Understanding biogas. (Alternative energy).

RICHARD MATTOCKS
VOLUNTEERS IN TECHNICAL ASSISTANCE
1815 NORTH LYNN ST., SUITE 200
ARLINGTON, VA 22209

Biogas is a by-product of the biological breakdown--under oxygen-free conditions--of organic wastes such as plants, crop residues, wood and bark residues, and human and animal manure. Interest in biogas as a viable energy resource has spread throughout the globe in the past two decades. Biogas generators or digesters operate throughout Asia, for example, with more than 100,000 reported in India, about 30,000 in Korea, and several million in China. Many more are operating in the Middle East, Africa, Oceania, Europe, and the Americas.

Biogas is known by many names--swamp gas, marsh gas, "will o' the wisp," gobar gas. It contains about 50-60% methane, the primary constituent of natural gas. Biogas is produced naturally from the degradation of plants in such situations as rice paddies, ponds, or marshes. Because it can also be produced and collected under controlled conditions in an airtight container, it can be an important energy source.

Ancient Chinese experimented with burning the gas given off when vegetables and manures were left to rot in a closed vessel. More recently, Volto, Beachans, and Pasteur worked with biogas-producing organisms. At the turn of the 20th century, communities in England and Bombay, India, disposed of wastes in closed containers and collected the resulting gas for cooking and lighting. Germany, the United States, Australia, Algeria, France, and other nations constructed such methane digesters to supplement dwindling energy supplies during the two world wars.

Biogas generators or digesters yield two products: the biogas itself, and a semi-solid by-product called effluent or sludge.

Biogas systems are most popular for their ability to produce fuel from products that might otherwise be wasted--crop residues, manures, etc. The fuel is a flammable gas suitable for cooking, lighting, and fueling combustion engines.

The digested waste--sludge--is a high quality fertilizer. The digestion process converts the nitrogen in the organic materials to ammonium, a form that becomes more stable when plowed into the soil. Ammonium is readily "fixed" (bonded) in soil so that it can be absorbed by plants. In contrast, raw manure has its nitrogen oxidized into nitrates and nitrites, which do not "fix" well in soil and are readily washed away.

Moreover, biogas systems offer a means to sanitize wastes. Simply put, these systems are capable of destroying most bacteria and parasitic eggs in human and animal wastes, enabling the digested sludge to be applied safely to crops, Tests have shown that biogas systems can kill as much as 90-100% of hookworm eggs, 35-90% of ascarid (i.e., roundworms and pinworms), and 90-100% of blood flukes (i.e., schistosome flukes, which are found in water snails that commonly live in paddy fields and ponds).

Biogas systems are also capable of digesting municipal sewage, which is a major source of pollution. Using biogas systems in this way substantially reduces the potential for environmental pollution.

Finally, agricultural and animal wastes, the major raw materials for biogas production, are usually plentiful in rural areas. People living in rural communities, who are often subjected to the price and supply fluctuations of conventional fuels and fertilizers, can benefit directly from biogas systems.

It should be noted that, while this article focuses on the production of biogas for fuel, in some applications the gas is considered to be the by-product of the process. Some digesters in China, for example, are used primarily for treating sewage and producing fertilizer, and only secondarily for producing fuel.

Biogas generation is a process that takes place in an oxygen-free environment. It uses anaerobic bacteria--bacteria that live only in the absence of oxygen--to break down complex organic compounds in fairly well-defined stages. The process is called anaerobic digestion. It produces biogas, a gas composed of approximately 50-60% methane, 40-50% carbon dioxide, as well as water vapor and a small quantity of nitrogen, sulfur, and other trace compounds. Biogas is flammable, which is what makes it useful, but it has a relatively low heat content, approximately 6.1 calories per liter (around 600 BTU per cubic foot). Compare this with pure methane, which has a heat value of 995 BTU per cubic foot, or natural gas with over 1,000. Nevertheless, biogas can be an important fuel source for many applications.

