durable. The simplified floating drum type has the disadvantage of losing roughly a

quarter of its produced gas. Therefore the option of fixed drum type will also be presented.

Floating drum type

The installation consists of a tank, with a slightly smaller tank in it. As the amount of biogas

produced increases the smaller tank will be filled up with the gas. This will cause the inner

tank to telescope out of the outer tank. As biogas is used the inner tank will move down into

the outer tank. The inner tank will represent a lid and as a storage for the gas. The gap

between the inner and outer tank has to be narrow to prevent any gas loss and to stop the

oxygen from entering the digester. significant amount of oxygen would kill the

methanogenic bacteria.

The advantages are a simplified and easy to understand process. The volume of the gas can

be regulated easily as its amount is directly visible. The gas pressure is maintained constant,

due to the weight of the gas holder. Furthermore the construction is easy and materials are

inexpensive. Mistakes from construction won’t lead to major problems in its operation or gas

yield.

To increase the temperature the biogas digester could be painted black, as it will increase

the absorbance of solar radiation.

Fixed dome type

A closed tank will act as a digester and as a gasholder. To maintain the gas pressure there

will be a valve like mechanism, with the use of a counterweight, the overproduced gas will

escape automatically. With this design there will be no loss of gas as with the floating drum

type and it will save costs because there is no need of a second tank.

A water tank will be used along with some piping to create the bio gas digester. For the

transportation of the gas a plastic tube will be used along with an outlet, also used in

plumbing. The design can be seen below.

The inlet pipe is secured in the inside of the tank, to make it more durable. It needs to be

installed approximately 100 mm above the bottom of the tank to make sure the fresh slurry

is deposit under the older slurry to ensure it works optimal.

The outlet pipe is put somewhere above the middle, this is the outlet for the slurry. The

outlet is put in this place to make sure there is still room for digesting and room for gas

storage.

To prevent too much gas pressure in the tank, the gas needs to be able to move into the gas

tube but also there need to be a safety valve. The safety valve will work as a counterweight,

when the pressure gets too high the valve will open.

The pipes can be secured in the tank by making a hole equal to the diameter of the pipe.

Afterwards it can be made airtight by mixing sand and glue and place this around the edges.

As for materials PVC can be used, PVC will not be affected by the slurry. For the gas outlet a

plastic valve as well as metal valve can be used.

Insulating the bio gas digester

To prevent the bio gas digester from drastic temperature changes the bio gas digester need

to be insulated. Drastic temperature changes will have a bad influence on the digesting

process. The digester can be insulated by construction cotton or with straw, depending on

the availability of the material since there is no research on which insulates better.

If, after your digester has been producing for a while, it slows production, valve off some

effluent and reenter it at the charging entrance. This will tend to buffer the slurry inside the

digester and production should soon resume. If proper C/N ratios are maintained with

proper slurry viscosities, this continuous flow digester should operate indefinitely providing

that it is regularly charged.

Gas pipe, valves and accessories

Defect gas piping accounts for almost 60% of non- functional biogas units. Therefore proper

construction needs to be executed. Single sizes for all pipes, valves and accessories can ease

the installation and maintenance. Biogas contains hydrogen-sulfide and is 100% saturated

with water vapor. This means that these components can’t consist of metals, as corrosion will

degrade these pipes, valves and accessories quickly. Galvanized steel pipes and plastic

tubing, made from PVC or rigid PE, are suitable to use. If gas pipes are laid in the open must

be UV-resistant.

Features

Fertilizer from the biogas digester

As biogas is produced the digested organic material is left behind, this exits the system and

can be used as fertilizer for the fields. This liquid anaerobic “compost” still contains all the

minerals and other soil nutrients of the waste, including the nitrogen that can be lost

through aerobic composting, making it high-quality fertilizer. While there are suitable

inorganic substitutes for the nutrients nitrogen, potassium and phosphorous from organic

fertilizer, there is no artificial substitute for other substances such as protein, cellulose, lignin,

etc.. They all contribute to increasing a soil’s permeability while preventing erosion and

improving the agricultural conditions. Organic substances also constitute the basis for the

development of the microorganisms responsible for converting soil nutrients into a form that

can be readily incorporated by plants.

Parameters and process optimization

Substrate temperature

Anaerobic fermentation can take place between temperatures of 3°C and about 70°C.

The temperature range for different bacteria can be differentiated into three ranges:

● The psychrophilic temperature range Below 20°C

● The mesophilic temperature range Between 20 - 40°C

● The thermophilic temperature range Above 40°C

Unheated biogas plants work effectively, when mean annual temperatures are around 20°C

or above. The production of methane by the methanogenic bacteria will increase with

increasing temperature. Within 20-28°C mean temperature range the gas production will

increase over-proportionally. The amount of free ammonia will also increase with

temperature increase, but this could inhibit or reduce the bio-digestive performance.

