How much FC is really needed

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sbe

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Sep 4, 2008
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I've been reading all the discussions regarding CYA and it got me thinking about how much FC is really required. I'm not talking about what one measures with a test kit but actual free chlorine is available for disinfection. Unfortunately I hadn't been able to locate a chart that plotted ORP against FC for very low concentrations until tonight. Here it is...

http://www.aquariustech.com.au/pdfs/tec ... Disinf.pdf

Assuming that anything more than 600 mV is considering sufficient then for a pH of 7 roughly .1 ppm of FC should be acceptable. Allowing for pH up to 7.5 FC would rise to .2 ppm and for 8.0 it would be .4ppm.

First, I'm wondering if that seems correct or if I've gotten something wrong. I'm ignoring the practical issues of how one might accurately measure or reliably maintain such a value. I just want to make sure I'm interpreting the theory correctly before I post where I'm going with this.
 
woodyp, this is The Deep End :shock: Make sure you brought your life preserver :cool:

sbe, Yes, that is kind of more or less true. We aim for an HOCl level of closer to 0.034 ppm. A couple of things to keep in mind.

First, the change in HOCl concentration with PH is no where near as large as you list if there is CYA in the water. Once there is any reasonable amount of CYA, the PH can essentially be ignored. See this post for details.

Second, a concentration of even 0.2 ppm of HOCl is impossible to maintain uniformly throughout the pool without CYA. Extremely low FC levels will simply vanish in the sunlight without any CYA, and even indoors, local chlorine demand from someone getting into the water can far exceed 0.2 ppm, leaving substantial volumes of water free of any sanitizer.

Third, ORP is not a good measurement to use as a guide to derive other numbers. The data is not completely conclusive, but it appears that HOCl concentrations are a much better guide to water safety than ORP is. ORP levels can vary dramatically in ways that have nothing to do with sanitation.
 
Got it. I've read all the papers referenced but haven't completely digested every nuance yet, particularly with regard to the stated concern regarding ORP but you alluded to what is the heart of my question.

At a pH of 7.4 half the FC will be available for disinfection at the target level of .34 ppm would translate into a FC level of .68 assuming zero CYA. You said this level would be impossible to maintain and mentioned two distinct mechanisms one small scale (the bather), the other large (UV).

I have a couple of questions.

With regard to overall HOCl levels can you flush out a bit more what makes you believe this couldn't be maintained? Is your concern a practical one such as because the rate of exhaustion varies there is no testing mechanism that could accurately check the level or perhaps that there isn't a dosing system that could respond fast enough to a large change in load?

As for local depletion wouldn't it be correct that in the absence of CYA wouldn't this purely be a function of the HOCl diffusion rate? Since I haven't seen any mention of how fast this occurs I'm wondering if you know its really a problem? I'm also thinking it isn't clear the situation is any worse for a body of water without CYA vs one with. The difference would be that in the former the chemical would have to migrate from adjacent water where in the latter it would be a combination of migration and release from the stored state due to the CYA. Since kill rates at the low HOCl levels we are assuming are often measured in seconds or even substantially longer is localized depletion even an issue?
 
ORP is flaky
Take a look at this paper which was done roughly in the same timeframe as the others they reference. Basically, the paper is just concluding that the OTO test sucks, which we already knew, and that the Free Chlorine (FC) measurment even with DPD isn't useful when chlorine is combined with other substances such as CYA, which we also already know. ORP does not correlate better than FC when there is no CYA present and when CYA is present they didn't try calculating the theoretical HOCl concentration. Don't get sold into the ORP hype that is only coming from ORP manufacturers.

