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Thread: How often to backwash a sand filter when fighting algae?

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    JasonLion's Avatar
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    How often to backwash a sand filter when fighting algae?

    How often should you backwash a sand filter when fighting algae? It seems well agreed that once the water is mostly clear the dirty filter effect helps you get the smaller particles that are left. What about when the water is a total green swamp?

    I have seen advice about this go both ways, backwash frequently to keep the flow higher, and let your filter get dirty so it filters better. My theory is that a clean filter allows more flow, which means more large stuff filtered out. Other people appear to say no algae is small and you need the dirty filter effect to filter it out.

    There are really two different definitions of "filters better" going on here. Normal day to day operation you want to filter the smallest particles you can, so that is better. But with a green swamp you want to get the largest volume of junk out and are willing to leave the small particles for later, so speed is better.
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    Re: How often to backwash a sand filter when fighting algae?

    Jason, that is an intuitive post and I am with you completely. The "dirty filter filters better" is, IMHO, somewhat of an urban legend in that while it makes some sense, there are very valid arguments agains it. Carried to an extreme, why wouldn't filter manufacturers simple tell you to fill your filter with dirt!!!

    I think filter pressure is the single most important guide. If you get somewhere around a 20% rise in filter pressure, I believe it's time to backwash/clean and, like you say, get your volume back up.

    I think that guideline should apply under any circumstance, algae or not. I also think, based on several years of reading posts about this, that cartridge and DE filters will tend to need cleaning more frequently than sand....simply because they will catch more junk and will tend not to have the capaqcity of sand filters. My two cents.
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    If you are looking for better cleaning from your sand filter why not add a little DE to it. Many people who run sand filters on this site add a little DE to get better cleaning from their sand filter.
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    Quote Originally Posted by sunnydays
    If you are looking for better cleaning from your sand filter why not add a little DE to it. Many people who run sand filters on this site add a little DE to get better cleaning from their sand filter.
    Today seems to be the day for me to rant. I have not seen any empirical evidence that adding DE is worth doing. It may be, but I have tried it three times now, and see virtually no difference in the clarity of my pool water.

    The only test that I see as valid is to look at the suspended particles in the water in front of your pool light at night. Using DE, it appeared to me that my water had exactly the same amount of particles as when I using sand alone.

    There is certainly no harm in applying DE but I question it's real value. A sand filter, set up properly and with your water in balance and properly chlorinated, will get your water clear enough to see heads or tails on a coin in the deep end.....that's good enough for me.

    Just thought I'd stir the pot a little. It's gonna' be 96 in Raleigh today and that makes me pretty cranky!!
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    Mod Squad JohnT's Avatar
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    My theory on algae fighting is that while you have live algae, circulation is more important than filtration, so I backwash whenever the gauge starts to climb. Once the algae is dead, then you want to remove it from the pool, so I go a little longer between backwashes.

    Dave S.: Days like today are the reason you have a pool. Go soak your head along with the rest of you!
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    I didn't mean that DE is the end all in solutions for clean water. I just thought since some people were have luck with it I would suggest it. I would rather have a filter with some DE to help clean the water than a sand filter filled with algae to clean the water better.
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    JohnT......... (head soaking as I type!!)

    Sunny.........my post was in no way intended to discount your advice but rather to put up an alternative opinion based on what I have experienced. You are certainly not alone with the DE theory.....I do think it's a good topic to bat around, however. I hope folks with personal, positive experience with DE that is somewhat quantifiable will post up soon
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    One of the benefits of DE filters is "supposed" to be the removal of smaller particles than sand filters. I'm not saying that's true, but IF it is true, then it "seems" logical that a coating of DE in your sand filter might also do the same. Of course, a lot of things that "seem" logical to me don't really pan out...
    Currrent pool under construction: DIY Kafko steel/vinyl IG 26kgal 20x40 Kidney, sand filter, Triac spillover spa, Aqua Logic SWG
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    I'm fairly sure medvampire said he did centrifuge tests on pool water after a week either with or without a DE layer on his sand filter and got noticably less stuff with the DE. But then waterbear said that that was with old sand, that DE just got you back to where you would have been with new sand. I wish I could find the reference, it was quite entertaining. With my old sand I have a visible, though only subjective, improvement with a DE layer.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Gathering real data on that may prove difficult. That you see improvement, even without any data, is pretty interesting to me and may well mean I didn't give it enough chance. I hope others will chime in soon with their personal experience. I think KurtV and I had a discussion a year or so ago and he was ambivalent about DE's effectiveness.
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    My Two Cents, and now with the Rates, its actually 2 cents, not a penny as it use to be compared to the US. While I don't have alot of experience, I have read tons and asked lots of questions.

    I am saving up to buy a robot, all the reading I have done suggests these lil babies filter down to 1 to 2 Microns, where most filters only achieve a level of around 5 or so. I have been told by my pool boy, if one wanted, they could forego the filter an pump in essence and use a robot twice a day. Look at the numbers, these lil buggers go through twice the water that a normal filter will in shorter time frame.

    Am I right or wrong, curious about those who have robots an what their opinions are?

    Rik

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    Well, we're getting off topic, but your post brings up a truly interesting point. The robots claim to pump (depending on size) somewhere in the 60-80gpm range. WOW!! How the heck do they do that with a little 24v motor that can't hold a candle to the horsepower of a pool pump!!

    I had never made that connection before (I have been looking at robots, too) and I think their claims require some investigation and thought. It isn't logical to me that they can move that much water with so little energy consumed. We may have to start another thread or move your post and mine somewhere else but that is a really eye-opening claim. Skepticism has hurt me from time to time but has usually served me well when reading manufacturers specs. I'll be interested in others' opinions.
    Dave S. - Forum owner
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