Which of the following is primarily a physical characteristic of water or sewage?

Question

Which of the following is primarily a physical characteristic of water or sewage?

1. A. pH
2. B. COD
3. C. BOD
4. D. Turbidity (Correct answer)

Correct Answer

Option D — Turbidity

Detailed Solution & Explanation

The correct answer is Turbidity.

Key Points

• Water and sewage characteristics divide into three families, and the question tests that division:
  • Physical — turbidity, colour, odour, taste, temperature, total solids.
  • Chemical — pH, COD, alkalinity, hardness, chlorides, heavy metals.
  • Biological — coliforms, BOD, pathogens.
• Turbidity is the cloudiness caused by suspended and colloidal particles scattering light. It is measured optically, involves no chemical reaction, and is therefore a physical characteristic.
• pH and COD are chemical determinations; BOD is a biological one, since it measures oxygen consumed by living micro-organisms.

Additional Information

• Turbidity is measured by nephelometry, comparing light scattered at 90° against a formazin standard, and is reported in NTU. The Indian drinking-water standard (IS 10500) sets an acceptable limit of 1 NTU, relaxable to 5 NTU where no alternative source exists.
• It matters beyond appearance: suspended particles shield micro-organisms from disinfectant, so high turbidity undermines chlorination. This is why filtration must precede disinfection.
• BOD is classified as biological rather than chemical because the oxidising agent is a population of bacteria, whereas COD uses a chemical oxidant, potassium dichromate.
• Temperature is the physical parameter that quietly governs the others — it sets dissolved-oxygen saturation, reaction rates and biological activity.