Water Treatment MCQ with answer and solution

Concept

Characteristics of slow sand filters include biological action via the zoogloeal layer, large area requirement, low filtration rate, and high bacterial removal efficiency.

Step-by-step solution

Step 1: Statement 1 is correct because slow sand filters develop a biological slime layer (schmutzdecke) on the sand surface. Step 2: Statement 2 is incorrect because slow sand filters have a low rate of filtration, typically 100 to 200 L/h/m² of filter area (unlike rapid sand filters which have a high rate). Step 3: Statement 3 is correct because the low filtration rate necessitates a large land area for water treatment plants using slow sand filters. Step 4: Statement 4 is correct as they are highly efficient in removing pathogenic bacteria (98-99%). Step 5: Therefore, statements 1, 3, and 4 are correct.

Explanation

Slow sand filters form a biological gelatinous layer (schmutzdecke) on top of the sand bed, which helps in the efficient removal of bacteria (about 98-99%). Due to their very low rate of filtration (100 to 200 L/h/m²), they require a large surface area for treating substantial quantities of water.

Why the other options are wrong

B: Statement 2 is incorrect as slow sand filters have a low filtration rate. C: Statement 2 is incorrect and statement 1 is missing. D: Statement 2 is incorrect.

Shortcut

Remember: Slow sand filters are slow (low rate, large area) but biological and very clean (high bacterial removal).

Common mistake

Confusing slow sand filter properties with rapid sand filter properties (rapid sand filters have high filtration rates and require less area, but lower bacterial removal efficiency).