Which of the following indicates that a sand filter needs to be backwashed?

A high pressure gauge reading is a clear indicator that a sand filter needs to be backwashed. This reading suggests that there is an accumulation of debris and contaminants within the sand filter media, which impedes water flow and increases the pressure required to push water through. When the pressure gauge indicates a level significantly higher than the normal operating range, it typically means that the filter is getting clogged and needs to be cleaned to restore optimal flow and filtration efficiency.

In contrast, normal pressure readings, clear pool water, and low water levels do not signify the need for backwashing. A normal pressure gauge reading suggests that the filter is functioning correctly and is not yet obstructed. Clear pool water implies that the filtration system is effectively doing its job, and there is no immediate need for backwashing. A low water level in the pool might cause other issues but does not specifically correlate with the need for a filter backwash. Thus, the high pressure gauge reading is the most direct sign that backwashing is necessary to maintain proper filtration performance.