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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 one: An introduction

This handbook describes biogas system theory, design, construction, and operation principles that are appropriate to the resources and needs of rural communities and small businesses in developing nations. In order to involve and benefit as much of the community as possible, new combinations of proven biogas concepts have been brought together and emphasis has been placed on several aspects of biogas technology that are often overlooked.

A common reason for being interested in biogas is to reduce the cost of fuel used for cooking. Clay stoves cost less, are easier to build, operate, and maintain than biogas systems. When compared with open cooking fires, clay stoves use less of the same fuels, and there is no smoke to get in the cook's eyes.

Profitable biogas systems are small factories that make a fuel that is best used to run stationary engines for mechanical and electrical power, and a fertilizer for fish ponds, gardens, and farm crops. There are many good books on simple wood conserving stoves such as the Lorena clay stove. Volunteers In Technical Assistance (VITA) and Volunteers in Asia have books on how to build and operate these stoves. Their addresses are in the Appendix.

A successful biogas operation, one that makes or saves more money than it costs, is a business operation. The biogas digester is only part of a biogas system: a system that should include a separate gas storage tank, engines to use the gas, ponds, the use of plant as well as animal waste, the production of fertilizer as well as gas, and business as well as technical skills.

What do you want a biogas system for?

· Which biogas purpose is most important to you: gas, fertilizer, or sanitation?

· Will the digester be fed manure, plants, or both?

· Is there enough water and organic wastes to feed a digester that will produce enough gas to run an engine?

· Are there ready uses and/or markets for a fertilizer that is mostly liquid?

· Should a biogas system be built for business, cooperatives, or family needs?

· Should a biogas business also build biogas systems for other people?

These are just some of the questions you will need to ask, and answer, before you can build a biogas system that will meet your needs.

There are many short "how-to" books on biogas. Why is this one so long? There are several reasons.

· With just a few pages of information, a digester can be made that produces biogas, but costs will be greater than profits or savings.

· To invest time and money in biogas is to invest in a business; it is that complicated. But a well run biogas business can be as profitable as any well run business.

· If you understand your business, you can find a solution to almost any problem. If you have enough information, you can make intelligent decisions.

Biogas manuals are often made short and simple in hopes of making biogas systems inexpensive and popular. The result is that many people are attracted to biogas believing that is no more complicated than building and operating a septic tank with a gas pipe connected to a stove. That is a frightening underestimation of the demands of what is in reality a small-scale business, not a backyard hobby. With a short, simple manual as a guide, a biogas digester can be built and operated, but it is very unlikely that it will be profitable. Discouraging people with a long manual is preferable to the disappointment and costs suffered when a project that was presented as simple turns out to be complicated.

Four general ideas guided the design of this handbook.

1) Ordinary words should be used as much as possible. Language special to a technical field is an easy short-cut for the expert, but it confuses the beginner. What is common sense to one person is brand new and unfamiliar to another.

2) One method of introducing a new concept is to repeat it in different ways.

3) All important aspects of a subject should be explored, social as well as technical.

4) Reasons should be given, not just how-to instructions. Understanding the why of a process leads to being able to make intelligent decisions on how to improve the process and solve problems not covered in directions.

Several books and magazines were used in writing this book. Unless otherwise noted, these sources are not quoted word for word. When a section is primarily from one source, that source will be acknowledged either at the beginning or end of the section. Ideas within sections are often illustrated by information from experience or other sources, and all source material is edited with the overall viewpoint and purpose of this book in mind. The Sources + Resources section of the Appendix describes the significant books and magazines used in writing this book and the Vocabulary section of the Appendix defines the technical words.

This handbook's start came from a year and a half's experience with a small 0.4 cubic meter capacity demonstration model biogas digester at the home of Doctor and Doctora Mercado in Butuan City, Agusan del Norte, Philippines, while I was a Peace Corps Volunteer. Many experiments, victories, disappointments, and surprises have been produced by that two-oil drum digester and two-oil drum gas storage tank. The digester can produce enough gas to cook rice three times a day plus more fertilizer than can possibly be used in the garden. Much was also learned from research, and studying several other working and non-working biogas digesters in the Philippines.

The following is adapted from the book, Biogas and Waste Recycling--The Philippine Experience by Felix Maramba, the developer of one of the world's most successful, popular, and profitable biogas systems.

The proliferation of biogas systems will uplift the social and economic life in the rural areas.

· It will improve the living conditions by controlling the pollution of the air and waters, and by promoting sanitation.

