Consider the following statements regarding self-purification and parameter interpretation:

Question

Consider the following statements regarding self-purification and parameter interpretation:

I. Satisfying oxygen demand in a stream promotes self-purification.

II. High temperature always reduces the self-purification capacity of a stream.

III. Complete oxidation of organic matter by microorganisms produces CO2, H2O, and inorganic salts.

Which of the statements given above is/are correct?

1. A. I and II only (Correct answer)
2. B. I and III only
3. C. II and III only
4. D. I, II and III

Correct Answer

Option A — I and II only

Detailed Solution & Explanation

The correct answer per the official JKSSB final key is I and II only.

Key Points

• Statement I is correct. Self-purification proceeds as the stream's oxygen demand is satisfied — micro-organisms stabilise the organic load aerobically, and the water recovers.
• Statement II asserts that high temperature always reduces a stream's self-purification capacity. Warmer water holds less dissolved oxygen, and since oxygen availability is normally the binding constraint, the net effect on a stream's assimilative capacity is usually a reduction.
• Statement III describes complete aerobic oxidation yielding carbon dioxide, water and inorganic salts.

Additional Information

• This question was among the ten reviewed by JKSSB after objections, and its key was revised from B (I and III only) to A (I and II only). It is worth knowing both readings, because the science and the final key sit awkwardly together.
• On the strict science, complete mineralisation of organic matter does yield $\text{CO}_2$, $\text{H}_2\text{O}$ and inorganic salts — that is the textbook description of aerobic stabilisation — while statement II's "always" is the kind of absolute that usually marks a statement false, since higher temperature simultaneously accelerates biological breakdown.
• The defensible reading behind the revision is that temperature's effect on dissolved-oxygen saturation dominates: saturation falls from about 9.1 mg/L at 20°C to 7.6 mg/L at 30°C, so the oxygen budget tightens faster than the reaction rate helps.
• For examination purposes the final key governs the marks. For understanding, hold on to the underlying mechanism — self-purification is limited by the balance between deoxygenation and reaeration, which is what the Streeter-Phelps oxygen sag curve describes.