Pool maintenance is a necessary evil if you want to lounge in clean, cool water on a sweltering summer day. It's frustrating to find that your pump is putting dirt back in after cleaning. The problem could be as simple as a faulty valve.
The sand in your filter might be too fine and is passing through your filter and into the pool. Sand that is at least .45 to .55 mm is required for most sand filters.
The sand in your filter might be old and needs to be replaced. As sand breaks down, it becomes finer and can pass through the filter.
A lateral – one of the small tubes in the bottom of your filter – could be cracked, allowing sand to pass through it.
The filter control valve might be malfunctioning, notes Eco Pools. Multiport valves, if worn or if the gasket is slipping, allow the water being pulled from the swimming pool to skip over the filter and go directly back into the pool without being filtered.
Your pump might be too large for the size pool you have. If the pump is too large, it will push the water too hard through the filter, pulling dirt with it.