Water treatment plants treat water from various sources like rivers and lakes. Water from these sources has to be purified to remove floating objects like sticks and other solids of larger dimensions, finer particulate matters, color, odor, pollutants, and harmful bacteria and microbes.

The water entering the treatment plant passes through intake screens to remove floating objects and larger insoluble materials. Back-flushing of the screen with air is done periodically to clear the screen and maintain the effectiveness of the screen.

Coagulants are added to the water to facilitate the subsequent sedimentation process. Water containing the rest of the impurities is taken to a sedimentation tank containing sand filters to remove suspended solids. Sand is then recovered and cleaned so that it can be reused. The next process is the bacterial disinfection and degradation of the organic compounds by treatment with ozone gas. Ozone, being unstable, is produced onsite by the use of oxygen in an electric discharge unit. The ozone thus produced is bubbled through water. Residual ozone is then converted to oxygen gas and vented out to the atmosphere.

Water will still contain some turbidity and other organics. An activated carbon filter is used to remove these impurities along with any remaining colored materials and odor-causing chemicals. Final polishing of this water is carried out by sand filters, and this water is taken to storage vessels. A small amount of chlorine can be used to treat this water to prevent bacterial contamination before the supply to the distribution system.

Thus, water from natural sources can be fully purified and disinfected and then stored in underground storage tanks.