How Your Treatment Plant Works
1. INTAKE
The untreated water from Lake Cochichewick flows by gravity from the intake structure in the lake directly into the Low Lift Pump Station.~ The water passes through a traveling water screen, which removes any floating debris, and into the wetwell.~ The water is pumped from the wetwell to the plant for processing.
2. OZONATION
The water from the Low Lift Pump Station is pumped into the ozone contact tank.~ Ozone gas is generated on site and is bubbled through the water in the contact tank.~ Ozone is a strong oxidant and disinfectant.~ The ozone disinfects the raw water prior to the remaining treatment processes in the plant.~ The use of ozone has also been found effective in iron and manganese precipitation, taste and odor control, color removal, and flocculent aid potential.
3. RAPID MIX
From the ozone contact tank the water enters the plant's two double stage rapid mix basins.~Various chemicals are added at this location in the plant to assist in the removal of impurities from the water.~ Mechanical mixers uniformly distribute the chemicals throughout the raw water.~ Potassium hydroxide is added to adjust the pH of the water prior to the water entering the rapid mix basins. Alum and polymer are added in the basins.
4. FLOCCULATION
From the rapid mix basins, the water flows to four double stage flocculation basins where the water and chemicals are gently agitated, allowing the particles that are present to cluster together and trap impurities.
5. SEDIMENTATION
The flocculated water enters the four sedimentation basins where the solids settle to the bottom, forming a thin layer of solid particles.~ The settled solids are periodically removed from the bottom of each basin by a moving siphon and transferred by gravity to a storage tank.~ The siphon system contains no moving parts in the water, and therefore does not agitate the solids, providing for a more effective and efficient means of the solids removal.
6. FILTRATION
The water then flows to eight deep bed dual granular activated carbon and sand filters that remove any remaining impurities.~ The water flows down through the filter media, which trap and adsorb remaining impurities.~ To complete purification, sodium hypochlorite is added for disinfection to the filter effluent.
7.~ STORAGE AND DISTRIBUTION
The filtered or "finished" water is stored in a 620,000 gallon clearwell, located under the plant.~ From there it is pumped into North Andover's water distribution system.~ The plant is equipped with four finish water pumps, with a pumping capacity of 12 million gallons per day.
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