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An Introduction to Zeolites

The word "zeolite" comes from the Greek meaning "boiling stone", a name that stems from the property of many natural zeolites to liberate water when heated. Zeolites are porous aluminosilicates of the alkaline and alkaline earth metals (mainly sodium, potassium, magnesium and calcium). Created by volcanic activity, there are about 45 naturally-occurring zeolites and upwards of 150 that have been artificially synthesized.(1)

Natural zeolites were first used by the Romans to filter their drinking water, whilst the first significant synthetic zeolite was prepared by Linde, a division of Union Carbide, in 1949. Zeolites came to prominence industrially when Mobil patented the synthetic ZSM-5 catalyst in 1977 for the conversion of methanol to petroleum.(2) Since that time, natural and synthetic zeolites have found application in a wide variety of industries, ranging from petroleum refining and detergent manufacture, through to water- and ammonia-retentive additives for soils. Zeolites have also been used in livestock feeds and human health food supplements and, more recently, the Pool and Spa Industry has seen the introduction of zeolites as a filter medium to supplement or completely replace sand and DE.

Zeolites as Pool Filter Media

The specific type of zeolite marketed for swimming pool filtration is known as clinoptilolite (from the Greek meaning "oblique feather stone", a reference to its the "feathery" appearance of its crystals). A naturally-occurring zeolite mined from volcanic deposits in several countries including the US, Canada, Germany, Russia, Japan and, notably, Australia and New Zealand, it can absorb a significant amount of water upon drying and retains its network structure even when heated to temperatures that would melt glass.(3) Bio-Lab is aware of two clinoptilolite products being sold as pool filter media in Australia and NZ, namely Zelbrite (Zeolite Australia Pty Ltd)(4) and Activated Zeolite Pool Filter Medium (SuperSorb Environmental NL).(5) A third product Zeotec Filter is currently being pursued by NZ Natural Zeolite.(6)

Generally, pool owners are advised to supplement part of their existing filter medium (usually sand) with zeolite. The higher the percentage of clinoptilolite, the greater the filtering capability. According to the marketing information supplied with Zelbrite, one gram of clinoptilolite has a surface area of 20 to 30 square metres.(4) As water passes through the zeolite particles, debris are trapped within the highly porous material. Clinoptilolite can filter out particles down to 3 microns;4 sand can usually manage down to 15 microns, whilst DE can accommodate 5 microns. Clinoptilolite also has a high affinity for ammonia present as ammonium ions which become trapped in the micropores. The ability to trap ammonia helps reduce chloramine formation and, hence, has a significant impact on decreasing chlorine demand.

When replacing sand with a zeolite, it is important to first cover the laterals, then fill the tank to the original sand level. As the zeolite is lighter than sand, it requires around 20% less zeolite by weight to fill the volume formerly occupied by the sand. After adding the zeolite, the filter will need to be backwashed for at least 40 minutes and preferably longer. As usual, don't forget to ensure an appropriate water level in the pool prior to backwashing.

When ammonia and other wastes build up in the zeolite, it is necessary to regenerate the filter. This is done by soaking the medium in a 10% w/w salt solution for 6 - 12 hours. The sodium ions replace the ammonium ions in the zeolite micropores by a process known as ion exchange. After soaking, a normal backwash in usually all that is required to flush the filter clean of the liberated ammonia. Whilst extremely stable, however, zeolite filter media will eventually clog up with material that can't be removed using a salt solution. In the normal course of events, it has a similar operational lifespan to sand and ideally should be replaced every 5 to 7 years.

Incompatibilities

While this filter medium sounds appealing, it does have some drawbacks. A major concern is product compatibility. Any products containing polymers or flocculating agents cannot be used. This includes most algaecides (eg. copper polyacrylate-based like MSA II, Back-Up II and Protector, and polyquats like Polygard and Algiguard), clarifiers (eg. Polysheen Plus, Super Clear Tabs) and flocculants (eg. Power Floc, Water Sparkle). Using these products will clog the porous material - sometimes irreversibly - resulting in inadequate filtration. Using zeolites as a sand replacement may also void some filter manufacturers' warranties, so it is prudent to check this before adding clinoptilolite to a filter.

References

1. "Zeolites: Industry Trends and Worldwide Markets in 2010" (Review of Report, www.newsletters.com/map/prod/722220.html), Technical Insights, Inc.
2. F.G. Dwyer, F.V. Hanson and A.B. Schwartz, US Patent 4,035,430, "Conversion of Methanol to Gasoline Product", Mobil Oil Corporation, New York, NY (Dated 12 July, 1977).
3. "The Mineral Clinoptilolite" (mineral.galleries.com/minerals/silicate/clinopti/clinopti.htm), Amethyst Galleries, Inc., Ponte Vedra Beach, FL.
4. "Zelbrite" (www.zelbrite.com/), Zeolite Australia Pty Ltd, Melbourne, Victoria.
5. "Activated Zeolite Pool Filter Medium" (www.supersorb.com.au/PDF/pool.pdf), SuperSorb Environmental NL, Albany, Western Australia.
6. "Swimming Pools" (www.zeolite.co.nz/pools.html), NZ Natural Zeolite, Matamata, New Zealand.

The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein