WEST BAY COUNTY REGIONAL WASTEWATER TREATMENT PLANT
Treatment Process
The West Bay County Regional Wastewater Treatment Plant, also known as Bay County Wastewater Treatment Plant (Bay County WWTP), is a secondary treatment facility utilizing the activated sludge process to clean the water. All water receives treatment and is recycled by discharging into the Saginaw River.
As an introduction to the detailed unit process descriptions, the following is a description of the treatment system listed in the following categories:
Preliminary Treatment – Equalization Basins – Primary Treatment
Secondary Treatment – Post treatment – Solids Handling
Preliminary Treatment
Preliminary treatment at the Bay County WWTP includes three separate physical processes. All wastewater entering the plant first flows through one of the two screening channels and a mechanical operated coarse screening system. This system removes coarse solids contained in the wastewater. The wastewater then flows into the wet-well and is pumped up (40 feet) by one of seven influent pumps to the grit chambers. As the wastewater flows through the grit chambers (one of two units) the velocity of the wastewater flow is slowed down and air is added to aid in the physical process of settling the inorganic material (sand, coffee grounds, egg shells etc.) to the bottom of the chamber. The wastewater then flows through the fine screen systems (one of two) which removes the smaller solid material from the wastewater. All material removed through the three preliminary processes are collected mechanically and transported to the landfill for disposal.
Equalization/Storage Basins
The Bay County WWTP has the total wastewater storage capacity of over 53 Million Gallons (MG). In 2010 the facility added a 50 MG storage pond. Excess wastewater flow that cannot be process immediately can be diverted to the 50 MG storage pond until the Bay County WWTP can process the water. Typically, this pond is utilized during wet weather events. All wastewater that is stored in the pond is brought back into the headworks of the plant for processing. In addition to the storage pond the Bay County WWTP has a 3.25 MG storage basin. Water can be diverted to this basin either before preliminary treatment or after such treatment process providing the Bay County WWTP flexibility to divert flows during adverse condition. Wastewater stored in this basin can be returned to the headworks or primary treatment for processing.
Primary Treatment
After preliminary treatment all wastewater flows through one of 6 primary tanks. The flow velocity through the tanks is reduced to allow suspended solids which are heavier than water to settle to the bottom of the tanks forming a sludge blanket. Floatable solids float to the water surface forming a scum layer. The sludge that settles to the bottom of the tanks is gathered by sludge collectors and pumped to the Digestion system for stabilization and recycling or disposal. The scum that forms on the surface of the water is removed mechanically, collected and transported to the landfill for disposal.
Secondary Treatment
After the wastewater has gone through primary treatment the water flows into the settled sewage wet-well where it is pumped into the aeration tanks and receives secondary treatment. The secondary treatment process is a biological treatment system that uses micro-organisms to clean the water. The aeration tanks (6) are where the micro-organisms come in contact with the pollutants in the water. During this process the micro-organisms reproduce and clean the water by utilizing the pollutants for their food source and creates a mixture of solids and wastewater referred to as Mix Liquor Suspended Solids (MLSS) . After the water flows through the aeration tanks it flows into the final clarifiers (4) where the MLSS settles to the bottom of the clarifiers forming a sludge blanket. This sludge contains high concentrations of micro-organisms that is returned back into the aeration tanks. A portion of the sludge flowing back into the aeration tanks is removed and pumped to the solids handling system where it is stabilized and processed for recycling or disposal. The clean water that separates from the sludge in the final clarifiers, overflows the weirs of the clarifiers and receives disinfection treatment before the clean water is recycled back into the Saginaw River.
Post Treatment
After the cleaned water overflows the final clarifier weirs the water flows into the chlorine contact chambers. This final water treatment process involves injecting chlorine gas into the water and allowing contact time to disinfect all water before being discharged. Prior to being recycled to the Saginaw River all water is de-chlorinate using the chemical sodium bi-sulfite to ensure the chlorine residual is measured at less than .038 mg/l. All recycle water is sampled and analyzed daily verifying such cleaned water meet or exceeds National Pollutant Discharge Elimination System (NPDES) permit standards. Of the Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS) pollutants received at the Bay County WWTP 97% – 99% are removed.
Solids Handling
Solids removed during the wastewater treatment process are either recycled to farm field to utilize the fertilizer and organic values or disposed of at the landfill. Any of the sludge removed during primary or secondary treatment are treated in the anaerobic digester system. This system is a biological process that stabilizes the solids by reducing the volatile organic contents of the sludge and destroying viruses that could be present. The majority of the sludges stabilized are recycled on farm land by subsurface injection of this by-product. All solids recycled are tested in accord to Environmental Protection Agency (EPA) guidelines and reported annually to the Michigan Department of Environmental Quality (MDEQ). The Bay County WWTP has over 2 million gallons storage capacity for recyclable sludge. The Bay County WWTP will also dewater and dispose of sludges in the local landfill during times of limited storage ability. This process requires Bay County WWTP staff to process the sludge through a belt press for dewatering and trucking such solids to the landfill.