City of Saginaw MI -
This part of the treatment process starts with the sewage passing through a fine bar screen (1/4 inch spacing), which removes sticks, leaves and other debris. After screening, the water flows through grit collectors, where centrifugal force is used to separate the heavy solids (grit) from the wastewater. Wastewater and the lighter organic solids continue on to the primary settling tanks. The grit is conveyed into a roll-off container and hauled to a landfill. Screenings collected are also put in this roll-off for disposal.
After screening and grit removal, the sewage is routed to six primary settling tanks, each with a capacity of 770,000 gallons. Floating materials are skimmed off the top by rotating skim arms, while the heavier residual solids, settle to the tank bottoms. The settled residuals are pumped out of the tanks, mixed with lime for stabilization and stored for application to farm land as a fertilizer and soil conditioner. From the primary settling tanks, the wastewater flows to the aeration tanks.
The second stage of the wastewater treatment is a biological process. This process takes place in the aeration tanks. There are four aeration tanks, each with a capacity of 1.5 million gallons. Three are normally used and the fourth is used during high flows.
In the aeration basins conditions are optimized to promote microbiological growth through the addition of oxygen and food (wastewater). The microorganisms metabolize the dissolved organics in the water and convert them into cell mass and a floc which can be separated from the water by settling.
From the aeration tanks, the water and floc flow equally to six final settling tanks. Each of the final tanks has a capacity of 1.27 million gallons. In these tanks the solids (floc) in the water settle to the bottom and are then pumped back to the beginning of each aeration tank to maintain a supply of microorganisms to continue the biological portion of the wastewater treatment process.
The clean water then flows from the final settling tanks to two chlorine contact tanks for disinfection. The tanks have a combined capacity of 800,000. Chlorine is added at the beginning of the two tanks to kill harmful organisms left in the treated water.
Any residual chlorine that may be left is removed by adding sulfur dioxide to neutralize it before the water is discharged to the river.
The residuals pumped from the bottom of the primary settling tanks are mixed with liquid lime to stabilize them. Lime raises the pH of the residuals to 12 in order to kill pathogenic (disease causing) organisms. The stabilized residuals, now called biosolids, are pumped to one of five individual compartments of a 12 million-gallon underground storage tank. The biosolids are stored in this tank from December through April when the ground is either frozen or too wet for subsurface soil injection.
In the spring, through the summer and into the fall, the biosolids are hauled to farm fields in tank trucks, where an applicator injects the material below the soil surface. Biosolids provide beneficial nutrients to promote crop growth (fertilizer) and act as a conditioner for poor soils.
Learn more at: the Water Environment Federation
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