A biogas digester is the device in which the digestion process occurs. The organic feedstock, which is called the substrate, may consist of soil, manure, crop or kitchen residues, or similar materials. The substrate is usually diluted with water and is thoroughly mixed into a slurry; crop residues and vegetation are usually cut or chopped into small, fairly uniform pieces. It is then fed into the digester and permitted to undergo degradation in a sealed oxygen-free chamber. When digestion is completed, the material is discharged, or removed from the digester. The biogas is collected for direct usage or pressurized for subsequent use. The discharged material is called effluent, or sludge.

The actual breakdown of organic material inside the digester is a three-stage process that leads to the production of methane.

The principles of anaerobic digestion are the same regardless of the digestion vessel. Organic material is loaded into a fairly warm, temperature-controlled, oxygen-free environment and methane is produced after acclimatization. The makeup or quality of incoming material to be digested, the vessel, and the surrounding environment influence the digester efficiencies. The production of gases is greater when the digester is operated at a relatively high temperature, when the substrate is stirred or otherwise agitated, and when system conditions are kept fairly constant. The important objective to keep in mind when operating a biogas digester is the production of the greatest volume of biogas in the shortest possible time.

Performance influences

Researchers are only now gaining a better understanding of the metabolic process in biogas digesters. They do know, however, that methane-producing organisms (called methanogens) "prefer" to channel energy, or calories (derived by breaking down incoming substrate), to methane rather than use the energy to construct or satisfy internal cellular needs. As such, methanogens do not adapt well to changes in their environment that may require them to increase their numbers or adjust their internal mechanisms. If the environmental changes are significant enough, the methanogens may slow or even stop their work.

Changes that may affect the behavior of the bacteria and thus the performance of the digester include variations in the substrate, presence of certain toxic chemicals, gas pressure, temperature, and the amount of time the material remains in the digester.

Other factors that could have a major impact on the operating performance of a biogas digester include biological balance/acidity, solids concentration, agitation, feedstock, pretreatment, and the carbon-to-nitrogen ratio.

Biological influences

Methanogens--methane-producing organisms--live in a syntrophic, or complementary, relationship with certain other microorganisms that consume the feedstock and produce simple acids as part of their metabolism. The simplest acids are essential to the metabolic processes of the methanogens. As acid-producing organisms tend to choke in their own acetic by-products, methanogens cooperate by consuming these by-products in the methane-producing process.

Given sufficient time to establish the proper ratio of methane-producing organisms to acid-producing organisms, a homeostasis, or stability, will occur with a pH of about seven in a digester. A digester fed poultry or high nitrogen waste may stabilize at a pH of eight or greater.

The objective here is to create a stable working relationship among the microbial population in the digester. This implies the need for fairly constant operating temperatures and feedstock characteristics. Conversely, any rapid variations of these conditions will cause the microbial population to shift dramatically and possibly upset the overall system balance in the digester. For example, if the methane-producing organisms become dormant due to, say, temperature fluctuations, the pH will drop so low as to incapacitate them.

Maintaining a stable pH requires stabilizing the feedstock as well as the operating temperature in the digester. If this proves impractical, adding lime or other buffering compounds to the digester will prevent the pH from falling. Note that the correct amount and type of buffering compound can be determined only on a case-by-case basis.

Operating temperature

Operating temperature is another factor influencing digester efficiency. A digester can operate in three temperature ranges: (l) the low temperature, psycrophilic bacteria range, which is less than 90[degrees]F; (2) the medium temperature, mesophilic bacteria range, which is 85-105[degrees]F; and (3) the high temperature, thermophilic bacteria range, which is 135-140[degrees]F. Organic material degrades more rapidly at higher temperatures because the full range of bacteria are at work. Thus, a digester operating at a higher temperature can be expected to produce greater quantities of biogas. The disadvantage of an elevated-temperature digester is that even minor changes in system conditions could offset digester efficiency or productivity. Moreover, an additional source of energy will likely be required to maintain the digester contents at a constant higher temperature.

Though operating temperature is critical, stabilizing the temperature and keeping, it stabilized are even more important. Variations of plus or minus 1[degrees]C in a day may force the methane-producing organisms into periods of dormancy.

