Biogas generation and making an anaerobic digester

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.

Best Fuel

Food sources generate the most fuel or methane. Pure sugar generates the greatest methane. When starting the biodigester for the first time, do not use a food source. It will generate too much acid.


pH Levels

pH should be maintained in the range of 7-7.2. If less than 7, it is too acidic, add lime or calcium bicarbonate. If above 7.2, it is too much of a base, add more waste to make more acidic.

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.



Keep waste material temperature between 20-45oC. Check temperature of waste at the Outlet.
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).

Parts of An Anaerobic Digester 

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:

First Materials

Cow or horse dung is  the first type of material to place in the biodigester for four weeks to generate methane. They are low energy foods, which will prevent the pH from getting to acidic. Rotting food should be avoided. Rotting food will make the pH to acidic. It is too high in energy.
  • The Indian Floating Drum
  • The Chinese Fixed Dome
  • Poly Plug Flow 
  • Continuous Flow Stirred Tank
All digesters have the following parts: 
  1. Inlet for placing the organic matter--cattle dong, human feces, plant matter, rottening food; 
  2. Gas outlet for the methane to get out of the digester
  3. Outlet for the slurry or effluent which can be used for fertilizer 
  4. Fermentor is where material is broken down by bacteria-inside the container, where the organic matter reside

Slurry, Sludge, Effluent

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.

Works Cited

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