|The Biogas/Biofertilizer Business Handbook (Peace Corps, 1985)|
I. Purpose and Scale
A. The subject of this book is one approach to building and operating biogas systems, not biogas digesters. Biogas systems include raw material preparation, digesters, separate gas storage tanks, use of the gas to run engines, and the use of the sludge as fertilizer.
B. The systems can be profitably operated as cooperatively or privately owned labor-intensive small businesses. They are designed to meet local fertilizer and fuel needs, using local resources and skills.
C. Like rice and corn mills, ice plants and general stores, biogas businesses have supply, production, distribution, and management responsibilities. Operated as businesses, biogas systems have the potential to make a rewarding return on investment and a contribution to the wealth of the community.
II. Raw Materials
A. Animal waste.
1. Manure is the most common raw material.
B. Plant waste.
1. It is useful when shredded, ground or pulped, and partially composted before being put in the digester.
C. Digester slurry.
1. Slurry is raw material mixed with water or the liquid portion of used slurry.
2. Slurry is mostly water; it should be only eight to ten percent solid.
3. Slurry should have a 30 to 1 carbon to nitrogen ratio and a neutral to slightly base (alkaline) pH.
4. No floating matter, dirt, or sand should be in the slurry.
5. A 40 day digester detention time is recommended, with slurry added once or twice a day.
III. The Digester
A. Horizontal, above ground digesters with a two to three percent tilt down from inlet to outlet are recommended in this book.
B. A digester length-to-diameter ratio of five to one is recommended.
C. Ferrocement, mild steel, and galvanized iron, are the recommended digester and gas storage tank construction materials.
D. Separate gas storage tank(s) are recommended in this book.
E. 10 to 30 cubic meter capacity systems may be practical, but 50 cubic meter capacity and larger systems are practical.
F. A continuously maintained digester temperature of 35 degrees centigrade (96 fahrenheit) is recommended.
G. An annual cleaning of the digester to remove scum and dirt is recommended.
A. Biogas can be produced by anaerobic digestion at rates of one to two cubic meters of gas per cubic meter of digester space per day in the digesters described in this book.
B. Biogas composition is 60 to 70 percent methane which burns, 30 to 40 percent carbon dioxide which does not burn, and a trace of hydrogen sulfide which smells like rotten eggs.
C. Condensation traps are needed to remove water vapor from Biogas pipes.
D. Biogas (methane plus carbon dioxide) or methane alone can be used for several purposes.
1. The gas can be used for cooking, gas lights, and gas refrigerators.
2. The position taken in this book is that the best use for the gas is as fuel for stationary spark-ignition and dual-fuel diesel engines for mechanical power and with generators for electrical power.
3. It may be possible to use the gas to heat boilers that power simple "rankine-style" engines for mechanical and electrical power.
E. The carbon dioxide in biogas may have some uses.
1. Carbon dioxide should not be removed from biogas unless there is a practical use for it. The process of separation must be simple and low cost.
2. Carbon dioxide can be used in greenhouse operations to increase crop yields, and it may be practical to freeze the carbon dioxide to make dry ice.
F. The waste heat from engines fueled by biogas or methane must be used to maintain digester temperature at 35 degrees centigrade.
G. Only when digester capacity is big enough to make the use of an engine practical is it likely that the system will be profitable.
A. Biofertilizer is used digester slurry and is often referred to in this book as sludge.
B. Only when the sludge is used as a fertilizer can a biogas system be profitable.
C. After the sludge has been exposed to the air for a couple of weeks, it makes an excellent soil conditioner and organic fertilizer for crops and fish ponds.
D. The liquid portion of the sludge (90 percent) can be recycled and used in place of water to dilute fresh raw materials.
VI. Secondary Biogas System Projects
A. Flat-plate solar collector water heaters can be used to heat digesters and in a dual-fuel system with biogas to power rankine-cycle engines.
B. Composting can be a useful and necessary part of biogas systems.
C. Bioinsecticides can be a safe, simple method of pest control. When bioinsecticides are used with organic fertilizers, there should be an increase in crop yields and a decrease in crop losses.