Described by some as the bullet proof filter, sand filters are the most commonly used filter for both domestic and commercial pools. Pool water passes through a tank filled with finely graded sand. You could say it operates somewhat like an espresso machine. As water passes through, the impurities attach themselves to the grains of sand. The larger particles are trapped at the surface of the sand bed and the finest particles are collected deep inside the bed where the grains are more tightly packed. One of the benefits of the sand filter is its simplicity. When the bed of sand is clogged with dirt, the pressure inside will register on a gauge, which indicates the need to flush or backwash the filter. Switch off the pump and turn the valve to ‘backwash’, which will reverse the water flow and send the unwanted particles down the waste line. The water should be clear after a minute or so.