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Thread: Sand filters and salt water pools

  1. Back To Top    #1

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    Sand filters and salt water pools

    There is a thread going in the build section where the OP wanted feedback on the type of filter he should get, sand, cart, or DE. A couple of people chimed in that one of the disadvantages of a sand filter was the chemical dilution that can occur after backwashing and adding top off water. One responder said that with a salt water pool, it could be that a sand filter may not be the best choice since salt would have to be replenished, etc, etc. No doubt that backwashing and refill will reduce the chemicals in the water. But, it got me to thinking about what the real numbers are in terms of gallons lost from backwash and salt loss. For now, I only did an analysis of salt loss. You can do something similar for CYA, and calcium, but I thought the salt would be a good place to start.

    First, there have to be some assumptions. You need to figure in the following:

    1) g/min lost in each backwash
    2) how long do you backwash
    3) Optimum salt level

    I used 40 gallons/minute for the backwash rate. Obviously this will vary on filter size, pump size, head loss, etc. But I figured 40 would be a decent starting number. How long you backwash in another variable that can/will change depending on a variety of factors. For my example is used 5 minutes (4 minute backwash, 1 minute rinse) since this is usually what I do. Actually, 4 minutes is a lot of backwash time, IMO. I also used 3400 ppm salt as an optimum since most SWCG use salt in this range.

    So, 40 g/min for 5 min =200 gal

    200 gal=757 L

    3400 ppm salt = 3.4 g/L

    3.4 x 757 =2574 g of salt


    = 2574 g of salt lost per backwash. That equals 31 ppm lost (assuming a ~20,000 gal pool) from the original 3400 ppm assuming one replaces the 200 gallons lost.
    2574 grams is 5.7 pounds.

    So, for a 40 pound bag of salt at ~$10 a bag, that roughly $1.47 worth of salt per backwash. Not a lot of money lost per backwash. Plus, 31 ppm is not enough salt loss to even come close to changing the salt concentration enough to stop chlorine generation because of low salt even if you didn’t add any.

    I invite others to chime in if my math is off someplace or my assumptions are off or to generally throw sticks. My thought with this was to see if the belief that sand filters were not the greatest thing for salt water pools had some merit. As disclaimers, I did not take into account the cost of water to refill after backwash. For most, 200 gallons is a small cost (note I said most, not all).


    Just a point of reference for how many inches 200 gallons represents. An 18x32 pool is 576 sq. ft. 200 gallons is around 1/2 inch of water in a pool that size.
    14,000 gallon IG, Vinyl. Hayward 3/4 hp superpump, Penatair IC40 SWCG, Pentair automation, Hayward sand filter, Aqua Comfort heat pump, Hayward 400k Lo-Nox LP heater.

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    MikeInTN's Avatar
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    Re: Sand filters and salt water pools

    Well, they never said it was a big disadvantage, did they?

    *trying to find the throwing sticks icon*

    24' x 52" AGP - approx 13,500 gallons
    Pentair Optiflo 1 hp/2sp pump w/ Swimpro Voyager 150 sq ft cartridge filter
    Intex 8110 SWCG
    "Fear the Schnauz!"

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    frustratedpoolmom's Avatar
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    Re: Sand filters and salt water pools

    Are you mad cuz I pushed a cart? :P LOL
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
    150 Sq ft. Clearwater Cartridge Filter; Former and DISSATISFIED "Pool Frog" owner
    http://www.PerfectlyClearPoolService.com

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    Re: Sand filters and salt water pools

    Quote Originally Posted by frustratedpoolmom
    Are you mad cuz I pushed a cart? :P LOL
    14,000 gallon IG, Vinyl. Hayward 3/4 hp superpump, Penatair IC40 SWCG, Pentair automation, Hayward sand filter, Aqua Comfort heat pump, Hayward 400k Lo-Nox LP heater.

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    Re: Sand filters and salt water pools

    bk, you're beating your head against a wall. I'll bet $50 that the ones who warned of the potential disadvantage of backwashing were cart owners.

    I don't have an SWG, but I have never found that backwashing significantly alters my pool chemistry, and I use significantly more water than the scenario you propose. IMO this is just another facet of the ongoing filter debate which will never be settled even if your calculations were exactly precise. I refuse to enter this argument... CARTS SUCK AND SAND IS FOR WIMPS! DE FILTERS RULE!!!
    20K gal IG plaster pool, Manually chlorinated with 6% bleach, 1.5 HP Sta-Rite Dura-Glas II pump, Pentair FNS Plus 48 DE filter, Polaris 280

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    Re: Sand filters and salt water pools

    20K gal IG plaster pool, Manually chlorinated with 6% bleach, 1.5 HP Sta-Rite Dura-Glas II pump, Pentair FNS Plus 48 DE filter, Polaris 280

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    frustratedpoolmom's Avatar
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    Re: Sand filters and salt water pools

    Beez, now that's funny!
    The part about DE filters rule
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
    150 Sq ft. Clearwater Cartridge Filter; Former and DISSATISFIED "Pool Frog" owner
    http://www.PerfectlyClearPoolService.com

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    Re: Sand filters and salt water pools

    It's a bit less that what you compute as salt is being "added" by sneaky swimmers all the time.

    Uncle Dave
    20K Gallon Gunnite pool/900 Gallon in ground Spa/2HP WhisperflowSpa
    Pentair Master temp 400/Hayward Sand WZeobrite/Jandy RS-8 Aqualink/Jandy 2HP Epump
    12 - 4x10 Panels/Zodiac G3-T5 / BBB

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    Re: Sand filters and salt water pools

    Quote Originally Posted by Davegvg
    It's a bit less that what you compute as salt is being "added" by sneaky swimmers all the time.

    Uncle Dave
    14,000 gallon IG, Vinyl. Hayward 3/4 hp superpump, Penatair IC40 SWCG, Pentair automation, Hayward sand filter, Aqua Comfort heat pump, Hayward 400k Lo-Nox LP heater.

  10. Back To Top    #10

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    Re: Sand filters and salt water pools

    Quote Originally Posted by bk406
    Quote Originally Posted by Davegvg
    It's a bit less that what you compute as salt is being "added" by sneaky swimmers all the time.

    Uncle Dave
    Don't worry bk, it's from the perspiration... You know, how beer makes you sweat a lot? :P
    20K gal IG plaster pool, Manually chlorinated with 6% bleach, 1.5 HP Sta-Rite Dura-Glas II pump, Pentair FNS Plus 48 DE filter, Polaris 280

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    Re: Sand filters and salt water pools

    Yeah...thats it... perspiration.

    Dave
    20K Gallon Gunnite pool/900 Gallon in ground Spa/2HP WhisperflowSpa
    Pentair Master temp 400/Hayward Sand WZeobrite/Jandy RS-8 Aqualink/Jandy 2HP Epump
    12 - 4x10 Panels/Zodiac G3-T5 / BBB

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    JasonLion's Avatar
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    Re: Sand filters and salt water pools

    bk406, I agree not especially significant.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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