What is the purpose of an underdrain in a filtration system?

The purpose of an underdrain in a filtration system is to collect the filtered water. After water passes through the filter media, where impurities and particles are captured, the underdrain allows for the flow of the clean, filtered water to be directed away from the filter bed and towards the next stage of treatment or storage. This component is essential for maintaining the efficiency of the filtration process because it ensures that the water is effectively removed from the filter after passing through, preventing overflow and allowing for continuous operation.

In the context of other options, while sediment removal is an important function of filtration, it is primarily the filter media itself that performs this task, not the underdrain. The underdrain does not add chemicals to the water; that would be the role of a different system or process. Additionally, while filter media needs support, the primary function of the underdrain is not to provide that support but to facilitate water collection. Thus, the function of collecting filtered water is the principal role of an underdrain within the system.