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
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