Sheboygan Regional Wastewater Treatment Facility

        Solids Handling


Sludge Digestion (Stabilization)

Primary sludge (with the waste activated sludge) is pumped to three heated primary anaerobic (without oxygen) digesters. In the primary digester, the sludge is stabilized by anaerobic bacteria that convert the sludge to methane gas (65%), carbon dioxide gas (30%), hydrogen sulfide gas (5%), water and inert organic matter. After the sludge is stabilized it is transferred to one secondary anaerobic digester for final stabilization. The primary and secondary digesters have a total volume of 4,855,000 gallons with a detention time of 30 days. While the sludge is in the primary digesters, the sludge is heated and mixed to create the appropriate environment for the anaerobic bacteria to stabilize the sludge. The methane gas produced is used as a fuel to heat the digesters and to fuel 10 30 kW Capstone Micro-turbines that produce 2,300 megawatts of electricity annually. Excess methane gas can be used to fuel a caterpillar engine that powers one influent lift pump.

Sludge Dewatering/Handling

Digested sludge is thickened from 2.5% to 6.0% solids using 2 Meter and 3 Meter Gravity Thickener Belts to reduce the sludge volume. The thickened liquid sludge is transferred to two 2,000,000-gallon Aquastore tanks that were added to the plant in 1996. The liquid biosolids are stored in (2) 2MG Aquastores until it can be dewatered/dried or applied on farmland located throughout Sheboygan County. The two sludge holding tanks provide 180 days of storage capacity so the sludge can be injected into the soil during spring, summer and fall. The liquid sludge is used as fertilizer on agricultural land and saves the farmer approximately $250.00 per acre in fertilizer cost.