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: Analyzing Statement A.
The solubility of gases in water depends on temperature. Cold water holds more dissolved oxygen than warm water. Consequently, dissolved oxygen levels fluctuate seasonally. Statement A is accurate.

Step 2: Analyzing Statement B.
Biochemical Oxygen Demand (BOD) signifies the oxygen consumed by bacteria during organic waste decomposition. High sewage content (high BOD) results in significant oxygen consumption, leading to low dissolved oxygen (DO) levels. BOD and DO are inversely related. Statement B is accurate.

Step 3: Analyzing Statement C.
Chemical Oxygen Demand (COD) quantifies the oxygen needed to chemically oxidize all organic matter (both biodegradable and non-biodegradable). BOD only measures the biodegradable portion. Therefore, for sewage water, COD is always greater than or equal to BOD. The statement correctly indicates COD is higher. Statement C is accurate.

Step 4: Analyzing Statement D.
Electrical conductivity in water is due to dissolved ions (salts). Saline water, with its high salt concentration, exhibits higher conductivity than less saline water. The statement incorrectly claims lower conductivity. Statement D is inaccurate.

Conclusion: Statements A, B, and C are accurate.