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HOW WASTEWATER IS TREATED
The Bristol Water Reclamation Facility (WPCF) is owned and operated by the City of Bristol. The City’s Facility presently treats up to 10.75 million gallons of domestic and industrial wastewater, serving 90% of the City’s approximately 61,000 residents.
The Facility is an activated sludge system, CT State permitted for treatment of an average daily flow of 10.75 million gallons per day (MGD.)However, the Facility has the ability to treat peak flows of up to 33 MGD.
Treatment consists of grit removal, primary settling, aeration, nitrification, denitrification, final settling and ultraviolet disinfection. Solids collected from the primary and secondary systems may be co-thickened in the primary clarifiers. The thickened solids are temporarily stored in a 300,000-gallon facility, until they are dewatered through a belt filter press to a concentration of 25% solids by weight. The solids are then transported to a final disposal site by a private hauler. The treated plant effluent is discharged into the Pequabuck River. Effluent flow to the river must have a monthly average of less than 30 parts per million (PPM) total suspended solids (TSS) and 25-PPM biochemical oxygen demand (BOD.)
The following describes the specific processes used to reclaim wastewater in the Bristol Water Pollution Control Facility:
PRIMARY TREATMENT (Click for Schematic)
SCREENING
The inlet channel to the WPCF is equipped with bar racks that automatically screen and remove debris, such as sticks, rocks, rags, paper, etc., which might damage the treatment plant equipment. These screenings are removed to containers, which are trucked to a waste-to-energy incineration facility.
GRIT REMOVAL
The screened wastewater then flows into grit removal chambers. The chambers cause the water to flow in a circular motion around the chamber, creating a vortex that accelerates the settling out of the water of heavier particles like sand, glass and other small debris.
PRIMARY SETTLING
The screened flow is measured and channeled to three circular primary settling tanks. Each tank holds one third of a million gallons. The wastewater slowly flows through the tanks allowing lighter, but still settlable material to accumulate at the bottom of the tank. These are the organic wastes or, wastewater sludge.
Floatable materials like grease and oils are skimmed off the surface of the tank.
Each tank is equipped with a radial skimming mechanism and sludge rake, which removes floatable greases and scrapes sludge and any remaining grit from the bottom of the tank. The skimmings are pumped to a dewater device and then transported for final disposal. Sludge is pumped to a storage tank. It is held until dewatering through a belt filter press to a solids concentration of 25% solids. Solids are then transported to a final disposal site by a private hauler.
Until the 1940’s, this would have been the extent of the wastewater treatment process, providing a 30% reduction in the strength of the wastewater.
SECONDARY TREATMENT (Click for Schematic)
AERATION BASINS
The flow from the Primary Clarifiers is then mixed with liquid containing an abundance of microorganisms produced in the subsequent treatment process. The combined flow is called Mixed Liquor Suspended Solids, and is pumped up to the aerated tanks. The organisms returned at this step are used to “re-seed” the bacterial population.
In the aeration tanks, the suspended waste particles become the food source for the microorganisms. Air is blown into the tanks to mix the material, prevent settling and provide oxygen for the organisms to respire or breathe. The microorganism produce a substance that causes the small waste particle to come together into larger and heavier particles, called “Floc,” which will separate and settle out of the treated water in subsequent settling tanks.
After aeration, the mixed liquor flows over effluent weirs into a collection channel where it flows to a distribution chamber and then to one of three final settling tanks, known as Secondary Clarifiers.
SECONDARY CLARIFIERS
The aeration tank effluent flows to one of three 110 foot diameter circular secondary sedimentation basins allowing the bacterial floc particles formed in the aeration tanks to settle out of the flow. Each tank is equipped with a radial skimming mechanism and bottom rake. The skimmer removes floatable biological residue and the rake scrapes bacterial floc particles (known as “activated sludge”) from the bottom of the tank. Treated effluent flows from the clarifier over effluent weirs, through a flow meter, and on to the ultraviolet disinfection units. At this point approximately 95% of the contaminants have been removed from the water.
A measured portion of the activated sludge is removed from the floor of the secondary clarifiers and is returned to re-seed the aeration tank bacteriologic population. This material is called “Return Activated Sludge” or RAS. The remaining excess sludge in the Secondary Clarifier, known as “Waste Activated Sludge” or WAS, is pumped from the floor of the secondary clarifiers back to the primary clarifiers, where it co-settles with the primary sludge.
ULTRAVIOLET DISINFECTION
Effluent from the secondary clarifiers flows to the Facility’s ultraviolet disinfection system. The system allows the treated water to flow past numerous lamps that produce ultraviolet light, which disinfects the majority of any microorganisms that may not have been removed during the treatment process.
The water can now be discharged. The reclaimed water is then conveyed through a 48” diameter pipe to a post-aeration facility approximately 1 mile away from the Plant. The post-aeration facility consists of a mechanical aerator and a series of six large concrete steps that the water cascades over, freshening and dissolving additional oxygen into the water before flowing into the Pequabuck River.
FACILITY ANALYTICAL QUALITY CONTROL LABORATORY
The laboratory is responsible for the collection and analysis of samples taken from all aspects of the treatment process. The results provide Operations and Management with the information necessary to determine the effectiveness of the treatment process, and the process control strategy. Data provided by the laboratory is also necessary for compliance monitoring and reporting required by the State and Federal permitting agencies. Supervising the Laboratory is a Connecticut State Licensed Environmental Laboratory Director. The Laboratory has an established EPA-approved Quality Assurance / Quality Control Program. Analyses include Biochemical Oxygen Demand; Total Suspended, Volatile, and Settleable Solids; pH; Conductivity; Ammonia; Chemical Oxygen Demand; Fecal Coliform; and digital imaging, microscopic examination of the activated sludge.
Click here for additional information on the micro organisms cultivated to treat municipal waste water.
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