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Methane Digester

Biogas or methane is the product of anaerobic bacteria as they break down organic waste. Methane can be burned in a stove or used in a generator as a fuel similar in use to Natural Gas. In fact, Natural Gas is mostly methane. Biogas production can be a great asset in a self-sustainable homestead because it provides high Btu energy and uses waste as an input. The byproduct of the production process can be used similar to other composts.
Feedstock for the Biogas process can be any organic matter, although close attention to the carbon to nitrogen ratios must be observed for maximum production. A 30:1 ratio is ideal. Most large systems use animal manure as a major feedstock, but vegetable, animal, and human wastes can be used. As an added benefit, pathogens are rapidly digested in this process, however, some weed seeds may not be destroyed.
The methane production process consists of a large tank or digester where the methane is produced, a temperature control, an agitator or stirrer, and a gas storage tank. Most home systems are of the continuous feed type, as you can add to them daily. This will be the system we will explore in this article.
The continuous feed system is sized so that the daily input is digested over 30-60 days. The daily gas requirement dictates the amount of input daily. So, say you need 1,200 Btu daily for cooking. This would amount to about 2 cubic feet of biogas (600 Btu per cubic ft).So, your daily input needs to be able to produce 2 cubic feet of biogas. This is difficult to calculate, because each feedstock has different rates of production. One pound of cow manure can produce about one cubic foot of gas. However, chicken manure is more potent, and can produce more gas per pound, but must be mixed with carbon content. 
So, once you figure out how much input is required a day, then you can size your digester. Daily volume multiplied by 40 is a good figure for digester size. This will give the material 40 days to digest. You will also have output daily in this same volume, so you should design a catch on some dry straw or high carbon content to help absorb odors and excess moisture. You will need to add water to the input daily as well, so if you could reasonably reuse some of the water that is discarded daily, this would increase the efficiency of the system.
The digester must be kept at a constant temperature to avoid the little critters going into shock. This is best done with good tank insulation and a solar water heater. The best temperature is the temperature of your body, as these little guys live in you as well. So, between 90 and 100 degrees F is best. As a backup, you could burn some of the methane to keep the tank warm.
As the gas is produced, it needs to go somewhere. It is fairly simple to make a water float gas tank. The gas will be stored in very low pressure, which is fine, and safe. You can then route the gas from the storage tank to the house using black poly pipe or pvc. Since we are working with low pressures, expensive pipe is not needed.
You can add meters and additional components as you get used to the system. The system will take some time to get a decent population of bacteria started in the tank, and this can be helped along by adding cow and/or human manure at the beginning, as both contain the bacteria needed.
This system can be sized to fit any need, but is most efficient for cooking and/or heating water. It could be used for electricity generation through a gas generator, but you would need to produce about 18 cubic feet per horsepower per hour. 225 cubic feet of biogas is equivalent to a gallon of gasoline and about 7Kw of electricity. If you have a bunch of animals, this might be a good form of constant electrical production.


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