BIOGAS PRODUCTION TECHNOLOGY
1. THE BIOGAS PLANT-SOME TECHNICAL CONSIDERATIONS
2. HOW TO INSTALL A POLYETHYLENE BIOGAS PLANT
1. THE BIOGAS PLANT-SOME TECHNICAL CONSIDERATIONS
The biogas plant consists of two components: a digester (or fermentation tank) and a gas holder. The digester is a cube-shaped or cylindrical waterproof container with an inlet into which the fermentable mixture is introduced in the form of a liquid slurry. The gas holder is normally an airproof steel container that, by floating like a ball on the fermentation mix, cuts off air to the digester (anaerobiosis) and collects the gas generated. In one of the most widely used designs (Figure 2), the gas holder is equipped with a gas outlet, while the digester is provided with an overflow pipe to lead the sludge out into a drainage pit.
Figure. 2. Diagram of Gobar-Gas Plant Used to Obtain Methane from Dung by
Anaerobic Fermentation (After Prasad et al. [20]1
The construction, design, and economics of biogas plants have been dealt with in the literature (13 - 21). For biogas plant construction, important criteria are: (a) the amount of gas required for a specific use or uses, and lb) the amount of waste material available for processing. Fry (17)
Singh (21), and others (1, 3) have documented several guidelines for consideration in the designing of batch (periodic feeding) and continuous (daily feeding) compartmentalized and non-compartmentalized biogas plants that are of either the vertical or horizontal type. In addition, Loll (18) has recently dealt with the scientific principles, process engineering, and shapes of digestion reactors, and with the economics of the technology.
Digester reactors are constucted from brick, cement, concrete, and steel. In Indonesia, where rural skills in brick making, brick laying, plastering, and bamboo craft are well established, clay bricks have successfully replaced cement blocks and concrete. In areas where the cost is high, the "sausage" or bag digester (14) appears to be ideal (Figure 3). The digester is constructed of 0.55 mm thick Hypalon laminated with Neoprene and reinforced with nylon. The bag is fitted with an inlet and an outlet made from PVC. Even if imported from the United States, the cost of the digester and the gas holder (both combined in one bag) is only 10 per cent of that for a concrete-steel digester. Another advantage is that it can be mass produced and is easily mailed. In rural areas, the whole installation is completed in a matter of minutes. A hole in the ground accommodates the bag, which is filled two-thirds full with waste water. Gas production fully inflates the bag, which is weighted down and fitted with a compressor to increase gas pressure.
Environmental and operational considerations
Raw Materials (19)
Raw materials may be obtained from a variety of sources - livestock and poultry wastes, night soil, crop residues, food-processing and paper wastes, and materials such as aquatic weeds, water hyacinth, filamentous algae, and seaweed. Different problems are encountered with each of these wastes with regard to collection, transportation, processing, storage, residue utilization, and ultimate use. Residues from the agricultural sector such as spent straw, hay, cane trash, corn and plant stubble, and bagasse need to be shredded in order to facilitate their flow into the digester reactor as well as to increase the efficiency of bacterial action. Succulent plant material yields more gas than dried matter does, and hence materials like brush and weeds need semi-drying. The storage of raw materials in a damp, confined space for over ten days initiates anaerobic bacterial action that, though causing some gas loss, reduces the time for the digester to become operational.
Influent Solids Content (16, 19, 21)
Production of biogas is inefficient if fermentation materials are too dilute or too concentrated, resulting in, low biogas production and insufficient fermentation activity, respectively. Experience has shown that the raw-material (domestic and poultry wastes and manure) ratio to water should be 1:1, i.e., 100 kg of excrete to 100 kg of water. In the slurry, this corresponds to a total solids concentration of 8 - 11 per cent by weight.
Loading (14, 19)
The size of the digester depends upon the loading, which is determined by the influent solids content, retention time, and the digester temperature. Optimum loading rates vary with different digesters and their sites of location. Higher loading rates have been used when the ambient temperature is high. In general, the literature is filled with a variety of conflicting loading rates. In practice, the loading rate should be an expression of either (a) the weight of total volatile solids (TVS) added per day per unit volume of the digester, or (b) the weight of TVS added per day per unit weight of TVS in the digester. The latter principle is normally used for smooth operation of the digester.
