WASTEWATER COLLECTION AND TREATMENT
HISTORICAL PERSPECTIVE
Manchester constructed a sewer collection system between 1912 and 1914 to service the densely populated center of town. This original system operated by gravity flow to a pump station along Manchester Harbor behind the current hardware store. The sewage was pumped out into the ocean (probably without any treatment) via a one and one half mile cast iron outfall pipe. The gravity system continued to grow gradually. As new subdivisions were constructed during the 1960’s, it was necessary to install sewer lift stations to pump sewage from low lying areas into the gravity sewer collection system.
Manchester’s first secondary treatment plant went on line in 1971 in the same location as the original pump station. This treatment facility had problems accommodating the fluctuating sewer flows resulting from infiltration/inflow (clean water entering the sewer system from rainfall or groundwater) and was often in violation of the EPA discharge permit.
A new outfall sewer was constructed in the early 1990’s and roughly followed the route of the original outfall. The outfall pipe discharges in approximately 35 feet of water that studies have proven to provide complete dispersion. The complete dispersion is not affected by wind direction of tidal flow.
SEWAGE TREATMENT
Manchester underwent a major upgrading and expansion of its wastewater treatment plant from 1997 through 1999. The plant operates under the following EPA permitted flow limits:
1.20 million gallons per day monthly average December through May
0.67 million gallons per day monthly average June through November
0.67 million gallons per day annual average (Ocean Sanctuaries Act Limit).
Raw Sewage enters the headworks building, passes through a “Muffin Monster” grinder and a grit removal cyclone before entering the influent wet well. Grit is separated from the raw sewage and is dewatered by a grit screw and accumulated for disposal. The raw sewage is pumped from the influent wet well to the aeration tanks where the biological treatment occurs. Sewage exiting the aeration tanks flows to two 48 foot diameter clarifiers where the sludge is settled out. Effluent from the clarifiers is chlorinated and discharged through the outfall pipe into outer Manchester Harbor.
Waste sludge from the clarifiers is accumulated in waste sludge holding tanks until sufficient volume is present to warrant operating the rotary drum thickener. The rotary drum thickener increases the percent solids of the sludge to approximately five (5) percent by removing excess water from the sludge. The thickened sludge is stored in a tank and then pumped into a tanker truck and transported to Upper Blackstone Water Pollution Abatement Facility in Millbury, MA for additional treatment.
The plant includes an odor control system that removes odors from the headworks building, the sludge holding tanks and the rotary drum thickener room using a negative pressure fan system. The odorous air is piped into a biofilter which uses a mixture of organic materials and biological action to remove the odors.
The upgraded plant was designed to deal with the flow fluctuations we have historically observed. The plant is designed for an average daily flow of 1.2 million gallons, a maximum daily flow of 3.0 million gallons and an instantaneous flow of 5.0 million gallons.
Although the plant is designed to treat 1.2 million gallons per day, the plant operates under an Ocean Sanctuaries Limit of 0.67 million gallons per day annual average which effectively limits expansion of the sewer collection system.
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