What is a key feature of slow sand filters?

A key feature of slow sand filters is that they are built in watertight basins. This construction is essential for their operation, as it allows for the collection and retention of water while facilitating the slow filtration process. The watertight design ensures that the water does not leak out and that a consistent water level can be maintained, which is critical for the formation of the biological layer known as the hypogeal or biofilm, where the actual filtration occurs.

In slow sand filters, water flows through a thick layer of sand, and the biological activity in this layer helps to break down pollutants and remove contaminants. The basin's watertight nature contributes to efficiency by maintaining the necessary conditions for the biofilm to thrive and effectively filter the water. The other features mentioned in the choices do not accurately describe slow sand filters or their operating principles, emphasizing why the watertight basin construction is a defining characteristic.