Regenerative media filters
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.
- Subject: Regenerative media filters
- Date: April 2014
This note reviews the functionality of regenerative media filters (RMFs). It is based on research including site visits, pool water tests, and discussions with major manufacturers. PWTAG’s work on this subject has spanned more than two years.
Non-regenerative (or pre-coat) filters have been used for some time – more abroad than in the UK. They can be effective, and take up less space than traditional sand filters, but their practical limitations have made it impossible for PWTAG to recommend them on large, busy public pools. They demand more skill from operators than sand filters.
In the last few years, about a dozen UK pools (and many more abroad) have installed a regenerative variation on this theme – of which there are at least three brands. The powdery filter coating is not replaced; instead, it is shaken down from the nylon filter wands at intervals (a process known as bumping) and allowed to reinstate. Filter media is replaced only after many such operations (commonly about every six weeks). It is important that operators use only the grade of filter medium recommended by the manufacturer. RMF installations may also have backup from UV and reverse osmosis (as may conventional filters).
Two immediate advantages are sometimes claimed over traditional sand filters: that they take up less floor space, and that significantly less water is used. The first is demonstrably true – although their use in a new pool means that any subsequent choice for replacement would be restricted to similar filters as there is unlikely to be enough floor space for conventional sand filters. They are also taller than sand filters. RMF filters are being promoted for use in any pools – including large busy ones.
The water saving issue is discussed here, as are other considerations.
PWTAG’s site inspections confirmed that pools with RMFs can provide good quality water, but like any filter there can be problems. There are also issues about the principle of the filters’ operation.
- The wands may not always completely re-coat after bumping. There remains a very significant filtration area which is covered, and the effect of the gaps in cover is uncertain. But there must be a possibility that pollution, including microorganisms, would pass through.
- The buildup of media on the filter wands can bridge between wands. There is some disagreement, even between manufacturers, about how much of a problem this is, but any bridging would reduce the effective filter area.
- The buildup of grease on wands will inhibit the adhesion of filter media (as might hardness scale and other chemical deposits). So wands have to be cleaned and degreased as necessary – sometimes as often as every six weeks. This may require an alkali wash, taking several hours, during which time the filter is out of action. High-pressure hosing is an alternative, although health and safety issues would need to be addressed.
- Filtered material, including microorganisms, remains in the filter and forms part of the mainly filter media mix that re-coats wands during bumping. Organic matter held on the filters has the potential to release disinfection byproducts into the pool water. Sand filters also retain such materials, but they are routinely backwashed to waste at least once a week. RMFs, by contrast, retain these materials until the medium is replaced (perhaps after six weeks).
- Microbiological tests are only a monthly snapshot of pool hygiene. Results PWTAG has obtained from pools with RMFs have been patchy, as they can be with any pool. In at least one pool, poor results were associated with gaps in filter media coverage.
- Replacement of wands and general refurbishment need to be considered carefully. Such issues are more challenging than with sand filters.
PWTAG is concerned that some pool operators believe the use of RMFs will result in cost savings associated with water and heat because there is no backwashing. But water replacement (dilution) in swimming pools is not only to replace backwash water but also to reduce the buildup of bather pollutants and chemical breakdown products from everyday water treatment processes. PWTAG recommends a water replacement rate for this purpose of 30 litres per bather irrespective of the filter type. This is widely accepted as important for good pool water treatment: without adequate dilution dissolved solids and chloramines can build up. Pools with regenerative media filters have been found to have total dissolved solid (TDS) levels very much more than 1,000mg/l above source water because dilution was being neglected. Chloramines, too, have been a problem in some cases.
As this note clearly implies, PWTAG’s recommendation for TDS is that it should not be allowed to rise more than 1,000mg/l above source water. That is the figure on page 99 of our book, Swimming Pool Water. Unfortunately a table on page 95, from a previous edition of the book, mentions a maximum of 3,000mg/l. This is not longer what PWTAG recommends and should not be quoted in support of regenerative media filters or any other process.