If your filter is returning debris to the pool, you definitely have a problem because filters are supposed to remove that stuff from the water, not recirculate it back into the pool. Here are some things to look for in each type of filter.
The great thing about cartridge filters is that they are so simple. We will assume that you have the proper cartridges in your filter
Worn cartridges – your cartridges may have holes in the fabric or they may be compromised in some other way. If the bands around the cartridge are broken, it is probably time for new cartridges anyhow.
Filter not assembled properly – it is possible that you may have not put the filter together right or may have left some parts out. It is pretty hard to mess this up, but it has happened before . . .
DE filters provide slightly better filtration, but they are also significantly more complicated to work on. Here are some things to look for if you have debris or DE returning to the pool.
Broken grids or manifold – this is the most common cause. The grids can break down to old age or due to abnormally high pressure. To check for this, you will need to do a filter clean (or have it done by Pool Stop) and check the integrity of the components.
Missing standpipe o-ring – the DE powder is so fine that something as small as a missing standpipe o-ring can create a path for the DE to return back to the pool. Again, this can only be checked by disassembling the filter.
Missing backwash o-ring – if the backwash o-ring closest to the backwash port is missing or damaged, you will lose a LOT of water quickly. However if the other backwash o-ring is missing or damaged, it will simply allow debris and DE to bypass the filter and go back to the pool.
Sucking in DE in backwash mode – if you accidentally add DE powder while the filter is in backwash mode (oops), then VOILA . . . you will dump all that DE back into the pool when you put the filter back in filter mode.
Most residential pools do not use sand filters, but if you are one of the lucky ones with a sand filter, here are some things to look for if you are returning debris to the pool.
Old sand – as filter sand ages, the particles lose their “edge” and no longer trap particles as they should. This usually results in cloudy water, but not in large debris returning to the pool.
Channeled sand – over time, the sand against the inside walls of the filter tank can become mixed with dirt and create channels along the inside wall of the tank through which water and debris passes.
Broken laterals – if you are seeing filter sand return to the pool, then you may well have broken laterals, or perhaps the air relief tube has become dislodged and sand is returning through it.
The solution for issues 1-3 is to change the filter sand and inspect for signs of wear. This is a labor intensive process, which is one reason that most people prefer cartridge filters on home pools.
Faulty backwash valve – if the o-rings or gasket on the backwash valve has failed, then it will allow water to bypass the filter, and in most cases also allow water to go straight to the drain. The solution is to disassemble the backwash valve and inspect for worn or missing o-rings or gaskets.