What system is responsible for collecting the filtered water in a rapid sand filter?

The correct answer is the under drain system, which plays a crucial role in the operation of a rapid sand filter. This system is located at the bottom of the filter and consists of a network of pipes or drainage elements designed to collect the filtered water after it has passed through the filtration layer, which is typically made up of sand or similar materials.

The purpose of the under drain system is to ensure that the clean water that has been filtered flows away efficiently without causing disruption to the filtration process. It helps maintain the appropriate flow rate and ensures that the pressure within the filter does not become too great, which could potentially disrupt the filtration bed or lead to bypassing of unfiltered water.

Other systems mentioned, such as the filtration layer, play a vital role in the filtering process itself but do not function as the collection mechanism for the treated water. The collection basin may store the filtered water, but it does not directly interface with the rapid sand filter to collect filtered water. The appurtenance system generally refers to additional components that support the functioning of the entire water treatment plant, but it does not specifically collect filtered water.