WEST BAY COUNTY REGIONAL WASTEWATER TREATMENT
The West Bay County Regional Wastewater Treatment Plant is a secondary treatment facility utilizing the activated sludge process.
As an introduction to the detailed unit process descriptions, the following is a description of the treatment system broken into the following categories:
The raw wastewater flows from the sewer system into the bar screen chamber where mechanically operated bar racks remove coarse solids contained in the wastewater. The material that is collected is transported to a landfill. The flow then continues to the raw sewage wet well. The sewage is then pumped up to the grit chamber. As the wastewater flows through the grit chambers (two available), the velocity of the wastewater is slowed enough for the organic material to settle to the bottom of the chamber. The settled grit is collected mechanically and transported to a landfill. The wastewater then flows through Channel Monsters, which shred any objects that remain in the waste stream.
The equalization basin is a dual chamber structure with a total storage capacity of approximately three million gallons. The incoming wastewater flow is regulated to the subsequent treatment systems by an overflow structure, magnetic flowmeter, butterfly valve, and equalization basin. As the wastewater flow approaches the overflow structure, the predetermined steady flow rate is allowed to flow to the primary treatment system. Excess flow is forced to the equalization basin by the automatic positioning butterfly valve. Excess water stored in the equalization basins is returned to the primary system for treatment when flow to the plant subsides.
The equalization basin is not used during dry weather periods. The equalization basin also provides a minimum of primary treatment for the entire plant flow in the event of a temporary loss of primary power. Also, in the case of a slug load in the plant influent, the flow can be diverted to the equalization basin. The diversion would prevent the slug load from reaching the secondary biological process.
As wastewater flows through the primary tanks, suspended solids, heavier than water, settle to the bottom of the tanks, forming a sludge blanket. Floatable solids float to the water surface, forming a scum layer. The sludge is gathered by collectors from the bottom of each basin and pumped to the solids holding tanks. Scum is skimmed from the surface, collected and transported to a landfill. The effluent from the primary settling tanks flow by gravity to the settled sewage wet well.
The secondary process reduces concentrations of dissolved and colloidal organic matter in the wastewater. Primary effluent is pumped from the settled sewage wet well into the aeration basins, where the wastewater is mixed with return activated sludge and aerated to maintain a dissolved oxygen concentration. During the aeration period, wastewater solids are used as food for microorganisms and much of the organic material is consumed. As a result, more activated sludge is produced. Following aeration, the activated sludge is separated from the wastewater stream. The activated sludge is allowed to settle in the final clarifiers. The wastewater, free from the settled activated sludge, flows over the final clarifier weirs and flows to the chlorine contact chamber.
A portion of the activated sludge is returned to the aeration basins to repeat the secondary process. Because more activated sludge is produced in the process, some activated sludge must be wasted. The waste activated sludge is pumped to the Dissolved Air Flotation (DAF) units. The two DAF units are located in the Bio-solids building. The DAF units thicken the sludge. The thickened sludge is pumped to the sludge holding tanks.
Effluent from the final clarifiers flows through the chlorine contact chamber(s) where chlorine gas is added for disinfection. Prior to discharge into the Saginaw River, sodium bisulfite is added to ensure that the chlorine residual is less than .036 mg/l.