Sludge is digested aerobically in a separate concrete basin structure. Sufficient volume is provided for approximately 80 days solids retention time and almost 20 days hydraulic retention time at design waste solids production. The basin is divided into two compartments to facilitate operation for sludge thickening. Thickening is achieved by shutting off the air supply and raw sludge feed to one of the digester compartments while the second compartment continues to operate. Gravity separation will settle the sludge and allow excess water to be decanted and returned to the aeration basins for treatment. Coarse bubble diffusers are used for aeration with process air supplied by a branch line from the main aeration basin air supply header. Manual valves are used to direct sludge and process air to either compartment and to isolate a compartment for thickening. Decanting is controlled by manual valves with decant returned by gravity to the aeration basins. After suitable digestion and thickening, the stabilized sludge is drawn off to the sludge drying beds. Manually operated valves also control this operation.