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close this bookThe Biogas/Biofertilizer Business Handbook (Peace Corps, 1985)
View the document(introduction...)
View the documentMain Points of the Handbook
View the documentPreface
View the documentChapter one: An introduction
View the documentChapter two: Biogas systems are small factories
View the documentChapter three: The raw materials of biogas digestion
View the documentChapter four: The daily operation of a biogas factory
View the documentChapter five: The once a year cleaning of the digester
View the documentChapter six: Tanks and pipes: Storing and moving biogas
View the documentChapter seven: The factory's products: Biogas
View the documentChapter eight: The factory's products: Biofertilizer
View the documentChapter nine: The ABCs of safety
View the documentChapter ten: Conclusion: Profiting from an appropriate technology
Open this folder and view contentsAppendix

Chapter eight: The factory's products: Biofertilizer

With all the talk about digesters as biogas producers, it is easy to forget that digesters produce two main products. The second product is organic fertilizer, in other words, biofertilizer.

Some background on fertilizers and plant growth is in order here. Of necessity, it will be brief, but this is a subject of great importance to anyone trying for greater self-reliance, and it is financially important to anyone concerned with making a biogas system profitable.

Fertilizer is perhaps the single most important factor in agricultural plant growth. One-fourth of the world's food supply is the result of the use of chemical fertilizers, but chemicals have no bulk, no fibers; they cannot hold the soil. In the American state of Illinois, approximately 3.9 tons of topsoil per acre of land are eroded away each year. One of the main reasons for this is farming methods that use only chemical fertilizers. The value of organic fertilizers (compost and digester sludge) is not only in the nutritional value of its chemicals (such as nitrogen), but also in the anti-erosion qualities of its humus (plant fibers).

Estimates show that organic wastes from animals, plants, and people could supply developing nations with six to eight times more plant nutrients than they now get from manufactured chemical fertilizers. Crops grown in soils fertilized with organic matter tend to be larger, healthier, less troubled by insects and plant diseases, and the soil suffers less from compaction or erosion.

In India, where there is more than 20 years of experience with wide-scale use of biogas systems, more than twice as much value is placed on the fertilizer than is placed on the gas.

This first section is based in large part on the introduction to biofertilizer in The Compleat Biogas Handbook.

The modern method of agriculture usually compares fertilizers by using their chemical analysis, particularly their relative amounts of the nutrient elements: N. P. and K (N-nitrogen, P-phosphorus, and K-potassium). This is because of the large quantities of these three nutrients (that which causes growth and development) in plants, in comparison to the quantities of other elements in plants. Also present in plants in large quantities are carbon, hydrogen, and oxygen; but these elements are easily gained by plants from air and water. Often chemical fertilizers are spoken of as "5-2-2" or "10-5-2." These numbers refer to the percentages of N. P. and K in the fertilizer.

In chemical terms, the dried solid sludge from digesters is a poor fertilizer. But when used in the large volumes that digesters produce sludge in, the whole sludge, liquid plus solid, is a good to excellent fertilizer in terms of its chemical analysis. The difference lies in the fact that most of the fertilizer value is in the liquid portion of the sludge and is in the form of ammonia-nitrogen (and related compounds), which will rapidly evaporate or wash away when the solid portion is dried. The local use of liquid sludge (with or without the solid portion) should not be too much of a problem if the sludge is used in fish ponds and in nearby irrigation systems. Large biogas businesses might think about buying old water or (cleaned-out) gasoline tank trucks to distribute and sell the liquid fertilizer.

It has been proven many times that the nitrogen in the waste that goes into a digester remains in the sludge that comes out to a greater degree than it does with composted waste. In NPK terms, the biogas process produces a better fertilizer than the compost process does. Basicly, the whole NPK valve of the original plant and animal waste remains and is usable by the growing crops. One study claimed that biogas sludge has three times more nitrogen than the fertilizer produced by the best compost process.

