How are solids removed in a slow sand filter?

In slow sand filtration, solids are removed primarily by the formation of a biological layer known as a "schmutzdecke," which develops on the surface of the sand bed. This layer consists of a mixture of microorganisms, organic matter, and fine particles that collect on top of the sand. As water passes through the filter, these solids get trapped in this layer as well as in the sand itself, effectively purifying the water as it permeates through the filter bed.

This method relies on the natural processes of sedimentation, biological degradation, and mechanical straining. The schmutzdecke is essential for capturing and breaking down organic material and pathogens, thus enhancing the filter's effectiveness over time, particularly in removing colloidal particles and bacteria. Therefore, the correct answer highlights the role of this thin layer on top of the sand bed in facilitating the filtration process.