No filter is designed to have dirt go through the filter and re-enter the swimming pool. As the filter sucks in the water from the skimmer hose it filters the dirt and debris, releasing clean water back through the pool's return jet. If dirt is making its way back into the pool, try a few remedies.
The three types of pool filter systems--sand, diatomaceous earth and cartridge filter--all rely on a pressure gauge to indicate when the filter is too dirty and needs to be cleaned. if dirt is escaping back into the pool, it could be because the filter is dirty and clogged, which means it needs to be backwashed. When the pressure gauge rises eight to ten pounds per square inch (PSI) above it respective normal operating range, backwashing is necessary. The pressure inside the filter builds up when it becomes clogged with too much dirt and debris from its normal operations.
Sand filters utilize a sand tank that filters dirt and debris as small as 20 microns in size. The sand inside the tank is protected by laterals. The sand filter laterals are a network of tubes that extend from a central PVC core and separate the sand and dirt from the water. The water passes through these laterals, which block the sand from entering the water. If dirt is escaping from the filter, it could be a sign of a broken lateral. in this event, you will most likely have sand coming from the return jet as well.
If backwashing doesn't work, the cartridge filter needs to be replaced. Cartridge filters use a pleated polyester cartridge that is encased in the filter housing. This cartridge is what captures the dirt and debris during the filtering process. The cartridges, which only need cleaning about once per month, may need to be replaced. Cartridges need to be replaced on a filter about every three to five years as the polyester becomes more porous and less likely to filter properly.
Diatomaceous earth filters are the most efficient on the market, capable of trapping dirt as small as three microns in size. The DE filters work with the help of diatomaceous earth powder that is added through the skimmer after the filter is backwashed. This powder attaches itself to filter grids, which then work together to capture the necessary dirt and debris. In some cases, the filter grid may crack or tear, allowing dirt to pass through during the filtering process. In this case, disassemble the filter and have it repaired by a licensed professional.