|The Biogas/Biofertilizer Business Handbook (Peace Corps, 1985)|
The raw materials of biogas digestion are organic plant and animal matter. That organic matter can be animal manure, crop waste, weeds from lakes and rivers as well as from land, or the organic waste from restaurants, market places, slaughter houses, and factories that use a fermentation process.
Decayed organic matter is the chief basis of all fossil fuels such as coal, oil, and natural gas (methane)--which are in turn only a small fraction of the remains of all the plants and animals that have lived over the ages. As a result of special conditions, fossil fuels have been preserved and are now being used at a rate that increases every year. Most of the organic matter that was formed in past ages has long since been converted back into carbon dioxide and water (Fry, 1974).
What and How to Feed Digesters
Organic waste can be divided into two groups: carbon-rich such as grass and crop stalks, and nitrogen-rich such as urine, human feces, and chicken manure. The carbon-rich waste contains a lot of carbon cellulose, which promotes biogas production, and the nitrogen-rich waste provides nutrients which promote the growth and reproduction of anaerobic bacteria. Experiments have shown that biogas production can be increased if the various organic wastes can be fed into the digester in correctly balanced proportions.
Organic waste materials for gas and fertilizer production include crop wastes, grass, leaves, weeds, urine, and the manure of people, pigs, cattle, and chicken. If the digester inlet is connected directly to toilets and animal pens so that the manure can flow through drain pipes into underground or downhill digesters, no special management is required, but many experts say it is very important to mix the waste before putting it into the digester.
If the wastes are mixed before going into the digester, more gas and a better fertilizer will be produced, because there will not be any undigested lumps of manure going through the digester. The solids to liquids ratio can be kept closer to the ideal ratio of one to ten. There will be less danger of overfeeding the digester with waste or flooding it with water. One solution might be to have the drain pipes empty into a mixing basin instead of doing directly into the digester.
Sometimes collecting waste for a digester calls for creative problem solving. How would you collect the combination manure and urine droppings of chickens? One way would be to let the chicken droppings fall on leaves, grass, or water lilies spread under the chicken cases where the whole mess could be swept up on a daily basis. The manure plus plant waste could then be used to make a digester slurry.
In order to raise the biogas production level, all plant waste (but not manure) must be compost for a short time before it is put into a digester (using the composting method described in the Appendix). Plants must first be composted for seven to ten days so that the biogas bacteria will be able to digest the plants and produce biogas. The alternative is undigested plant waste floating on the surface of the slurry, forming a scum layer which will not decompose and will stop biogas from getting out of the slurry.
When plants have been composted with lime or an enzyme for a short time before being put in a digester, the waxy surface layers of the plants are broken down, which in turn speeds up the breakdown of the fibrous material in the plants. Shredding, grinding, or pulping plants into very small pieces before they are composted increases the amount of plant surface area which is exposed to the air, making it easier for the compost rotting process of aerobic decomposition to break the plant fibers down enough so that the biogas rotting process of anaerobic decomposition can produce biogas.
Another benefit of partial Composting is that it brings down the carbon/nitrogen ratio of the plants, which is often up around 60/1 to 100/1. After one week of Composting, the carbon/nitrogen ratio can be reduced to between 16/1 to 21/1, carbon nitrogen ratios that are much closer to the ideal environment for methane producing bacteria to live and grown in.
It cannot be emphasized too strongly that the raw materials of biogas digestion should not have specific gravities less than that of water. In other words, the waste must not float on water, as most plants can. The reason is very simple. If it floats in water, it will almost certainly float as scum inside a digester. It will not mix with the rest of the slurry. Scum is often the single biggest problem in a digester. It must be avoided at all costs. Even most animal manure will have pieces of plant matter that will float. Grinding or chopping up plants before they are used as animal feed will result in a manure that is not full of large plant fibers which will become scum inside digesters.
Cud-chewing (ruminant) animals such as cattle, goats, and sheep are different from animals such as chickens and pigs. The manure of cud-chewing animals, if allowed to dry, will not absorb water again, it will float. Even grinding the dry manure into powder will not make the manure absorb water--it will always float.
It is an unavoidable restriction; the manure of cud-chewing animals must be collected in a naturally wet state and kept wet until put in a digester. Do not avoid using the manure of cud-chewing animals. When the manure is wet, it causes fewer scum problems than manure-from noncud-chewing animals, because cud-chewing animals grind and break down plants more completely than other animals can.
Different plants and different times of the year require different amounts of partial composting to get them ready for digesters. In general, Composting, should take only seven to ten days during hot weather (30 degrees centigrade and above) and ten to 15 days during cool weather (20 degrees centigrade and below). Too much Composting, and too little Composting, will both decrease biogas production levels; so the best thing to do is to try different lengths of time and use what works best.
Almost as important as not wanting scum floating on top of the slurry is not wanting dirt and sand taking up valuable space on the bottom. That some undigestible dirt will get in with the organic matter is unavoidable, but try to keep it to a minimum. Manure should, if at all possible, be collected off of concrete floors, not the ground, and a dirt trap like those shown in Diagrams 7, 8, and 9 should be used. The dirt and sand that is separated from the Blurry is not useless. It should be added to compost piles.
