It contains a high concentration of organics and is rich in nitrogen and phosphorus. It must be returned to the wastewater treatment cycle at a point where it will have the least effect. In this system, waste sludge is subjected to extended aeration for a period of 12 to 25 days. During this time, the microorganisms in the sludge break down the organic material. As the supply of nutrients in the sludge decreases, the organisms consume their own cell material and reduce the organic content even further.
Aerobic digestion is accomplished in one or more tanks mixed by diffused aeration.
Multi-stage systems use the first tanks to accomplish most of the digestion. The last stage is used for solids concentration. A supernatant is produced during this step and must be recycled. The major advantages to aerobic sludge digestion include the production of a more stable sludge, fewer odor problems, and no safety problems related to the production of methane and carbon dioxide. However, aerobic digestion produces a sludge which is difficult to thicken, resulting in disposal problems.