Cartridge filter
Also called: cartridge pool filter, filter cartridge, filter element.
A cartridge filter is a pool filter built around a pleated element, like a large paper coffee filter folded into a cylinder. Water passes through the pleats and dirt stays behind. There is no multiport valve and no backwashing: you pull the element and rinse it with a hose.
Why it matters
It is the simplest filter to live with and it wastes essentially no pool water, which matters on a well or under water restrictions. Clean it on pressure, about 8 to 10 PSI over the clean baseline, aiming the hose down between the pleats. Never use a pressure washer: it drives grit deeper into the fabric and splits pleats. What a hose will not shift is oil, and oil needs a soak, degreaser first and acid second. A cartridge tank is a pressure vessel, so shut off the pump and open the air relief valve before opening it. Above ground, a cartridge suits pools up to about 10,000–12,000 gallons.