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POOL PLANT – FILTERS


Filters Under The Microscope


I


t cannot be over emphasised that the principles of good housekeeping recommended by STA and PWTAG are essential in the formula for continuously producing safe, clear, hygienic water. Over and above this is the requirement that the pool operator is aware of the potential dangers and instigates regular checks of the filtration system.


The potential dangers relate to, firstly, the understanding and maintenance of breakpoint chlorination (or bromination) at all times. Any levels below the recommended two parts free chlorine to one part combined chlorination will possibly be the only opportunity that pathogenic (harmful to humans) micro- organisms will need to multiply successfully and cause a threat to health. Secondly, in relation to the filtration system the introduction of continual flocculation is a must – this ensures that the disinfectant resistant eggs of the pathogenic organism cryptosporidium are prevented from passing through the filter.


Regular inspection and maintenance of filters are essential, and this should include: • Sampling of media to identify any colonisation by microorganisms


• Visual inspection of filter media – checking to see there is no caking or balling of the bed


• In spa and hydrotherapy filters, additional study of the bed to discover if there is fatty residue building up


• Observation to see if the bed is completely active – or are there dormant areas • Checking filters to see if hydraulic and/or air scour is effective.


Ineffective portions of the sand bed that lie dormant with no active disinfectant reaching them, can cause a breeding ground for unwanted bacteria. These will discharge into the flow when conditions are conducive to their


multiplication i.e. low sanitiser, heat and plentiful nutrient.


Perhaps at this point it is pertinent to recommend that future design of filters should include easy access for inspection with viewing portholes located at the sand- water interface so that bed activity can be monitored. Another feature which can aid observation is lighting within the filter. Modern technology allows the bio swabbing of filter outlet samples to ascertain any colonisation of the filter by harmful bacteria. It is common that a great percentage of commercial pool operators still use the strainer box to dispense coagulant after backwashing – this is not recommended by STA or PWTAG. Furthermore this can be rectified by the introduction of a simple dosing arrangement – this allows for continual


Recent information by microbiologists has led to the discovery of possible problems within medium to high rate pressure sand filters – the most common types used in pool spas – with different uses producing specific problems. Robbie Phillips and Richard Lamburn, STA’s Technical Pool Plant Team, take a closer look at the issues


flocculation during bathing, which enhances the filter efficiency. Perhaps, with more research, the value of chlorine dioxide with flocculation and/or disinfection together with UV radiation will solve a serious problem. In summary, it is essential that breakdown in disinfection is treated as a major incident and rectified immediately, with consideration of pool closure. It is also necessary that


filters be inspected regularly with the objective of identification of possible process malfunction. Spa and hydrotherapy pools should be inspected every six months and conventional pools every year. Suitable risk assessment for both types should be undertaken before and during opening.


There are defined bed depths with medium rate filtration. Often these are not adhered to resulting in a breakdown in filtration. This highlights the role and responsibility of designers and installers.


Poorly managed filtration systems are a source of significant dangerous bacterial growth.


STA 01922 645097 www.sta.co.uk


Extreme caking of sand filter surface layers on a poorly managed unit (caused by ineffective back washing)


A view of the filter bed breaking up as the air blower is switched on


www.swimmingpoolnews.co.uk SPN April 2015 49


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