Biogas is the breakdown of organic matter into methane gas(CH4). Organic matter can be from cattle, human excrement, and old rottening food. Leftover food material seems to be a more efficient fuel than cattle or human excrement. Biogas can be used for cooking, lighting, and other energy applications. The second bi-product, slurry or effluent, can be used as fertilizer. With the biodigester everything modern society discards as waste is not waste but fuel. Rotting left over food, rotting fruits/vegetables, cow dung, dog poop, human feces, all can be used as material to generate fuel--methane.
For organic matter to breakdown, there must be an absence of oxygen, so the anaerobic bacterias can perform their function- anaerobic digestion. The process of methane generation begins with hydrolasis, where complex molecules are broken into smaller molecules, like fats being broken into glucose. The acetic bacteria then breaks these molecules into acetic acid. Methanogenic bacteria breaks acetic acids into methane or carbon dioxide into methane. Biogas is typically 60% to 80% methane.
Anaerobic process occur in a temperature range of below freezing to above 135° Fahrenheit (F) (57.2° Centigrade . The best temperature for the anaerobic process is about 98°F (36.7°C) (mesophilic) and 130°F (54.4°C) (thermophilic). Biogas generation decreases between about 103° and 125°F (39.4° and 51.7°C) and gradually from 95° to 32°F (35° to 0°C).
Biogas is generated in a anaerobic digestor. Digesters come in numerous shapes, sizes, and constructions. Four types of common digesters exist, with thousands of users. They are:
All digesters have the following parts:
The slurry that comes out of the digester, if it is from human fecal matter must be composted for a year or more, to be used for fertilizer. To compost, mix green plant matter, dried plant matter, and ash to the slurry compost. The compost mixture should be left alone for at least a year, to be broken down. After one year the compost must look and smell like soil. The compost should also have worms. The secretion of worms provides micro-nutrients to the compost making the it ideal for fertilizer.
Water is added with waste. When adding waste material a certain ratio is to be observed. Adding food waste requires a ratio of 1 part water and 1 part food waste. Adding animal manure requires 1 part water and 3 part manure.
Relevant Article: Compost Toilet--manage your poo for health, safety, and fertilizer