The filters in this system are high-rate sand filters. Sand filters consist of a large tank, made of fiberglass, concrete or metal, containing a thick bed of special-grade sand, which has a squarish shape.

The filters in this system are high-rate sand filters. Sand filters consist of a large tank, made of fiberglass, concrete or metal, containing a thick bed of special-grade sand, which has a squarish shape.

During filtering operation, dirty water from the pool comes in through the filter’s inlet pipe, which leads to the water distribution head inside the tank. While pulls the water down through the sand, the tiny sand particles catch any dirt and debris. At the bottom of the tank, the filtered water flows through the pick-up unit and out the outlet pipe.

Over time, the collected dirt and debris in the sand slows down the water flow. This means it’s time to backwash the filter. To backwash, the custodian adjusts a number of valves to redirect the water flow. He or she closes the return pipe leading to the pool and opens the drainage pipe, which lead to the sewer system. He or she adjusts a valve at the filter to connect the pipe from the pump to the outlet pipe and connect the drainage pipe to the inlet pipe. With this arrangement, water from the pump pushes up through the sand, dislodging the dirt and debris. At the top of the filter tank, the dirty water flows out through the inlet pipe and into the sewer

In place of a sand filter, some pool systems use a diatomaceous earth filter or a cartridge filter. In a diatomaceous earth filter, water from the pool passes through filter grids coated with diatomaceous earth, a fine powder made from the chemically inert, fossilized remains of sea organisms called diatoms. In a cartridge filter, dirty water passes through a filter made out of polyester cloth or corrugated paper. Instead of backwashing, you simply remove the filter and hose it off. After a few years (or as many as eight years), it’s time to discard the old filter and put in a new one.