It may seem counter-intuitive, but fine sand is actually an excellent cleanser for pool water. The sand particles are ground to the consistency of .45 to .55 in diameter--rough enough to trap debris, but fine enough to allow water to filter through.
The pool water enters a tank through a diffuser, which draws the water down into the bed of sand. The particles of debris are caught in the sand bed, and the water that exits from the bottom of the filter is returned clean to the pool.
Anything in the water that is smaller than 40 microns will most likely not be filtered by the sand. The cleansed water will return to the pool through a set of lateral tubes at the base of the filter. To kill bacteria and mildew, you will need to add pool chemicals to the filtered water, but the sand filter will reduce the need for harsh chemicals.