Team Measuring Building for Water Line Installation

We have designed a simple methane digester comprised of two or more discarded 55-gallon drums. Manure is added and mixed with water to produce methane gas.  This gas can then be stored and burnt to generate energy for cooking or other household uses. We hope that this very simple solution will someday alleviate the wholesale cutting of firewood and making of charcoal.

What is Methane Generation?

Livestock manure contains a portion of volatile (organic) solids, which are fats, carbohydrates, proteins and other nutrients that are available as food and energy for the growth and reproduction of anaerobic bacteria.

The anaerobic digestion process occurs in two stages. The volatile solids in manure are initially broken down to a series of fatty acids. This step is called the acid-forming stage and is carried out by a particular group of bacteria, called acid formers. In the second stage, a highly specialized group of bacteria, called methane formers, convert the acids to methane gas and carbon dioxide.

The anaerobic process depends on methane formers because they are more environmentally sensitive than acid formers. Methane bacteria are strict anaerobes and cannot tolerate oxygen in their environment. They function best at 95 degrees Fahrenheit; therefore to obtain maximum gas production, heat usually must be added to a digester.

Methane bacteria are slower growing than acid-forming bacteria and are extremely pH-sensitive (pH 6.8-7.4 optimum). The acid formers will grow rapidly if an excess of organic material is fed to a digester, producing an excess of volatile acids. The accumulated acids will lower the pH, inhibiting the methane bacteria and stopping gas production. To help buffer the system against increases in acids, high alkalinity must be maintained. Lime has been added to digesters during start-up or periods of slug loading to maintain pH control.

A variety of materials can become toxic to anaerobic bacteria — salts, heavy metals, ammonia and antibiotics. Bacteria require minimum amounts of salts for optimum growth. However, if salts are allowed to accumulate beyond bacterial requirements, they can become toxic and inhibit digestion.

Soluble heavy metals (copper, zinc, nickel) may be toxic to digester bacteria. Most heavy metals can be precipitated out with sulfides and will cause no problems in the sludge. Livestock feeds containing significant amounts of heavy metals may require special attention.

Most livestock manure (particularly swine and poultry) contain appreciable amounts of nitrogen, which will be converted to ammonia in the digester. Most of the ammonia will accumulate in the digester material and will become toxic if not controlled. Ammonia toxicity is a major concern in the anaerobic digestion of livestock manure. To avoid the problem, loading rates must be carefully controlled.
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