Seeding (14, 19)
Common practice involves seeding with an adequate population of both the acid-forming and methanogenic bacteria. Actively digesting sludge from a sewage plant constitutes ideal "seed" material. As a general guideline, the seed material should be twice the volume of the fresh manure slurry during the start-up phase, with a gradual decrease in amount added over a three-week period. If the digester accumulates volatile acids as a result of overloading, the situation can be remedied by reseeding, or by the addition of lime or other alkali.
pH (14, 19)
Low pH inhibits the growth of the methanogenic bacteria and gas generation and is often the result of overloading. A successful pH range for anaerobic digestion is 6.0 - 8.0; efficient digestion occurs at a pH near neutrality. A slightly alkaline state is an indication that pH fluctuations are not too drastic. Low pH may be remedied by dilution or by the addition of lime.
Temperature (13,14,19, 21)
With a mesophilic flora, digestion proceeds best at 30 - 40 C; with thermophiles, the optimum range is 50 - 60 C. The choice of the temperature to be used is influenced by climatic considerations In general, there is no rule of thumb, but for optimum process stability, the temperature should be carefully regulated within a narrow range of the operating temperature. In warm climates, with no freezing temperatures, digesters may be operated without added heat. As a safety measure, it is common practice either to bury the digesters in the ground on account of the advantageous insulating properties of the soil, or to use a greenhouse covering. Heating requirements and, consequently, costs, can be minimized through the use of natural materials such as leaves, sawdust, straw, etc., which are composted in batches in a separate compartment around the digester,
Nutrients (13,17,19, 21)
The maintenance of optimum microbiological activity in the digester is crucial to gas generation and consequently is related to nutrient availability. Two of the most important nutrients are carbon and nitrogen and a critical factor for raw material choice is the overall C/N ratio.
Domestic sewage and animal and poultry wastes are examples of N-rich materials that provide nutrients for the growth and multiplication of the anaerobic organisms. On the other hand, N-poor materials like green grass, corn stubble, etc., are rich in carbohydrate substances that are essential for gas production. Excess availability of nitrogen leads to the formation of NH3, the concentration of which inhibits further growth. Ammonia toxicity can be remedied by low loading or by dilution. In practice, it is important to maintain, by weight, a C/N ratio close to 30:1 for achieving an optimum rate of digestion. The C/N ratio can be judiciously manipulated by combining materials low in carbon with those that are high in nitrogen, and vice versa.
Toxic Materials (13,14,19)
Wastes and biodegradable residue are often accompanied by a variety of pollutants that could inhibit anaerobic digestion. Potential toxicity due to ammonia can be corrected by remedying the C/N ratio of manure through the addition of shredded bagasse or straw, or by dilution. Common toxic substances are the soluble salts of copper, zinc, nickel, mercury, and chromium. On the other hand, salts of sodium, potassium, calcium, and magnesium may be stimulatory or toxic in action, both manifestations being associated with the cation rather than the anionic portion of the salt. Pesticides and synthetic detergents may also be troublesome to the process.
Stirring (13,14,17 - 19, 21)
When solid materials not well shredded are present in the digester, gas generation may be impeded by the formation of a scum that is comprised of these low-density solids that are enmeshed in a filamentous matrix. In time the scum hardens, disrupting the digestion process and causing stratification. Agitation can be done either mechanically with a plunger or by means of rotational spraying of fresh influent. Agitation, normally required for bath digesters, ensures exposure of new surfaces to bacterial action, prevents viscid stratification and slow-down of bacterial activity, and promotes uniform dispersion of the influent materials throughout the fermentation liquor, thereby accelerating digestion.
Retention Time (19, 21)
Other factors such as temperature, dilution, loading rate, etc., influence retention time. At high temperature bio-digestion occurs faster, reducing the time requirement. A normal period for the digestion of dung would be two to four weeks.