ORP is very flaky and all over the map between different sensors from different manufacturers. I describe how terrible this situation is in this post. I tried to do a correlation analysis using data in the Aquarius paper and was having a heck of a time because the graphs within the paper itself are terribly inconsistent! For example, reading the ORP off of the "ORP millivolts versus Free Chlorine" on page 1 at 2 ppm FC and pH 9 gives around 625 mV while the "ORP millivolts versus pH value" graph on page 4 gives around 540 mV. I find that I can get a decent correlation if I ignore the graph on page 1 and use instead the two graphs on page 4 (the first having detailed data for a pH of 7.5) and I have added that to my PoolEquations spreadsheet. However, note that Aquarius has the steepest ORP mV change per doubling in HOCl at 46 mV per doubling compared to most other sensors which have around 20-28 mV per doubling (Sensorex is a weird exception with 84 mV per doubling).

See this post for how ORP correlates with calculated HOCl in real pools. In this study of 620 samples from 194 pools, comparing ORP readings from a portable Oakton ORP measuring device vs. the built-in ORP controllers in many of the pools, 30 out of 130 (23% of those that had built-in ORP controllers) were off by more than 100 mV from each other measuring the same pool water at roughly the same time! To depend on ORP in any absolute sense is pretty much ridiculous. It's OK for process control when manually adjusting a setpoint to get to the FC level you want, but other than that it's not very useful. Oxidizers such as MPS will register high ORP, but are not very fast at killing pathogens while the presence of reducing agents will lower ORP even if they don't react with chlorine or react very slowly. ORP is measuring a thermodynamic value (even then, it doesn't match theory in the case of HOCl) and not reaction rates so it is easily fooled. The presence of hydrogen gas from SWG systems can affect the readings (though not actual kill times) as can sunlight.

The World Health Organization (WHO) and others have set 650 mV as the disinfection standard for water in spite of all the problems noted above. At a pH of 7.5 (and 77ºF temperature), 650 mV is achieved with no CYA in the water with 0.023 ppm FC for the Chemtrol sensor, 0.082 ppm FC for the Oakton sensor, 0.25 ppm FC for the Aquarius sensor, and 0.62 ppm FC for the Sensorex sensor. So which one do you choose? It's nuts, yet no manufacturer cops to this problem.

Low chlorine levels are difficult to maintain
Trying to maintain FC levels too much below 0.5 ppm is difficult in pools because a relatively small chlorine demand can readily consume that amount of chlorine locally thereby creating regions with no chlorine for longer than one would like. In higher bather load pools, it may be difficult to circulate the water well enough and fast enough to replenish a chlorine level of, say, 0.2 ppm FC (diffusion is slow in still water; it is physical circulation/movement of the water that distributes the chlorine). This is one of the big advantages of using CYA in the water since it is an HOCl buffer so can hold a chlorine reserve while maintaining a low HOCl level that is still fast at killing pathogens. The chlorine is released from CYA very quickly -- the half-life of one species is 0.25 seconds so half of the reserve can release in that time (normally far less is needed from the reserve). This is far faster than the many minutes it would take for circulation to get chlorine from the controller to any particular spot in the pool or from one part of the pool to another.

