Surface Water Treatment Process

The sequence of water treatment units in a water treatment plant mostly remains same, as the principle objectives are to remove turbidity and disinfection to kill pathogens.

  • The first treatment unit in a water treatment plant is aeration, where water is brought in contact with atmospheric air to fresh surface water and also oxidizes some of the compounds. Many Water Treatment Plants do not have aeration system.
  • The next unit is chemical addition or flash mixer where coagulant (mostly alum) is thoroughly mixed with raw water by way of which neutralization of charge of particles (coagulation) occurs.
  • This water is then flocculated i.e bigger floc formation is encouraged which enhances settlement.
  • The flocculated water is then taken to sedimentation tanks / clarifiers for removal of flocs and from there to filters where remaining turbidity is removed.
  • The filtered water is then disinfected, mostly with chlorine and then stored in clear water reservoirs from where it is taken to water distribution system.
  • Sludge from clarifiers and filter backwash water are generally discharged into the nearby drain, however, there is a trend now to reuse / treat these wastes.

Brief review of the operations and processes involved in Water treatment:

      Operation / Process


Micro strainer

Remove algae and plankton from the raw water


Strips and oxidizes taste and odour causing volatile organics and gases and oxidizes iron and manganese. Aeration systems include gravity aerator, spray aerator, diffuser and mechanical aerator.


Provides uniform and rapid distribution of chemicals and gases into the water.


Application of oxidizing agents such us ozone, potassium permanganate, and chlorine compounds in raw water and in other treatment units; retards microbiological growth and oxidizes taste, odor and colour causing compounds


Coagulation is the addition and rapid mixing of coagulant resulting in destabilization of the colloidal particle and formation of pinhead floc


Flocculation is aggregation of destabilized turbidity and colour causing particles to form a rapid-settling floc


Gravity separation of suspended solids or floc produced in treatment processes. It is used after coagulation and flocculation and chemical precipitation.


Removal of particulate matter by percolation through granular media. Filtration media may be single (sand, anthracite, etc.), mixed, or multilayered.


Destroys disease-causing organisms in water supply. Disinfection is achieved by ultraviolet radiation and by oxidative chemicals such as chlorine, bromine, iodine, potassium permanganate, and ozone, chlorine being the most commonly used chemical