Gravelly Pond has been in use as a water supply since the early 1900s. It is located in Hamilton along side Chebbacco Road. The pond has a maximum depth of around 57 feet and is approximately 49 acres in size. It has an estimated capacity of over 360,000,000 million gallons, equal to approximately twice the Town's annual usage. Over the past few years it has provided approximately 60 percent of Manchester's water.
Water from the pond is pumped out and treated by the Gravelly Pond Water Treatment Plant which was completed in May 1997. Treatment provided at the Gravelly Pond Water Treatment consists of filtration, disinfection, fluoridation, and corrosion control.
Filtration is the primary treatment measure at the facility, and it is a required treatment technique for surface water supplies such as Gravelly Pond. Filtration is used to remove particulates in water and is primarily intended to remove micro-organisms. Filtration is provided by a Trident Microfloc package treatment system. This package system is a modular, premanufactured treatment units consisting of two basic treatment components. The first component is an upflow clarifier used to remove the larger particles in the water. The second component is the filter itself which removes any remaining particulates in the water. With both components, the plant is able to reduce the turbidity of the water by a factor of ten.
Disinfection is another required treatment technique for surface water. Disinfection is an additional measure of eliminating micro-organisms which may pass through the filtration process. At the treatment plant, disinfection is accomplished by adding sodium hypochlorite, a form of chlorine, to the filtered water and then storing the water for a time to allow the chlorine to complete the disinfection. Through this process, the plant is considered to provide over 99.9 percent inactivation of certain types of micro-organisms.
Disinfectant is also added to provide a residual level of disinfectant in the water mains. A residual level of disinfectant is needed because micro-organisms are ubiquitous and can even grow within a water distribution system, and a residual level of disinfectant helps prevent the re-growth of any micro-organisms within the piping. The target level of disinfectant residual for water leaving the facility is approximately the same level as that recommended by the American Water Works Association.
Fluoridation is an optional process used as to help prevent tooth decay. Fluoridation is accomplished at the treatment plant by adding Hydrofluosilicic Acid to the filtered water.
Corrosion control consists of adding chemicals to reduce the corrosivity of the water and to prevent lead and copper from entering the drinking water. At the Gravelly Pond Treatment Plant, two chemicals are used. The first, potassium hydroxide, is used to raise the pH of the water. The second, zinc orthophosphate, is used to provide a thin coating on the pipes.
Gravelly Pond Water Treatment Chemicals
Chemical |
Use |
Dose |
Potassium Permanganate |
Oxidizing agent used to reduce iron & manganese |
0.2 - 1.5 mg/l |
Aluminum sulfate |
Coagulant aid. |
5 - 15 mg/l |
Polymer |
Coagulant aid. |
1.5 - 4.5 mg/l |
Hydrofluosilicic Acid |
Fluoridation |
3.5 - 4.5 mg/l |
Sodium Hypochlorite |
Disinfecting agent |
2 - 4 mg/l |
Potassium Hydroxide |
Strong base used to adjust the pH. |
5 - 15 mg/l |
Zinc Orthophosphate |
Corrosion inhibitor |
2 - 3 mg/l |
All of the chemicals used at the plant are certified by the National Sanitation Foundation for use in potable water systems and meet or exceed American Water Works standards. Material Safety Data Sheets for the chemicals are available from the DPW.
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