IN-POND AERATION/CIRCULATION:

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The treatment process starts in Chase’s pond. In August of 2016 an aeration/circulation system was installed in the 15 acres of the pond nearest the dam to improve in-pond water quality and to attempt to prevent future algal blooms. It is these 15 acres of the pond that is thought to be the seeding/starting area for algal blooms. The aeration system consists of eight four-foot round discs, each with 100 feet of coiled plastic pipe with air release holes punched in them connected to rubber hose that runs back to an air blower system in the screen house building at the water’s edge. The eight discs sit just off the pond bottom where the air pushed to each disc is released.

There are multiple and significant benefits to the release of this air. In the area of the pond with this system installed, the air released allows increased oxygen throughout the entire water column. Maintaining an oxygen rich environment prevents poor water quality from forming under low/no oxygen conditions as well as the associated conditions that can produce algal blooms. As the air rises from the discs at the bottom of the pond toward the surface it creates an upward current that mixes the oxygen rich water (circulation) in a large area surrounding each disc. The mixing/agitation itself is thought to also discourage algal blooms by causing the break-up of fragile nuisance algae and also not allowing them to swim and maintain themselves in the areas that are most beneficial to their growth. There is also a secondary benefit to fish and other aquatic life in the pond and over time this system can continually improve overall water quality by reducing the bottom muck layer.

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STRAINING:

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Pond water is withdrawn from Chase’s Pond through a pipe located in an underwater intake structure protected by a concrete box located just off the bottom of the pond. The pond water travels through the pipe into the nearby screen house building near the dam which has large screens to physically prevent and remove large pieces of debris (leaves /sticks/ chunks) and living species (fish and amphibians) from entering and potentially clogging the treatment process. This is essentially “straining” the water as it first enters the treatment process.

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PRE-TREATMENT:

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During challenging treatment problems such as algal blooms and pond turnover events, the strained water is pre-treated to improve removal of impurities and maintain the production of the high-quality water. The amount or need for “pretreatment” varies significantly and therefore pretreatment chemicals are added to the process only where needed. The entire Direct Filtration treatment process utilizes only those chemicals which are approved for use in drinking water by NSF International. The two pretreatment techniques periodically used are; chemical oxidation and adsorption. Potassium Permanganate chemical is sometimes added to break down (oxidize) tough to remove impurities so that they might be transformed to less problematic ones or those that might be removed in the remaining treatment processes. Powdered Activated Carbon (PAC), specific types of wood ash treated to very high temperature, may be added during algal blooms to remove (adsorb) tastes and odors that can produce unpleasant water quality. Adding PAC is one of the most common techniques used to remove taste and odors because it is non-specific in adsorbing a broad range compounds that produce taste and odors. These pretreatment processes either destroy the problematic compounds or greatly improve the ability to remove them in the next steps of the treatment process.

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COAGULATION AND FLOCCULATION:

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The strained and sometimes pretreated water exits the screen house and flows by gravity through a thirty-inch underground ductile iron pipe to the Josiah Chase Water Filtration Plant (JCWFP). In order to remove impurities in pond water it must be destabilized by adding a varied but adequate amount of chemical coagulant. Coagulants destabilize the water so that impurities and particle solids present might be removed by filtration. This process is called “coagulation”. Aluminum Sulfate coagulant, commonly known as Alum, is utilized to destabilize and start removal of particles and impurities. Many times, a low dose of a second coagulant chemical polymer is needed to jump start the destabilization process. This secondary coagulant is particularly needed in cold water where chemical reactions are naturally slowed. These coagulants are added under optimum treatment conditions, such as in a specific pH range, to ensure best treatment. To get to and maintain the optimum pH range, sodium hydroxide chemical is added. The destabilized water particles being formed must be brought to a large enough size so that they can be removed by the next processes. This is done by agitating the water with passage through a cork-screw like pipe called a static mixer. When the particles bump into each other during mixing they grow larger in size so that they might be efficiently removed. This growth in size is known as “flocculation” and the particles solids formed known as “floc”. With formation and increase in size of these particles, most barely visible to the human eye, the water becomes cloudy or turbid.

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CLARIFICATION:

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The larger floc particles or solids formed are then removed (water is clarified) in a process called “clarification”. There are numerous techniques to clarify water before filtration. The District’s treatment process uses upflow clarifier technology (2) which act as large particle filters, catching the vast majority of larger particles formed as the water flows upward through about five feet of this filter material. The material is pea sized plastic beads varying in size and shape. A small amount of a sticky polymer chemical is added to enhance removal of the formed floc in the upflow clarifier material. With removal of the larger particles, the clarity of the treated water is greatly improved.

