- Sludge accumulated at the plant during the primary and secondary sedimentation processes contains a high concentration of organic material that must be stabilized in order to be used as a raw material for organic fertilizer.
- The sludge is stabilized with the use of eight closed digester tanks.
- These tanks operate at a mesophilic temperature, i.e., a fixed temperature of about 35°C is maintained.
- Sludge stabilization takes place in an anaerobic biological process, i.e., the organic material is decomposed with microbes in the absence of oxygen.
- The anaerobic decomposition process is two-stage – different microbes are used at each stage during which methane and carbon dioxide are released. The sludge remains in the digester tanks for about two weeks.
- The digested sludge leaving the digester tanks contains water produced during the biological decomposition processes and is therefore diluted and contains only 2-3% solids.
- The digested sludge is transferred to the dewatering machines in order to reduce its volume and facilitate conveyance of the sludge for further treatment at composting sites outside of the plant.