When using many organic wastes, especially manure, aerobic composting will result in a 25 percent loss of nitrogen as compared with anaerobic digestion. Aerobic compost will have more of its nitrogen tied up in a slow release form that is not as easily released as the ammonia-nitrogen of anaerobic sludge. For use on pastures, grasses, and crops such as rice, corn, and other grains, the ammonia-nitrogen of biogas sludge is best. For use on tree crops (fruits and nuts), legumes, and most vegetables, the slow release nitrogen of compost is best.

Operating large aerobic compost systems can be as time consuming as operating large biogas systems. The equipment needed for biogas systems will be more expensive than the equipment needed for making aerobic compost, but then composting does not produce methane. And even a well-run composting operation cannot kill as many disease-causing organisms as the anaerobic process can in a well-run, continuous-fed horizontal biogas digester.

Some soils respond better to liquid sludge than others. Open, porous soils (sandy or loamy) will in general be more capable of remaining loose and tillable than silt or clay soil when digester sludge is used as a fertilizer.

The nutrients in biogas sludge encourage the growth of soil bacteria, an event which can have many benefits for the soil structure and humus content. But if too much sludge is used too often, the soil will become clogged with the products of this growth, and slime organisms will begin to grow.

Percolation, the ability of water to move through the soil, is then dangerously reduced, and the carbon dioxide released by aerobic decay in the soil cannot leave the soil, causing the soil to become acidic, which is usually not very good for the plants. A close check on the soil pH will provide evidence that this is happening.

The use of liquid sludge could probably be increased if the soil is first tilled (harrowed or plowed), with the sludge applied a few days later. Compact clay soils respond to sludge by clogging more rapidly than sandy, open soils, so plowing before using the sludge can be very important. The use of an agricultural lime or dolomite before the sludge is applied will, to some degree, lessen the acidifying tendency of biogas sludge. The real cure is to keep the soil open, so that carbon dioxide can escape, and oxygen can enter.

Both digester sludge and compost have something that agricultural lands need and which chemical fertilizer do not have. That thing is humus. Humus is rich organic matter which, in addition to having value as a fertilizer, helps hold soil together, reducing the damage that can be caused by wind and rain.

Key elements in the following section were adapted from the book, Biogas and Waste Recycling--The Philippine Experience.

From a farmer's point of view, it might be better to call them biofertilizer digesters instead of biogas digester. The second product of digesters is a sludge which is a very watery liquid with usually only eight to ten percent solids floating in it. It has the appearance of a dark, but thin, mud (if it has been in the digester long enough), but on closer look, small plant fibers and bits of organic matter can be seen floating in it or settling to the bottom.

When newly discharged from the digester, it still lets off some biogas. The original offensive odor of manure is replaced by an entirely different smell, that of hydrogen sulfide which disappears as the sludge is exposed to the aid. The volume of the sludge is large, because there is very little volume decrease brought about by the biogas digestion process.

The ten percent of the sludge which is solid has a known fertilizer value for crops. A rather unexpected benefit comes when the dried sludge is used after sterilization, as a food component for pigs. Not only does the sludge satisfactorily perform as feed, it also definitely hastened weight increase. While some of the favorable effect on pig growth can be attributed to the presence of amino acids and trace elements in the sludge, it is more probable that the large effect on pig growth by so small an amount, just ten percent of the feed, is due to vitamins, B-12 in particular, and what is called unidentified growth factors. More information on the use of solid sludge as a feed supplement can be found in the book, Biogas and Waste Recycling--The Philippine Experience. (Dried solid sludge has also been found to make a very sanitary bedding for animals kept in pens.)

The liquid sludge is about 90 percent of the total sludge. This large volume can be a big disposal problem. Of the ten percent solid sludge, it can be said that the problem has been satisfactorily solved through its utilization as a livestock feed ingredient or as a fertilizer and soil conditioner (humus). The solid sludge is small in quantity, and can be dried in the sun and stored, used immediately, or sold. By contrast, the 90 percent liquid sludge is very large in quantity. Sludge collected in ponds showed no indication or tendency for ill effect on plants due to salinity (salts). But, since the sludge is the end product of an anaerobic process, it takes weeks to get atmospheric oxygen to dissolve in the liquid. The big problem is the practical use of such a big volume of liquid. There are several good possibilities.