· It will raise the standard of living by providing the means for economic advancement.

· By utilizing wastes and local materials to serve farming needs, and by making the land more productive through recycling systems of farming, it will create a pattern of rural living that can lead towards self-reliance.

Although perfecting biogas technology requires experimentation, no expensive or complicated equipment is needed to build and operate biogas systems. Biogas systems are made-to-order for farm communities. Plant and animal waste is continuously produced on farms, hence there is a reliable and unending supply of raw materials. Biogas systems are well suited for agricultural power and fertilizer requirements.

After the biogas has been produced, the plant and animal waste is removed from the biogas digesters as a watery sludge. A sludge that retains all of the nutrients contained in the original plants and animal manure. In the Philippines, at Liberty Flour Mill's Maya Farms, it was found that when the manure of four sow units (one sow unit = the sow plus all offspring up to eight months old) is used as the raw material for a biogas digester that sufficient fertilizer will be produced for three crops on one hectare of crop land and 200 square meters of fish pond. The only fertilizer element of which there is not enough is potash, and extra potash can be supplied by the ashes of burnt crop waste.

Last but not least biogas systems control pollution caused by the manure and other farm wastes. Sanitary conditions are promoted by eliminating the manure which breeds flies and spreads diseases. It is known that the use of chemical fertilizers has contributed greatly to the pollution of streams. This pollution can be minimized if organic fertilizer from biogas digesters and compost piles is used as a replacement for chemical fertilizers.

Biogas technology is a new concept, and as is the fate of new ideas, it will encounter initial resistance. It costs money to construct and maintain biogas systems. It requires new techniques in operation. How well will it control pollution and promote sanitary conditions? How good is the fertilizer and feed value of the sludge? How good is the biogas as a fuel? Are biogas systems economically feasible and socially acceptable? A deeper understanding of these questions will go a long way toward general acceptance of biogas systems.

Biogas Biology

What follows is an introduction to the biology of biogas. It helps explain how and why plant and animal waste can become a burnable gas and a quality fertilizer. Understanding the why of biogas will make it easier to understand how to operate a profitable system.

Millions of years ago the primitive air was composed mostly of carbon dioxide, water vapor, and methane. There was little or no oxygen in the air, and all life lived and moved in a world which would not allow us to survive. We are aerobic, that is, we need oxygen in the air we breathe. It is called free oxygen because it is not combined with any other element. Whatever primitive life existed in the dawn of prehistory was anaerobic, that it, it did not need or use free oxygen in its life processes.

An interesting question is, where was all the oxygen? Answer: It was locked up in iron oxide (oxide: oxygen combined with another element such as iron) deposits, locked up in carbon dioxide, locked up in hydrogen oxide (also known as water), and happily combined with whatever was available. Another interesting question is, why is the air so full of oxygen today? Answer: Green plants.

Photosynthesis means using light (photo), to make (synthesis) the chemicals necessary for life. Plants take in carbon dioxide and "break" it into its parts. They keep the carbon and release the oxygen into the air. Animals take in oxygen and release carbon dioxide. Life is one big balanced circular process--it is very intelligently designed.

Life on the primitive Earth was very simple, there were no animals, there were no photosynthesis plants, and so there was little or no free oxygen. The only important source of oxygen is the activity of green plants. (Protect your local forest!) Slowly, photosynthetic types of plant life developed and covered the Earth, but it took a long time for the oxygen level to build up to any great degree in the air.

As conditions changed on Earth, those life forms which once could live in the open air could not survive the gradually increasing oxygen levels in the air. Today these organisms can only survive in places where the ancient no-free-oxygen conditions still exist, such as in biogas digesters and the bottoms of swamps.

These organisms (mostly bacteria) are still important. In nature everything eventually returns or cycles, and these anaerobic organisms help to return complex organic matter such as plants and animals back to simple organic matter that plants and animals need in order to live and grow.

Plant food comes from the air and the soil. Plants take basic elements such as carbon, oxygen, hydrogen, nitrogen, phosphorus, and potassium from the air and soil to make their proteins and carbohydrates. People get proteins from meat, fish, and beans, and carbohydrates from rice, corn, and wheat.

When plants and animals die, their remains, made up of complex molecules, are decomposed (broken down) into simple molecules by organisms such as bacteria and returned to the soil and air. In airless places such as swamps, lakes, and slow stream bottoms, the only way plant and animal remains can be broken down is by becoming food for anaerobic types of life.


