When should a rapid sand filter be backwashed?

A rapid sand filter should be backwashed when it becomes dirty or exhibits a significant loss of head, typically in the range of 6 to 10 feet. This loss of head indicates that the filter media has accumulated enough contaminants, particles, and debris that flow through the filter has been substantially impeded.

Backwashing serves to reverse the flow of water through the filter, dislodging and removing the trapped solids that cause the buildup and loss of head. This process not only restores the filter's efficiency by allowing water to flow freely again but also helps maintain the quality of the treated water by ensuring that the filtration process is effective.

The remaining options do not align with standard operational practices for backwashing a rapid sand filter. For example, backwashing based solely on the filter being less than half full does not address the critical factor of filter performance; timing weekly does not consider the actual condition or performance of the filter; and backwashing based on water temperature is not a recognized practice as the cleanliness of the filter is the primary concern.