The liquid sludge, with or without the solid portion, can be used as a combination irrigation water and fertilizer. Using the liquid sludge in this way has been very successful. Blue-green algae grow in great abundance in rice fields irrigated with biogas sludge, and the algae increase the fertilizer value of the sludge by taking nitrogen from the air and making it available to the crops (nitrogen is 78 percent of the air). It is also likely that the sludge contains plant hormones and other plant growth factors, because the dark green leaves and general robust appearance of sludge fertilized plants cannot be explained by the chemical composition of the sludge, which is low in nitrogen.

The liquid sludge can be valuable in aqua-culture fish ponds. When the sludge is exposed to the brilliant tropical sunlight, it supports a profuse growth of plankton (miniature water plants and animals). Fish, particularly tilapia (silver carp), feed on plankton. The rice fields at Maya Farms are now being replaced by tilapia ponds.

Fresh Air + Sludge = Fertilizer

Literature on biogas is generally silent about the toxic effect of biogas digester sludge. The sludge is hailed as an excellent fertilizer. That all is not well, however, is hinted at in the statement that the sludge must be allowed to ripen before it is used as a fertilizer. Indeed, the freshly discharged sludge is very toxic to fish and plants, even when greatly diluted with water. Tilapia, a fish noted for its ability to survive in very hostile environments, dies in fresh sludge. A hardy water-loving plant, kang-kong, fails to develop, and even grass withers when watered with fresh sludge.

The cause of this toxicity could be the lack of oxygen in fresh sludge, the osmetic effect of high salt levels in sludge, or the presence of toxic substances. Recent work on rice cropping in India and at the International Rice Research Institute in the Philippines points to hydrogen sulfide as a material which is toxic to rice. If the cause of toxicity of the sludge is hydrogen sulfide, then detoxification can be achieved by oxidizing the hydrogen sulfide. In other words: expose the sludge to air for 15 to 30 days before using it.

Well-aerated sludge is free from the sulfide odor and is no longer toxic to fish or plants. Even before it dries, aerated solid sludge attracts ants, insects, chickens, etc., a sure sign of detoxification. It is also a sign that the sludge may have feed value. When dissolved in water, hydrogen sulfide is, within weeks, broken down by the oxygen in water--hence the need to create conditions for effective aeration (exposure to air), such as bubbling air through the sludge and/or exposure of large surface areas of sludge to air.

A sludge aging pond is usually a biogas system necessity; algae and fish ponds are also included if they serve the purposes of a biogas system's design. If all the ponds are down hill from the digester, gravity will carry the sludge from pond to pond, and pumps or water wheels will not be needed to lift the sludge from pond to pond.

The first pond, the aging pond, can be used to reclaim some or all of the liquid portion of the sludge for mixing with fresh waste to make new slurry. This could be done with filters made of straw, gravel, or screens. Another way to separate the solid from the liquid sludge is to rake out the solids and drain off the liquid portion. The solids can then be aired for use in gardens and fields, used in compost piles, or sold in bags.

The purpose of the first pond must be to make sure that all of the sludge gets enough exposure to enough air. The ponds should have thin concrete or plastic bottoms and sides to keep the valuable fertilizer from being soaked up by the ground.

The first ponds should only be a few inches deep so that there is a large surface area exposed to the air and the pond must be divided into sections like those in the aging pond in Diagram 1. The dividers will make sure that all of the sludge spends enough time in the pond. The total volume of the aging pond (not counting the space taken up by the dividers) should be at least three-fourths the volume of the digester that the sludge came from, so that all of the sludge will be in the pond for approximately 30 days. For example, a six cubic meter digester will need a 4.5 cubic meter aging pond with approximate dimensions of 3.0 meters by 5.0 meters by 0.3 meters. The final dimensions cannot be figured until the space taken up by the dividers is known.

