N1-Drinking Water Treatment Plant Processes

Table 1.0 – Summary of the Water Treatment Unit Operations

Unit Operation Definition
Raw Water Supply Water supply from a natural water source. This also includes all screens and pumps. In some instances, facilities have installed chlorination lines to protect the water intake lines from micro and macro biofouling.
Pre-Sedimentation Basin Basin or lagoon that allows heavy suspended solids to settle.
Rapid Mix Chamber This unit mixes chemicals with intake water. Usually chemical coagulants are added just upstream of this unit in order to remove suspended solids. In some instances, chlorine (and sometimes along with acid) is added just upstream of the coagulant injection point (ie. prechlorination). This is meant to aid coagulation, reduce amount of chemical coagulant used, and disinfect water.
Flocculation Basin Water from the Rapid Mixer now enters the Floc Basin. Some floc basins are multistage. Mixing paddles gently mix the water so that flocs are formed and settle.
Sedimentation Basin Water from the floc basin now enters the sedimentation basin. Flocculated material will settle out.
Filters Filters are used to remove any remaining suspended solids. The filters usually consist of a combination of sand and gravel. These units usually have the capability to automatically backwash the filter bed when they are dirty. In some situations they may use turbidity and differential pressure across the filter for backwash control purposes. Sometimes chlorine is added before and after the filters.
Granular Activated Carbon Beds (Optional) Granular activated carbon beds are used for removing organics. These units usually consist of a combination of activated carbon and gravel. These units usually have the capability to automatically backwash the activated carbon bed when they are dirty. In some situations they may use differential pressure across the unit for backwash control purposes.
Ion Exchange Units (Optional) Resin media contact units are used to remove cationic and anion chemical species from water supplies. A cationic exchanger can reduce hardness as well as problem metals such as Fe, Mn, and Pb. Anionic exchangers can reduce nitrates and sulphates.
Disinfection Contact Chamber Disinfection of the water with either a chemical oxidant or ultra violet light is required prior to sending water to distribution and/or the treated water storage tanks.

For example, chlorine is added to the water at the contact chamber (post chlorination) to achieve a required total residual chlorine concentration (TRCC). If the residual is too high a reducing agent is added to decrease the TRCC. Sometimes it is necessary to add ammonia just prior to the post chlorination injection point in order to make sure all free residual chlorine is converted to combined residual chlorine. The combined form of chlorine residual is more stable and is able to maintain effective residuals for disinfection throughout the water distribution system.

Storage Tank Water now enters the storage tank for distribution to the general public. For chlorinated water supply systems, the TRCC is monitored to ensure there is high enough concentration of residual chlorine for disinfection. If the TRCC in the storage tanks drop below an acceptable TRCC concentration for disinfection an additional dose of chlorine is applied to the storage tanks.

Notes: Some water treatment facilities have utilized membrane units (nano-filtration or reverse osmosis units) just down stream of multimedia filters and activated carbon beds (if installed) to remove extremely fine particles and organics. Usually will eliminate need for water ion exchange units.