Appropriate active chlorine level for sanitation
As for what level of active chlorine (hypochlorous acid) is needed for decent sanitation, that is a legitimate debate. Kill times are usually quoted by using the product of chlorine concentration in ppm (C) with time in minutes (T) at a given kill rate (log reduction or % kill) for a given condition of pH and temperature. Most heterotrphic bacteria have a 99% (2-log reduction) kill at room temperature (77ºF) at a pH of 7.5 with a CT of 0.04. See the PDF file with a table in this link at the CDC to see more difficult pathogens. With a minimum FC that is 7.5% of the CYA level, this is equivalent to an FC of around 0.06 ppm FC with no CYA and has an ORP (at 77ºF) of 682 mV with Chemtrol, 638 mV with Oakton, 557 mV with Aquarius, and 373 mV with Sensorex. Pathogens with a CT of 0.04 get 99% killed in 0.04/0.06 = 0.67 minutes or around 40 seconds. As for uncontrolled bacterial growth, one looks at the 50% kill rate in 15 minutes which is the fastest generation rate for bacteria. I write about how to calculate this in this post. So the 0.06 ppm FC would kill pathogens faster than they can reproduce if they had a 2-log (99% kill) CT of 0.06*15*2/0.301 = 6 or lower. In the CDC table, pretty much the only pathogens not killed reasonably quickly are the bacteria Burkholderia pseudomallei, Vibrio cholerae (rugose strain; smooth strain is easy to kill), Yersinia enterocolitica, and all of the protozoa though most especially Cryptosporidium parvum (the more recent CT value for Crypto is 15,300 as this table is a bit out of date). In practice, it is only Crypto that is a problem in chlorinated pools. Even Giardia lamblia with its CT of 15 is reasonably handled since protozoan oocysts do not reproduce in swimming pool water -- they only reproduce in your gut when the cysts are broken open. A 99.9% kill with a CT of 15 is equivalent to a 99% kill with a CT of 7.5 and that takes 7.5/.06 = 125 minutes or around 2 hours to achieve at the FC/CYA ratio minimum for manually dosed pools recommended on this forum. That won't prevent person-to-person transmission during a fecal accident, but it will prevent widespread outbreaks. This is one reason I think the standard for commercial/public pools could be higher with an FC/CYA ratio of around 0.2 (i.e. an FC that is 20% of the CYA level -- 6 ppm FC with 30 ppm for outdoor pools and 4 ppm FC with 20 ppm CYA for indoor pools) which will do a 99% kill of Giardia in just under 40 minutes.

There is a downside to not using any CYA at all and having higher FC levels such as found in most indoor pools. The rate of production of volatile and irritating nitrogen trichloride is theoretically much higher in such pools since it is roughly proportional to the active chlorine (hypochlorous acid) level as I describe in this post.

Richard
 
Thanks for such a detailed post, including the kill times. Your conclusion regarding the adequacy of the recommended FC levels with the possible exception of Crypto is consistent with what I was coming up with and is at the heart of what I'm stewing over. Is what's driving the use of CYA convenience.

The advantages of CYA seem pretty clear. The buffering properties create a chemical system that easily maintains a constant low level of FC without the need for continuous doping. The disadvantage is it is hard to raise the Chlorine level beyond a certain level.

The accepted wisdom seems to be CYA is worth the tradeoff. If you'd indulge me I'd like to understand that in a bit more detail because I'm wondering if what is right for a commercial pool or a backyard owner who doesn't want to have to learn very much is the the right answer for someone who is committed to learning the chemistry and is attentive.

For example imagine a residential pool without CYA is monitored and dosed every evening to say 1 ppm FC. Wouldn't that just be equivalent to super chlorinating daily? If the pool were covered and used 4 hours per day in bright sunlight under light load wouldn't one expect to end the day at about .06 ppm even without any sort of continuous doping?

I know it flies in the face of everything out there but I wonder why this is so awful? You mention the problem of irritants but putting that aside for a moment (if only because I haven't noticed any) are there other reasons not to take such a route, other than the higher consumption of Chlorine (which I think could be substantially solved with better dosing)?
 
Without CYA and only adding chlorine once a day there is essentially no protection for person to person transmission in the afternoon. The chlorine will be essentially gone from sunlight, and bathers will consume all of the remaining chlorine in their local patch of water all the while shedding bacteria and viruses. A subsequent swimmer passing through that patch of water will be exposed.

There are also challenges should your additions of chlorine miss a day. On the second day there will be no chlorine to start and algae will have a whole day to get hold. WIth CYA, there will still be chlorine around from the previous day that has a reasonable chance of preventing algae from getting started.
 
JasonLion said:
Without CYA and only adding chlorine once a day there is essentially no protection for person to person transmission in the afternoon. The chlorine will be essentially gone from sunlight, and bathers will consume all of the remaining chlorine in their local patch of water all the while shedding bacteria and viruses. A subsequent swimmer passing through that patch of water will be exposed.
Click to expand...

