- 2072 Magh · 8 marks
Methods of sewage disposal
Short answer
Disposal by dilution discharges treated effluent into water bodies, requiring adequate flow and self-purification; it is cheap but can pollute. Land treatment applies sewage on soil for irrigation, recovering nutrients and water but needing large land area and risking groundwater contamination.
What are the various methods of sewage disposal? Describe with their advantages and disadvantages.
Sewage (after suitable treatment) is finally disposed of by two natural methods.
Sewage disposal
|-- Dilution (into a river, lake or sea)
'-- Land treatment (irrigation / sewage farming, broad irrigation, filtration)
1. Disposal by dilution
Sewage is discharged into a river, lake or sea, where the large volume of water dilutes it and natural self-purification (with the oxygen dissolved in the water) oxidises the organic matter.
Essential conditions: a river with enough flow and DO; no downstream water supply intakes near the outfall; good mixing; no unsightly floating matter; dilution factor according to the table (as per common practice, Garg):
| Dilution factor (river flow / sewage flow) | Treatment needed |
|---|---|
| more than 500 | none (raw sewage allowed) |
| 300-500 | primary treatment |
| 150-300 | secondary treatment |
| less than 150 | complete treatment |
Advantages: cheap, simple, uses natural resources, no land needed. Disadvantages: pollutes the river if flow is small; odour and floating matter; harmful to downstream users; not feasible in dry season of small streams; sea disposal may affect the coast.
2. Disposal on land (land treatment)
Sewage is applied on the land (sewage farming/irrigation). Organic matter is oxidised by soil bacteria, the plants use nutrients, and the water is partly evaporated and partly percolates.
Conditions: sufficient land at a distance of several hundred metres to 1 km, porous soil (sandy loam), low groundwater table, suitable climate, and a skilled operation. Sewage must first be pre-treated (sedimentation) to avoid clogging ("sewage sickness" occurs when the soil pores clog).
Advantages: recovers manure, nutrients and water for crops, an income to the municipality, good removal of BOD and bacteria, no river pollution. Disadvantages: needs large land area (the rate is 1 ha for 250-500 persons or about 50 m³/ha/day); smell and mosquitoes; risk to health of workers and consumers of vegetables; contamination of groundwater; not useful in the rainy season or on frozen ground; sewage sickness.
Methods of land application: broad irrigation (sewage farming), surface/spray irrigation and intermittent sand filtration.
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