The self-purification of a natural stream after discharge of sewage is retarded by:

Question

The self-purification of a natural stream after discharge of sewage is retarded by:

1. A. Low dissolved oxygen and high organic load (Correct answer)
2. B. High sunlight and temperature
3. C. Moderate temperature
4. D. High flow velocity

Correct Answer

Option A — Low dissolved oxygen and high organic load

Detailed Solution & Explanation

The correct answer is Low dissolved oxygen and high organic load.

Key Points

• Self-purification is a stream's natural capacity to recover from an organic discharge, and it is fundamentally an aerobic process — micro-organisms need dissolved oxygen to stabilise the waste.
• It is therefore retarded when DO is already low and the organic load is high: demand outstrips the oxygen available, the reach turns anaerobic, and stabilisation slows to a crawl while foul odours develop.
• The other options assist recovery rather than hinder it. High flow velocity promotes turbulence and reaeration; sunlight drives algal photosynthesis, adding oxygen; moderate temperature keeps saturation reasonably high while sustaining biological activity.

Additional Information

• The competition between the two governing rates is captured by the Streeter-Phelps oxygen sag curve: deoxygenation from the waste pulls DO down, reaeration from the atmosphere pushes it back up, and the lowest point of the curve is the critical deficit where aquatic life is most at risk.
• The recovering stream passes through recognisable zones of degradation, active decomposition, recovery and clean water, each with its own characteristic organisms.
• High temperature is genuinely double-edged: it speeds biological breakdown but lowers oxygen saturation, so hot-weather discharges are the most damaging.
• Self-purification has a limit, which is the entire justification for treating sewage before discharge rather than relying on dilution.