After going through the aging pond, the sludge, with or without the solid portion, should flow into algae and fish ponds, or it can go into an irrigation system to fertilize crops. Palm-leaf roofs to protect the sludge ponds from being diluted or flooded by rain, greenhouses to use the sludge during cold weather, and storage ponds to save the sludge during times of the year when there is not a large demand for fertilizer are just three of the many solutions that have been thought of to answer the many questions that come up when working with liquid fertilizer.

Algae ponds can be very useful. Algae release oxygen that mixes with the sludge, making it less toxic to fish and other plants. The water of algae ponds has been found to be a good high protein drinking water for all kinds of farm animals. In several cases the algae is harvested with rakes, dried, and feed to chickens. At Maya Farms, ducks get up to 50 percent of their food from the sludge of algae ponds, though at first Maya Farms had to add other feeds to the ponds to get the ducks interested in eating the digester sludge and algae.

Growing plants need certain nutritional elements. Carbon, hydrogen, and oxygen are obtained by plants from the air and water. The other essential elements are supplied through the soil. The elements in the soil may be greatly reduced after severe cropping and through leaching, that is, the elements may be washed away by water seeping through the soil. Unless these nutrients are replaced, future crops may give increasingly poorer yields. Nature's way of keeping the soil fertile is by recycling the organic waste of old plants by returning all organic matter back to the soil.

Biofertilizers and Chemical Fertilizers

Continuous use of chemical fertilizers, which are in part made from increasingly expensive oil, can deplete the soil of many nutritional elements. The Guidebook on Biogas Development states that chemical fertilizers are effective in the soil for one year while biogas fertilizers are effective for three years. While half of the nitrogen in digester sludge is in the form of fast-acting ammonia-nitrogen, the sludge in general is a slow release fertilizer which will continue to fertilize the soil for three years.

Another disadvantage in using chemical fertilizers is the water pollution that can occur when excess chemicals find their way into rivers and streams. But farmers have persisted in the exclusive use of chemical fertilizers in spite of the warnings that complete abandonment of organic fertilizers would cause the soil to become less fertile and in spite of the pollution caused by chemical fertilizers and unused plant and animal wastes. The disadvantages have been overshadowed by the easy-to-use nature of chemical fertilizers and the immediate but short-term increase of crop yields. Over the years increasing quantities of chemical fertilizers have to be used to maintain high crop yields.

The natural fertilizers such as compost have not been totally abandoned. New ways are being developed to shorten composting time and to improve the quality of compost. These developments, together with the increasing prices, unreliable supply, and pollution problems of chemical fertilizers have started a return to traditional organic fertilizers (see Compost section of the Appendix).

The biogas process is an improved treatment for plant waste and animal manure. In aerobic decomposition of organic matter, ammonia and carbon dioxide are lost to the air. There is also considerable leaching of plant nutrients that can dissolve in water. The resulting compost has less food value for crops than was in the original organic material. In the biogas process, the organic material is decomposed in waterproof, airtight tanks. Only the elements carbon, hydrogen, and oxygen in the form of methane and carbon dioxide molecules are lost. Almost all the other essential elements are retained in the sludge.

The nutrient value of biofertilizers vary with the type of organic waste used to make them. When only animal manure is used, the phosphorus content is usually less than the nitrogen and potassium. Depending on the soil analysis and the crop to be grown, it may be necessary to supplement the biofertilizer, be it biogas sludge or compost with small quantities of chemical fertilizers to meet the requirements of normal plant growth.

Solid Biogas Fertilizer

Solid biogas sludge is composed mainly of humus (organic matter) and plant nutrients. It has a carbon to nitrogen ratio of approximately 13 to 1, which is good, because of its closeness to the C/N ratio that is found in land which is good for farming. This means that the sludge can be used directly (after aging) as fertilizer. The sludge will not compete with crops and decay organisms in the soil for the available nitrogen.

As an organic fertilizer, solid sludge plays an important role in plant nutrition and soil conservation.

1) It improves the physical condition of the soil by improving texture, moisture holding capacity, and aeration.

2) It increases the pH buffering capacity of the soil.

3) It combines with inorganic soil compounds to prevent their loss by leaching and releases them for use by the plants.

