Because they were not built for reverse water flow, cartridge filters can't be "backwashed" simply by reversing the water flow in the pool's filter system. These filters must be cleaned by hand when they are dirty. A good cleaning with the garden hose is often enough to restore filter function, but a chemical cleaning is required if you find yourself having to clean your filter every 3 or 4 days. Although pool pump construction varies slightly, the basic process of accessing and cleaning the filter is essentially the same on all equipment.
Cut the power to the filter, shut the valve from the filter outlet that goes to the pool inlet, and shut the valve that goes from the skimmer to the pump inlet. If you don't have valves at these connections, insert a plug into the skimmer to stop the flow of water into the pump and another plug at the pool inlet to prevent the water from flowing back into the filter housing.
Open the filter drainage valve. The drainage valve is located at the bottom of the filter housing. The goal is to empty the trapped water from the filter housing so it does not recontaminate your freshly cleaned filter when you reinstall it later.
Take the top off the cartridge filter to access the cartridge. Depending on the type of cartridge, you may have to unlock a safety latch. Check with your manufacturer's instruction manual for directions on how to unlatch and remove the top of the filter housing.
Lift the filter from the housing and hose it off with a garden hose. It's best to set the garden hose to the most powerful setting to remove any debris caught in the cartridge filter. Hose down the inside of the filter housing, too.
Prepare a chemical bath for your filter in a container large enough to hold the entire pool filter. To make your solution, combine 1 cup of trisodium phosphate (TSP) and 4 cups of water, repeating this ratio until your filter is completely covered in the solution.
Allow the pool cartridge to soak overnight in the TSP solution before rinsing it off with your garden hose.
Replace the cartridge filter and place the top back on the filter housing. Make sure the O-ring on the filter housing is properly seated and that the housing is locked. Replace the O-ring if it is flattened, cracked, or decaying.
Close the drainage valve, then open the filter and pump inlet valves. If you plugged the skimmer and pool return inlets earlier, unplug them now.
Turn on the filter and open the air relief valve on the top of the unit. When the water begins to spray out, shut the air valve. Your cartridge filter is now ready to clean your pool water one again.