Bottlenecks, considerations, and research and development
Bioconversion of organic domestic and farm residues has become attractive as its technology has been successfully tested through experience on both small- and large-scale projects. Feeding upon renewable resources and non-polluting in process technology, biogas generation serves a triple function: waste removal, management of the environment, and energy production. Nevertheless, there are still several problems (14, 19, 20) that impede the efficient working of biogas generating systems (Table 5).
TABLE 5. Considerations Relating to Bottlenecks in Biogas Generation
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Aspect |
Bottlenecks |
Remarks |
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Planning |
Availability and ease of transportation of raw materials and processed residual products |
Use of algae and hydroponic plants offsets high transportation costs of materials not readily at hand. Easily dried residual products facilitate transportation. |
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Site selection |
Nature of subsoil, water table, and availability of solar radiation, prevailing climatic conditions, and strength of village population need to be considered. |
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Financial contraints: Digester design; high Transportation costs of digester materials; installation and maintenance costs; increasing labour costs in distribution of biogas products for domestic purposes |
Use of cheap construction materials, emphasizing low capital and maintenance costs and simplicity of operation; provision of subsidies and loans that are not burdensome. |
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Necessity to own or have access to relatively large number of cattle |
Well-planned rural community development, ownership and biogas distribution schemes necessary. |
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Social contraints and psychological prejudice against the use of raw materials |
Development of publicity programmes to counteract contraints compounded by illiteracy; provision of incentives for development of small- scale integrated biogas systems. |
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Technical |
Improper preparation of influent solids leading to blockage and scum formation |
Proper milling and other treatment measures (pre- soaking, adjustment of C/N ratio); removal of inert particles: sand and rocks. |
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Temperature fluctuations |
Careful regulation of temperature through use of low-cost insulating materials (sawdust, bagasse, grass, cotton waste, wheat straw); incorporation of auxiliary solar heating system. |
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Maintenance of pH for optimal growth of Methanogenic bacteria C/N ratio |
Appropriate choice of raw material, regulation of C/N ratio and dilution rate. Appropriate mixing of N-rich and N-poor substrates with cellulosic substrates. |
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Dilution ratio of influent solids content |
Appropriate treatment of raw materials to avoid stratification and scum formation. |
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Retention time of slurry |
Dependent upon dilution ratio, loading rate, digestion temperature. |
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Loading rate |
Dependent upon digester size, dilution ratio, digestion temperature. |
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Seeding of an appropriate bacterial Population for biogas generation |
Development of specific and potent cultures. |
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Corrosion of gas holder |
Construction from cheap materials (glass fibre, clay, jute-fibre reinforced plastic) and/or regular cleaning and layering with protective materials (e.g., lubricating oil). |
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Pin-hole leakages (digester tank, holder, inlet, outlet) |
Establishment of "no leak" conditions, use of external protective coating materials (PVC, creosotes |
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Occurrence of CO2 reducing calorific value of biogas |
Reduction in CO2 content through passage in lime-water |
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Occurrence of water condensate in gas supply system (blockage, rusting) |
Appropriate drainage system using condensate traps |
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Occurrence of H2S leading to corrosion |
On a village scale, H2S removed by passing over ferric oxide or iron filings |
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Improper combustion |
Designing of air-gas mixing appliances necessary |
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Maintenance of gas supply at constant pressure |
Regulation of uniform distribution and use of gas; removal of water condensate from piping systems; appropriate choice of gas holder in terms of weight and capacity |
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Residue utilization |
Risks to health and plant crops resulting from residual accumulation of toxic materials and encysted pathogens |
Avoid use of chemical industry effluents; more research on type, nature, and die-off rates of persisting organisms; minimize long transportation period of un-dried effluent |
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Health |
Hazards to human health in transporting night soil and other wastes (gray-water) |
Linkage of latrine run-offs into biogas reactors promotes non-manual operations and general aesthetics |
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Safety |
Improper handling and storage of methane |
Appropriate measures necessary for plant operation, handling, and storage of biogas through provision of extension and servicing facilities |
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Rural communities using the integrated system are appropriate examples of recycled societies that benefit from low-capital investments on a decentralized basis and such communities are attuned to the environment. The technology thus seeded and spawned is, in essence, a populist technology based on "Nature's income and not on Nature's capital."