4) It stimulates the growth of micro-organisms, retards the irreversible fixation of nutrients, and helps prevent soil erosion.

Liquid Biogas Fertilizer

The liquid sludge contains less concentrated quantities of nitrogen, phosphorus, and potassium than solid sludge does, but considering the fact that a large amount of water is needed for irrigation and the large quantities of liquid sludge, these nutrients can build up to become too much of a good thing. Liquid sludge promotes the growth of large quantities of nitrogen-fixing algae wherever it applied. Since algae have a short life cycle, the decaying algae increase the nitrogen content of the sludge--a good thing only if it is controlled.

In addition to the major plant nutrients: nitrogen, phosphorus, and potassium (NPK), biogas sludge also contains fertilizer elements such as calcium, sulfur, magnesium, and the essential trace elements copper, zinc, manganese, and others.

When the liquid sludge was used at Maya Farms as irrigation water for flooded rice (IR-26), the available phosphorus content of the irrigated soil increased by 80 to 500 ppm (parts per million), while available potassium increased by as much as 300 ppm. In addition to phosphorus and potassium, a sufficient nitrogen supply was apparent from the appearance of the crop.

The effect on plants caused by the continuous use of biogas sludge should be continuously watched. Use of the sludge should be regulated in order not to subject the soil to excessive amounts of nutrient elements. Without regulated use, unbalanced proportions of nutrients may result in abnormal plant growth or yield. For example, an excessive amount of phosphorus represses the availability of trace elements like iron and zinc, which are essential to plant growth. Unregulated use of the liquid sludge for irrigation also has the tendency of making the pH level of the soil slightly alkaline, although alternate flooding and draining tends to bring the pH somewhat lower.

One study found that an uncontrolled supply of nutrients, especially nitrogen, may depress crop yields. When biogas sludge runs directly into rice fields, there is a large increase in the growth of blue-green algae. These algae absorb nitrogen from the air, which is good for the crop. But there can be too much of a good thing.

If the supply of sludge is not stopped halfway through the growth cycle of the rice, flower initiation is harmed. The rice plants continue to grow, but the rice grains do not develop.

In the Philippines and other countries with hot, humid climates, the decomposition and reuse of organic matter in the soil is much faster than in colder countries. In the tropics the soil is generally low in organic matter content. The soil can easily lose the plant nutrients which are the fertility of the soil. The surface soil also becomes easily eroded when there is a lack of organic matter. The use of the organic fertilizers, biogas sludge and compost, is very important, particularly in tropical countries.

Experiments are being conducted to find out if the biogas process can make the sludge completely free from parasites and pathogens (disease causing organisms). Preliminary results are encouraging, but more work is necessary to establish a definite conclusion. A lot depends on how long the waste stays in the digester: the detention time. If the digester is fed too much slurry too often, the amount of time any one portion of organic (and disease carrying) waste stays in the digester will be shortened. Overloading can change a safe 40 day detention time into an unsafe 20 day detention time very easily.

How to Use Biofertilizer in Fields

1) Fresh sludge from digesters contain compounds such as hydrogen sulfide which can kill plants and fish. Symptoms of this have been observed at Maya Farms when fresh sludge went directly to the rice fields. Aging and aeration of the sludge in a shallow pond for 15 to 30 days brings down the concentration of these toxic compounds and brings up the concentration of oxygen.

2) Continuous use of biofertilizer may result in too much growth of nitrogen fixing algae. The excessive nitrogen may cause weak roots and stems. This is particularly dangerous at the heading stage of rice.

3) Continuous application of sludge in clay soils may cause problems around the root areas of the crops.

4) Too much sludge in fish ponds can cause too much algae growth. Since algae has a short life cycle, the decaying algae, if not racked out and used as food for animals or biogas digesters, can use up all the oxygen supply in water and cause fish to die.

5) Continuous use of sludge on fields can make the soil acidic or alkaline, depending on the pH of the organic waste used in the digester and the original pH of the soil. If the soil becomes too acidic, a little limestone can be added to the sludge, but only if and when needed.

