Sewage Disposal Methods
Sewage disposal is carried out by two principal methods
1. Dilution or the disposal of sewage in water.
2. Irrigation or the disposal of sewage on land. This may be further classified as:
a) Broad irrigation or sewage farming and
b) Sub-surface irrigation or land infiltration.
DISPOSAL BY DILUTION:
THE PROCESS: Disposal by dilution is the process whereby treated sewage or the effluent from treatment plants is discharged into bodies of water or watercourse. Sewage is principally got rid of by its dilution or dispersion into the body of water with large volumes of water contained in it.
CONDITIONS FAVOURING DILUTION: The conditions favorable for dilution are
1. Where sewage is comparatively fresh.
2. Where sewage has been mostly removed of the floating and settle able solids.
3. Where it is possible to thorough mix or diffuses sewage through diluting water.
4. Where diluting water high in dissolved oxygen (D.O.) content is available.
5. Where currents are favorable causing no depositions, nuisance or destruction of aquatic life.
STANDARDS OF DILUTION:
a) Where available dilution is 500 times, crude sewage without treatment may be let into the body of water.
b) Where available dilution is between 300 to 500 times, the sewage effluents should not contain suspended solids more than 150 ppm.
c) Where available dilution is between 150 to 300 times, the sewage effluents should not contain suspended solids more than 60 ppm.
d) Where available dilution is less than 150 times, the sewage effluent should not contain more than 30 ppm of suspended solids and the 5-day BOD at 18.3°C should not exceed 20 ppm.
The various actions involved are:
a) DILUTION: As the putrescible organic matter is discharged into the following water, it is rapidly dispersed or diluted in it, the action resulting in diminishing the potential nuisance of sewage.
b) SEDIMENTATION: This also helps in self-purification by the separation of the settleable solids in sewage in the form of sludge deposits.
c) OXIDATION: As soon as the organic matter meets the water, it starts getting oxidized owing to the development of the oxidizing organisms in water.
d) REDUCTION: It occurs due to the hydrolysis of organic matter either chemically or biologically.
e) SUNLIGHT: It is effective through its stabilizing and bleaching effects on bacteria and through photosynthesis.
In “Stream Water Dilution” only limited quantity of water is available for dilution where as in “Sea Water Dilution” infinite quantity of water is available for dilution. Hence any amount of the crude sewage (without any treatment) can be disposed into the sea directly.
DISPOSAL BY IRRIGATION:
broad irrigation: In this process, sewage is caused to flow over Cultivated lands, from which a Part of the sewage evaporates and through which the remainder percolates ultimately to escape into surface drainage channels* as sewage waters the land and adds to its fertilizing value, because of the presence in it of nitrogen, phosphate potash etc., crops like cotton, sugarcane, plaintain, potatoes and grass can be profitably grown. This is called ”Sewage farming" It is found that it is possible to have 33% or more yield under sewage irrigated crops than under the well or canal irrigation.
As in the case of dilution, purification here is due to a physical action (aeration at, the Surface and filtration underneath the soil), chemical action (i.e. oxidation) and bio-chemical Action due to soil-bacteria, converting sewage matter into plant food to be taken up by the roots of the Irrigated crops.
SEWAGE APPLICATION METHODS: Following three methods are generally used.
(i) FLOODING: In which sewage is distributed over a ploughed and level area, enclosed by dykes. The depth of dose varies 2 »5 - 60 cm
(ii) surface irrigations- In which sewage is allowed to overflow from channels or ridges over the surface of land into which it sinks or over which is overflows into a cross-ditch placed lower down. This ditch conducts sewage to a point of disposal or into another lower area requiring irrigation.
(iii) RIDGE and furrow method: Consists in ploughing a field into ridges and furrows, filling the furrows with sewage while crops grow on or between the ridges.
STANDARDS: The standards laid down as per “Royal Commission Report" on sewage disposal are
a) Broad irrigation without cropping on good soil and sub—soil such as sandy, loamy overlying gravel and sand, can handle 2,80,000 litre of sewage per day per hectare.
b) Broad irrigation with cropping on good soil and subsoil, such as sandy, loamy overlying gravel and sand, can handle 78, 500 - l340'00 litres of sewage per day per hectare.
c) Broad irrigation with cropping on heavy soil overlying clay sub soil can handle 56,000 litres of sewage per day per hectare. -
d) Broad Irrigation with cropping on stiff clayey soil overlying dense clay can handle 34,000' litres of sewage per day per hectare.
SUB SURFACE IRRGATIONS: -
It is also termed, as land infiltration is the, application of sewage into the land through a system of open jointed pipes or drains, laid near the surface of the ground enabling sewage to percolate into the surrounding soil. Actions involved in purification are Alteration, which removes the suspended matter and biochemical action, aerobic near the surface and an aerobic below.
CONDITIONS FAVOURING SUB-SURFACE IRRIGATION ARE: -
a) Where dilution water is not easily available.
b) Climate is dry favouring drying up conditions.
c) Land is cheap and plentiful.
d) Sub-surface strata are porous favouring large in filtration.
e) Rainfall is meager and demand for irrigation water is heavy.
Sub soil-water is low.