The pre-treatment of raw water for the Mitchell Water Treatment Plant happens at Lake Brandt with the addition of liquid sodium permanganate (NaMnO4), an oxidizing agent that helps remove iron, manganese, and objectionable tastes and odors. When iron and manganese are oxidized, they become insoluble in water and may be removed by the sedimentation and filtration processes.

At the Townsend Plant, pre-treatment happens with the introduction of a little hypochlorite inside the raw water pipe. Sodium permanganate cannot be used because of the low contact time.

In the coagulation and flocculation processes, very fine suspended particles are joined together to form larger particles that can be settled and filtered out of the water. These include fine silts, bacteria, color-causing particles, and viruses that may not otherwise settle for days, months, or even years. While individual particles are largely invisible, their combined effect often displays as cloudiness or turbidity in the water. These particles are small enough to pass through the treatment process if not properly coagulated and flocculated. They could adversely affect not only the clarity of water, but also its taste and odor and the efficacy of chlorine disinfection.

During coagulation, ferric sulfate* is added to the incoming raw water in the flash mixer. The water is stirred vigorously to assure its quick and uniform dispersion. The iron-based coagulant reacts rapidly with compounds in the water that contain carbonates, bicarbonates, and hydroxides to produce a jelly-like substance, which absorbs impurities. At the same time, the positively charged ferric sulfate neutralizes the negatively charged natural particles and draws them together to form small particles called micro-floc.

Water moves from the flash mixer to the flocculation basins, which contain a vertical system of flocculators. The basins provide a gentle, constant mixing of micro-flocs formed during coagulation. The stirring promotes the formation of larger and heavier flocs. After 20-30 minutes, the floc particles become visible and look like tiny tufts of cotton, separated by clear water. After flocculation, water moves into the sedimentation basins.

* Ferric sulfate (Fe2(SO4)3) effectively removes natural organics from raw water and maintains its effectiveness through a broad pH range.

Sedimentation removes solids from water by gravity settling. Settling basins are designed to hold large volumes of water for several hours. Their design slows the water's velocity and turbulence so it no longer transports the flocculated solids and they settle to the bottom of the basins. The solids that settle out are removed through the Trac-Vac system, a giant blade that scrapes the basin's bottom.

At the end of settling basins, a hydrated lime/caustic* is added to the water to increase its pH and create the oxidation reaction. Afterward, chlorine* (sodium hypochlorite) is added for disinfection, to oxidize iron and manganese, and to prevent aquatic growth on filters during the filtration process.

*Lime (Ca(OH)2), lime slurry, or sodium hydroxide (NaOH) are alkaline compounds added to adjust water's pH and alkalinity.

*Chlorine (Sodium Hypochlorite) (NaOCI) is the treatment plant's primary disinfectant and works as a monochloramine when mixed with ammonia (NH3) for secondary disinfection throughout the distribution system.

Water flows from the sedimentation basins into filters that layered material of graduated size (called media), which remove solids from the water. The first layer of media is anthracite (coal) and the second layer is sand.

Under the sand there is a layer of stone or plastic composite. At the bottom of the filter there is an underdrain system to support the media.

Water flows by gravity onto the filters from the sedimentation basins, which leaves behind suspended particles as water passes downward through the filters. After a period of time, the accumulated solids begin to clog the media. Once the filters are clogged, they must be backwashed. Backwashing is the reversal of the direction of the flow through the filters. A rapid upward flow lifts the particles and keeps them in suspension until they can be washed out.

Following filtration, water flows into a storage tank called a clearwell. 

Water leaving the clearwell is fluoridated, stabilized, and chlorinated. 

Fluoridation is a safe and economical process endorsed by public health groups worldwide to reduce tooth decay. Fluoride* is added to the line leaving the clearwell so doses can be precisely controlled.

Orthophosphates are added to stabilize metal water pipes. They prevent corrosion and metals from leaching into drinking water, especially lead and copper.

Chlorine* is the primary disinfectant used in the treatment process inside the plant. Chloramines are secondary disinfectants used in the distribution process to keep water safe for drinking without forming substantial amounts of disinfection-bi-product (DBP). Sufficient amounts of sodium hypochlorite and liquid ammonium sulfate* are added to the finished water to form monochloramine. A specific ratio is maintained so a minimal amount (1 part per million) of monochloramine remains until the water reaches the customers' tap. 

*Fluoride (hydrofluorosilicic acid, HFS) maintains fluoride concentration in drinking water at levels known to reduce tooth decay in children. At optimum levels, fluoride can reduce the incidence of tooth decay among children by 65%. The Environmental Protection Agency (EPA) and North Carolina Department of Environmental and Natural Resources-Public Water Supply Section regulate fluoride usage. Based on the EPA’s recommendations, NC regulators recommended control limits of 0.7-1.0 mg/L of fluoride. Fluoride is received as 25% of hydrofluorosilicic acid and the solution is applied at the final treatment step before water enters the distribution system.

*Chlorine (Sodium Hypochlorite, NaOCI) disinfects water. It is the primary disinfectant used inside the plant and works as monochloramine when mixed with ammonia for secondary disinfection throughout the distribution system.

*Liquid ammonium sulfate (LAS) is mixed with chlorine to form monochloramine at the final treatment before water leaves the plant. Monochloramine remains with the water and acts as secondary disinfectant throughout the distribution system. Unlike chlorine, it does not react with organics to form disinfection biproducts such as total trihalomethanes.

After the final addition of fluoride, corrosion inhibitors, and chloramines, water is pumped into the pipes that lead to customers through storage tanks. The pumping system includes electric and diesel pumps to ensure uninterrupted water supply, even during power outages or other distribution challenges.