Preliminary Treatment begins with the collection and pumping of raw sewage. This happens at the Influent Pumping Station where the City of Manchester accepts wastewater from Manchester, Bedford, Goffstown, and Londonderry.
The influent (wastewater) enters the pump station, which is 90 feet below the ground. Here, raw sewage passes through one of five channels. Three channels have mechanical bar racks for coarse material removal where rags, sticks and large objects that flow with the sewage are removed through screening.
From here the wastewater flows to the sewage pumps that are run by 450 hp motors. These pumps can each move 33 million gallons of sewage daily. The sewage is then pumped up to the grit channels.
The grit channel is a rectangular chamber that slows the speed of flow and allows inorganic materials (minerals, sand, gravel, egg shells, glass, coffee grounds, etc.) to fall to the bottom of the chamber. The flow is still fast enough to keep organic materials (decayed food, shredded toilet tissue, toilet wastes, sugars, fats, and grease) in suspension and assures it floats to the next process which is primary treatment. Grit buckets are connected to chains that run along the bottom of the chamber and scoop the grit from the bottom of the tank.
Two conveyor belts move the grit that is dumped from the grit buckets to a container that is hauled away by a truck to a secure landfill. The treatment plant has four (4) chambers. Each chamber is 70 feet x 10 feet x 10 feet; 7,000 cubic feet each or 52,360 gallons.
The grit is removed from the wastewater to protect pumps from abrasion and wear. Grit removal also reduces the clogging in the pipes to the primary clarifiers.