Treatment Process
PRIMARY TREATMENT
This part of the treatment process starts with the sewage passing through a fine bar screen ( one inch spacing), which removes sticks, leaves and other debris. After screening, the water flows through grit collectors, where the velocity is slow enough to allow heavy solids (grit) to settle out, but still fast enough to carry organic solids through to the primary settling tanks. The grit is removed from each collector channel by a series of buckets attached to chains traveling along the bottom of the channels and lifting the grit up to an overhead hopper. From the hoppers, 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 four primary settling tanks, each with a capacity of 770,000 gallons. Floating materials are skimmed off the top by rotating skim arms and diverted to skimpits, while the heavier remaining solids (sludge) settle to the tank bottoms. The rotating arms also divert the settled sludge across the sloping bottoms of the tanks to a drawoff pit near the center of each tank. This pit is the point where the sludge is drawn out of the tanks, mixed with lime for stabilization and stored for future application to farm land. When the heaviest materials have settled out, the wastewater flow then is collected from the primary tanks and sent to the aeration tanks.
SECONDARY TREATMENT
There are four aeration tanks, each with a capacity of 1.5 million gallons. Three are used normally and the fourth is used during high flows, normally during rain events. The aeration tanks are where most of the secondary treatment takes place.
Secondary treatment is a biological process by which air (dissolved oxygen) is induced into the wastewater to stimulate the growth of bacteria and other organisms. Oxygen is needed for the organisms to consume most of the waste materials present in the wastewater. Five mechanical aerators per tank lift the water and splash it against a backplate. This process duplicates the same natural process that occurs in lakes, rivers and streams.
From the aeration tanks, the flow moves through a transfer channel, dividing it equally to five of the six final settling tanks (one is rotated out of service to allow for tank maintenance and cleaning). Each of the final tanks has a capacity of 1.27 million gallons and a detention time of 5½ hours at average flow rates. In these tanks the solids in the water (mixed liquor) settle to the bottom and are then pumped back to the beginning of each aeration tank to maintain a supply of organisms (bugs) to continue the process of waste removal.
The water then overflows the final settling tanks and flows through an underground pipe to two chlorine contact tanks for disinfection. The tanks have a combined capacity of 800,000 gallons and a normal detention time of 45 minutes. We add chlorine at the beginning of the two tanks at an approximate rate of 1.00 mg/L (250 pounds per day).
As the water passes through the tanks and around the baffles, the chlorine kills the harmful organisms. Any residual chlorine that may be left is removed by adding sulfur dioxide at the discharge end of the tanks before the water is allowed to reach the river. Although chlorine is very beneficial, the dechlorination process is needed because any excess can and may become harmful if it were allowed to reach the rivers' ecosystem.
BIOSOLIDS DISPOSAL
The heavier solids that settle to the bottoms of the primary settling tanks are known as primary sludge. The primary sludge is pumped from the primary settling tanks to one of two 500,000 gallon capacity above-ground storage tanks. During this pumping, liquid lime is added to stabilize the sludge and raise the pH above 12 in order to kill pathogenic organisms. The stabilized sludge, now called biosolids, is then 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 soil injection. As the biosolids further settle in this tank, some excess water (supernatant) can be decanted off the individual compartments, raising the solids content of the product.
When field conditions allow, the biosolids are hauled to farm fields in tank trucks, where an applicator injects the material below the soil surface.