6) Sludge should be plowed into the soil about one week before sowing seeds for best results with non-irrigation crops. Grain crops can be fertilized once or twice more before harvesting. Vegetable crops should not be fertilized again.

How to Use Biofertilizer in Fish Ponds

The basic source of food for pond fish is plankton: the large number of very small plants and animals found near the surface of bodies of water. The plant population of plankton consists mainly of algae. The animal population of plankton consists of many kinds of one cell organisms, miniature shell fish, and the larvae form of larger animals.

Poultry and livestock manure can be used directly in fish ponds to grow plankton but the odor of the manure has a tendency to have a bad effect on the taste of the fish. This is particularly true in the case of Bangus (milk fish/Chanos chanos).

A better way to fertilize a fish pond is to use the manure in a biogas digester. Then use the digester sludge to fertilize the fish pond--after the sludge has been aged. There are several advantages to this method.

1) The sludge does not smell bad and will not give fish a bad taste.

2) The sludge promotes the growth of plankton better than manure does.

3) Fish need oxygen, and aged sludge leaves more oxygen in the water than manure does.

4) When the biofertilizer is made, biogas is also made.

5) Fish ponds are probably the easiest year-round use for biogas sludge.

6) Only a relatively small amount of sludge is needed to fertilize a fish pond.

Maya Farms raises Tilapia, a hardy fish which thrives in fresh and brackish water conditions. Tilapia feed primarily on plankton but also eat other organic matter. With nothing but plankton and pig feed sweepings as fish food, the fish ponds yield about two tons of Tilapia per hectare every three months.

The culture of Tilapia can be a very profitable operation. Once started, the maintenance costs are minimal. The sludge from a biogas digester is enough to support the growth of plankton in fish ponds and the undigested solids in the sludge can serve as direct feed supplement for the fish. There is no problem with the cost or supply of fingerlings because Tilapia reproduce very fast. They reach sexual maturity in about three months and breed as often as once a month. In fact, there is need to control the Tilapia population in fish ponds, otherwise overcrowing will cause stunted growth and poor harvests of large fish.

Overcrowding can be controlled by maintaining separate male and female ponds for breeding, and stocking the other ponds with only manually selected male fingerlings. The male Tilapia can be identified by examining the "urogenital papillae," a finger-like structure behind the anus on the belly of the fish. The male has one opening at the tip of the papilla while the female has two openings, one for the exit of eggs and the other for urine. Another difference is that the papilla of the male is pointed while the papilla of the female is somewhat brown and round.

For raising Tilapia on plankton, the best depth for the fish pond is 75-100 cm (30-40 inches). The recommended stocking rate is about one to two fish per square meter of water surface. The formula for figuring surface area is length times height.

1) A 29 cubic meter biogas system in the Philippines for 50 cows included a 52 square meter sludge aging pond and a 80 square meter fish pond from which 20 kilograms of Tilapia were harvested every two months.

2) Compared to the traditional method of fertilizing the fish pond, digester sludge can increase the size of the fish harvest by 25 percent in two-third the usual time.

A biogas system on a farm in Thailand uses sludge to fertilize gardens and fish ponds. After two or three harvests, it switches the gardens to fish ponds and the fish ponds to gardens. After another two or three harvests, it switches the fish ponds back to gardens and the gardens back to fish ponds. The purpose of this continuous switching of land back and forth between use as fish ponds and gardens is to benefit more completely from the fertilizer value of biogas sludge.

Well-operated fish ponds, as part of biogas systems or as separate businesses, can be very profitable. VITA and the Peace Corps has an excellent (long) book, Freshwater Fish Pond Culture and Management, that studies the subject of fish ponds in great detail.

Sales of inorganic fertilizers, especially nitrogen, have increased in recent years resulting in a depletion of humus in the soil and therefore still heavier applications of inorganic fertilizers leading to unavoidable ecological imbalance. This lack of humus is causing serious problems and concern.

The availability of a natural fertilizer, such as biogas sludge, offers the farming world a means to return to the classic use of organic fertilizer (compost) in a new form. The choice is made more attractive by the continuing price increases of inorganic fertilizers. The average international price of ammonia (nitrogen) fertilizer increased nine times between 1972 and the end of 1974 (Barnette, et al., 1978).