In the example I quoted that isn't the case, however I understand your point. But I think you are missing mine so lets assume one of the various methods of injecting Chlorine so there is a small steady flow. The FC levels are still going to vary significantly because I haven't allowed for a controller and feedback but one could go that next step too.

So long as the FC level is at least as high as what it would have been using CYA I'm having trouble seeing any health risks. I agree that Chlorine usage would almost certainly will be higher but that is because the FC is higher.

What I'm getting at is that with CYA you keep the low level FC level consistent but the price is an inability to raise the FC level beyond a certain point. That point is low enough to open the door to a number of problems...Crypto for example. So you get something, but give up something. It doesn't come for free. There is a risk.

On the other hand if you run without CYA it is very difficult to control the low level FC level except over a rather wide range but it is trivial to raise the level more or less as high as you want. Again nothing is free. You choose your poison.

It isn't obvious which is the better tradeoff for any given situation (commerical/residential, covered/uncovered, etc).

The assumption seems to be that tightly controlling the low level FC level is so important that its worth the accepting the downsides. I'm just wondering if anyone has considered if that is really true for all installations.
 
You can't say "So long as the FC level is at least as high as what it would have been using CYA" and also have anything resembling low levels of chlorine and zero CYA, it won't work out that way. If you magically guarantee the FC level, then sure that will work by definition, but trying to build that in a practical pool is extremely difficult. FC gets used up in local areas. The chlorine demand from one person entering the water exceeds 2 ppm in the water near their body. Without CYA the FC level does not recover from that for some time, until there is sufficient mixing, which could easily be half an hour or more. There are ways to avoid this using extreme circulation systems, but they get expensive.

Likewise, don't under estimate the disadvantages of even medium FC levels and zero CYA causing over chlorination, chlorine breakdown byproducts which cause respiratory issues, and swimsuit/hair damage. All of which can again be controlled by careful management of the FC level, but again you quickly start wandering into extreme circulation systems trying to produce those uniform levels. Without the extreme circulation, operators start raising the FC level to compensate for one kind of problem, and create a different problem with high FC side effects.

Crypto is a real issue, but even having zero CYA doesn't make it easy to solve, it is still going to be a huge pain. In a residential environment, where waiting 24 hours is reasonable and the odds of crypto being in the water are way way lower, you can do about as well with reasonable CYA levels as commercial pools can do in a few hours with zero CYA. But the real solution to crypto in the long term is going to have to be some other chemical (or UV and those extreme circulation systems again).

The ideal approaches for commercial vs residential and indoor vs outdoor are all going to be different. The regulatory environment around commercial pools puts significant constraints on the systems you can use and the higher bather load presents a whole array of issues we haven't even touched on. For the moment commercial outdoor pools should probably aim for very low CYA levels (but not zero), which requires more total chlorine than current CYA using practice and higher end test equipment to maintain (standard CYA tests are wildly imprecise at low CYA levels), stuff that residential users wouldn't normally consider.
 
(wrote this as Jason was responding; some duplicate info; some additional info)

If you had a continuous dosing system, it would have a hard time maintaining 0.2 ppm FC with no CYA in the pool. The reason is that this isn't very much chlorine so localized chlorine demand can fairly easily wipe that out, even in nooks and crannies of pool surfaces where circulation is poor and where algae could grow. If instead you maintained a 1 ppm FC chlorine level, then you get around this problem, but create another where you have an active chlorine concentration that is 5 times higher than necessary (compared to 0.2 ppm; over 10 times higher compared to what we propose for residential pools). This not only can create more disinfection by-products, but it oxidizes skin, swimsuits and hair and potentially corrodes metal and oxidizes other materials 5 times faster. Essentially, you are over-chlorinating. On the other hand, this is what occurs in most indoor pools that don't use CYA. My wife has personal experience with this difference where her swimsuits would degrade (the elasticity gets shot and then some fading occurs) over just one 5-month winter season at an indoor pool with 1-2 ppm FC with no CYA and her skin was flakier and hair frizzier while in our own outdoor pool over multiple 7-month summer seasons the swimsuits show minimal sign of degradation and her skin and hair fare much better.

