Utilities Division The City of Wooster utilities division extends a commitment to excellence in public service through the proper treatment, protection, and preservation of the cities water resources.
Secondary Treatment
After passing through the primary clarifiers, the settled sewage goes into the four aeration tanks. These tanks have a network of plastic tubes on the bottom with perforated membranes covering the openings. The picture shows a tank which has been emptied for maintenance. Normally, the tank is full of settled sewage. Air is pumped into these pipes and bubbles out of the membranes. This aerates the sewage, causing rapid bacterial growth. The bacteria break down the sewage. Sludge from the final clarifiers is brought back to the center return tank, where some of it is distributed to the aeration tanks, while the rest is pumped to the head (beginning) of the plant.
Under normal flow conditions, the sewage next goes into the two older final clarifiers. These work just like the primary clarifiers. The solids settled and are collected by an automatic scraper and pumped to the return tank, while the final effluent is pumped into the ammonia towers. During high flow conditions, the sewage goes from the aeration tanks into all four final clarifiers.
The final effluent is pumped to the tops of the two ammonia towers. It is distributed by rotating arm assemblies onto corrugated plastic media. The final effluent picks up oxygen as it flows down the towers. Bacteria on the media break down the ammonia into nitrates and further purify the final effluent. At this point, the final effluent is either sent to the two newer final clarifiers to remove bacteria from the towers, or it is sent to the chlorine contact tank. The removal of ammonia and postaeration of the final effluent is considered to be advanced secondary treatment. Some sewage plants have tertiary treatment, which may include chemical treatment to remove phophorus, and sand filters for addtional solids removal.
