Operations

Treatment plant operations

The plant utilizes an activated sludge process with the addition of ferric chloride for phosphorus removal. The effluent from the plant is disinfected with chlorine and then treated with sulfur dioxide to remove any residual chlorine prior to discharge. Biosolids are stabilized with lime for pathogen control and then land applied to agricultural ground to recycle nutrients.

Preliminary Treatment

Wastewater first enters the treatment plant at the headworks building.  The first stage of treatment utilizes an in-channel grinder to macerate any solid material in the wastewater.  If the channel grinder is out of service staff can divert flow through a bar screen to remove some solids.  The bar screen requires manual raking and transporting the debris to a dumpster.  This is strictly a backup system.  The wastewater is then pumped to the primary treatment system.

Primary Treatment

Raw wastewater is pumped to two 60-foot diameter primary clarifiers after preliminary treatment.  The primary clarifiers main purpose is to remove settleable and floatable solids from the wastewater.   Settled solids are removed from the bottom of the clarifier by a sludge pump and stored for further biosolids treatment.  Floatable solids are removed by a skimmer arm and deposited in a pit for later pumping.  Approximately 50% of solids and BOD have been removed at this point.  The primary effluent flows from the primary clarifiers to the aeration cells to begin secondary treatment.

Secondary Treatment

Secondary treatment starts in the aeration cell.  Primary effluent mixes with activated sludge and air (via centrifugal blowers) to promote the biological treatment of the wastewater.  Microorganisms present in wastewater consume phosphorus and ammonia, nutrients that are harmful to the receiving waters.  The addition of ferric chloride at the end of the aeration cell contributes to the removal of phosphorus as well.  After leaving the aeration cell the partially treated wastewater flows to two 60-foot diameter secondary clarifiers.  These clarifiers operate in the same fashion as the primary clarifiers, with the exception of how the settled solids are handled.  The settled solids in the secondary clarifiers, known as activated sludge, contain the microorganisms vital to the biological treatment process.  This activated sludge is returned to the aeration cell to blend with the primary effluent.  The effluent from the secondary clarifiers is treated with chlorine gas for disinfection and then sulfur dioxide gas to remove any residual chlorine.  After disinfection and de-chlorination, the fully treated water is discharged into the Kalamazoo River.

Biosolids

Primary sludge is pumped to a holding tank to await further processing.  Once or twice per week primary sludge is thickened with the addition of an anionic polymer which binds the solids and allows excess water to be return to the headworks to be treated along with raw wastewater.  The thickened sludge is then stabilized by the addition of lime.  The lime raises the pH above 12 to neutralize pathogens.  The sludge, now called Class B Biosolids, is pumped to long-term storage to await land-application on farm fields.  The biosolids are utilized by farmers to fertilize their land.  To learn more about biosolids and how they can be safely utilized on agricultural land, click this link to the State of Michigan Biosolids web page.

collection system operations

The City of Marshall operates and maintains approximately 50 miles of sanitary sewer.  The collection systems conveys wastewater from the residential, commercial, and industrial areas of town to the wastewater plant.  There are also 10 wastewater lift stations located throughout the City and four at Lyon Lake.  Wastewater Staff operates and maintains the 14 lift stations and the Department of Public Works crew maintains the sanitary sewer system.