Water Treatment Process

Plant operators carefully monitor every step of the treatment process to produce your drinking water. They make sure that when you turn on the tap at home or at work, the water you get meets or exceeds all federal and state safety standards.

Terminal Reservoirs & Treatment Plants

Raw (untreated) water from Durham’s 2 reservoirs/lakes is pumped into terminal reservoirs located at the city’s 2 water treatment plants. The Brown Water Treatment Plant (WTP) terminal reservoir holds approximately 90 million gallons and the Williams WTP terminal reservoir holds approximately 45 million gallons. These volumes represent a 2 to 3 day supply of water for the 2 treatment plants. The storage in the terminal reservoirs provide a constant supply of raw water for the treatment plants should any interruption in raw water delivery occur due to either line breaks or routine maintenance.

Coagulation, Flocculation & Sedimentation

Raw water from the terminal reservoirs flows into clarifiers where sediment and other particulate matter are removed by processes known as coagulation, flocculation and sedimentation. Coagulation occurs with the help of chemicals known as coagulants that use their positive charges to attract negatively charged particles like sediment and organic matter present in raw water. Flocculation then occurs as these particles clump together and form flocs, or clumps of sediment and particulate matter. These flocs then settle to the bottom of the clarifier (sedimentation) where rake-like structures slowly scoop the solid matter out.

Clear, sediment-free water leaves the clarifier from the top and flows onto the next step in the treatment process. Currently, both treatment plants use a chemical called ferric sulfate as a coagulant.


After water has gone through the flocculation and sedimentation process to remove the sediment/particulate matter, it flows through filters composed of layers of crushed anthracite coal, sand and gravel. The filtration process removes additional minute particles which are not removed by the flocculation and sedimentation process.

Disinfection & Chemical Addition

Several chemicals are added to the water throughout the different processes to facilitate water treatment. They include chemicals for disinfection (chloramines), corrosion inhibition (orthophosphate), pH balance (sodium hydroxide) and dental health (fluoride). The concentrations of each of the majority of these chemicals must meet EPA standards.

Find out more about the chemicals we use to facilitate water treatment.

Storage & Distribution

Once treated and disinfected, drinking water is stored in covered tanks called clear wells. The city stores several millions of gallons of treated water in clear wells on the treatment plant sites ready for distribution. Treated water is also stored in elevated and ground level water storage tanks located throughout the city. Levels in the towers are monitored remotely and generally are filled each evening using off peak pumping strategies. Towers and elevated tanks help maintain pressure in the distribution system so that each household and business has sufficient flow.

Filtration Chamber
Sedimentation Chamber
Terminal Reservoir