Question

The role of slow sand filters in water treatment facilities is:

• A. To remove water borne microbes upto a set standard.
• B. To remove excess ammonia present in the polluted water
• C. To evaluate the microbiological characteristics of water.
• D. To neutralize the increased pH of the water.

Hint:

The key to understanding slow sand filters is the "schmutzdecke" – the "dirty layer" on top. This biological layer is what "eats" the bad microbes. So, the main job of the filter is biological purification, i.e., removing microbes.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Slow sand filtration is a type of centralized water purification system. The question asks for its primary role in the water treatment process.
Step 2: Detailed Explanation:


A slow sand filter works by passing water slowly through a bed of fine sand.

The primary mechanism of purification is not just physical straining. A complex biological layer, called the schmutzdecke, forms on the surface of the sand.

This schmutzdecke is a gelatinous matrix of bacteria, fungi, protozoa, rotifera, and algae. It is this biological layer that is responsible for most of the purification.

The microorganisms in the schmutzdecke trap, prey upon, and outcompete pathogenic microbes (bacteria, viruses, and protozoan cysts) in the water.

As a result, slow sand filters are extremely effective at removing waterborne microbes, often reducing turbidity and microbial counts by over 99\%. This brings the water quality up to a safe, potable standard.

While some ammonia removal can occur through nitrification, it is not the primary role (B). Filters are a treatment step, not an evaluation method (C). They do not significantly neutralize pH (D).

Step 3: Final Answer:
The main role of slow sand filters is to remove waterborne microbes to a standard safe for consumption.