Activated sludge

The activated sludge process relies on a dense microbial population being in mixed suspension with the wastewater under aerobic conditions. With unlimited food and oxygen, extremely high rates of microbial growth and respiration can be achieved resulting either in the utilization of the organic matter present to oxidized end products (i.e. CO₂, NO₃, SO₄ and PO₄) or the biosynthesis of new microorganisms (Section 10.2.3). The activated sludge process relies on five inter-related components: the reactor, the activated sludge itself, the aeration/mixing system, the secondary sedimentation tank and the returned sludge (Table 11.1 and Figure 11.1). However, there is an increasing interest in replacing secondary sedimentation with either an internal or an external membrane filtration unit to separate the solids from the final effluent (Section 18.9.1).