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POOL PLANT – FILTERS


Key Facts To Filtration And Backwashing


HERE, ROBBIE PHILLIPS, STA’S SENIOR POOL PLANT TUTOR TALKS FILTERS AND EXPLAINS WHY THEY ARE SO IMPORTANT IN ENSURING POOL WATER IS SAFE AND HYGIENIC


clean the pool water, it is passed through a filtration system. If we didn’t do this, the source water would leave a cloudy sediment-rich pool.


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WHAT HAPPENS IN THE FILTER? Pool water is passed vertically through a bed of sand (or other media), which has regular, sharp angular grain sizes (0.5-1.0mm) so that there are specific, evenly sized gaps between the sand grains. By regulating the flow rate, very small particles are allowed to become trapped between the angled faces of the sand grains.


A typical mature pool filter traps particles as small as 10 microns but to trap organisms and other debris of even smaller size, such as cryptosporidium infective oocysts (eggs), flocculation or coagulation of 3-6 microns is necessary because this process binds smaller particles to a size where the resultant particle is not able to pass through the sand bed. The beds can be uniform grain size range or be a multimedia bed with layers of media that starts at the top of the filter bed with filter sand and increases in sand/pebble size as it goes down to the bottom.


An arrangement of fine slitted pipes lies at the bottom of the filter, which allows water and minute particles to pass but retains the filter media.


Current industry regulations recommend a filtration velocity of between 10 and 25m³/hr (m/hr), which should give a corresponding filtration rate of between 10 and 25 m³/m²/hr. This is medium rate filtration. However, the filtration potential is rather less effective immediately after backwashing because it takes a period of time for all the water to pass through the filter.


High rate is from 25 up to 50 m³/m²/hr, where flocculation and coagulation are not nearly so effective.


FILTRATION MODE As the pool water runs through the filtration mode, particles become trapped on the top of the sand bed. This results in a build up of pressure on the inlet side of the filter and a decrease in outlet pressure, which is known as a ‘pressure differential’. When


38 December 2013 SPN


wimming pool water contains various pollutants that are introduced by bathers, the pool operator and the source water supply. To purify and


more resistance occurs during the filtration mode, turnover times are affected and this is a clear sign that the filter needs to be cleaned or ‘backwashed’.


HOW DO WE BACKWASH? Backwashing involves reversing the flow of water through the filter to fluidise the bed and lift the particle build up to send it to drain. The trapped particles are much lighter than the sand so there is natural separation. To optimise this agitation of the bed, the backwash can be carried out in two parts: Air scour: the bed is subjected to a cascade of air to loosen the bed. Agitation: the bed is subjected to a flow of water at a specific rate, which causes the sharp sand grains to clash and ‘self clean’.


INEFFECTIVE BACKWASHING It is vital that any backwashing is carried out to the manufacturer’s guidelines, especially when it comes to water velocity through the filter. It needs at least 30m³/m² per hr to ensure the bed is lifted so that any pollutants are sent to the drain.


The design of filters is critical to safe and hygienic pool water. Ineffective backwashing can lead to problems that could compromise bathers’ safety, so a thorough understanding of this issue, gained through training and site based knowledge, is paramount. For further advice on managing pool plant operations and STA’s Level 2 and Level 3 Swimming Pool and Spa Water Treatment qualifications, please contact the STA. SPN


STA 01922 645097 www.sta.co.uk


• Backwash according to pressure differential and/or time.


• Until sight glass runs clear. • When it’s safe to do so.


“BACKWASHING INVOLVES REVERSING THE FLOW OF WATER THROUGH THE FILTER TO FLUIDISE THE BED AND LIFT THE PARTICLE BUILD UP TO SEND IT TO DRAIN. THE TRAPPED PARTICLES ARE MUCH LIGHTER THAN THE SAND SO THERE IS NATURAL SEPARATION”


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