The conclusion of almost all of the literature on pathogens (disease-causing agents) in the sludge from sewage biogas systems is that the pathogens are destroyed. If pathogenic bacteria do find their way through a digester, it is because of faulty design of the digester itself or from putting too much waste in the digester for its size (Fry, 1974).

An Important Question

There is an important question which every owner of a biogas system must answer. Should human waste--feces and urine--be used as fuel for digesters? And if human waste is used, should the sludge be used as a biofertilizer?

These are not all-or-nothing questions. Most of the sludge, the liquid portion, could be used instead of water to dilute the fresh waste. This would not only provide a use for the liquid sludge, it would also increase biogas production by 10 to 20 percent (a guess). Another possibility is to have digesters: one for human waste and one for all other organic waste. The human waste digester would be used for gas production only, with the sludge going directly into a septic tank (see Diagram 17).

Human waste can be safely used in biogas and biofertilizer production:

· If the waste is calculated to stay in the digester long enough for the sludge to become sanitary (see Facts and Figures section of Appendix for details).

· If a sanitary way can be found to collect the human waste, find its approximate weight, mix it into a slurry without lumps, when at all possible, and get it into the digester with the diluting liquid (water or liquid sludge) in a way that is not more trouble than it is worth.

· If no soap of any kind gets into the digester, because the chemicals in all soaps can kill biogas producing bacteria.

There are a few points to remember when considering building a combination digester and septic tank. The digester will most likely be underground, unless all the toilets are on the second floor, so the water table must be low enough at all times of the year to not be a danger to the digester.



-Digester capacity should be one cubic meter for every ten times per day each toilet is used.
-To clean this digester, either buckets or a pump will be needed. Like all digesters, it should be cleaned once a year.

The best gas production rate for an unheated, underground horizontal digester, such as a biogas-toilet system, will probably be about 0.8 cubic meters of biogas per cubic meter of digester space per 24-hour day. The drains of the toilets should come out from under the building (it would not be safe to build a digester under a building) at a 45 percent angle to the underground digester. The digester should be large enough to take the largest possible quantity of daily waste and still maintain a 40-day detention time.

The pipe going from the digester to the septic tank should have a large diameter to keep it from getting blocked. The top of the pipe to the septic tank should be 15 cm (6.0 inches) below the bottom of the digester roof to reduce the chance that scum will be able to block the gas pipe. The digester's inlet and outlet pipes must have the same large diameter to keep the slurry moving safely from toilets to digester to septic tank.

A removable top on the digester will be necessary so that the digester can be cleaned at least once a year. The septic tank will still needs its own removable cover. If the biogas is not used fast enough, one of the openings through which the gas might push digesting waste will be the toilets. For this reason, it is very important to regularly check water condensation traps and gas storage tanks to make sure they are working correctly. If a metal gas storage tank is used, do not forget to include a lightning rod because the gas tank will probably be near homes.

Because human waste has a very low carbon to nitrogen ratio, another organic waste with a high carbon to nitrogen ratio, such as newspaper, after it has been partly composted and turned into a watery newspaper mud, should be added in order to get a biogas production level that is closer to the digester's potential. It will probably not take much newspaper (or other high carbon waste), but experimenting will be the best way to find out just how much.

Concluding Thoughts on Biofertilizer

The following observation from the book, Economics for a Developing World, is an appropriate conclusion to a chapter on fertilizer: "In addition to land, fresh water and energy, supplies of fertilizer are becoming more and more scarce as a result of higher prices and alternative uses for the petroleum inputs into fertilizer production. The process of manufacturing chemical fertilizers requires large amounts of costly energy inputs. In the face of these rising costs and the enormous expansion of world demand for fertilizers, it is clear that future (chemical fertilizer) prices will continue to rise....

It becomes all the more urgent, therefore, for...populous food-short intensify their efforts to promote labor-intensive and energy-saving small and medium-scale commercial farm development. This is where the greatest and least costly output potential lies."