One way to overcome the problem of lower temperatures is to dilute the daily incoming waste material with preheated (solar-heated) water. Or you can construct a greenhouse or compost pile around the digester.

Note that the amount and type of waste to be degraded as well as the operating temperature are two important factors governing digester size.

Solids concentration

The moisture content of the waste that is diluted should be in the range of 5-12% total solids. The percentage of total solids should include a minimum of inorganic sands and soils. Incoming waste products may have to be diluted to a consistency of slightly thick cream. A rule of thumb for diluting cattle waste is 2.5 parts water for every one part of relatively dry waste or one part water for every one part of fresh manure.

Stirring the contents

The microorganisms degrading the waste material are living, metabolizing creatures that produce their own metabolic by-products. To prevent the bacteria from stagnating in their own waste products, and thus to promote a more rapid digestion, stir or agitate the digester contents by paddle, scraper, piston, or in more sophisticated settings, by gas recirculation.

Agitation also helps to minimize the build-up of internal fibrous scum on top of the digestion liquor. Failure to break the scum may result in excessive gas pressures forcing substrate out of the openings instead of permitting the gas to escape through gas transport lines. The scum may also plug the digester. Digesters that are fed higher volumes of fibrous waste may require special design considerations.

Feedstock pretreatment

Feedstocks sometimes require pretreatment to increase the methane yield in the anaerobic digestion process. Pretreating the feedstock (with alkaline or acid treatments, for example) breaks down the complex organic structures into simpler molecules that are then more susceptible to microbial degradation.

Thus, you may want to pretreat any incoming substrate whose volatile solids are not readily degradable. Note that microorganisms do not readily act upon rice hulls or sawdust.

Fibrous wastes also require special handling. Wastes with long fibers such as straw should be chopped or broken. Any given waste will digest more rapidly, and possibly even more completely, when broken into bits. Thus, the finer the waste is shredded, ground, or pulped, the easier the digestion process will be.

Carbon-to-nitrogen ratio

If the carbon-to-nitrogen ratio is either too high or too low, or fluctuates substantially, the digestion process will slow or even stop. To act efficiently on the substrate, microorganisms need a 20-30:1 ratio of carbon to nitrogen, with the largest percentage of the carbon being readily degradable. Digesters have efficiently operated on poultry waste with a 5-7:1 ratio. The key here is to keep the quantity as well as the characteristics of the incoming substrate constant.

One note of caution: some carbon compounds resist being broken down. Lignin, for example, which all land plants use to help stiffen and support themselves, is the least readily degradable carbon compound. The amount of lignin increases proportionally with plant age. Thus, old grass contains more lignin than new grass, and wood contains more of it than do leaves. Thus, horse dung and mature vegetative waste material are probably not good feedstocks, because they contain a high fraction of non-degradable lignin.

Presence of toxins

Certain medications (e.g., antibiotics used in animal feeds or injected into animals), feed additives, pesticides, and herbicides may have adverse effects on anaerobic bacteria, particularly the methanogens.

Digester size

Digester design depends basically upon the availability and type of waste to be fed to the digester, as well as the amount of gas and/or fertilizer required.

A distinct advantage of small digesters over large ones is that their contents require less vigorous and less frequent stirring (only several times a day) to prevent scum buildup and thus increase the production of biogas. A principal disadvantage of these digesters, on the other hand, is that their operating temperatures tend to fluctuate more often and to a much greater degree.

Nevertheless, feeding a biogas digester--regardless of its size--any number of individual or combined feedstocks or organic materials will result in the production of biogas as long as the proper conditions exist and are kept fairly stable. These conditions were researched initially for sewage treatment plants and more recently are the subject of intense investigation toward meeting the waste management needs of various agricultural and specialized industries.

Raw waste material

Husbandry practices can influence the quantities of manure available for use in the digester. For example, cattle in pasture will scatter their waste over a large grazing area, making waste collection difficult and will likely contain a lot of soil or grit, which will eventually clog the digester, and thus not be suitable for the production of biogas. Conversely, a herd that spends most of the day in a confined area (e.g., a corral) will deposit waste in a concentrated area, making it possible to collect waste more easily.

