Consider the following statements regarding physical characteristics of water and sewage:

Question

Consider the following statements regarding physical characteristics of water and sewage:

I. Turbidity is a measure of light-scattering due to suspended solids.

II. Colour in water is mainly a biological characteristic related to algal blooms.

III. Temperature affects the solubility of gases and the rate of biochemical reactions.

Which of the statements given above is/are correct?

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

Correct Answer

Option B — I and III only

Detailed Solution & Explanation

The correct answer is I and III only.

Key Points

• Statement I is correct. Turbidity is measured by the scattering of light by suspended and colloidal particles, which is why it is determined nephelometrically and reported in NTU.
• Statement II is incorrect, and it is the error the question rests on. Colour is a physical characteristic, not a biological one. It may arise from dissolved organics such as tannins and humic acids, from iron and manganese, or from industrial dyes — algal blooms are only one possible cause among many.
• Statement III is correct. Temperature governs the solubility of gases and the rate of biochemical reactions, making it the physical parameter that quietly controls most others.

Additional Information

• Water colour is reported as apparent colour (measured on an unfiltered sample, so it includes suspended matter) and true colour (measured after filtration, so it reflects only dissolved substances). IS 10500 sets an acceptable limit of 5 Hazen units.
• The temperature effect runs in two opposing directions, which is what makes it interesting: warmer water speeds up microbial breakdown but holds less dissolved oxygen, so a warm stream both digests waste faster and asphyxiates sooner.
• Oxygen saturation falls from about 14.6 mg/L at 0°C to roughly 9.1 mg/L at 20°C and 7.6 mg/L at 30°C — worth carrying, since it explains why summer discharges are the most damaging.
• Statement questions in this topic almost always hide the error in a misclassification, as here, or in an absolute word.