Consider the following statements regarding chemical characteristics:

Question

Consider the following statements regarding chemical characteristics:

I. COD usually gives a higher value than BOD for the same wastewater sample.

II. High hardness in water is mainly due to dissolved Ca^2+ and Mg^2+ salts.

III. BOD is used to estimate the concentration of inorganic salts in water.

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 is I and II only.

Key Points

• Statement I is correct. COD always exceeds BOD for the same sample, because a chemical oxidant attacks both biodegradable and non-biodegradable organics while micro-organisms consume only the former.
• Statement II is correct. Hardness is caused chiefly by dissolved calcium ($\text{Ca}^{2+}$) and magnesium ($\text{Mg}^{2+}$) salts.
• Statement III is incorrect. BOD measures organic matter, not inorganic salts. Inorganic content is assessed through TDS, conductivity or specific ion tests — a straightforward misattribution.

Additional Information

• Hardness splits into temporary (carbonate) hardness, caused by bicarbonates and removable by boiling or by lime treatment, and permanent (non-carbonate) hardness, caused by sulphates and chlorides and requiring lime-soda softening or ion exchange.
• It is conventionally expressed as mg/L as $\text{CaCO}_3$; IS 10500 sets an acceptable limit of 200 mg/L, relaxable to 600 mg/L in the absence of an alternative source.
• Hard water is not a health hazard but it is an economic one: it consumes soap, and it deposits scale in boilers and pipes, reducing heat transfer and eventually causing failure.
• The BOD/COD relationship is worth stating as a rule with no exceptions — if a laboratory reports BOD above COD, the analysis is wrong, not the water.