The amount of manure produced per animal per day varies. For example, one may expect about 60 pounds per day from a 1,000 pound beef or dairy cow and about nine or 10 pounds per day from 1,000 pounds of broiler chicken. Remember, increased gas production is directly proportional to the amount of volatile solids in the raw waste used.

Under optimum collection conditions (i.e., when animal is confined), you get:

* 4 lbs. of manure per 100-lb. Sheep

* 80 lbs. of manure per 1,000-lb. dairy cattle

* 60 lbs. of manure per 1,000-lb. beef cow

* 10 lbs. of manure per 200-lb. pig

* 45 lbs. of manure per 1,000-lb. horse

* 0.2 lb. of manure per 4-lb. poultry layer

The rule of thumb here is that the waste material from two adult cattle will usually supply the gas required for cooking food for a family of four. Comparable quantities of other waste may produce slightly more or slightly less gas.

If you are considering relying on the use of a significant amount of vegetable waste in your digester, you need to know when such material will be available in the greatest quantities. For example, water hyacinth may be available year round in some climates, while grain straw or other crop residues will be most plentiful only at harvest.

Wilted or semi-dried vegetation may require the addition of water in order to maintain optimum solids concentration. Freshly-cut young vegetation may require less dilution than freshly cut older plant material.

Organic loading rate

The organic loading rate refers to the number obtained when the weight of the volatile solids loaded each day into the digester is divided by the volume of the digester. ("Volatile solids" refers to the portion of organic material solids that can be digested. The remainder of the solids are fixed. The fixed solids and a portion of the volatile solids are non-degradable. Organic material may also contain a substantial amount of water.)

Loading rate is an important parameter, since it tells us the amount of volatile solids to be fed into the digester each day. At high loading rates, the feeding has to be more nearly continuous (perhaps hourly). At lower loading rates, the biogas digester needs to be fed only once a day.

Digesters are designed to receive and treat from 0.1 to 0.4 pounds of volatile solids per cubic foot of digester volume. Although the actual loading rate depends on the type of wastes fed to the digester, 0.2 pounds of volatile solids per cubic foot of digester volume is a frequently used design parameter. This means a digester used to process mainly manure should be designed to accommodate from 20 to 120 cubic feet of digester volume per 1,000 pounds of live animal. (The actual amount varies from species to species.) Here, it is important to remember that a digester must be designed on the basis of the amount of waste that can be collected and actually fed to the digester, not on the quantity of waste produced.

For illustration, the following estimates are useful:

* 1 lb. of volatile solids per 200-lb. pig per day

* 1 lb. of volatile solids per 100-lb. sheep per day

* 0.04 lb. of volatile solids per 4-lb. poultry layer per day

* 6 lbs. of volatile solids per 1,000-lb. beef or dairy cattle per day

* 9-10 lbs. volatile solids per 1,000 lbs. of poultry layer

The percentage of water in animal waste on a unit volume basis is around 75-95%. Of the solids in the waste, about 70% are volatile. Percentage of water in vegetable and plant wastes varies from 40-95%. Of that, the percentage of volatile solids varies from 50-95%. The amount of biogas produced from vegetable and plant waste varies because various crops have differing biomass production rates.

With time, constant temperature, and a uniform incoming substrate, a digester will stabilize. The rules of thumb for any digester include:

1. Incoming substrate 5-12% total solids

2. 0.2-0.5 pounds volatile acids per cubic foot of digester volume

3. 1-2 pounds raw manure per cubic foot of digester space per day

4. 0.2-1.0 unit volume of biogas produced per unit volume of digester

The actual amount of biogas that will be produced can be determined by experimentation with various types of waste, the amount of water used to dilute an incoming waste, operating temperature, and loading frequency.

A source of potential confusion in determining digester size is the means to measure gas production. When reading literature on biogas digesters, make sure that the gas production under discussion is in comparable units. Gas produced in a digester is biogas, of which 50-60% is methane; the remainder is carbon dioxide and other gases. Biogas volumes are distinct from methane volumes. Other ways of quantifying gas include: gas volumes per volume of digester, gas volumes per 1,000 pounds of live weight of an animal species, gas volumes per pound of volatile solids added, and gas volumes per pound of volatile solids destroyed.