Biogas generated from locally available waste material seems to be one of the answers to the energy problem in most rural areas of developing countries. Gas generation consumes about one-fourth of the dung, but the available heat of the gas is about 20 per cent more than that obtained by burning the entire amount of dung directly. This is mainly due to the very high efficiency (60 per cent) of utilization compared to the poor efficiency (11 per cent) of burning dung cakes directly.
Several thousand biogas plants have been constructed in developing countries. A screening of the literature indicates that the experience of pioneering individuals and organizations has been the guiding principle rather than a defined scientific approach. Several basic chemical, microbiological, engineering, and social problems have to be tackled to ensure the large-scale adoption of biogas plants, with the concomitant assurances of economic success and cultural acceptance. Various experiences suggest that efficiency in operation needs to be developed, and some important factors are: reduction in the use of steel in current gas plant designs; optimum design of plants, efficient burners, heating of digesters with solar radiation, coupling of biogas systems with other non-conventional energy sources, design of large-scale community plants, optimum utilization of digested slurry, microbiological conversion of CO2 to CH4, improvement of the efficiency of digestion of dung and other cellulosic material through enzyme action and other pre-digestion methods, and anaerobic di gestion of urban wastes
We may summarize some of the research and development tasks that need to be undertaken as follows.
In basic research:
a. Studies on the choice, culture, and management of the micro-organisms involved in the generation of methane.
b. Studies on bacterial behaviour and growth in the simulated environment of a digester (fermentation components: rate, yield of gas, composition of gas as a function of variables - pH, temperature, agitation - with relation to substrates - manure, algae, water hyacinths).
In applied research:
a. Studies on improving biogas reactor design and economics focusing on: alternative construction materials in stead of steel and cement; seeding devices; gas purification methods; auxiliary heating systems; insulator materials; development of appropriate appliances for efficient biogas utilization (e.g. burners, lamps, mini tractors, etc.).
b. Studies for determining and increasing the traditionally acknowledged fertilizer value of sludge.
c. Studies on quicker de-watering of sludge.
d. Studies on deployment of methane to strengthening small-scale industries, e.g., brick-making, welding, etc.
In social research:
a. Effective deployment of the written, spoken, and printed word in overcoming the social constraints to the use of biogas by rural populations.
b. Programmes designed to illustrate the benefits accruing to rural household and community hygiene and health.
c. Programmes designed to illustrate the need for proper management of rural natural resources and for boosting rural crop yields in counteracting food and feed unavailability and insufficiency.
d. On-site training of extension and technical personnel for field-work geared to the construction, operation, maintenance, and servicing of biogas generating systems.
e. Involvement and training of rural administrative and technical personnel in regional, national, and international activities focusing on the potentials and benefits of integrated biogas systems.
Table 6 shows a number of the benefits of biogas utilization, set against the related drawbacks of presently used alternatives.
