**Question:** Most important layer of a slow sand filter is -

A. Gravel layer
B. Filter fabric or screen layer
C. Sand layer
D. Compost layer

**Core Concept:** Slow sand filters are a type of biological filtration used in water treatment. They are composed of multiple layers that help in removing contaminants from the water.

**Why the Correct Answer is Right:** The most important layer in a slow sand filter is the **C. Sand layer**. The sand layer serves as the primary filtration component, where most of the contaminants are trapped and removed. The sand provides a surface for the growth of beneficial bacteria, which further aid in the removal of contaminants through a process called biological degradation.

**Why Each Wrong Option is Incorrect:**

A. **Gravel layer**: Although the gravel layer helps in distributing the water evenly and preventing clogging, it is less crucial compared to the sand layer as it only provides structural support.

B. **Filter fabric or screen layer**: This layer is used in some types of filters (e.g., membrane filtration) but not in slow sand filters. Its purpose is to prevent larger particles from passing through, but the sand layer still plays a more significant role in removing contaminants.

D. **Compost layer**: Compost is primarily used in composting systems and is not a standard component of slow sand filters. The primary function of a slow sand filter is biological degradation, which occurs within the sand layer.

**Clinical Pearl:** In a slow sand filter, the **C. Sand layer** is the most important component for removing contaminants and facilitating biological degradation. This process is crucial in producing clean water for consumption and preventing waterborne diseases.