Hydraulic retention time 

Hydraulic retention time (HRT) is the average number of days a unit volume of substrate is to remain in the digester. Put another way, HRT is the volume of material already in the digester divided by the average amount of incoming daily feedstock, or the average age of the digester contents. The HRT will vary from 10 to 60 days, and is an important parameter because it influences the efficiency of the biogas digester.

Closely controlled digesters will average about 20-25 days retention time. Shorter retention times will create the risk of washout, a condition where active biogas bacteria are washed out of the digester at too young an age, making the population of bacteria unstable and potentially inactive. Daily conversion of organic material to methane will continue to increase per unit increase of weight (i.e., age) of bacteria up to a certain point. Thereafter, methane production will drop off per unit weight (or age) of bacteria.

Note that a longer retention time requires a larger digester and more capital for its construction. However, that the smaller the digestion vessel, the less time the methane-producing bacteria will have to act on the available substrate and the more likely the biogas system could malfunction. One should consider all these factors carefully before choosing a system.

Variations in design

There are two general design characteristics of digesters: batch feed and continuous feed. The batch digester is loaded, sealed, and after a period of gas collection, emptied. A batch digester can essentially be any suitably sized container or tank that can be sealed and fitted with a means to collect the biogas. The continuous feed digester receives substrate on a continuous or daily basis with a roughly equivalent amount of effluent removed. There are many possible design variations for continuous feed digesters.

Continuous feed digesters

The design variations for continuous feed digesters can be divided into four distinct types: the Indian design, the Chinese design, the sewage treatment plant, and the hybrid design.

Indian design: The Indian, or Khadi, design is based on the principle that gas produced will lift a bell-shaped dome located above the digestion vat. Substrate enters one side of the digester and displaces effluent out the other side. As gas is produced, it is collected under the dome, forcing it to rise. The dome descends as gas is forced out of the digester into the gas transport lines.

Chinese design: The gas storage chamber in the Chinese design characteristically has a fixed top. Substrate enters one side; effluent exits the other side. Gas produced accumulates under the dome and above the vessel contents. Increases in gas volume displace digester contents into the displacement, or overflow, chamber. The materials forced into the displacement chamber will, by virtue of gravity, attempt to flow back into the digester. The attempt by the displaced liquor to flow back into the digestion vessel creates the pressure to force the gas into the gas transport line. As the gas is used, materials displaced into the displacement chamber will flow back into the vessel.

Sewage treatment plant: Though the designs associated with treating sewage or industrial wastes follow the same basic principles of the Indian and Chinese designs, they are much more complex and more efficient. The digester content is stirred either by paddle or gas recirculation. Temperature controls are much more stringent and digester content may be heated. The effluent exits the plant and is thickened prior to final disposal. Gas is tapped from the digester, possibly pressurized, and used for heating purposes or flared; it may be used for process heat in the digester. The sewage treatment plant principles may be employed on a much smaller scale with lower levels of technology.

Hybrid designs: Hybrid digesters imitate the principles employed in other designs, except that digestion vessels conform to the least expensive, most readily available construction materials. They can be built from available scrap materials, plastic bags, or covered troughs. A very simple design is the end-to-end welding of 55-gallon oil drums to create a long, narrow, small-volume continuous feed digester. With hybrid digesters, care must be taken not to let construction economy offset digester efficiency or productivity.

Comparing digesters

The more sophisticated biogas digesters require skilled people to build, operate, and maintain them. Such digesters will likely be more economically feasible if they are used to process large quantities of waste. Although a high-technology digester does produce considerably more gas than either the Indian or the Chinese design, it has higher capital and operating costs and requires careful monitoring on a daily basis.

The Indian and Chinese designs are less expensive and easier to build and operate, but those benefits are countered by fairly inefficient gas production. Moreover, leakage may become a problem if the digesters are not maintained well. Although the Indian design produces slightly more gas than the Chinese design, it is slightly more expensive and has the added maintenance requirements associated with the floating dome.

