Waste water treatment

Fittings, Fixtures And Services 3(1+2)

Lesson 10 : Water supply system, drainage system and drainage bye laws

Waste water treatment

Private Waste Treatment
In some localities, a usually rural area, a house is not connected to a municipal sewerage network and is necessary to have their own sewage treatment on site. Two common methods of sewage treatment are the Cesspool, which simplify stores sewage until it is emptied by road tanker, and a Septic tank, which processes the sewage.

A Cesspool is a brick or concrete water-tight tank of about 20 m 3 capacity. It is sited at least 15 meter from the house and away from any wells, springs or other sources of drinking water.

A Septic Tank is much smaller than a cesspool as it stores water for periods of a few days only, during which it is digested by anaerobic bacteria. Sludge settles at the base of the tank and this must be removed periodically. The treated water, possibly with additional filtration, is then discharged into a ditch or stream.

Municipal Sewage Treatment

Waste water, soil and rain water is collected and transported in a network of sewers. In some cases there are combined sewers which take all of these wastes. Alternatively, there may be separate sewers for dirty water, which needs treatment and for surface (storm) water, which can be discharged directly into a river without any treatment.

Treatment of sewage can be carried out in a number of different ways.

The First stage is either a screening of communition process. If screening is used solids can be removed and treated separately. Communition is the reduction in size of solids.

The Second Stage is a sedimentation process which allows 30-50 % of suspended solids to be deposited at the base of a tank. The partially treated water from this process then undergoes a bacteriological treatment which is designed to reduce its oxygen demand. If it were discharged into a river in this condition it would cause deoxygenation of the water which would kill fish and produce foul smelling water.

The bacteriological treatment can be either through trickling filters or using an activated sludge process. Filtering is carried out through beds of stones which are covered in a film of bacterial material. Air passes up through the filter. In the activated sludge process, air is bubbled through the dirty liquid which is mixed with microorganisms. This is followed by a sedimentation process, the deposited solids being returned to the aeration tank.
If the treated water is to be discharged into water which is to be used for recreational purposes or for hot water supply, it is treated with chlorine to destroy harmful bacteria.

Last modified: Monday, 13 February 2012, 9:07 AM