I am doing research for a story and I wonder if anyone can point me to research on turnover? In particular, has any research been done that demonstrates what percentage of a given pool's water actually goes through the filter during "a turnover?"


Near as I can tell, "turnover" is theortical, and the proof that the correct turnover rate has been chosen is in the clarity of the pool?


Any citations, links, explanations, graphs, charts, drawings on a napkin, etc. would be most appreciated!


Thanks for your help,


Kirstin Pires

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  • This is a great thread, I re read it today all the way through and I can see both points of view having just cleared a Pea green pool and used the bottom drain method to remove most of the crud as quickly as possible but I would say if the filtration isn't up to the job it pretty much a waste of electricity whichever way you do it but at least with a sand filter you can suck to waste for at least 80% of the crud quickly meaning the chlorine won't be used up quite so fast and the fresh water lowers the CYA level so helping with the original problem.

    When running normally I would say that the skimmer is the better way because that is the way the dirt etc enters the pool and providing you get that out in a reasonable time it won't sink but if you follow Rex's ideas about revolving the flow and 24h run times then virtually everything is filtered out before it sinks.

    When people vacuum their pools a little dust cloud is usually stirred up so by the time they have finished the water isn't quite as clear as before they started. With Rex's idea and 24h pump runs there is no dirt on the bottom to stir up, it isn't there so 1 NTU is about the clarity of the water when the system is running and not when others have let the dirt settle and then test it.  Of course the filtration still needs to be up to the job.

    I plumb the main drains as returns when in normal running and that together with the side returns ensures a good mix of the chlorine so green areas are non existent.

    I plumb the main drains to do both jobs and that allows the best of both worlds for pool maintenance.

  • Interesting thread, with lots of different opinions! 

    A little off of the topic, but maybe some insight that might prove helpful:  We have found that when we sink the pickup hose on our RO rig we read the dirtiest water (in terms of TDS) at the bottom of the pool.  Initially we were allowing the pool pump to run and "blend" the clean water coming back with the dirty water in the pool.  Our thought (incorrect) was that we could possibly create cleaner water faster by allowing the pool water to blend together, therefore lowering TDS faster. We had been told for so long that all the water was the same, and that we were smoking something if we thought "heavier" water sat at the bottom of the pool! 

    We now set the pickup tube relatively low in the pool and return the clean water quietly at the surface, with minimal disturbance.  This allows for a clean water layer to form, eventually dropping and achieving the lower TDS level that we are looking for (the customers pump, unless commercial, is shut off to stop blending of good and bad water).  Needless to day, we do not run as long when we are able to pull dirty water through the system and return clean at two different levels.

    I can definitely see the debate regarding skimmer pool percentage versus main drain percentage pools and cleaning.  Add a pool cleaner (roving main drain) and I see an even greater blending of water possible as the cleaner may climb walls and pick up water at various levels throughout the pool.   Return water coming back disturbs the water from (typically) 18" below the surface upward, and if the skimmer is the main suction point, then I can see where the top 18" is clean while the rest of the pool may suffer.

    Cool debate!  Turnover is a pretty subjective topic, and I wonder how many people really take the time to think it out.  I would venture to say that even pools mandated by the Health Dept. could be achieving the proper turnover rate, but yet are not the safest pools to swim in.  Just because a pool theoretically hits the designated turnover rate does not mean that  all of the water has been filtered.  As indicated by the discussion here, it may not even be close!

  • Since you can easilly set up a scenario where you can turn over the water over 50 times and do nothing but create a band of clean water in an otherwise dirty pool by either returning all of the water in one direction at the bottom, and having all of the suction from the bottom, or reverse the process and return and suck only from the top, potentially making it impossible to get cleaning and sanitation to 90% of the pool regardless of turnover rate, and since you can clean all of the water in 24hours if you do not do ANY water mixing and return all of hte fresh water to the top and close to the walls, and all of the suction from the drain, you have the difference between infinite turnovers and no clear water and one turn over and clean water. How does this mean that "Turnover rate" has anything to do with water clarity?

         If you "Mix" the water instead of displacing dirty with clean, you will require turning the water over at least 10 more times, not because the turnover rate causes it, but because you are mixing and diluting the dirty water with clean. Again, turnover rate does not determine water quality, but how you mix, or don't mix your water with both suction and returns.