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Present problems |
Benefits of Biogas |
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Depletion of forests for firewood and causation of ecological imbalance and climatic changes |
Positive impact on deforestation; relieves a portion of the labour force from having to collect wood and transport coal; helps conserve local energy resources |
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Burning of dung cakes: source of environmental pollution; decreases inorganic nutrients; night soil transportation a hazard to health |
Inexpensive solution to problem of rural fuel shortage; improvements in the living and health standards of rural and village communities; provides employment opportunities in spin-off small-scale industries |
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Untreated manure, organic wastes, and residues lost as valuable fertilizer |
Residual sludge is applied as top-dressing; good soil conditioner; inorganic residue useful for land reclamation |
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Untreated refuse and organic wastes a direct threat to health |
Effective destruction of intestinal pathogens and parasites; end-products non-polluting, cheap; odours non-offensive |
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Initial high cost resulting from installation, maintenance, storage, and distribution costs of end-products |
System pays for itself |
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Social constraints and psychological prejudice to use of human waste materials |
Income-generator and apt example of self-reliance and self- sufficiency |
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2. HOW TO INSTALL A POLYETHYLENE BIOGAS PLANT
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HOW TO INSTALL A
POLYETHYLENE By
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1. WHAT IS A BIOGAS PLANT ? A biogas plant is commonly known as a BIODIGESTER. The biodigester is a technology that takes advantage of excrement from animals and humans in order to transform it into BIOGAS (methane gas) as a source of fuel, and ORGANIC FERTILIZER. Biogas is produced by bacteria that decomposes animal manure. The residual material is known as effluent and is very high in nutrients, thanks to the bacterial action and absence of oxygen. These factors help to eliminate bad smells and pathogens as well. A plastic bag, an outgoing biogas valve and a security valve compose a biogas polyethylene plant. |
Fig 1. The biogas plant. Gampaha, Sri Lanka |
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2. BENEFITS OF BIOGAS TECHNIQUE Biodigesters can be fed with animal and human excrement. For this reason, biogas plants help to diminish illnesses and reduce the population of insects like flies and mosquitoes. Thanks to biogas production, it is no longer necessary to use firewood to cook. The use of firewood affects the environment because it promotes the destruction of the forest and it also causes respiratory illnesses to people who cook with it. Biogas also saves money, replacing electricity or liquid gas as sources of energy for cooking. Effluent from the biogas plant can be used as organic fertilizer, and the application of chemical fertilizers can therefore be reduced. This way, farmers save money, and pollution of the atmosphere by chemical inputs is avoided. The same effluent can be used in fish ponds and to produce aquatic plants. Aquatic plants produce enough material to feed animals and/or to make compost. |
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3. STEPS TO INSTALL A BIODIGESTER FOR A RURAL FAMILY Step 1: Materials The recommended biodigester length for a small family is 10 meters. To install a biogas plant of this size, the materials listed below are needed:
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Figure 2. Location where the biogas |
Step 2: Biodigester location Once all the materials are
obtained, the biodigester location needs to be decided. The biodigester
should be close to the animals, and it is also recommended to connect any
human latrines to the biogas plant to eliminate another source of pollution.
Lastly, the location should be near the kitchen where the biogas will be
used. An inclined gutter should be dug at each end of the grave. It should be the same width as the cement pipes. It is necessary that the floor of
the grave is in no way sloped, or else, the system will not work. Also, the
walls should be totally flat and free of stones or roots that can break the
bag. |
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Figure 4. Details of the dug that should be dug at each end of the grave. Source. Botero et al., 1999. |
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Fig. 5. Preparing the plastic bag.
A person pulls one
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Extend the 28 m plastic bag on a flat floor that does not have any stones that can break it. It is better if this process is carried out in a hall room of the community centre. Then, fold the plastic in half and cut it in two 14 meters pieces. Next, a person (with no belt, rings or anything that might break the bag) introduces the second bag into the first one by pulling it through the 14 m length of the bag. When the second bag has been pulled through, the system is now a double bag, 14 meters in lenght. Now, the bag is ready and should be folded and stored in a safe place until the day of installation.
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Fig 6. Sequence of the installation
of the outgoing biogas valve. Four meters from one end of the double bag (nearest to the kitchen), make a small cut (¾inch) to introduce the 1 inch PVC screw. Between the screw and the double bag, place a plastic disk and a rubber disk (piece of used tire). Then, outside the double bag, place another rubber disk, then a plastic disk and the PVC adapter. Next, place a 10 cm PVC tube, the
PVC elbow and the second 1 inch PVC tube. Finally, place the plastic bag with
the outging valve in the grave and pass the two ends of the double bag
through the 1 meter cement tubes. Instead of cement tubes, you can use
plastic buckets. Remove the bottoms of three buckets and connect them
together to make a pipe. . You will need 4 buckets at each end (8 buckets
total). Step 5: Security valve Next, the security valve should be placed according to the following: |
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Figure 8. The security valve is tied to a wooden post. |
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ATTENTION: Make sure the 20 cm PVC tube, is not glued to the "T". Every six months, the Steel wool needs to be changed, without dismantling the rest of the system. Insert the valve into a transparent plastic bottle, full of water, and tie it to a wooden post (next to the exit valve). Make small holes in the plastic bottle to ensure that the 20 cm PVC pipe is 8-10 cm under the water level. Finally, the outgoing biogas and security valves are connected through the transparent plastic hose. Step 6: Fill up the biogas plant with exhaust and water Tie one end of the double bag with rubber belts, and insert a hose into the other end. Connect this hose to the exhaust pipe of a car, making sure it is well attached. Start the engine, and let it run until the bag is full. When the outgoing biogas valve starts to bubble, that indicates that the biodigester is at its maximum capacity. |
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Next, through the same hose, fill the biodigester with water, until it reaches the level of the buckets, and the gas can no longer escape. This is a critical step, because if it is not carried out correctly, the bell where the biogas will be stored will not form.
