The Water Treatment Process
Raw Water Intake
Water is drawn into the plant via the use of pumps from the river or lake through large metal grills called screens and trash racks. These block large objects such as wood, leaves and other forms of debris from entering the treatment plant along with the water.
It is at this stage the chemicals Alum or Aluminum Sulphate (a coagulant), liquid polymer (a flocculant) and chlorine (disinfectant) are first introduced. The headworks is designed to ensure rapid mixing and uniform distribution of the chemicals with the raw water. The Alum reacts rapidly with the water’s alkalinity to produce a gelatinous (jelly-like) precipitate of Aluminum Hydroxide called microfloc that entraps and absorbs impurities. The liquid polymer aids coagulation by enlarging the floc particles through bridging. Chlorine is sometimes added at the headworks to prevent algae growth on the wall of the flocculation and sedimentation basins.
The design of the flocculation basin facilitates the gentle, constant mixing of the microfloc formed during coagulation. This stirring promotes contact and the formation of larger and heavier floc at a faster rate.
Sedimentation involves the removal of solids from water by gravity settling. Water flow is greatly reduced allowing the heavy floc to settle to the bottom of the basin where it is referred to as sludge.
This sludge is channeled to sludge lagoons where further settling takes place. Supernatant water from the lagoons is then returned to the headworks.
After leaving the sedimentation basin, the water is then filtered. This facilitates the final and complete removal of any finely divided suspended matter, plus any floc carryover that remain after the coagulation and sedimentation processes.
The filtration system is a monotype constant rate gravity filter system. The filter media consist of six feet of sand. Periodic backwashing of the filters is required to remove any accumulated suspended materials.
The water leaving the filter flows to the clearwell where it is disinfected and stored. Chlorine is added to the water to remove and destroy any bacteria or viruses present in the water
Sometimes it is necessary to add lime (Calcium Hydroxide)to the filtered water in order to maintain a pH level of 7. A sequestering agent, which offers scale and corrosion control, is also added to the water before distribution.