Question

Choose the correct statements.
A. Dissolution of oxygen in surface water changes with change in temperature during extreme summer and winters.
B. Dissolved oxygen and BOD have inverse relationship in sewage water.
C. Chemical oxygen demand is always higher than BOD in sewage water.
D. More saline water will have less conductivity.

• A. A and B only
• B. A, B and D only
• C. A, B, C and D
• D. A, B and C only

Hint:

Remember these key water quality relationships: - Temperature & DO: Inverse (Hot water, less oxygen). - BOD & DO: Inverse (High waste, less oxygen). - COD vs BOD: COD \(\ge\) BOD (COD measures more stuff). - Salinity & Conductivity: Direct (More salt, more conductive).

Answer 1

The Correct Option is D

Step 1: Analyze statement A.
The solubility of gases in water is temperature-dependent. Cold water can hold more dissolved oxygen than warm water. Therefore, dissolved oxygen levels change with the seasons. Statement A is correct.

Step 2: Analyze statement B.
Biochemical Oxygen Demand (BOD) is the amount of oxygen consumed by bacteria while decomposing organic waste. In water with high sewage content (high BOD), the decomposition process consumes a large amount of dissolved oxygen, leading to low DO levels. Thus, they have an inverse relationship. Statement B is correct.

Step 3: Analyze statement C.
Chemical Oxygen Demand (COD) measures the oxygen required to oxidize all organic matter (both biodegradable and non-biodegradable) chemically. BOD only measures the biodegradable portion. Therefore, for sewage water, COD is always greater than or equal to BOD. The statement says COD is higher, which is generally true. Statement C is correct.

Step 4: Analyze statement D.
Electrical conductivity in water is due to the presence of dissolved ions (salts). Saline water has a high concentration of dissolved salts, and therefore has a higher conductivity than less saline water. The statement says it will have less conductivity. Statement D is incorrect. Conclusion: Statements A, B, and C are correct.