Glass

All the principals of high-rate sand filtration apply to glass, with a few exceptions. Unlike sand, which traps debris with its jagged edges, glass has a slight negative charge on the surface to attract fine particles, which cling to the media like static-charged socks to a sweater. This weak charge releases the particles upon back washing for a more efficient cleansing. Manufacturers say glass has another unique characteristic: filtration takes place through the entire bed depth, unlike sand, in which gunk commonly builds up within the top 6 or so inches. This allows for a larger load of contaminant.

This is because glass, which is less dense than sand, doesn’t pack as tight. Proponents also say glass has a tighter turbid catch, with an ability to filter down to 9 microns. Some brand claims go as low as 3 microns. This DE-quality filtration can be achieved without chloramine-producing flocculants and shock treatments, says one manufacturer, which is one reason glass is most commonly found at indoor pools where air quality is a top concern.