Water Treatment

The Wise County PSA obtains the bulk of it’s water from the Clinch River via the Carfax Water Treatment Plant. Below is a diagram showing the basic steps in the treatment process:

Water Treatment Plant Process
(Click to view larger image)

The essential steps are as follows:

  1. Water is withdrawn from the Clinch River. It flows through fine screens that keep any large debris, heavy sediment, or aquatic life from entering the raw water pumps.
  2. The raw water is then pumped from the intake station to the water treatment plant.
  3. The first step in treatment is adding chemicals to aid in treatment. Raw water has suspended solids, organic matter, and microscopic organisms (like algae, bacteria, cryptosporidium, giardia, etc.), that need to be removed. Sodium permanganate is added to aid with oxidation of organic matter. Polyaluminum chloride (PAC) is added to aid in settling suspended solids. Additional chemicals are added later in the process.
  4. After adding chemicals, they are mixed, either with a rapid stirring mechanism or a flow tube called an inline mixer. This gets the water and chemicals thoroughly mixed.
  5. The next step is called flocculation. This process uses relatively slow-moving paddle wheels to stir the water and chemicals together. This allows the solids and the PAC to form what is called “floc”. This floc are larger clumps of solids and chemical that are heavier than individual particles and will settle out of the water faster.
  6. After flocculation is sedimentation. This is essentially a stilling basin where the water is allowed to move very slowly and the floc can settle to the bottom. Floc water is fed into one end and clear water is removed off the top of the basin at the other end. At this point, the water is clear and free of sediment
  7. After sedimentation is filtration. The filters are layers of anthracite, sand, and gravel. Before entering the filter, disinfection chemical (in our case, sodium hypochlorite) is added to the water. Water is put on top of the filter and percolates to the bottom. Filtration removes any particles that may not have settled in sedimentation and also aids in removal of larger microorganisms (such as cryptosporidium and giardia) that are difficult to kill with disinfectant alone.
  8. After filtration, the water is almost ready for treatment. Before sending the water to the clearwell (a large storage basin holding treated water ready to be pumped into the system), some additional sodium hypochlorite is added to prevent regrowth of harmful organisms in the distribution system. Fluoride in the form of hydroflourosilicic acid is also added and acts as a dental aid. Finally, phosphate is added as a corrosion inhibitor. This chemical acts to form a barrier on the inner surface of pipes that prevents the water from leaching out copper and lead into the water.
  9. The water is now sent into the distribution system via our service pumps.
  10. This process does generate some waste material. Two steps generate this – sedimentation and filter backwash. The sedimentation basins will fill with solids and periodically have to be removed from service to have solids removed from the bottom and be washed down. This is done every few months depending on the season and how much solids we get from the river. Every few hours, the filters have to be backwashed to remove material that is filtered out in the various layers. They are removed from service and water is pumped backward through them. This water, along with the sedimentation basin waste, is send to a pair of treatment lagoons where the waste material is allowed to settle and the decanted water is returned to the river via Bull Run Creek. This discharge is regulated by the Department of Environmental Quality.

Our water treatment operators are licensed by the state Department of Professional and Occupational Regulation and our treatment processes and operations are regulated/monitored by the Virginia Department of Health Office of Drinking Water.