So having CYA lets you get the best of both worlds. I'm missing your point of the "downsides" with using CYA. What is the problem? If you use too much CYA, then yes it can be difficult to shock a pool and especially difficult to superchlorinate for Crypto, but in a commercial/public pool (indoors and outdoors) you could use 4 ppm FC with 20 ppm CYA which not only is equivalent to 0.2 ppm FC with no CYA, but lets you superchlorinate to 30 ppm FC if you wanted to which is roughly equivalent to 10 ppm FC with no CYA assuming you first lower the pH to around 7.0 before superchlorinating (so that the pH rises to around 7.5). That level for 25 hours meets the Crypto CDC killing guidelines. Of course, you could just use 1 ppm of chlorine dioxide created by adding sodium chlorite to the pool overnight for 8 hours and achieve the same Crypto kill, but that isn't approved by the EPA. Realistically, every commercial/public pool of moderate to high bather load should have supplemental oxidation to reduce disinfection by-products (non-chlorine shock or ozone or UV) and should have some method for reducing the risk from Crypto (ozone or UV; coagulation with microfiltration might also be OK).
 

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First off thank you both for taking the time to answer my questions. I really appreciate your walking me through your logic.

I've got a couple of more questions if you will indulge me. Jason can you tell me how you arrived at a figure of a half hour or more for a localized area to recover from depletion? Also you quote the amount of FC used from a person entering the water in ppm for an unknown volume. I would think that what is really needed is the total FC likely to be consumed (or the ppm in a known volume which is the same thing). Would you happen to have that information.

Richard, you make a very good argument for localized buffering with CYA but I'm still not quite willing to give up understanding what a strategy where the FC level is raised overnight (when there is little UV and no load) then allowed to drift down to a level not wildly higher than your target with CYA during the day would look like. If nothing else its amusing to consider an alternate scenario and see how it plays out. I can't help it as its part of the engineering mind.

Your concerns with the concept seem twofold. First, such a low level would be difficult to maintain overall and second that it would be impossible to maintain locally (nooks and such). Focusing on the second concern first in light of a proposed regiment of elevated levels overnight (minimum 8 hours) with pump running and no bather load can you tell why you think this would result in an algae problem? I don't mean to be thick but my inclination would be the reverse. The elevated levels would reduce the risk.

As for during the day, I take both your points, but the issue here isn't algae its 1) overall load and 2) localized demand. You each have the same concern about recovery time for localized depletion but if you could tell me how you came up with the stated recovery time I'd really appreciate it.

I can see where you are coming from on CYA. It seems like a pretty simple, straight forward solution so why would I question it? Partly its just my nature to need to understand things by challenging assumptions and considering alternatives. But there is also the concrete nature of CYA. A decision to use it is not easily reversed so my tendency to make an even greater effort to understand the tradeoffs better than I might otherwise compared to something that could simply undone.

Hopefully you don't find my questions tedious. I apologize if they are. Alternately if you find my probing interesting in that it causes you to think about the issues in a different way, even if the best solution still turns out to be the same that would wonderful.
 
[EDIT] Note that I have redone the following calculation as I originally incorrectly used the sweat content instead of urine content. That mistake woefully underestimated the actual chlorine demand. [END-EDIT]

Take a look at this WHO document in Section 4.3 on bather-derived chemicals and Table 4.1 giving the composition of typical sweat and urine. Ignoring the amount of sweat and using the low-end of the urine estimate of 25 ml (5 teaspoons) per bather, this is 12,220 mg/L * (25 ml / 1000 ml/L) = 305.5 mg Nitrogen. Ammonia (and likely urea as well) requires at least 7.6 times the amount of chlorine to get oxidized (it's 1.5 times in molar units and another factor slightly higher than 5 of chlorine Cl2 vs. ammonia N units). So that's 305.5*7.6 = 2321.8 milligrams of chlorine minimum per bather. In reality, it's a function of how long one swims, but lets ignore that for now and also note that I'm using conservative low-end estimates at each step so actual chlorine demand will likely be quite a bit higher than this.

