Water Treatment Process
Water Treatment General
Typically, there are three water treatment processes in our industry, and WCSA employs all three. First, if there is a groundwater source that is not under the influence of surface water, generally, all the utility must do is add chlorine for disinfection. Second, if there is a groundwater source that is under the influence of surface water, the utility must do more than add chlorine. However, most groundwater sources are too pure (clean) for conventional water treatment to be effective. For that reason, membrane (filtration) technology is employed. Third, if there is a groundwater source that is under the influence of surface water and not very clean, or if the water source is surface water, conventional water treatment is typically the most effective.
This groundwater source is an artesian (groundwater under positive pressure) spring that flows by gravity from the community of Taylors Valley. Chlorine and fluoride are the only treatment required to meet or exceed water quality standards. From this location, this water is used to serve Taylor’s Valley, while the remainder flows by gravity along the Virginia Creeper Trail to nearby Damascus, Alvarado, Bethel, and other areas along the Route 58 J.E.B. Stuart Highway corridor, from Damascus to the Middle Fork Drinking Water Plant. From this location, any excess water can be pumped toward Glade Spring, the Town of Abingdon or the Bristol area. It is believed that this source was developed in the 1930s under the Works Progress/Projects Administration. For redundancy, this area, except for Taylor’s Valley, can also be served by the Middle Fork Drinking Water Plant.
Mill Creek Drinking Water Plant
The Mill Creek Water Treatment Plant is a membrane filtration plant that is jointly owned by WCSA and the Town of Chilhowie. The plant has a capacity of 2.5 million gallons per day.
Water is supplied to the treatment plant from three springs, or groundwater that was deemed to be under the influence of surface water. These sources are too pure for conventional treatment to be effective. Water travels from the three springs into a water storage tank for mixing and is then pumped into the treatment plant. Water travels through a prefiltration process before it is again pumped into the headworks of the membrane filtration.
Once water travels through the membrane filters, chlorine and fluoride are added, and the water is piped to two parallel water storage tanks located beside the water treatment plant. Water remains in the tanks until it has met the required disinfection time. Water is then pumped into the Town of Chilhowie’s and the WCSA’s distribution systems.
The Mill Creek Drinking Water Plant primarily serves Wideners Valley, Friendship, Glade Spring, Meadowview, and Emory, as well as the Town of Chilhowie.
Middle Fork Drinking Water Plant
The Middle Fork Drinking Water Plant is a conventional surface water treatment plant that was initially placed into service in 1977. Major upgrades have been completed that increased overall capacity from 4.6 million gallons per day to 12 million gallons per day.
Raw water is currently pumped from the Middle Fork of the Holston River to the plant; when the upgrade is complete, water from the South Fork of the Holston River will also be pumped to the plant for processing. Sodium permanganate is a liquid oxidant that is added to the raw water to address taste and odor that may occur naturally. Next, powder-activated carbon is added to the water to remove organic contaminants, as well as to improve upon any taste and odor that may be present.
Processes at the facility that remove particles and harmful bacteria from the raw water include: coagulation, flocculation, sedimentation, clarification and filtration.
Coagulation removes dirt and other particles found in the water. Chemicals are added to the water to form tiny sticky particles, called “floc,” which attract the dirt particles. The floc grows in size and weight through this slow mixing process; as the water begins its three-hour journey through the sedimentation basins, the floc settles to the bottom of the basins. Static devices, called “tube settlers,” increase the settling capacity of the sedimentation basins, thereby enabling more particles to be captured at this stage of treatment. The water then enters the (adsorption) clarifier, where vigorous mixing captures and removes any remaining particles.
Most modern water-treatment plants utilize rapid dual-media filters following coagulation and sedimentation. A dual-media filter consists of a layer of anthracite coal above a layer of fine sand. The upper layer of coal traps any remaining floc, while the lower layer of sand traps smaller impurities. Eight of these filters, housed in concrete boxes, are employed by WCSA. A large tank, called a clear well, is built under the filters to hold the water temporarily. At this point, we add chlorine for disinfection and fluoride for dental health before the water is pumped to the water distribution system.
Each process mentioned here has an associated cleaning or backwash process. At the most optimal time, the impurities (sludge) removed in the process are conveyed to holding ponds or lagoons. Later, the sludge is moved to an open-air drying bed where water is evaporated. The goal is to dry the sludge as much as possible before it is properly disposed off-site.
The expanded Middle Fork Drinking Water Plant not only features more capacity, but it will demonstrate innovative energy recovery as well. At the point where raw water enters flocculation, pressure reduction was required for the South Fork water. Rather than wasting that energy through a large pressure-reducing valve, Francis turbines were added to generate electrical energy from water passing through the turbines. The electricity generated is then used to power other parts of the treatment process.
The Middle Fork Drinking Water Plant has received numerous awards, including the Virginia Department of Health Optimization Program Gold Award. This award recognized the facility as one of the top performing water plants in Virginia. The mission of Virginia’s Optimization Program (VOP) is to encourage waterworks to provide water with a quality that exceeds minimum regulatory standards and to operate water systems in an exemplary manner. VOP attempts to accomplish this mission by establishing optimization goals, communicating the goals to affected waterworks, and measuring performance. The Virginia Department of Health believes that when waterworks owners and operators are aware of enhanced performance goals and track specific performance measures, they will improve the finished water quality delivered to their consumers and enhance public health protection. Water treatment plants are graded on six criteria. To meet gold standards, a water treatment plant must meet or exceed goals at least 95 percent of the time. From 2010 to 2012, the Middle Fork facility was ranked No. 1 out of 130 similarly sized facilities in Virginia.
The Middle Fork Drinking Water Plant is manned 24 hours per day, seven days per week, 365 days per year. The plant also serves as the after-hours call center for customers who have questions after normal business hours.
Areas served daily by the Middle Fork Drinking Water Plant include the Town of Abingdon, points north along and adjacent to Route 19, along Route 75 to Green Springs Road, and adjacent areas all the way to the City of Bristol, Va., and the City of Bristol, Tenn., and Scott County, Va. When necessary, the plant can also replace the Mill Creek and Reservation Spring sources (except for Taylor’s Valley).