Here, peak or higher flows are held to supplement low flow periods such as midnight to early morning. This allows for remaining treatment steps to be smooth and continuous for higher quality treatment at less cost. At this stage the wastewater still contains dissolved organic materials. Bacteria growth is artificially stimulated through injection of air, encouraging consumption of the dissolved wastes. A combination of biological and chemical processes remove ammonia, phosphates and additional solids.
Aluminum sulfate is added to help the solids bind together for later removal.
Bacteria again assist in the process by helping remove ammonia and other forms of nitrogen. The final step in advanced secondary treatment involves a natural settling of solids, in secondary clarifiers. The secondary solids, which are much more liquid than primary solids, are removed and pumped to sludge thickeners which remove excess water prior to continuing on to the digesters. In this final stage, the wastewater passes through coal filters to remove any remaining solids. Following filtration, the final step is to expose the water to ultraviolet light.