Two Ways of Producing 230,000 Tons of Nitrogen-rich Fertilizer

-The systems in this chart cost US$ 4,800 each in India (1975), and each produced 140 cubic meters of biogas per day.

The Layers Inside A Digester

Biogas Flame Temperatures

adapted from Lik-lik Buk, a rural development handbook

Another place where anaerobic bacteria help is in the digestive tracts of many creatures. Termites use them to break down the wood they eat. Cud-chewing animals such as cattle have many anaerobic "little bitty buddies" in their complex digestive tracts which help them to break down the plants they eat. The two main places where we find anaerobic life today are underwater and in digestive systems. A biogas digester is, in a way, an artificial digestive system.

Anaerobic metabolism, the internal life process of oxygenless bacteria, is not as efficient as aerobic metabolism. Anaerobic bacteria cannot use as much of the energy in their food as aerobic bacteria can from their food. Anaerobic bacteria lose much of their food energy when they give off methane gas--too bad for the bacteria, but just what we want.

When compost is made in the open air, aerobic bacteria take part in the rapid breakdown of organic matter. The temperature inside a compost pile is often as high as 70 degrees centigrade (160° F) during its most active period. Similar organic wastes, when placed in the airless world of a biogas digester, produce very little heat, decompose slowly, and as a by-product release most of the energy which was locked up in the organic molecules--still locked up as flammable methane gas.

This difference between aerobic and anaerobic metabolism, in regard to their ability to efficiently use biological energy, also shows up in the fact that the process inside a biogas digester is easier to upset than the process inside a compost pile. Changes in temperature, types of organic waste, and levels of toxic (poisonous) matter which would not harm the aerobic compost process, will slow down or even stop the anaerobic biogas process.

Understanding the breakdown of molecules for energy is really quite simple. Suppose there is a coil spring between your hands. When you force your hands together and lock your fingers together, the spring will try to push your hands apart, and your fingers will keep them together. It took force to bring your hands together, and now the spring stores potential energy, locked between your hands. In a similar way, atoms, which are the building blocks of all matter, are locked together to form molecules, and in doing so they store energy between them. When they are unlocked, energy is released.

When we put atoms like carbon and oxygen together, one carbon atom and two oxygen atoms, we get a molecule of carbon dioxide (CO2). Two hydrogen and one oxygen gives us a molecule of water (H2O). One carbon plus four hydrogen is a molecule of methane (CH4). These are very simple molecules (combinations of atoms), but nature often puts together hundreds of atoms of many different kinds and comes up with very complex molecules.

If a molecule is unstable, the locks in it are not very good, and it may break apart very easily. More stable molecules are harder to break apart, Just as your pushed-together hands would be hard to break apart if you had strong fingers, or if your hands were tied together with rope.

In everything that is or was alive, molecules are broken apart or formed, not by force, but with the help of enzymes. Enzymes are complex products of living organisms that cause or speed up chemical reactions. In the spring-hands-fingers model, a little grease or oil would act as an enzyme, causing the fingers to slip apart and the stored energy to be released. If the hands were tied together with rope, an enzyme would act like a pair of scissors, cutting the rope.

In a biogas digester, enzymes break complex molecules apart, step by step, into simpler molecules. The process has been compared to an assembly line, except that it is a disassembly line, where one group of workers take apart complex molecules, give the less complex molecules to another group of workers, who disassemble them further, and so on until the last group of workers breaks the molecules into the simple molecules: water (H2O), carbon dioxide (CO2) and methane (CH4).

A biogas digester is like a factory (you are the boss), filled with workers, busy manufacturing gas and fertilizer from organic materials. Inside the factory, decay happens in steps:

1) Aerobic: Oxygen enters with the manure, plants, and water. First aerobic bacteria use up the oxygen. They also do what they can to break the materials down. Carbon dioxide is released and some heat is produced.

2) Enzymes: In this stage, anaerobic bacteria releases enzymes which attack the large organic molecules in what was manure and plants in order to break them down into bite-size pieces.

3) Acid digestion: The bite-size molecules, still fairly large, are absorbed by bacteria and digested (eaten). The main products of this process are simple molecules, the majority of which are short chain fatty acids, hydrogen, and carbon dioxide.

4) Gas digestion: Now comes the part we have been waiting for. The fatty acids are used as food by the last group of bacteria. These bacteria produce water, hydrogen sulfide, carbon dioxide, and best of all, methane. It is methane, mixed with carbon dioxide and a few other trace gases, that we call biogas (House, 1978).


gas pipe to an engine and generator building and/or kitchen stove