DISPOSAL OF DOMESTIC WASTE WATER IN URBAN AND RURAL AREAS
The domestic waste water (sewage) begins to cause nuisance as it becomes stale. If it is possible to dispose off sewage within four to five hours after its production, the treatment required is less in magnitude. This lesson deals with the methods of disposing domestic waste water.
The methods of sewage disposal can classified as follows:
1. Disposal by dilution
2. Disposal by land treatment
1. Disposal by dilution :
In this process, the raw sewage or the partially treated sewage is thrown into natural waters having large volume. The sewage in due course of time is purified by what is known as the self-purification capacity of natural waters. The limit of discharge and degree of treatment of sewage are determined by the capacity of self-purification of natural waters.
Conditions favorable for dilution :
Following conditions are favourable for sewage to be disposed off by dilution into natural waters:
(i) It is possible only to provide primary treatment to sewage i.e., removal of floating matter and settleable solids.
(ii) Currents of flow of diluting waters should be favourable which means that nuisance should not be caused when sewage is discharged into diluting waters
(iii) Diluting waters are not used for the purpose of navigation for at least some reasonable distance on the downstream from the point of sewage disposal.
(iv) Diluting waters should not have habitation or they should not hage been used as source of water supply for at least some reasonable distance on the downstream from the point of sewage disposal
(v) Dissolved oxygen content of diluting waters should be high
(vi) The place is situated near natural waters having large volumes
(vii) The sewage is relatively fresh and it is possible to bring it to the point of discharge within four or five hours of its production.
2. Disposal by land treatment :
Here, the raw domestic waste water (sewage) is applied on the land. A part of sewage evaporates and the remaining portion percolates through the ground and is caught by the underground drains for disposal into natural waters. The sewage adds to the fertilizing value of land and crops can be profitably raised on such land.
The term sewage farming is also sometimes used for indicating disposal of sewage by land treatment. The design of a good land treatment system demands the services of environmental engineers, hydraulic engineers, irrigation engineers, agronomists, soil scientist, etc.
Conditions favourable for land treatment ;
(i) The area of land treatment is composed of sandy, loamy or alluvial soils. Such soils are easily aerated and it is easy to maintain aerobic conditions in them
(ii) The depth of water table is more even in rainy season so that there are no chances of pollution of underground water sources by land treatment
(iii) The rainfall in the area is low as it will assist in maintaining good absorption capacity of soil
( iv) There is absence of river or other natural water sources in the vicinity of disposal of sewage.
(v) There is demand for cash crops which can be easily grown on sewage farms
(vi) There is availability of large open areas in the surrounding locality for practicing broad irrigation by sewage.
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