If you have a swimming pool filter then the likelihood is that it is a sand filter. As the name suggests a sand filter uses sand to filter the impurities out of the water. Sand is put in the filter vessel and the water is forced under pressure through the bed of sand. The water molecules pass through the gaps between the sand grains but solids in the water are held back in the gaps between the grains.
You cannot use just any old sand in a swimming pool filter it has to be a special refined grade of sand known as 16/30 grade. The numbers refer to the size range of the grains in thousandths of an inch. In 16/30 grade sand the smallest grain will be 16 thousandths of an inch, there should be no grains smaller than this. The largest grain will be 30 thousandths of an inch, there should be no grain bigger than this.
This type of sand is not usually kept in stock in builders merchants or DIY superstores but you can buy it from PoolStore. It comes in 25 kg bags and a typical filter requires 6 or 7 bags. Rather than load them all in your car, we can deliver them to you door the next working day.
If the grains of sand are smaller than 16thou they will pass through the laterals at the base of the filter and end up back in your pool. If you see sand grains in your pool then it is probable that they have come out of the filter either because the grains are too small or because the laterals in the filter are broken.
If the grains are bigger than 30thou they will be too big to catch the fine solids in the water.
After a while the gaps in the sand fill up with dirt from the pool and you have to “backwash” the sand by forcing the water through it in the opposite, upwards, direction. This loosens up the sand allowing the dirt to be washed out and then you can restart the process again.