Sludge accumulated in the facility 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.
Sludge stabilization is performed with the use of eight closed digester basins with a floating roof. This floating roof precludes the presence of air above the sludge and makes the stabilization process more efficient.
These basins operate at a mesophilic temperature, i.e., a fixed temperature of about 35°C is maintained.
The sludge stabilization method is an anaerobic biological process, i.e., decomposition of the organic material is accomplished 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 basins for about two weeks.
The digested sludge leaving the digester basins 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 thickening machines in order to reduce its volume and facilitate conveyance of the sludge for further treatment at sites located some distance from the treatment facility.