Digested sludge (sewage technology)
In the wastewater treatment sludge through digestion tank stabilized sewage sludge and a waste product of wastewater treatment . It is a mixture of about 95 to 99% water and 1 to 5% solids accordingly. The solids are about half inorganic and half organic, for example residues of microorganisms from biological wastewater treatment. Under anaerobic conditions, i.e. without oxygen , bacteria , in the last phase methane bacteria , convert the easily putrefactive part of the organic matter in the original raw sludge, forming digester gas into digested sludge. Around a third of the solids present are converted into gases.
Like biogas, the digester gas mainly contains methane (CH 4 ) and carbon dioxide (CO 2 ) as well as small amounts of other gases such as hydrogen sulphide (H 2 S). Such sewage or biogases are used to generate electricity and heat. This type of conventional treatment of sewage sludge takes place in digestion towers within larger sewage treatment plants .
In Germany, stabilized sewage sludge from municipal wastewater treatment plants is used as nitrogen and phosphorus fertilizer in agriculture (56% - according to DWA 2005), which at the same time also improves the soil structure after strict control of the ingredients and application in accordance with the Sewage Sludge Ordinance (AbfKlärV). The remaining quantities are mainly incinerated after dewatering or drying (> 35%).