  • Bather load would only mix water in the area where there are bathers.  If the pool is deep, then one can certainly have little circulation / water flow in the depths in the deep end and I believe that is part of what Blatchley was seeing.  Now since most contaminants are coming from bathers, having worse circulation at depths may not be terrible, but there should be SOME circulation there just to ensure that it is still disinfected.  Also, dirt, leaves, and other objects that sink (perhaps after getting water-logged) would need to be removed.


    I agree that turnover is still a useful parameter.  One can have a separate discussion of how to make each turnover more efficient through different types of circulation patterns (i.e. mixing vs. displacement).

  • Michael,

    I'm in agreement with everything you just said about the possible deficiency scenarios of the calculated turnover requirements... in relation to circulation patterns playing a key role in how successful it is in clearing a pool. There is enough of those type scenario pools around, (gutter pools with wall returns pointing more or less up), to make your arguments somewhat valid.  But there is still the issue that bather loads seen in these pools would mix the pool anyway.

    Even if that pool scenario was the norm, which it is not, then that would still not make the turnover calculation a usesless endevor as you suggest. Setting circulation patterns aside, the turnover calculation in the code is still of importance, as it is part of the baseline criteria for determining minimum pump, pipe, and filter sizing requirements. It's all tied together. 

  • Al Neumann said:
    And with that in mind, the principal behind turnovers is the same, despite the semantics and individual percentages  of the actual experiments... and that is the greater number of turnovers, the better quality of water. In the end, when all is said and done, this is true for either calculation, am I basically right?


    So for Kirstens question of if there is any science behind turnovers, the answer is probaly yes. And my answer of what is most often cited of Gage and Bidwell is still correct, but probaly for the wrong reason as you pointed out. Is it just the correctness of the science that you have an issue with, or is it the principal of turnovers in general? I'm assuming it is just the correctness of the science.


    Yes, regardless of calculation, the greater number of turnovers, the greater the quality of the water.  Though as Michael correctly points out that specific quality improvement for each turnover depends on how the water is flowing through the pool (i.e. whether is it mixed or displaced and what proportion of water is moved in that way) and in a case with dead spots you can reach a limit of improvement.  It seems that the pool/spa industry has mostly focused on trying to achieve mixing without dead spots whereas Michael is proposing more of a displacement model.  Obviously, the success of either depends on not having dead spots at all.


    And yes, there appears to be science used by the industry but to the extent that Gage and Bidwell is being quoted, it is sometimes mis-applied and is not a good model to use in the first place.  I do not have any problem with the principle of turnovers -- only the incorrect use of the Gage and Bidwell numbers as meaning percent of water going through the filter for the number of turnovers -- something that is wrong even in the CPO course and that I wish were corrected there and elsewhere.

  • From my understanding, turnover refers to the number of times the amount of water that is in the pool goes through the equipment. In other words, regardless of whether the only suction is from the skimmer, and the returns return water to the surface as well, you still get "Turnovers", and the same number as you will with any other adjustment. Turnover rate does not change just because you alter where the water is pulling from or returning to. That was my point. You can "turnover" the water, i.e. run the volume of the water that is in the pool a thousand times and never get the water cleared. You can thouroughly mix the water, and require more than 10 "Turnovers" to get the water as clear that you can get the water with ONE turnover using replacement. What you really want to measure is the percentage of "Dirty" "Unfiltered", "Unsanitized" water that you filter out "Per Turnover". In the surface skimming / returning scenario, we get ZERO filtration of dirty water, even though we get as many turnovers as we want. If we have primarily surface returns of clean water, and primarilly surface suction for taking to the water to the filter / sanitation unit, we have EXACTLY that scenario. If your primary suction is at the surface, edges of the pool, you want your primary returns to be at the bottom middle of the pool, and to do as little "Mixing" with the dirty water as is possible. Less "Mixing", and more displacement / replacement means that you get better water clarity, and better filtration of dirty water in "One" turnover, than you get with 10 turnovers with mixing, and better than a thousand turnovers if you both suck the water and return the water from the same levels in the pool. Thus, "turnover" rate is a waste of calculation. More importantly, how much of your circulation is replacement, how much is mixing, and how much is just re filtering the water you already filtered. The higher the amount of refiltering already cleaned water, the more turnovers you need to get water clarity, to the point where it will take infinite turnovers without any improvement on water clarity and sanitation.

