Zeolite: alternative to sand media
Zeolite is a natural alternative to silica sand used in high-rate sand filters. It is a microporous, aluminosilicate mineral that is commonly used as a sand replacement because it has better filtration capabilities and is capable of ion exchange, a fact that will be explored in more detail.
Zeolite has superior filtration capabilities relative to sand for two reasons. On a grain-by-grain basis, zeolite particles are larger than sand particles, and each particle is highly porous. Under a magnification, a grain of zeolite looks like a mountain range, with a significantly higher surface area than silica sand. For this reason, zeolite has dual filtration capacity, trapping debris not only between the particles, but also within each particle. Compared to sand, which filters particles of about 30 microns, zeolite can filter particles of about 5 microns, making it comparable to DE or cartridge filtration.
Furthermore, zeolite is capable of ion exchange. With its unique ability to adsorb ammonia, zeolite can play a role in water purification, in theory. Ammonia enters the water from bather waste and the negatively charged surface of the zeolite can release an ion in favor of the ammonia molecule.
Then, once the zeolite has reached its capacity for adsorbing ammonia, it must be recharged. This involves soaking the zeolite particles in a salt solution to replace the ammonia ions with sodium ions. It is generally recommended to make a 10 percent salt solution (about 1 oz of salt to 10 fl oz of water), allowing the zeolite to sit overnight. Then backwash to waste.
It should be noted that chlorine competes with zeolites to capture ammonia, and chlorine wins. So, while it is true that zeolites are capable of ammonia adsorption, ammonia will combine with chlorine to form chloramines far more quickly than it takes for an ammonia molecule to reach the filter. Therefore, while zeolites are capable of ammonia capture in theory, in water that contains chlorine, the chlorine will dispense with the ammonia long before the zeolites do. Thus, zeolites do not decrease chloramines formation.
Zeolite filter media is considerably more expensive than sand. However, its increased filter capacity may extend back washing cycles, a convenience that may outweigh the cost.