In an SBR process, each cycle starts with the reactor nearly empty except for a layer of acclimated sludge on the bottom. The reactor is then filled up with the wastewater and the aeration and agitation are started. The biological degradation process begins during the filling step and proceeds, once the reactor has been filled up, until a satisfactory level of degradation of the pollutant is achieved. Then the aeration and agitation are stopped, and the sludge begins to settle. Depending on the time allowed for the sedimentation, anaerobic reaction can occur, which may reduce the organic content of the sludge. Once the sludge has settled, the clear top layer of treated wastewater is discharged and a new cycle can hegin. Anaermhic sludge digestion may also he included as one of the steps in the cycle