If your pool has sand in it during normal operation (not just during or immediately after a backwash) then you probably have broken laterals in the sand filter. To understand how this happened and what needs to be done, how sand filters function must be explained a bit.
To keep it simple: A pool’s sand filter is just a large container full of sand (and sometimes gravel). Water from the pool comes enters at the top, where it falls through the sand. This collects the detritus from the pool. It literally gets caught in the sand. At the bottom of the sand filter, buried in the sand are plastic pipes called “laterals”. Imagine the spokes on a wagon wheel. These are all connected to a standpipe in the center of the filter. These laterals have many perforations in them, which allows the water to return to the pool, but are small enough that the sand cannot enter. The water enters the perforations, flows to the center pipe, and returns to the pool, “cleaned” by the sand filter.
Over time, the detritus builds up in the sand, increasing pressure. Periodically the sand filter has to be cleaned out. This is done by backwashing. It reverses the flow of water through the sand filter. Instead of water flowing through the sand and into the laterals and then the center pipe, now it’s backwards. All the debris that has collected on the sand is now forced back out and down a waste pipe.
Over years, the laterals will wear out. The tiny perforations that keep the sand out, will eventually wear due to the water and sand grinding against them. Chlorine will also degrade the plastic over time. Long periods of very high chlorine levels will exacerbate it.
Other causes include dumping sand on top of the lateral during past maintenance and damaging the laterals, over-pressuring, slamming open/close valves and creating a water hammer event.
To correct, the sand must be removed from the filter, exposing the laterals. Shopvac the sand out, being very careful around each lateral. Each lateral will need to be inspected and the broken ones replaced. Look for cracks or breaks in them. Inspect the center standpipe for damage as well, as if it’s cracked sand could be forced in there as well. The laterals screw into the standpipe and there should be no gap between the lateral and standpipe. If unsure which laterals are the culprit, replace all of them. Typically, pool sand lasts 5-10 years, so unless the sand is very old, it can be reused. That said, pool sand is not expensive typically, and if already removing the sand to service the laterals, it may be a good time to replace the sand. When replacing the sand, don’t just dump a bag in. Add some water to the filter and carefully place the sand around the laterals until above them. Dumping a bag of sand in could possibly damage the new laterals.
After replacing the laterals, some residual sand and sand moved into the standpipe may cause some sand to initially come out of pool return. Run under normal operation for several hours to confirm that the laterals are fixed.