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The municipal sewage treatment process varies from one plant to another, but the basic outline is the same in the majority of plants. The raw sewage is pumped into a grit chamber where the course suspended solids and course sands are removed. The remaining liquids/solids flow slowly through a primary settling tank where the fine suspended solids and fine sands settle to the bottom and are removed. The next tank is an aeration tank where bacteria are introduced along with air to feed on the sewage. This mixture is called "activated sludge". This activated sludge is then pumped into a final settling tank where the remaining solids are removed. The remaining effluent is then sent to advanced treatment and disinfecting. The part we are interested in, and where the major difference in plants exists, is in what they do with the sludge. There are several common different ways of disposing the sludge. At some plants the sludge is trucked off to the land fill in loads of up to 25 tons. At the Christchurch City Treatment Plant some of the sludge is utilized to create methane which is then used to heat the sludge, unfortuanly not all of the sludge is utilized and the rest is sent out into a sludge lagoon and then used for fertilizer. Here in Daytona the sludge from each station is piped to a central station and then sold to Enviro Inc. who treats it and sells a portion of it as a fertilizer, the rest is then disposed of in the landfill. If a power plant such as the one in Germany were combined with a sewage treatment plant then it could easily be utilized to create a biogas. In Devon England there is an existing power plant on the site of a sewage plant that utilizes biogas created through anerobic digestion of the sludge. |