Water Pollution Control Facility Plant Description
The Water Pollution Control Facility (WPCF) is a conventional secondary treatment waste activated sludge facility and is designed to treat sewage. It has the following characteristics:
Average Flow=1.83 million gallons per day (mgd)
Biochemical Oxygen Demand (BOD)= 7,440 pounds per day
Total suspended Solids (TSS)= 6,000 pounds per day
BOD = the quantity of Oxygen utilized in the biochemical oxidation of organic matter under standard laboratory procedures.
TSS = the total suspended matter that floats on the surface of, or is suspended in, water, wastewater or other liquids, and which is removable by laboratory filtering.
The NPDES permit requires that the facility maintain a minimum of 85 % removal of both BOD and TSS. Historical data show a removal efficiency of greater than 90%.
The WPCF services all sewered residences,commercial and industrial businesses and public buildings in each Montague village except Millers Falls. The WPCF also services the Riverside section of Gill, and accepts septage from all towns in Franklin County. Approximately 10% of the collection system remains a combined sewer system, receiving flows from catch basins as well as receiving sanitary sewerage. Treatment is ongoing 24 hours per day, 365 days per year. In addition WPCF staff operate and maintain 8 remote pump stations, 5 grinder pump stations, one diversion structure/combined sewer overflow, the Millers Falls flume and canal crossing heaters.
The production of an effluent which meets the criteria of the NPDES Permit entails several processes necessary to separate water from the waste via settling, biological treatment, preliminary, primary, secondary treatment and chlorination:
X. Preliminary treatment takes place in the facility headworks. This is an enclosed area where large materials such as sticks, rags, stones and grit are removed from the raw waste stream. These materials would interfere with other treatment processes and so are the first to be removed. The equipment used for preliminary treatment is the bar screen, aerated grit chamber and grit washer. Only a small fraction of the raw waste is removed in this process.
XI. Primary treatment process entails the physical separation of wastes from the water due to some material being heavier or lighter than the water carrying it. The equipment used is called primary clarifiers that are located outside. The waste stream following preliminary treatment flow to these two rectangular clarifiers where the flow slows due to the volume of the primary clarifier tanks. Slowing the flow allows certain wastes to separate from the flow, to be collected and removed from the system and needs to be disposed of. The material so removed is called primary sludge. At this point in the treatment process only 30 to 60 percent of the raw waste has been removed, not enough to meet the discharge limits of the facility, minimum 85% removal. The waste stream following primary clarification is now called primary effluent. Due to the location of the facility and the topography of the land it is necessary that the primary effluent be lifted to a height from which it can then flow by gravity through the remainder of the process to the Connecticut River. This lifting or pumping is accomplished through the use of a system of two large internal-lift screw pumps sized to meet facility flow capacity of 1.83 mgd and a sustained and instantaneous peak flow of 4.65mgd.
XII. Secondary treatment involves a biological process, which converts non-settleable or floatable waste remaining untreated by the primary process to a material that will settle in circular outdoor concrete tanks called secondary clarifiers. Once the primary effluent has been lifted and discharged into a wet well it continues on to the secondary treatment system. This step of treatment involves the use of single celled organisms and bacteria in an environment allowing the organisms to come into contact with the primary effluent stream. Treatment occurs in the aeration tanks so named due to the air that is continually diffused into the tanks. The air is required by the organisms for respiration and also acts as a mixing mechanism. The aeration tanks are large having a volume of 230,000 gallons for each of the two tanks. The material and organisms produced in the aeration tanks is termed mixed liquor suspended solids (MLSS). Immediately following aeration the MLSS flows to two circular secondary clarifiers, 176,000 gallons each, where the flow is slowed and the MLSS allowed to settle. The substance, waste activated sludge or biosolids, which is separated from the carrying waters, is then mechanically removed and needs to be disposed of. A portion of the waste activated sludge is returned to mix at the head of the aeration tanks to begin the biological process of secondary treatment again. The carrying waters now termed secondary effluent meet the required 85% removal criteria for discharge to the Connecticut River and yet during a number of months must be processed through one more step, chlorination.
XIII. Chlorination involves disinfection of the facility effluent prior to discharge to the river. The clear water overflowing from the secondary clarifiers passes to the chlorine contact tanks. For the months of April through October the NPDES permit requires that we treat the effluent to reduce the presence of pathogenic or disease causing organisms. This is accomplished by introducing chlorine to the secondary effluent just prior to entering the 2 chlorine contact tanks, 25,000 gallons per tank. The volume of the tanks provide for a detention time to allow the chlorine to accomplish the pathogen reduction. It is after this is accomplished and the effluent overflows the chlorine contact tanks that the Connecticut River receives flow from the Montague WPCF.
From both the primary and secondary steps, a biosolids is produced that is removed from the facility. After the biosolids are drawn from both the processes it is discharged into a circular gravity thickener, 37,700 gallons. In this tank the sludge is thickened from an estimated 1% solids to as high as a 4.5% solids. This accomplishes a reduction in the volume of liquid that contains the biosolids. This is then put into Sludge hold tanks and then to the rotary press for more dewatering. A sludge cake is produced which varies between 18-30% solids. The sludge is either shipped out or composted for reuse.