If you are trying to maintain only 0.2 ppm (mg/L) of chlorine, then that gets consumed by one bather with their sweat and urine in a volume of 2321.8/0.2 = 11609 liters which is 3067 gallons. If someone urinates, then that would be a much larger volume of chlorine required to oxidize it. The above was for 25 ml but 1 cup of urine would be 236.6 ml or 9.5 times as much chlorine demand so would consume the chlorine in around 29,000 gallons (if you only had 0.2 ppm FC). That is, one person urinating in the pool could consume all of the chlorine in the pool (in practice, it consumes all of it in a localized area as it takes time for the urine to circulate).

In reality, one also sweats and there is also chlorine oxidation of skin, swimsuits, etc. though I've never seen an estimate of these factors.

You also need to consider the breakdown from sunlight which will be half of the chlorine lost roughly every hour for pools of typical depth (it's every 35 minutes closer to the surface). Chlorine will also react with organics blown into the water at a rate that is not only related to the chlorine (and organics) concentration, but also to temperature, but in a warm pool it's usually more than 0.5 ppm FC per day. However, in a commercial/public pool, the primary chlorine demand is from bather load. With only 0.2 ppm FC in the pool you have virtually no margin for error if the demand fluctuates because you have eliminated your chlorine buffer (i.e. CYA).

However, such total chlorine demand is not really the issue in a residential pool since it's really not that high for the pool as a whole in that case. The issue is whether you can actually control such low levels of chlorine. How would you regulate it? ORP isn't sensitive enough to properly distinguish such low levels of chlorine (it does much better at higher ORP mV levels which is why it doesn't do well when there is a lot of CYA in the water because the hypochlorous acid levels are too low to regulate easily). How would you ensure that areas of poor circulation in pools, including rough pool surfaces, corners, etc. do not get locally depleted of chlorine? At 2 ppm FC you've got 10 times the amount of chlorine (mostly in reserve) than at 0.2 ppm so for whatever circulation or diffusion that there is, it would take 10 times the chlorine demand to run out of chlorine locally. How are you going to make up for this with only 0.2 ppm FC?

You say that you will shock at night. That's sort of like using Trichlor pucks and then shocking weekly to try and make up for getting behind with too low an active chlorine level. Why subject your pool and all equipment and surfaces to 5-10 times the active chlorine on a regular basis just to make up for too low a chlorine level in some places during the day? Yes, you could try this and see what happens, but I don't think you will be able to regulate it easily during the day and besides, what do you do if your "shock" up to 1-2 pm FC doesn't drop far enough if the pool opens before the sun has had time to lower the chlorine level -- do you then add a chlorine neutralizer or do you expose people to the higher chlorine levels (similar to what they get today in many indoor pools)? This just sounds like it's getting too complicated and has too many ways it can fail.

In Europe there is a German DIN 19643 standard that specifies 0.3 to 0.6 ppm FC (or 0.2 to 0.5 ppm FC if ozone is used) with no CYA because they have activated carbon in the circulation system as part of this standard and this removes not only chloramines but also all chlorine that must then be re-injected downstream. The purpose of this is to minimize disinfection by-products by removing them and oxidizing precursors that are stripped of chlorine. However, such systems have a heck of a time regulating anything lower than 0.5 ppm FC so that's pretty much where most of them operate. So what you propose is certainly not unheard of, but even at 0.5 ppm FC they are still at least 2.5 times higher than they could be and they know they want to be as low as possible to reduce the rate of production of disinfection by-products which is why the low-end of their standard is so low.

