Industries use water that is obtained from the water treatment system for a variety of purpose such as for manufacturing goods, for heating, for cooling, as carrier of raw material, as carrier of waste matter, as a solvent etc.

The resulting water is then classified as waste water.

The indiscriminate discharge of these waste water streams into the environment can render soils sick, pollute the receiving bodies of water, cause air pollution by generating obnoxious gases.

Discharge untreated waste water into the domestic sewer system makes the tasks of treating domestic sewage a very difficult and costly exercise.

To prevent any health hazards caused by discharging waste water into the environment and protect domestic sewage, the waste water must be treated before discharge.

Important contaminants of concern in waste water treatment are: suspended solids, nutrients, priority pollutants, refractory organics, heavy metals, dissolved inorganic etc.

The common types of waste water treatment methods are:

• Physical unit operations: Treatment methods in which the application of physical forces predominates. Screening, mixing, flocculation, sedimentation, floatation, filtration and gas transfer are typical unit operations.

• Chemical unit processes: Treatment methods in which the removal or conversion of contaminants is brought about by the addition of chemicals or by other chemical reactions. Precipitation, adsorption and disinfection are the most common examples used in waste water treatment.

• Biological unit processes: Treatment methods in which the removal of contaminants is brought about by biological activity. Biological treatment is used primarily to remove the biodegradable organic substances and nutrients from waste water. Basically this substances are converted into gases that can escape to the atmosphere and into biological cell tissue that can be removed by settling.