|The Biogas/Biofertilizer Business Handbook (Peace Corps, 1985)|
What design should be used in building a biogas digester? There are dozens of variations on two basic kinds of digesters. The design described in this book-an above ground, continuous-feed displacement digester with a separate gas storage tank--was chosen for several reasons. This design costs more to build than some other designs, but it produces more gas and a more sanitary fertilizer than most. It is also relatively easy to build, operate, repair, and to make profitable.
Digesters can be designed for either batch feeding or continuous feeding. Batch digesters are completely filled with a mixture of organic waste and water to make a slurry. The digester is then closed and left to digest as long as a sufficiently high level of biogas is produced. When gas production has slowed or stopped, the digester is emptied and then refilled with a new batch of slurry. Batch digesters have advantages where the availability of organic waste is not continuous or is limited to coarse plant waste. Batch digesters require little daily attention, but they do require a great deal of work to empty and load, and the gas and fertilizer production is never constant. This problem can be solved by building several batch digesters that are filled on different days and are all connected to the same gas storage tank. This can be expensive, but it guarantees a relatively constant supply of gas. Unconfirmed experiments at the Indian Institute of Technology have discovered that the nitrogen-phosphorus-potassium fertilizer value of sludge (digested slurry) is 30 percent less for batch-fed digesters than it is for continuous-fed digesters.
With continuous-fed digesters the slurry is added at regular intervals, usually every morning, and an equal volume of sludge is removed from an outlet opposite the inlet at the same time. The rate of gas and fertilizer production from even one continuous-fed digester is more or less constant. Continuous-fed digesters are made in two basic designs: vertical and horizontal. Vertical digesters are usually round or square tanks built underground and are as tall as, or taller than, they are wide or long. Horizontal digesters are usually built above ground and are much longer than they are wide or tall.
The horizontal (above ground) design has several advantages over the vertical (underground) design:
1) In the vertical digester organic waste often escapes being "eaten" by the bacteria. Slurry added one day can easily be withdrawn soon afterwards at the nearby outlet, as incompletely digested waste. In horizontal digesters the slurry must pass an area of maximum digestion on its way from inlet to outlet, with no part of the slurry spending less time in the digester than any other part.
2) From a practical point of view, above ground digesters are easier to get at to repair and clean than underground digesters.
3) The problem of large scum layers is less for horizontal digesters because they have a larger slurry surface area than vertical digesters of the same size.
4) Horizontal digesters do not usually have to be repaired or cleaned as often as vertical digesters.
5) Given equal size and other factors, horizontal digesters will produce more biogas than vertical digesters (Merrill and Fry, 1973).
How big should the digester be? Some points to think about:
1) An average size family will need a two or three cubic meter capacity digester and a large family a three or five cubic meter digester, if the biogas is only used for cooking. Because of their small size, family digesters often cost more to build and operate than they are worth.
2) A limiting factor when deciding what size digester to build is the quantity and quality of available organic waste and water.
3) Plant waste, when prepared correctly, can produce biogas and biofertilizer without manure having to be used at all.
4) Gas production can be increased at the expense of fertilizer production by using the liquid portion of the sludge taken out of the digester, instead of water, to dilute the fresh waste going into the digester.
5) Unheated digesters will produce less gas and a less sanitary fertilizer during cold weather and rainy seasons, than they will during hot times of the year.
6) Biogas systems are more likely to be profitable when they are part of businesses such as piggeries, slaughter houses, mills, market places, restaurants, and agricultural cooperatives. These businesses have access to large quantities of organic wastes, and they can use or sell the fuel and fertilizer. They have the necessary management skills to run a biogas system as a business. A small business biogas system could be as small as ten cubic meters or bigger than 100 cubic meters.
The important question is not the size of the system, but rather: Can the needs be met by the resources in a way that does not cost more than it is worth?
Information on building concrete digesters is in the Ferrocement section of the Appendix. Ferrocement information is presented separately because it goes into extensive detail. It can be used for any concrete construction project, not just for biogas digesters.
Metal digesters and metal gas storage tanks can be made by the same companies that make metal water tanks. The digesters and gas tanks should be made at the site where they will be used, and the welding and painting of the tanks should be done with great care. There are more details on metal digester construction in the Appendix, and more details on metal gas storage tanks in Chapter Six.
Biogas digesters should be built above ground for several reasons. (At most, only a few inches of the digester should be underground.)
· The closer the temperature of the slurry (the mixture of organic waste and water) inside the digester is to 35 degrees centigrade/95 degrees Fahrenheit, the better it will be for the biogas producing bacteria.
· Underground digesters in hot climates will always be cooler than above ground digesters in the same areas, which means underground digesters will, everything else being equal, produce less gas.