Applications

Biogas can be burned directly as a fuel for cooking, lighting, heating, water pumping, or grain milling, and can also be used to fuel combustion engines. In larger applications where scale and skills warrant, biogas can be pressurized and stored, cleansed for sale to commercial gas suppliers, or converted to electricity and sold to power grids, to meet peak energy needs.

Gas transport lines are connected to the gas-collection chamber of the digester (the floating dome of the Indian-style digester). The gas has a high moisture content. It is necessary to devise a way to remove the moisture before the gas is used. One way is to slope the transport line back toward the digester so that the moisture will flow down the line back into the tank. If this is not practical, it will be necessary to install a sump, or chamber, in the gas line to collect the moisture.

Biogas is also very corrosive. It may contain dangerous amounts of hydrogen sulfide, a poisonous flammable gas that produces a highly corrosive acid when mixed with water. For this reason, gas transport lines must be corrosion resistant. Polyvinyl chloride (PVC) plastic pipe is a good choice for gas lines because it is durable, corrosion resistant, and usually economical. Because the gas is so corrosive, it may have to be cleansed before it is used, particularly in engines.

While biogas is an excellent fuel, it does have a fairly low energy value for its volume--500-600 BTUs per cubic foot--and the pressure in the distribution lines may be low. Lamps, stoves, refrigerators, and other appliances require specially designed jets to offset the low energy value and the low gas pressure. To stabilize the flame on a cookstove, for example, the jet forcefully shoots the biogas up through and out of the burner. Jets can either be purchased or built easily from locally available materials.

The amount of methane required daily per household will vary. About 0.5 to 1.0 cubic meter of biogas is required per family member for food preparation alone, and roughly one cubic meter of biogas is produced per 1,000 pounds of animal. Meeting one family member's cooking requirements, then, requires two of three healthy dairy or beef cows, or eight to 10 pigs (weighing 150 to 250 pounds each), or over 500 chickens. The amount of waste material produced by these animals varies with their health and diet and will influence the number of animals required. Collecting more than 30 to 40 pounds of waste daily per 1,000 pounds of live weight per animal will increase the amount of gas produced per animal.

The effluent leaving the digester at the end of the digestion period is spread on farmland much as the undigested manure. Research has been performed on using the digester effluent to feed cattle or to promote algal growth in fish ponds, as is done in some Chinese aquaculture installations.

Construction

The equipment and materials required for digester construction depend upon the level of technology employed. The basic Chinese design requires cement, sand, clay, lime, and bricks. Sulfate-resistant cement should be used if available due to the corrosive nature of the gas and slurry. The Indian design requires these same materials, plus some welding and iron works. The higher technology designs may require some specific machinery and electronics.

Small-scale, nonpermanent digesters can be constructed of oil drums or uniformly-supported plastic bags.

Skills required

The basics of a digester can be creatively adapted by competent, local craftspeople working with locally available materials. The Chinese design requires the skills of a competent mason. The Indian design requires the skills of a competent mason as well as an iron worker and welder. More sophisticated digesters for larger scale applications require plumbers and electricians.

Once constructed, the digester requires the daily attention of a semiskilled individual. Each day, the digester must be fed and agitated, and the effluent properly disposed of. Just as a caretaker tends to a herd of animals, the individual responsible for the digester must understand the operational procedures. This person must maintain not only the digester's physical plant, but also ensure that the gas transport line and gas utilization system are operative and in good repair.

Construction costs

Costs for construction are governed by the level of technology employed. They range from a few dollars for digesters built of readily available scrap to a few hundred dollars for a small family, Chinese-style digester, and from hundreds of dollars for a small-scale Indian-style digester to hundreds of thousands of dollars for a large-scale operation. A rule of thumb for comparable sized digesters is that the Chinese-style digester costs half that of a "drum"-style Indian digester. A more sophisticated digester will cost at least three times that of an Indian-style digester of comparable volume.

Biogas efficiency

The amount of biogas varies from 30 to nearly 100 cubic feet per 1,000 pounds of live body weight, so there is no universal formula to determine biogas efficiency.

