Final answer:
A filter must be backwashed when head loss across the filter increases to a set limit, indicating clogging which requires cleaning to restore its function.
Step-by-step explanation:
A filter must be backwashed when the head loss across the filter increases and reaches a pre-determined limit. This is typically because the filter has accumulated particles to the point where the flow is significantly impeded, requiring cleaning to restore function. The correct answer to the given question is D.
Backwashing is a process where water is passed in reverse through the filter, effectively removing the trapped particles and cleaning the filter media. This practice is common in water treatment processes to ensure effective filtration is maintained. When head loss increases, it indicates that the filter is becoming clogged with particles, and it becomes necessary to backwash it to prevent a decrease in water quality and to maintain system efficiency.