· In climates with cold weather, the extra expense of heating digesters will prove more profitable in the long run than avoiding some of the cold by building underground.
· High water tables and the chance of flooding is another problem for underground digesters, and underground digesters are harder to clean than above ground digesters.
· The main advantage to building underground is that the dirt will help support the digester walls. The walls do not have to be as strong or expensive as the walls of above ground digesters. But if an above ground digester is made well, the increased construction costs can be rewarded with increased biogas production and a higher quality fertilizer.
It is often easier to put slurry in an underground digester because the slurry can flow down into it. But again, one-time savings in construction costs do not outweigh the continuously higher savings or profits of an above ground digester's higher gas production rate. Slurry loading ramps can be built if a down hill site cannot be found for an above ground digester. But there is nothing that can be done for an underground digester that has production costs that are higher than the value of the gas and fertilizer.
Greenhouses are buildings with glass or plastic walls and roofs in which plants are grown when it is too cold outside. Greenhouses let in the sunlight and trap the heat of the sun. When a greenhouse is built around a digester, an unheated digester will produce more gas than it would have and a heated digester will need less heat to produce biogas at the maximum rate. Greenhouses should not be completely sealed; as a safety measure to allow an exchange of air, there should be vents at the top of greenhouse roofs from which the biogas could escape if there was a leak in the digester. Biogas digesters have traditionally been made with concrete (underground digesters with brick or hollow block). But metal, plastic, and fiberglass can also be used to construct digesters (and gas storage tanks) using the designs and operation methods described in this book.
There are advantages and disadvantages to all possible construction materials.
· Concrete (using the ferrocement method) may be the cheapest method, but concrete digesters cannot be moved.
· Concrete digesters have to be very carefully made if they are to be watertight and airtight.
· Concrete will stay warmer at night longer than metal or plastic, and that means more gas.
· Metal can rust; the welding and painting must be done perfectly.
· The zinc in galvanized iron can kill biogas producing bacteria, so the inside walls of metal digesters must be painted.
· Once made, plastic and metal digesters are less likely to leak. When empty, plastic bag digesters can be moved. e Plastic and concrete will not rust....
This list could go on and on--the choice of building materials should be decided by using the material(s) that are affordable, available, and best suited to local resources and needs.
It is important to keep the proportion of length to diameter (or surface area of a cross section--which is width x height) of a digester to within certain limits (see chart in Appendix):
1) If a digester is too long and thin, the fresh slurry will not mix properly with the active bacteria and the digestion process will be slow in starting. Fresh slurry should come into contact with the slurry of previous days, which in turn, should be in the active stages of decomposition leading to the final stage of methane production.
2) If a digester is too short or too wide, the physical and biological steps will not be spread out enough. Square and round digesters produce less gas and a less sanitary fertilizer than long digesters. Today's fresh slurry is mixed at random with previous slurry, some will be taken out before it has been completely digested, and some will stay in the digester long after it has been completely digested.
3) The proportions of diameter to length of a digester is not very critical. A ratio of five in length to one in diameter is best. Ratios between 8/1 and 3/1 length to diameter are the outside extremes of digester proportions. Any digester which is longer and thinner or shorter and fatter will not produce as much gas and quality fertilizer as a digester of the same capacity, but with a better shape.
All biogas digesters should be built with a two to three degree tilt, starting at the inlet and going downhill towards the outlet. With a tilt which is less than two to three degrees, the slurry will not move through the digester fast enough. With a tilt greater than two to three degrees, the slurry will race through the digester too fast (Fry, 1974).
A very difficult question to answer is, "How much does a biogas digester cost?"
· First of all, there is much more involved than Just a digester. There are also gas storage tanks, ponds, engines, generators, pipes, valves, tools, and so on.
· Then there are questions such as which building materials are going to be used and what are local labor costs?
· One thing is for sure, once a biogas system is built, the major expenses are finished. Each year after construction, the costs of producing fuel and fertilizer (as a percentage of investment), should go down. Commercial fuel and fertilizer prices will without a doubt go up.
The following cost information comes, in part, from an article on biogas in the October, 1980, issue of "VITA NEWS."
Cost estimates for biogas systems vary widely depending on design, size, location, building materials, labor costs, and the method used to figure the costs. The Chinese claim to be able to build their underground digester for less than US$ 100. However, this estimate has been challenged for not including true labor and material costs. Another problem with the Chinese digester is that it produces very little biogas. In China they are producing biogas at the rate of 0.2 to 0.3 cubic meters (per cubic meter of digester space per 24 hours) and one fourth that rate during the winter.