Temperature changes

The production of biogas is highly sensitive to temperature changes. For fermentation brief

fluctuation not exceeding the limits below will be un-inhibitory to the process.

● The psychrophilic range ± 2°C/h

● The mesophilic range ± 1°C/h

● The thermophilic range ± 0,5°C/h

For plants built underground temperature fluctuations between day and night will not pose

as a problem, as temperature of the earth below one meter stays almost constant.

Nutrients

Besides carbon and energy from organic substances, bacteria need mineral nutrients to

grow. They also need mineral nutrients in addition to carbon, oxygen and hydrogen. The

minerals needed are; nitrogen, sulfur, phosphorus, potassium, calcium, magnesium and trace

elements such as iron, manganese, molybdenum, zinc, cobalt, selenium, tungsten, nickel etc.

“Normal” substrates such as agricultural residues or municipal sewage contain adequate

amounts of the mentioned elements. When there are higher contents of any individual

substance in the mixture it usually has an inhibitory effect on the process. Therefore analysis

is recommended to define which nutrients have to be added.

Retention time

In the mesophilic temperature range the retention time for liquid manure is:

● Liquid cow manure 20-30 days

● Liquid pig manure 15-25 days

● Liquid chicken manure 20-40 days

● Animal manure mixed with plant material 50-80 days

When the retention time is too short the bacteria won’t be able to reproduce, as they are

“washed out” faster. The fermentation process will then come to a standstill. On average 40

days are used as retention time.

The pH value

The methanogenic bacteria thrives best under neutral to slightly alkaline conditions. After

stabilizations under anaerobic conditions the pH value will lie between 7 and 8,5. A pH value

below 6,2 of the mixture will be toxic to the methanogenic bacteria.

Nitrogen inhibition

All the substrates put in the digester contain nitrogen. When pH values increase a relative

low nitrogen concentration can have a inhibiting effect on the process of fermentation. A

nitrogen concentration of approximately 1700 mg ammonium-nitrogen (NH4-N) per liter

substrate creates a noticeable inhibition. However the methanogens are able to adapt to the

range of 5000-7000 mg/l substrate o nitrogen concentration when given enough time. This

is only possible if the ammonia level (NH3) does not surpass 200-300 mg NH3-N per liter

substrate. The ammonia level depends on the process temperature and the pH value of the

substrate slurry.

C/N Ratio

The metabolic activity of methanogenic bacteria can be optimized at a C/N ratio of roughly

8-20. The optimum point will vary from case to case, as this depends on the substrates

involved.

Agitation

To maintain process stability within the digester, mixing or substrate agitation is need. The

most important objectives of agitation are:

● Removal of metabolites (gas produced by methanogens )

● Inoculation (mixing of fresh substrate with bacterial population

● Prevention of sedimentation and scum formation

● Avoidance of pronounced temperature gradients

● Uniform bacterial population density

● Prevention of dead spaces formation in the digester (maintain effective volume)

1. Slow stirring instead of rapid agitation

2. Thin layer of scum in a digester completely filled with substrate, maintained

sufficiently wet won’t affect the process

3. Mechanical agitation is not needed for all biogas digesters. In the case of substrates

with such high solid content, no stratification occurs orr substrates consisting of only

solute substances.

Inhibitory factors

Heavy metals, antibiotics (Bacitracin, Flavomycin, Lasalocid, Monensin, Spiramycin, etc.) and

detergents from livestock husbandry can affect the process of biomethanation inhibitory.

Various inhibitors have their own limit concentrations before having a negative effect (see

table below) .

Start-up of a plant

Initial filling

When a new biogas plant is filled, the initial filling should be either digested slurry from

another plant or cattle dung. The substrate can be diluted with more water than usual to

allow the digester to fill up completely. The type of substrate causes a stable digestive

process to be achieved in several days to several weeks. Cattle dung can yield good gas

production within one or two days. The breaking-in period can be recognised by:

● Low quality biogas (more than 60% CO2)

● High odorous biogas

● pH value sinking

● Gas production is erratic

The first two gasholder fillings have to be vented, as residual oxygen can be an explosion

hazard.

References

Case study summary. (2016, December 20). Retrieved from

(http://www.ashden.org/files/SKG%20full.pdf

Frost, C. (2002). Technologies demonstrates echo: Horizontal biogas digester. ECHO

http://www.wcasfmra.org/biogas_docs/Horizonal%20Biogas%20Digester.pdf

Mattocks, R. (1984) .Understanding Biogas Generation.

http://pdf.usaid.gov/pdf_docs/PNABC935.pdf

Kossmann, W., Pönitz, UTA. Biogas Digest Volume 1 :Biogas Basics. ISAT

Kossmann, W., Pönitz, UTA. Biogas Digest Volume 2 :Biogas-Application and Product

Development. ISAT