Backwashing

Backwashing sends water backward through a sand or diatomaceous earth filter, lifting the trapped dirt and debris off the filter media and flushing it out through a waste line instead of back into the pool. It's typically triggered by the filter's pressure gauge reading 8-10 psi above its clean, freshly backwashed baseline, not on a fixed calendar schedule. Cartridge filters are the exception: they're removed and hosed off (or soaked) rather than backwashed, since there's no reverse-flow valve in that kind of system.