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.
Sludge
After the raw sludge has settled in the primary clarifiers, it is pumped to the sludge thickener or to the primary digesters.
Sludge from the primary clarifiers can be pumped into it, along with excess sludge from the return aeration tank. The thickener works just like a primary or final clarifier, except that it has a lid to cut down on odors. A blower vents the thickener dome, sending the exhaust air to the inlet for the aeration tanks blowers. An automatic scraper collects the settled sludge, which is then pumped to the thickener or to the three primary digesters. The thickener is intended to be an aerobic (with oxygen) process.
Our sludge digesters are anaerobic, which means without oxygen. The primary digesters have fixed covers. Heat from boilers which are fueled by either methane produced by the digesters themselves, or by natural gas, is used to raise the sludge temperature to about 100-105o F. As the sludge breaks down, it produces water, methane, and carbon dioxide. The water (supernatant) floats to the top and is sent to the return tank (or thickener when in use).
The sludge is then sent to the two secondary digesters. These have floating lids, which are held up by the pressure of the gas inside. Excess gas that is not burned in the boilers is burned at one of two burners. The sludge continues to break down, and excess water again goes to the return tank. The digested sludge is then pumped to the sludge lagoons.
The new sludge tank, which is not currently in use, is designed to mix sewage digested sludge from the secondary digesters with lime sludge from the new water plant. This mixture is thicker than sewage sludge, has a higher pH, and contains calcium, all advantages for the next process, land application. The mixed sludge will go into the sludge lagoons.
As the lagoon sludge settles, the supernatant which separates out is drawn off and sent back to the head of the plant. A sludge hauling company then pumps out the sludge, and applies it the the fields of farmers who wish to use it. In this manner, it help to fertilize the fields. Because we can not remove toxic metals from the sewage, it is important that the industries which have these metals in their waste are regulated and monitored.