DIAGRAM 7: DIRT AND SAND TRAPS
Note: If these methods are not used, some other method should be used so that sand and dirt does not get into the digester and "waste" valuable space. Also, make sure that nothing that floats gets into the digester.
REMOVING SAND AND DIRT FROM SLURRY BEFORE IT IS PUT IN DIGESTER Fry, 1974
The slurry mixing machine in Diagram 8 can be designed to hold all, 1/2, 1/3, or 1/4 of the daily slurry load filled up to the level of the top of the slope beside the beater. Too much or too little slurry will make the machine difficult to operate. If the radius at the front of the beater is not correctly adjusted, then the slurry in the machine will not circulate easily.
A piece of wood should be attached to one blade, protruding one inch and used as a measure. Covers are usually fitted over the beater to avoid splashing. For the collection and removal of dirt and sand, a channel is made in the floor near the inlet pipe to the digester (the lowest point). This channel runs to a two inch diameter hole in the wall which can be plugged with a piece of wood. The inlet pipe to the digester is opened after the slurry has been mixed.
In Diagram 9 the level of the bottom of the pipe to the digester is one-half to one inch above the floor of the mixing basin in order to reduce the amount of dirt getting into the digester. A valve or plug should be at the floor level of the basin, at the end of a channel that crosses the middle of the basin from one side to the other. This dirt will make a profitable addition to a compost pile.
Carbon and Nitrogen
The first requirement of the raw materials of biogas production is that they must contain organic carbon and nitrogen in quantities that have a certain relationship to each other. From a biological point of view, biogas digesters can be considered as a community of very small animals called bacteria, feeding on and changing organic matter into methane gas and carbon dioxide. The element carbon (in the form of carbohydrates) is the bacteria's rice and bread, and the element nitrogen (in the form of proteins) is the bacteria's meat and fish. The bacteria use the carbon for energy and the nitrogen for growing.
A digester's bacteria uses carbon about 30 times faster than it uses nitrogen. This is also true for people, we need a lot more rice and bread than we need meat and fish. A carbon/nitrogen ratio (C/N) of 30 (30/1 or 30 times as much carbon as nitrogen) will permit digestion and gas production to proceed at the best possible rate, if other conditions such as temperature are favorable. If there is too much carbon (C/N of 60) in the slurry, all of the nitrogen will be used up first, leaving a lot of unused carbon. This will make the production of biogas slow down. If there is too much nitrogen (C/N of 10), the carbon will soon be all used up, digestion will slow down, and the remaining nitrogen will be lost as ammonia gas which smells bad but does not burn. In addition to a lower biogas production rate, the loss of the nitrogen decreases the quality of the fertilizer.
DIAGRAM 10: pH LEVELS
To measure the acid or base condition of anything, the symbol "pH" is used. The liquid in your stomach, vinegar, Coke, and beer are all acid. Ammonia and lime are base. A neutral solution has a pH of 7.0, an acid solution has a pH below 7.0, and a base (also called alkaline) solution has a pH above 7.0. The acid-base balance has a very big effect on all living things. The maintenance of a stable pH is very important to all life. Animals cannot eat food, nor can plants live in soil that is too acid or too base. Both strong acids and strong bases can destroy anything they come in contact with (see Diagram 10).
Blood has an almost neutral pH of about 7.8. Most living processes take place in the range of pH 5.0 to 9.0. The pH requirements of a biogas digester are in a narrow range of pH 6.6 to 7.6. When the pH level drops below 6.6 or goes above 7.6, biogas production slows down and if the pH level goes 0.5 of a point above or below that range, biogas production is likely to stop. Maintaining a good pH level is an important factor in keeping the biogas production rate high. The pH of a digester should be a little on the base side of neutral; some say pH 7.0 to 7.2 and others say 7.0 to 7.8.
In order to maintain the necessary acid-base balance, one can check the pH level from time to time. The method of checking the pH level is simple. Drop a piece of litmus paper into the slurry, immediately observe the change in color of the paper, and compare it with a standard chart of pH colors to tell what the pH of the slurry is. (If the local drug stores do not have litmus paper, ask one to order a supply of it.)
A good way to find out what is happening inside a digester is to attach a strip of litmus paper and a thermometer to a long stick and put it down the digester inlet for five minutes, bring it out, read and record the results, change the litmus paper and shake down the thermometer, and take a second reading down the overflow pipe at the outlet end, then compare the two sets of readings. Because of the different types of biological activity going on at the beginning and end of digesters, there may also be a difference in readings between the two ends. Litmus paper is the easiest and cheapest way to measure pH levels, but it is not the most accurate method. Litmus paper is useful for approximate, but not exact, readings.
It has been observed that a red or yellow biogas flame often means that the slurry is slightly acid. Adding a little lime or ash to the slurry mix should help adjust the acidity and restore normal gas production. Usually the answer is not to give the digester any medicine but rather to check and see what it might be that you are doing wrong. If the bad practice can be stopped, the digester will heal itself, usually. The problem may be overfeeding of slurry or it may be a wrong balance of types of plants and manure. Using only sludge to feed the digester for a few days can help sometimes, but never add any acid to a biogas digester that has become too base (alkaline). Adding acid will only increase the production of hydrogen sulfide, which is of no use at all. For more detailed information on C/N ratios and pH levels, read the Facts and Figures section of the Appendix.