             Keep in mind that ANY AND ALL experiments on "Turnover rates" are going to be 100% reliant on how much mixing we are doing, what percentage of dirty water is being filotered or cleaned, and how much is replacement, i.e. almost 100% dirty water being filtered and clean water(unfmixed) used to push the dirty to the suction source. In other words, the "turnover rate", for getting pools cleared, will be completely different for every single pool, even those with the exact same gallonage, dirt load, bather load, same equipment and operation times, ... if the adjustments of mixing and replacement are changed. The exact same pool, with a different adjustment could be 100% clearer in 1/10th of the time with a different return / suction adjustment. That is why calculating turnover (The amount of time it takes to run a pools "Volume" of water through the equipment), means nothing in real life. You can have 1/10th the turnover and a cleaner pool, or a hundred times the turnover and a dirty pool.

  • Michael,

    I'm not discounting what you say about how well your displacement methodology works, or that it isn't logical, or that it may even be a better way of filtering... setting all codes aside. What I said before, and am still saying now, is that it may just be more suitable for  remedial actions rather than for long term use on our commercial, semi-public, and public pools. Primarily because the displacement method, as you describe, would likely have issues with continuous skimming gutters, and would have thourogh mixing anyway due to increased bather loads throughout most of the day. It just doesn't seem practical except for remediation of a problem, and if temporarily incorporated during a slow period. I'll give you your kudo's, and let's just say we agree to disagree on the rest. 

    By the way, your friend's short-circuiting example of having the discharge and suction a foot below the water may be a good example of layering, but trying to relate it to the fruitlessness of turnovers was a little weird, as it could have no other outcome... as the pools main drain and returns were not involved. The point you were making wasn't very logical.


    And Richard,

    Interesting. I'll be honest with you, I never really ever read Gage and Bidwell's experiment, and what it was actually all albout. I was just going by what is most often quoted, as turnover discussions was always pretty much a given. Kent's explanation always sounded good enough for me to understand the principal, and took his word on the actual science. In my defence, remember, I'm juts a pool guy, (not a chemist or a scientist), and often just try to understand the principal behind some of these things. I understand now where you are coming from.

    And with that in mind, the principal behind turnovers is the same, despite the semantics and individual percentages  of the actual experiments... and that is the greater number of turnovers, the better quality of water. In the end, when all is said and done, this is true for either calculation, am I basically right?


    So for Kirstens question of if there is any science behind turnovers, the answer is probaly yes. And my answer of what is most often cited of Gage and Bidwell is still correct, but probaly for the wrong reason as you pointed out. Is it just the correctness of the science that you have an issue with, or is it the principal of turnovers in general? I'm assuming it is just the correctness of the science. 

  • Al,


    Yes, that's the research I was thinking of.  By "circulation", I meant top/bottom mixing.  Blatchley's speculation that the contaminants at the surface were staying near the surface is what I meant.  So having water enter from the bottom and get removed through gutters would make sense to do (vice versa would also work).


    As for the theory of dilution, I'm going to quote from the article I linked to since it describes what I was talking about.  As stated in the abstract on PDF page 10 (paper page 1195) in this paper,


    It can readily be demonstrated by computation and by experiment that 7 turnovers are required to effect a removal of 99.9 per cent of the dirt present in the water. of the pool when recirculation was started. At the end of the first turnover the purification will be about 63 per cent, after two turnovers about 86 per cent, at the end of three turnovers about 95 per cent, after four turnovers about 98 per cent, after five turnovers 99.3 per cent, and after six turnovers 99.7 per cent. To accomplish a purification of 99.99 per cent 10 turnovers will be required.


    So note that Gage and Bidwell clearly understood the proper formula for perfect theoretical mixing which is 1-EXP(-x) as I had described earlier.  This is where one gets that 63% of the water goes through the filter in one turnover.  This is theoretical assuming that all the water coming in from returns it thoroughly mixed with all the water in the pool before it exits through a skimmer, floor drain, gutter, etc.


    Then Gage and Bidwell describe their dirt model which is again theoretical as follows:


    If the pool is used regularly by bathers further increments of dirt will be introduced into the water daily, and the removal of each successive daily increment will proceed according to the law. The result of the addition of such daily increments will be an increasing accumulation of dirt in the water up to a certain point, after which the dirt content of the pool water will remain practically constant, subject only to the fluctuations caused by the variations in the daily bathing load. The amount of this accumulation and the time required for the pool water to reach a condition of equilibrium depend upon the rate of turnover of the pool by the flowing through or by the recirculation system, and in the latter system is also dependent upon the efficiency of the filters.

