Methane Tank

(or digestion tank), a reinforced-concrete reservoir of considerable volume (up to several thousand cu m) for biological processing (fermentation) of the organic portion of solid sewage by means of bacteria and other microorganisms under anaerobic conditions. The decomposition of organic matter takes place in two phases. In the first phase fatty acids, hydrogen, and amino acids are formed from carbohydrates, fats, and proteins. In the second phase destruction of the acids occurs, with formation mainly of methane and carbon dioxide gas. A mixture of raw (fresh) sediment from the primary settling tanks and excess active slime from the secondary settling tanks after the air tanks usually passes into the methane tank. In the methane tank the mixture to be fermented is heated (usually with live steam) and mixed.

A distinction is made between mesophilic fermentation (at temperatures of 30°−35°C) and thermophilic fermentation (at temperatures of 50°−55°C). In thermophilic fermentation the process of decomposition proceeds more rapidly, but the fermented sediment gives up its water less readily. The mixture of gases produced during fermentation consists mainly of methane (up to 70 percent) and carbon dioxide gas (up to 30 percent). The methane, which is burned in the boiler, is used to produce the steam that heats the sediment.