Every 5 - 7 years you will have to replace the sand in your sand filter. The sand actually wears out. The sharp edges of the sand particles become worn and rounded and don't capture the contanimates as well. Also the sand will start to ball up or the water will create channels thru the sand. You will notice that your pool water is not as clean or the interval between required backwashes is shorter. This guide shows you the steps required to replace the sand in your sand filter.