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Article #17: Filtration vs. Oxidation

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    Monthly Manager Moments Article #17


    Part 2 of a Series on How Pools Work - Filtration vs. Oxidation

    Overview -

    Filtration is of course the term for filtering the water in a pool.  We'll briefly discuss filter types commonly used in commercial pools, and specifically the filters used here at EWU.  Oxidation refers to reducing and/or eliminating organic compounds that cause "chlorine smell" and eye irritation.  Oxidation is measured in ORP (oxidation reduction potential) by an electronic probe.  Chlorine is the most prevalent oxidizer in the pool industry, although other oxidizers are also used.  The most common enemy of oxidation is monochloramine  NH2Cl.  Although this compound of chlorine provides a little oxidation, it is dramatically less than than "free chlorine" which is hypochlorous acid or HOCl.  There are other forms of combined chlorine besides monochloramine, but we'll just refer to all forms as combined chlorine. 

    Why filtration vs. oxidation?

    It's because as an operator, we have to keep in mind that oxidation is about 65% of the formula in maintaining pristine, clear, sparking water; whereas filtration is really only about 35%.  The filters must work, of course, to keep particulates out of the water, but to get the sparkling quality that some pools have, it requires excellent oxidation.  When first filling your pool, crank up the chlorine to provide outstanding oxidation prior to the introduction of swimmers, who will also introduce organic contaminants that must be oxidized.  I recommend seeding your pool with granular chlorine (calcium hypochlorite) before you start your pump.  If you start with oxidation at a high level, you'll find that the filters are  able to clear your pool up very quickly.  This pool went from "Shrek's Swamp" to crystal blue in just one day of filtering, thanks to good filters, and high ORP levels at the time of filter start-up.  What's high ORP?  I recommend 850 mV or higher, to provide essentially constant breakpoint chlorination and outstanding clarity.  OK, then what's low ORP?  Anything approaching 700 mV is low indeed.  It takes just 650 mV to kill most bacteria, but well over 800 mV to get the outstanding clarity that you want.


    Pool filters

    Pool filters come in two main categories: vacuum filters and pressure filters.  Vacuum filters are those in which the pump pulls the water through them; and pressure filters are those in which the pump pushes the water through them.  As for types of filters, we see mostly hi-rate sand or DE in the commercial pool industry.  Smaller pools and spas may use cartridge filters, similar to your car's oil filter.  Regardless of the type of filter, it is either a vacuum or pressure system.  Let's look at them individually. 


    Oxidation is probably the most important, and sometimes the most difficult part of the pool clarity puzzle to maintain.  Our goal for oxidation is continuous breakpoint; which means that the water has so much oxidizing power that all organic compounds are constantly being eliminated/oxidized.  No build-up of combined chlorine is allowed to occur.  Combined chlorine is our main enemy, causing vitually 100% of eyeburn and irritation, plus the classic "chlorine smell" of many swimming pools.  Why is it difficult?  Because it depends upon many things, some of which you cannot always control.  To maintain high ORP, at 850 mV or better, the following conditions must exist:  low pH - around 7.3, 7.4 at most; zero or low cyanuric acid; zero or low combined chlorine; free chlorine of 3-5 PPM; predictable bather loads; reliable oxidizer feed system.  You can see that any one of these conditions may get messed up and reduce ORP below the desired levels.  When that happens, operators may need to superchlorinate their pool to again reach breakpoint chlorination level.  What does that mean, anyway?  Breakpoint is the point at which the oxidation has overwhelmed the combined chlorine and eliminated it, so only free chlorine remains.

    Glossary of terms

    Filtration - the process of filtering the pool's water to obtain clarity.

    Backwashed - the term for reversing the flow to the filters, to lift the dirt off the surface and send it to waste.  Traditional vacuum DE filters are not backwashed, but merely hosed down.

    Hi-rate sand filters - 20 GPM/Ft2 filtration rate, special grade #20 silica sand, creates a 3 dimensional filtering effect, due to the depth of the filtering ability.  Filters composed of sealed tanks through which the water must pass to get filtered.

    Bumping the filters - stopping the flow, or shaking the elements to redistribute the media and extend the filter cycle.

    Mud balls - clumps of dirt that become hard and form "rocks" that can penetrate the sand bed, creating channels.

    Channeling - see mud balls above.

    Tumbling - too much flow on backwash cycle that results in loss of sand, due to flushing it away to waste.

    Surge tank - a collection tank where surge is accommodated.  Here, it's also the filter tank.

    Vacuum DE filters - 2-2.5 GPM/Ft2  nylon filter sleeves coated with DE or other media.  Vacuum DE means that the pump is after the filters and the water is pulled through.  Sysem composed of a surge tank/filter pit in which the filters are positioned.  Water must flow through the filters to get to the pump's impeller.

    Vertical turbine pump - Tall, vertical pump system, with one or more impellers, typically associated with vacuum DE filters.

    Oxidation Reduction Potential (ORP) - the oxidizing power of the water.  Generally, the higher the better.  ORP is about 2/3 of the contribution to pristine water, and filtration about 1/3.

     Regenerative DE - a hybrid DE system, contained in a sealed vessel; with backwash capabilities.


    Questions about filtration or oxidation?  Send me an email!


    Greg Schmidt, AFOI

    EWU Aquatic Center Manager


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    Contact Information

    Greg Schmidt
    PAV 336
    Cheney, WA 99004

    phone: 509.359.4252

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