Figure 10. Photo of a biogas plant, filled with exhaust and water. |
Figure 11. Diagram of a
biogas plant, full of exhaust and water.
Water represents 75% of
the total volume of the bag.
The other 25% is full
of exhaust.
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At this point, open both ends of
the plastic bag. The bell should be formed and biogas will not escape. Step 7: Management of the biogas plant Daily, the biogas plant must be
fed with 18 kilos of FRESH excrement (from cows, buffaloes, goats, pigs,
etc.) blended with 90 kilos of water. This is approximately a mixture of 1
bucket of excrement with five buckets of water. This mixture is needed to
replace the water lost during the process and keep a flowing material inside
the biodigester. YOU SHOULD NEVER CHARGE THE BIOGAS PLANT WITH CHICKEN MANURE. This is not appropiate for biogas production. |
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Figure 13. The biogas plant shoud be fed every day with fresh manure. Manure is blended with water (1:5) and then through to the biodigester.
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Step 8: Protection of the biodigester and use of organic fertilizer In order to protect the plastic bag from sunlight and animals, a roof and a fence should be built. This will extend the life of the biogas plant (10 years Dig a hole next to the end of the
biogas plant to collect the effluent which can be used as fertilizer (see
figure). This fertilizer can be utilized for orchards or to produce aquatic
plants for animal consumption or composting, as well as in ponds for breeding
fishes. Fig 14. The effluents from the biodigester can be used directly as organic fertilizer or to produce aquatic plants, that then can be used to feed pigs or produce compost. |
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Step 9: Install the burner 30 days after the installation of the biogas plant, remove the PVC cap from the security valve and connect the end of the PVC pipe to the burner. All connections should be made with plastic or PVC pipe, because the biogas is corrosive. Only the pipe that will be used to burn the biogas is galvanized iron.
Through the application of biogas
technology the quality of life can be improved, save money and increase
productivity of the farm and the same time, while protecting our natural
resources. This simple and cheap technology can be applied everywhere in the
Tropics. 4. REFERENCES BOTERO, R.; AGUILAR, F.; PRESTON, T. 1999. The biodigester. In Toward a better use of our natural resources. EARTH, Guácimo, C.R. 28 p. BOTERO, R.; AGUILAR, F. X.; FERNÁNDEZ, J.; RUÍZ, P. 1997-1998. How to install a low cost biodigester. EARTH, Guácimo, C.R. Program of Community Development [videocassette] 16 min. BOTERO, R.; PRESTON, T.R. 1987 Low-cost biodigester for production of fuel and fertilizer from manure . Cali, Colombia. 30 p. DEUSTCHE GESELLSCHAFT FÜR TECHNISCHE ZUSAMMERNARBEIT. 1998 Biogas Basics. Information and Advisory Service on Appropiate Technology, Eschborn, Germany. ‹http://gate.gtz.de/biogas/basics/basics.htm›. FAO & (CIPAV) FUNDACIÓN CENTRO PARA LA INVESTIGACIÓN EN SISTEMAS SOSTENIBLES DE PRODUCCIÓN AGROPECUARIA. 1995. Plastic biodigester generator of biogas and organic fertilizer. Cali, Colombia. 18 p. PRESTON, T. R.; RODRIGUEZ , L.J. 1999. Manual for the installation of low cost biodigester. Fundación Universidad para la Agricultura Tropical. Vietnam. Cd-rom |