For example, efficiency varies depending upon how the biogas is used. Biogas plants use organic wastes, which, if not fed to a digester, are at best spread over land or at worst directly burned. Although direct combustion of dung or grasses yields at best 10% of the available energy, the nutrient values of such wastes are severely reduced. Biogas systems yield 40-50%, or better, of the thermal potential of organic wastes and yield a fertilizer of superior quality. Composting provides excellent fertilizer with no gas.

Efficiency varies with the type of digester, the operating conditions, and the type of material loaded into the digester. All else equal, the Chinese-style digester produces about half as much gas as the Indian-style digester, which in turn yields less than half the gas of more sophisticated units. The Chinese design, the Indian design, and the high-technology designs, respectively, yield about 0.2-0.3, 0.5-0.7, and 1.0-2.0 volumes of biogas per volume of digester. And, in general, digesters produce more gas with poultry waste (about 100 or so cubic feet of biogas per 1,000 pounds of live poultry weight) than they do with cattle waste (25 to 30 cubic feet per 1,000 pounds of live cattle weight).

Apart from these factors, the key to maintaining efficiency is to feed the digester a uniform feedstock daily, to maintain a constant operating temperature, and to agitate the contents regularly.

Maintenance

Biogas digesters require careful maintenance. Operators should be responsible for the following maintenance activities:

* Daily Activities: Collect and prepare the feedstock, and load it into the digester. Collect the liquid effluent from the digester. It may be spread over fields, used to fertilize fish ponds, or dried for later use.

* Periodic (at regular intervals) Activities: Remove the digester contents, including any solids that have accumulated at the bottom of the digester. Because of the potentially corrosive nature of the digester contents (slurry as well as gas), check all metal components of the digester to see whether they need to be resurfaced (e.g., the metal dome of the Indian-style digester).

* Occasional Activities: Check the digester, particularly Chinese-style digesters, for any gas leaks. Also, examine components in high-technology units such as pumps and mixers, which require occasional repair or replacement.

Finally, preventing sand, dirt, and gravel from mixing with dung as it is being collected, and protecting the dome of the digester with a metal or asphalt coating, will lengthen time between maintenance.

Currently

Extensive research continues with the various biogas generation plants operating worldwide. Various institutions throughout the world are Conducting research toward making maximum use of the biogas produced. This involves matching energy needs to gas production, and using equipment that burns or converts the gas more efficiently. Additional research deals with digester designs and design parameters; here, heat losses and maintaining an adequate, stable temperature in the digester are of prime interest to researchers in their efforts to maximize methane production. Other research efforts focus on improvements in the use of digester effluent to promote maximum growth of algae, fish, aquatic vegetation, and farm animals.

RELATED ARTICLE: Local regulations.

Consult local officials about any regulations and laws that may prevent you from building or using a biogas generator. On the positive side, some laws might work in your favor. For example, the governments of some developing countries provide investment incentives, grants, or low-interest loans to people who want to introduce a biogas plant. Such governments are actively pursuing national policies that would reduce dependence on imported fuels and so encourage the production of biogas as an environmentally safe fuel source.

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Shuler, M.L., ed. Utilization and Recycled Agricultural Wastes and Residues. Boca Raton, Florida: CRC Press, Inc., 1978.

Subramanian, S.K. Bio-gas Systems in Asia. New Delhi, India: Management Development Institute, 1977.

Targanides, P. "Anaerobic Digestion of Poultry Waste." World Poultry Science Journal 19 (1962):252-61.

Tatom, J.W. "Pyrolysis Experience in the Developing Countries." Proceedings, Big-Energy '80 World Congress and Exposition. Washington, D.C.: Bio-Energy Council, 198, pp. 180-85.

Understanding Biogas Generation, ISBN: 0-86619-204-2, [C]1984, Volunteers in Technical Assistance

Richard Mattocks is an environmental scientist. He specializes in the management of solid waste materials and the recovery of biomass products, and is currently researching various uses of biogas digester effluent, particularly its use as an animal feed source.

VITA is a private, nonprofit organization that supports people working on technical problems in developing countries. VITA offers information and assistance aimed at helping individuals and groups to select and implement technologies appropriate to their situations.