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Digester gas cogeneration

Sewage treatment or domestic wastewater treatment is a process of removing contaminants from wastewater and househould sewage, both runoff (effluents) and domestic. It includes physical, chemical and biological processes to remove physical, chemical and biological contaminants. The anaerobic digestion of sewage sludge involves fermenting the sludge in tanks at a temperature of 32 to 34 °C for about 25 days. The thermal energy generated by a combined heat and power (CHP) unit preheats the sludge and keeps the temperature of the digestion tank constant. The resulting biogas normally consists of 50 - 60 % methane, 30 - 40 % carbon dioxide and small quantities of residues. The gas is compressed and purified if it contains larger amounts of contaminants stored temporarily in a storage tank from which it is fed to CHP unit at constant pressure. A gas engine transforms the energy stored in the biogas into mechanical and thermal energy. It also powers a synchronous generator, which in turn generated electrical energy for the operation of the sewage treatment plant.

Anaerobic digestion destroys more volatile organic compounds and produces more gas than do traditional composing methods used for treatment of sludge. Because the process is contained, odor is also controlled and this can help meet premitted limits on emissions. During the process, less solid waste is produced, and what is produced can be used directly on fields as a mulch or soil amendment.

Both industrial facilities and municipal sewage treatment plants face the challenge of removing organic waste from water. They not only meet minimum standards of water quality in their process, but also deal with odor control and energy costs.

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