 Indicated by a pressure gauge on the pressure side of the filter reaching into the 'red line' area, the pool owner is alerted to the need to 'backwash' the unit.The sand in the filter will typically last five to seven years before all the "rough edges" are worn off and the more tightly packed sand no longer works as intended .Recommended filtration for public/commercial pools are 1 ton sand per 100,000 liters water (10 ounces avdp.