Standard High-Rate

This process is a modification of the standard rate process. The solids loading is much greater, and the sludge is mixed by gas recirculation, pumping or mechanical mixing (Figure 2(b)).

Two-stage

This method features two tanks. The first serves for digestion and is fitted with heating and mixing facilities, while the second is used for the storage and concentration of digested sludge and for the formation of a clear supernatant (Figure 2(c)).