The Sand Dollar filter is the perfect high performance sand filter. It’s incredibly simple to operate and maintain, and it’s built with long-term reliability in mind. Pool water is routed through the sand-filled pressure vessel. The sand traps particles so small they are undetectable to the naked eye. Over time, dirt accumulates in the spaces between the sand particles, causing the pressure in the vessel to rise. This signals you to “backwash” the filter. Simply reverse the water flow and “knock out” the dirt. All Pentair sand filters feature a special internal design that maintains the sand bed level and ensures even water flow. That results in the most efficient filtration possible.