Gravel is used in Water Filtration with water treatment filtration products Silica Sand, Anthracite, Activated Carbon, Birm, Pumice and Garnet in portable water treatment, surface water treatment, swimming pool water treatment, R.O. reverse Osmosis treatment plant and wastewater treatment also in conventional water treatment filters. Filter Gravel is an extremely effective filter media because of its ability to hold back precipitates containing impurities. Filter sand size, angularity and hardness are the important filter sand characteristics to ensure proper filtering. In slow sand filters, the gravel is normally on the bottom layer. We use them as channels for filtered water to make it to the outlet lines. Sand will often plug the holes in the outlet pipe and the gravel helps prevent that.