    Assuming a daily increment of dirt equal to that in the pool at the start and a filter efficiency of 100 per cent, with a daily turnover (T = 1) equilibrium will be reached at the end of the ninth day when the accumulated dirt in the pool will be equivalent to about 58 per cent of the amount present when recirculation and daily bathing was started. With two turnovers per day (T-=2) equilibrium will be obtained in four days with a dirt load of about 16 per cent, with T 3 a balanced load of about 5 per cent will be obtained on the third day and with T = 4 a balanced load of about 2 per cent will
    be obtained at the end of the second day. On the other hand, if the recirculation system is so small that it requires two days for each turnover, accumulation of dirt in the pool will continue for about nineteen days and the dirt load carried in the pool thereafter will be about 155 per cent of the amount present at the start.


    What they implied but didn't state explicitly is that their dirt model assumes that ALL of the dirt for the day is introduced at the start of the day.  Their model does NOT assume continuous introduction of dirt throughout the day with bather load which is something I calculated in my post.  That's why I called their model bogus because it is not at all realistic.  Also, their model does NOT say that 42% of the water goes through the filter in one turnover, but rather that the reduction of dirt in their model is 42% (so one ends up at 58% of starting level) in the steady-state, but not in the first day where there would be a reduction of 63% and then the second day more dirt would be introduced, etc.  So I wish the pool/spa industry would stop quoting Gage and Bidwell's dirt model as having anything to do with the amount of water going through a filter in a given number of turnovers since that is NOT what the model is calculating.


    As I indicated in my earlier post, Table 1 in this document says that the 42% for 1 turnover is the "Percent clarification of pool water after equilibrium is obtained" which is an accurate statement assuming a model of adding dirt at the start of each day and where the percent is relative to the amount of dirt added each day.  Again, it is NOT the amount of water that gets through the filter in a given number of turnovers.

  •          About 5 times a year I have the opportunity to take my 70 sq. ft. D.E. filter and 1 H.P. portable pump out to the field to clear up a pool that is pea soup. In other words, while the water level is at the middle of the tile, the water itself is so green as to make it impossible to see the top step.  

            After treating the water chemically to kill all algae, I have the suction side set up with a 3 way valve, and a piece of flex pipe to suck in the D.E. to charge the filter. Once charged, all of the suction comes from a pipe that I run to within a few feet of the main drain, and within 6 inches of the bottom as well. The return is set up to put all of the filtered water back at the very surface of the pool, as close to the wall as is possible to rotate the fresh water around the pool.

            Consistently, the surface water clears up first as it displaces the dirty / dead algae filled water downward. Since this is a D.E. filter setup, I have it set for 4 hours on 2 hrs off, repeated 4 times in a 24 hour period to allow the filter to recharge. Within 4 hours I can usually see the bottom step clearly, by the end of the second 4 hour filtration cycle I can usually see the main drain, but the top foot or so is usually crystal clear. By the time I arrive the next day, the entire pool is crystal clear, and all that is left is what has settled out on the bottom of the pool.

            One time I asked a friend to set everything up for me. I told him how I wanted everything set up, unfortunately, what I communicated, and what he understood were 2 different things. We ended up with the suction about a foot under the surface of the pool right next to the wall, and the return line right next to the wall as well, about a foot below the surface blowing the opposite direction. The equipment ran as it normally does, 4 hours on, 2 hours off, 4 hours on, 2 hrs off, for 2 full days. When I arrived back, the top foot of water was crystal clear, and the top step was clearly visible. The rest of the pool was still pea soup.

            I honestly believe that I could have left that setup exactly as it was for a year and never gotten that pool cleared up. Once I corrected the setup to pull strictly from the main drain, and return to the surface close to the edge of the pool, the water was perfectly clear the next morning after running 3 four hour filtration cycles. Understand that the filter was never changed, and the volume of water processed per hour, or per 24 hours, for all of these pea soup pools never changes much as the pool conditions are the same. In other words, the "Turnover Rates", regardless of setup, never changed. All that changed was that I wasn't filtering water that I already cleaned, and I used the clean water to move the dirty water down towards the suction for the filter.

