Water Processing

Water Processing

A Water Treatment Plant (WTP) must be designed to accommodate the raw water source variability over climatic seasons, as well as the seasonal demand fluctuations of the community. Their challenge is to consistently process raw water streams to produce safe, clean and good-tasting water. The typical water processing steps include:

1. Preliminary Treatment
2. Primary Treatment
3. Post Treatment

Preliminary Treatment

Prior to entering the plant, the raw water is passed through a screen to remove large debris such as sticks, leaves, plants, aquatic life and other large objects. Chlorine is typically added to the water prior to entering the plant to eliminate disease causing microorganisms and to prevent microorganisms and algae from growing in the plant during water treatment. Treatment processes that occur prior to the water entering the plant are part of Preliminary Treatment.

Primary Treatment

Water flows from the screening process to the first step of primary treatment – rapid mixing, where coagulants and auxiliary chemicals are added to the water to aid in particulate removal. In the rapid mix tank, the coagulants and other chemicals are rapidly mixed in the water to evenly distribute these materials. The coagulant breaks down repelling forces between small suspended particles and allows these particles to group together.

Water moves from the rapid mix tank to the flocculation tank. The flocculation tank has paddles or other means to mix the water very slowly. During the slow mix, the suspended particles begin to form larger groups of solids called floc. The formation of floc creates masses that can be separated from the base water. The process of developing these solid masses is called flocculation.

The water with floc masses slowly flow to the sedimentation tank where the larger, heavier, floc particles settle to the bottom. The water is retained in the tank so that the floc particles have time to drop to the bottom. The solids are collected at the bottom of the tank and are pumped to a clarifier to further collect solids. The water flows to a filter tank to filter out any of the foc that is not removed in the sedimentation process.

The filter media used in the filter tank will be dependent on the raw water being processed. Typically multiple layers of charcoal, gravel and sand are utilized; charcoal to improve the taste characteristics, gravel and sand to remove particles. A water treatment plant typically operates continuously and the filter is back washed to remove the particulate embedded in the filter media. The backwash water is pumped to a clarifier for solids separation.

Post Treatment

After filtration, chlorine or other disinfectants are added to the water to keep the piping system and storage reservoirs free from pathogens and to maintain a residual to the tap. In many communities, the water is fluorinated for dental health considerations. Chemicals are also added to adjust the pH and buffer the finished water to make it less corrosive to the piping system.