EDR is a high-tech process where minerals and other constituents in water are separated by an electrical charge. The resulting ions are transferred through membranes from a less concentrated to a more concentrated solution. By varying the amount of the electrical charge input to the system, it is possible to remove the dissolved solids in water to the extent desired. EDR is a process which can also be used in desalinating sea water. The dissolved solids removed are concentrated in a "brine" or salt solution. About 11 % to 12% of water pumped from wells ends up as brine waste.
Howard R Green Company was retained to design the new water treatment plant in September 1991. The design was completed and a letting was held in April 1992. The plant began treating water in January 1993, and final construction was completed in April 1993.
Washington obtains its water from three deep wells ranging in capacity from 600 to 780 8pm. Water can be withdrawn from any of the wells separately or in a combination of any two at one time. The untreated water first passes through the EDR units. There are three EDR units, each able to produce 285 8pm of finished water. Total EDR treatment capacity is 855 8pm (1.2 million ga11ons per day). The treatment plant normally operates between the hours of 8:00 p.m. through 11:00 a m. to take advantage of lower offpeak electrical rates. The average daily water usage in 1993 was 818,000 gallons per day. In order to meet peak flows, it is possible to bypass the EDR units with part of the untreated water and then blend treated and untreated water.
After the EDR units, the water flows through an aerator where odor-causing gases and carbon dioxide are removed. The removal of carbon dioxide raises the finished water pH and causes the water to be stable or slightly alkaline, resulting in some deposition of scale in the water mains. Aeration reduces the amount of caustic soda and chlorine used in the finished water. Thirty pounds of chlorine and ten gallons of caustic soda were injected per day into the finished water; now, no caustic soda and ten pounds of chlorine are used per day as a result of aeration. The hydrogen sulfide gas leaves the water as it passes through the aerator, and this loss of gas creates less chlorine demand. Total and free chlorine residuals are now detected in every water main of the town, whereas before, the residuals would not be detected in certain areas of Washington. The pH level of the water increases as it passes through the aerator by removing CO2; whereas before, caustic soda was added to raise the pH. Before aeration the pH is 7.1; after treatment it is 7.9. This gives a stable water quality, which is important because some water mains are 100 years old and have deposits in them. Phosphates have been cut back from seven pounds per day to one pound per day. Better water quality is now being achieved with fewer chemicals added to the finished water.
From the aerator the water falls into an underground concrete basin where chlorine (disinfectant) and polyphosphate (control for scaling) are added. Caustic soda (pH adjustment) and fluoride (dental protectant) may also be added, if needed. The water is pumped by either of two transfer pumps to the ground storage reservoir from where it is pumped again into the distribution system by three high-service pumps. The capacities of the pumps are 750, 1000, and 1200 8pm. The plant has the ability to remove varying percentages of dissolved solids depending upon the final water quality goal for plant flow. At the present time the EDR units remove about 50% of the total dissolved solids. Radium levels in Washington's finished water are now far below the maximum allowable drinking water standard.
The EDR equipment is housed in a 52' x 64' precast concrete building with a metal standing-seam roof over precast concrete beams. The building is low maintenance and is very energy efficient. For example, since the plant went on-line in January 1993, space heaters in the main operating room have seldom operated, due to the naturally warm water temperature and the heat given off by the equipment. Also, to further reduce costs, the plant is designed to run during off-peak electrical demand periods. Total cost of the facility, including improvements to the existing plant and ground storage reservoir was about $ 1,800,000. Operational costs have been less than anticipated.