Tribute to Dr Phil Penny

Glass filter media (TN42)

PWTAG Technical notes are updates or new material for the standards and guidance given in the PWTG book, Swimming Pool Water and the PWTAG Code of practice and should be read in association with these publications.

Glass has been used as a filtration medium since the early 1900s. But refined glass filter media have been available only in the last 20 years. Media currently available and widely used for pool filtration are either recycled, crushed glass or virgin glass beads.

Glass seems to function perfectly well as a filter medium. This technical note is about its practical application.

Specification and quality

As in the case of sand, there can be enormous differences in quality depending on the provenance of the material and its selection. The quality/consistency of the production process, and the quality control exercised by the manufacturer, will also have a huge influence on the performance of the end product.

It should be noted that many brands of glass media on the market are simply re-badged, recycled, scrap glass that would otherwise be destined for the re-melt or shot-blasting industries.

It is very difficult for the pool operator to evaluate the differences between glass media, but help should soon be at hand in the form of a European standard for glass filter media that is currently being developed by CEN/TC164/WG9. Then only media that conforms to this new standard should be considered for swimming pool use.

In the meantime, care should be exercised in selection of media and two selection criteria may be useful.

  • As glass media is a manufactured rather than natural product, it can be expected to be more consistent in quality. But such consistency can be expected only from manufacturers who practise and adhere to ISO 9001, 14001 & 18001 standards.
  • Drinking water certification (BS EN 12902, DWI certificate 31 and NSF 61 & NSF 50) can also be considered as useful quality assurance benchmarks.

Performance

These quality references relate largely to the cleanliness and safety in use of the media but have no bearing on their filtration performance. Some published material is available that plots the mechanical filtration performance of different media.

If these tests have been carried out by appropriate accredited laboratories and in accordance with swimming pool norms, including BS EN 16713, then PWTAG would have no reason to contest the results.

Care should nevertheless be exercised in comparing data from competing sources to ensure that the data is from an accredited test centre and that testing norms are the same or comparable.

In all cases, for sand or glass or any other filter media, it is important to comply with the manufacturer’s instructions on filtration rates, run phase and backwash.

As with sand media, effective coagulation will facilitate precipitation of organic material out of solution and improve the efficiency of fine particle removal. This contributes positively to bather safety and to chloramine control in swimming pools.

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