Question

Give a detail account on water recycling stages.

Answer 1

Water recycling stages:
Conventional waste water treatment consists of a combination of physical, chemical and biological processes which remove solids, organic matter and nutrients from waste water.
The waste water treatment involves the following stages:
Primary treatment:
Primary treatment involves temporary holding of the waste water in a tank. The heavy solids get settled at the bottom while oil, grease and lighter solids float over the surface.

• The settled and floating materials are removed. The remaining liquid may be sent for secondary treatment.

Secondary treatment:
Secondary treatment is used to remove the biodegradable dissolved organic matter. This is performed in the presence of oxygen by aerobic microorganisms (Biological oxidation).

• The microorganisms must be separated from treated waste water by sedimentation.
• After separating the sediments of biological solids, the remaining liquid is discharged for tertiary treatment.

Tertiary treatment:
Tertiary or advanced treatment is the final step of sewage treatment.

• It involves removal of inorganic constituents such as nitrogen, phosphorus and microorganisms.
• The fine colloidal particles in the sewage water are precipitated by adding chemical coagulants like alum or ferric sulphate.