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FILTRATION:

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Now that the majority of larger particle solids have been removed, the clarified water moves to the “filtration” step. Our filtration plant has four filters containing a mix of three filtration media. The filter materials consist of small grains of anthracite coal and two types of sand. These three small filter materials remove the smallest of particles down to approximately 5 microns, or approximately fifteen times smaller than the diameter of a human hair at 75 microns.

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PRIMARY DISINFECTION:

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The next step in the treatment process is “primary disinfection”. A solution made of free chlorine (bleach) is added to the water in a large holding tank under the filtration plant. This large holding tank is called a clearwell. A required amount of chlorine is added to the water for a particular amount of time to ensure that disease causing organisms are killed (disinfection). The chlorinated water travels in a twisting like path through concrete chambers from one end of the 300,000 gallon clearwell to the opposite end. This ensures that the water has had adequate contact time with the added chlorine for disinfection. This disinfected water is then pumped into our Distribution system to keep up with the daily demand of our customers. While being pumped into the distribution piping, additional chemicals are added for secondary disinfection and corrosion control.

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CORROSION CONTROL:

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To produce water that is optimized for “corrosion control”, specifically to reduce lead and copper and other metal corrosion in customers pipes and the distribution system, the District adds an orthophosphate corrosion inhibitor (as part of a blended phosphate chemical) to form a protective film on pipes. The blended phosphate chemical added contains orthophosphate for corrosion control and some polyphosphate to help maintain water clarity and prevent red tinted water when traveling through older pipes. The District also raises water pH with sodium carbonate which makes the water less corrosive. This combination has proven to appropriately reduce corrosion while maintaining stable high-quality water in the distribution system.

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SECONDARY DISINFECTION:

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The District is required to protect customers from potential issues such as main breaks within the distribution piping, not only through corrosion control but also by ensuring water has an adequate and persistent amount of chlorine available in the water to safely combat such issues. This is known as providing “secondary disinfection”. Secondary disinfectants must be a form of chlorine, whether free chlorine, monochloramine, or chlorine dioxide. Free Chlorine and Monochloramine are the most commonly used forms and are accepted as safe and best available practices.

The use of chlorine to disinfect water has nearly eliminated waterborne illness but the trade-off with its use is that Disinfection Byproducts (DBPs) can form when chlorine (any form of chlorine) added to treated water during primary and secondary disinfection processes react with organic matter naturally present in surface waters and some ground waters. Some DBPs are regulated by the US EPA as suspected carcinogens. The EPA has set limits or maximum levels in drinking water for the regulated DBPs Trihalomethanes (THMs) and Haloacetic Acids (HAAs). Monochloramine disinfectant is most commonly used at mid to larger water utilities and for those that use surface waters because it is proven to significantly reduce the formation of regulated Disinfection Byproducts (DBPs) and provide a stable chlorine residual in larger distribution systems thus playing an important dual role. Free chlorine which is more reactive, can form DBPs faster, and is more commonly used with ground water supplies (utilities that use drilled or dug wells as a water supply) where DBP formation potential is less because ground water tends to contain less natural organics which could form DBPs. In addition, most systems that use free chlorine are smaller in size so typically can add less free chlorine and maintain lower water age due to a smaller piping area, also tending to lead to lower DBP formation. The type of secondary disinfectant used is tailored and dependent on many individual system conditions.

To reduce the formation of regulated DBPs, the York Water District, along with most other medium to large water providers using surface water supplies in the area, adds a small amount of ammonia (equivalent to four drops in a 55 gallon drum) to treated water to form monochloramine for secondary disinfection. Over 25 years ago, YWD made the switch from free chlorine to monochloramine and was able to reduce the formation levels of regulated DBPs by almost 70%. Monochloramine has also been proven to produce improved taste and odor over free chlorine.

The District currently maintains around 100 miles of underground pipe serving our customers, four pumping stations to boost pressure and provide adequate supply of water, as well as two large standpipe tanks containing a maximum of 5 million gallons which is available at any time. These tanks store millions of gallons so that if there were unusual circumstances such as excessive water demand, a large fire, or large pipe break we, would not run out of drinking water. We also maintain two emergency interconnections which can be opened to provide water in an emergency in either direction; one with Kennebunk, Kennebunkport, and Wells Water District to the north and the other with Kittery Water District to our south.

See Distribution here.

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