The design suggested in this book can produce four to ten times as much gas per day than the Chinese design can in the summer. One reason the Indian digester design has not become more popular in India and elsewhere is its high initial capital costs when compared with the value of its products. A single family unit costs US$ 375. Although this is several times the average annual individual income in rural areas, the Indian digester is promoted as a family investment, not a business investment. Another fact to think about is that many, maybe even most, of the Chinese and Indian model digesters that have been built around the world have also been abandoned.
The high price of biogas systems has increased interest in building and using biogas systems on a business and cooperative scale, instead of on a single family scale. That business or cooperative could be a group of relatives, neighbors, or friends, or it could be a restaurant, market place, hospital, fish farm, or even a whole village.
There are at least six steps to making a cooperative or business biogas system profitable. The system must:
1) include fish ponds and/or other uses for the fertilizer that is produced,
2) be large enough to benefit from the addition of a stationary engine to the system, fueled by the biogas, and heating the digester with the excess engine heat,
3) consider making financial payments to the investors in the business or cooperative as an alternative to using the fuel and fertilizer as payments,
4) consider trading for or buying organic wastes for the digester,
5) consider building central piggeries, chicken coops, and cattle stalls animals owned by cooperative members or business partners in order to collect as much organic waste as possible.
Practical experience with small business and cooperative biogas systems is still limited. But there do appear to be real economies of scale. The bigger a digester is, the more profitable it can be.
Making a feasibility study can provide an organized structure within which the many important questions of a beginner in the biogas business can be answered. Questions such as: What types and quantities or organic waste are available locally? How big should the business be, in the first year, in the second year? Is there enough water available to make the slurry? What about the local infrastructure? Are there markets for the fertilizer? Will the local banks provide loans? Can local construction companies build a biogas system? Are there government programs that can provide financial or technical help? (An outline of a feasibility study is in the Appendix.)
Maya Farms, which has its own large scale biogas system in the Philippines, is now in the business of building biogas systems for other businesses. What follows is adapted from an article in the "Philippine Farmers Journal" of November, 1980, about the Maya Farms biogas systems construction business. Businesses interested in contact Maya Farms can write to them at the Maya Farms address given in the Appendix.
Foremost Farms, considered the biggest and the most modern pig farm not only in the Philippines, but also in Asia, has contracted Maya Farms to build a biogas system for them. The system when finished will produce up to 765 cubic meters of biogas every day.
The services of a Maya Farms biogas system contract include: site survey, biogas system design, construction, and the supervision and training of personnel who will later operate the system.
Maya Farms experts first survey the site to determine its topography, the water table, and the space available for a biogas system. Then they make the designs and plans. The farm owner has the option of hiring local contractors to build the biogas system, but they have to be supervised by an engineer from Maya Farms. The engineer lives in with the workers until the construction is finished.
It is also included in the contract for Maya Farms to train the people who will eventually operate the biogas system. Specifically, they teach the people involved how to make the necessary adjustments for appliances, engines, and generators which will be fueled by the biogas, and how to process the solid sludge into a feed supplement and the liquid sludge as crop irrigation or fish pond fertilizer. Maya Farms also handles contracts where the client operates the biogas system with a Maya Farms supervisor.
The size of the biogas systems depends mainly on the site and size of the farm. Most of the systems are continuous-fed, multi-digester systems, and they range in size from 255 cubic meters of biogas per day to 765 cubic meters of biogas per day. Although contract prices amount to thousands of pesos, it is very reasonable and worth the investment, considering that biogas systems can last for a lifetime, providing fuel, feed, and fertilizer.
To date, the Bio-Energy System Division of Maya Farms has 12 big clients. Of these, four are already operating their systems [Aries Agro-Industrial Dev. Corp. of Laguna, Multi-Farm Agro-Industrial Dev. Corp. of Cebu, Aveco Farms of Pangasinan, Cardova Farms of Batangas], five are under construction [Green Field Piggery & Agricultural Corp. of Bulacan, Reliance Agricultural Dev. Corp. of Bulacan, San Victores Dev. Corp. of Bulacan, Gold Star Piggery Farms of Bulacan, Monterey Farm Corp. of Lipa City], and three are in the planning stage [Console Farms of Bulacan, Remman Ent. Corp. of Batangas, Foremost Farms Inc. of Rizal].
DIAGRAM 5: ROUND BIOGAS DIGESTERS
Safety from Lightning:
Means should be provided on metal digesters and biogas holders to lead lightning away to the ground through conductors. Drive a metal stake deep into the ground near the digester and connect the digester and the stake together with thick electrical wire in order to make a good electrical contact.
DIAGRAM 6: RECTANGULAR BIOGAS DIGESTERS