What is the layer of solids and biological growth on a slow sand filter called?

• A. Biofilm
• B. Schmutzdecke
• C. Floc layer
• D. Sludge blanket

Answer: (B)

The layer of solids and biological growth on a slow sand filter is referred to as the schmutzdecke. This term originates from German and translates to "dirty cover." The schmutzdecke forms as water flows through the filter and organisms such as bacteria, protozoa, and other microorganisms establish themselves on the surface of the sand. This layer is crucial for the filtration process, as it helps in the removal of pathogens and particulate matter from the water by providing a large surface area for biological activity and physical filtration.

The presence of the schmutzdecke indicates a properly functioning slow sand filter. As the biological layer develops, it enhances the filter's ability to purify water, making it an essential aspect of sand filter operation. While other terms like biofilm, floc layer, and sludge blanket refer to various types of biological or particulate accumulations in water treatment processes, they pertain to different contexts and types of filtration. Therefore, schmutzdecke is specifically associated with slow sand filters, highlighting its importance in this particular filtration method.