             As an experiment on another pea soup pool, after replacing the grids on a damaged 48 sq. ft. D.E. filter, and recharging it, I put the floor heads on, put the skimmer on half way, adjusted the times as I do for my portable equipment, and left for 24 hours. ((4) 4 hour cycles) and came back. You could barely see the 3rd step, but still could not see the drain. I set all of the suction to the main drain, turned off the floor heads and adjusted all of the return lines to cause a ripple 3 feet away from each return, as close to the sides as is possible, and move the water in a circular motion. When I arrived back near the completion of (2) more 4 hour cycles, the water was crystal clear. Of course, the pump was a 2 H.P. verses my portable 1 H.P. so the "Turnover Rate", even with more plumbing, was greater than with my portable, but the issue is still this:

             Mixing requires many, many more times the turnover rate to clear a pool and keep it clear. Why? you are filtering more and more clean water with the dirty water. When the suction and the returns were at the same level at the surface of the water, ONLY the top 12 inches of the water got cleared. Floor heads mix the clean water with the dirty, so even with full drain suction, you will probably need to "Turn the water over" at least 10 more times to get the same water clarity.

            Don't take my word for it. Set up these same scenarios for yourself and test them. They work for me every time. You just need to be sure that you explain to the customer that you are going to take longer than normal to clear up their pool if you use floor heads, or if you use returns, return the fresh water to the surface, and only suck water from the skimmer. After 24 hours of only having the surface water cleaned, switch to full main drain and you can almost see the water clear up before your very eyes.

            The question is, if the surface foot of water was the dirtiest, and it stays the dirtiest, regardless of how the water flow is set up, why does the water ALWAYS clear up from the top down leaving the top foot of water crystal clear before the main drain becomes visible in a diving pool, and progressively getting clearer from the top down until even the water right over the drain is clear? If displacement of dirty water is not occuring, and if turnover rate doesn't change, why does "Mixing" the water take over 10 times as long, and 10 times the turnover, and the water is still not as clear as with the other method?

            Perhaps what we are looking for is the turnover rater of JUST dirty water, with as little mixing as is possible? If all the algae is filtered out because all the water got filtered sooner, without filtering more and more dirty water, the water should clear up quicker. Contaminants are contaminants, and getting those contaminants removed by getting them through the filter and sanitized is the target. Filtering and sanitizing water that has already been cleaned and sanitized, even if you "Re-clean" and "Re-sanitize", it turning over a volume of water a hundred times greater than a pool that only filters out dirty water and uses the clean sanitized water to move the dirty water tow the filter adn sanitation equipment will ALWAYS give superior performance. Like I said, I could have left the setup where only the water on the surface was filtered, and the clean water was returned to the surface, run indefinitely and the rest would never have gotten cleaned. This isn't "Mixing" either, and it clearly set up a layer of "Hyper cleaned" water over a pool that was pure gunk below it. when I moved the suction down by the drain, and left the return at the surface, the layer of clean water at the top was visibly thicker every ten minutes or so.

            Again, I understand that n many locations, ordinances prevent this setup. I also cannot see how adjusting things as I suggest would leave any "Layers" of unclean water near the surface, seeing as there never are any layers of algae or murky water between my layers of freshly cleaned water, indeed, returning the filtered water would seem to preclude this ever hapening, and the facts that I have oft seen seem to refute opposing conclusions as well. Perhaps those that believe that adjusting the returns, and the suction, as I describe here leaves a layer of uncleaned water can explain why I never see that when turning green pools clear. 

            Again, I undertand that many of the laws prevent this methodology from being applied, and thus, those pools must "Mix" the clean water in with the dirty, by law, and thus require 10 times the turnover rates to get the same clarity and sanitation because it is more of a coin toss as to which molecules of water (Clean or dirty) will get filtered, and as the dirt gets filtered out, the odds get stacked in favor of more clean water being filtered again than dirty, but that is different from deciding what is "Best" for more efficient filtration and sanitation.

            One more thing, I adjust my pools to be 80% drain and 20% skimmer, even though I will get 20% "Already Filtered" water going into the filter along with the dirtier water from the drain. Why? Because a lot of dirt and debris starts out floating, and the skimmer will get this and keep my pools cleaner.         

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