What it sounds like you want to do is similar to what is described in this link. Note that Mr. Kinch has never corrected his website (in spite of my writing to him) in the section on "Eliminating Cyanurics" where he still states that CYA inhibits nearly all the sanitizing power of chlorine (he doesn't mention that this depends on the FC/CYA ratio), that standard dye tests are not accurate (he doesn't mention FAS-DPD that measures to within 0.2 ppm via "count the drops"), that ORP is a measure of disinfection (not true; MPS registers strongly but does not kill pathogens quickly), etc.

Richard
 
Very interesting thread, I am currently experimenting with low level chlorine and Dryden Aqua's ACO, (no CYA) whilst testing is not complete I can say that trying to maintain a low level is very hard with sudden and dramatic variations in temperature and although my over night chlorine loss is only 0.08ppm during the daytime I am currently losing 0.9ppm during average semi bright cooler weather and 0.47 when the sun is strong and the temp is high so ACO seems to work when the sun is strong but not so well at other times so my pool is effectively over chlorinated and not controllable enough yet.

Richard what are your thoughts on amperometric sensor probes compared to ORP probes, are they more accurate?
 
sbe said:
Focusing on the second concern first in light of a proposed regiment of elevated levels overnight (minimum 8 hours) with pump running and no bather load can you tell why you think this would result in an algae problem?
Click to expand...
It wouldn't. Neither of us suggested that it would. I have been focusing on person to person transmission of bacteria and viruses. The only time that algae becomes an issue in your system is when you miss a night of elevated levels. In a residential situation that is practically guaranteed to happen and will be quite annoying. In a commercial situation that is much less likely, but instead becomes a cost issue caused by the increased cost of maintaining a process that is less resilient in the face of mistakes.

I come at it primarily from a practical standpoint, while chem geek primarily derives things by theory. I have measured water next to a bather and compared it to the bulk pool water, with no practical way of surveying the volume effected. 2 ppm is the worst case I have observed, actual numbers vary from there to zero. Likewise the mixing time has been measured based on place to place variation in dye levels after dye addition. Again, mixing time varies and 30 minutes is about the worst you normally see.

sbe said:
Hopefully you don't find my questions tedious. I apologize if they are. Alternately if you find my probing interesting in that it causes you to think about the issues in a different way, even if the best solution still turns out to be the same that would wonderful.
Click to expand...
I enjoy it. It is fun to run through our reasoning with someone who can follow the issues involved without getting completely lost. It allows us to double check our thoughts and brings up possibilities that we might have missed.
 
teapot said:
Very interesting thread, I am currently experimenting with low level chlorine and Dryden Aqua's ACO, (no CYA) whilst testing is not complete I can say that trying to maintain a low level is very hard with sudden and dramatic variations in temperature and although my over night chlorine loss is only 0.08ppm during the daytime I am currently losing 0.9ppm during average semi bright cooler weather and 0.47 when the sun is strong and the temp is high so ACO seems to work when the sun is strong but not so well at other times so my pool is effectively over chlorinated and not controllable enough yet.

Richard what are your thoughts on amperometric sensor probes compared to ORP probes, are they more accurate?
Click to expand...
First off, one or both of your numbers is probably off since the 0.9 ppm during less bright cooler weather is higher than the 0.47 when the sun is strong and the temp is high. I'd be interested in the correct numbers -- were these two reversed?

An amperometric sensor if by far more accurate in terms of measuring chlorine levels. Such probes, however, are generally quite a bit more expensive. I'm not sure how finicky they are in terms of adjustment/cleaning/calibration. An even better probe, in theory, would be one using a selective membrane for hypochlorous acid (Chemtrol has such a probe), but in the past those have been unreliable possibly because the membrane gets affected by something else in the water over time but perhaps these issues have been worked out.

Your chlorine losses are interesting since in theory if you were able to truly maintain 0.2 ppm FC throughout the day and night you would lose around half as much chlorine during the day as one would using CYA at our recommended levels. In practice, keeping a more readily controlled 0.5 ppm FC with no CYA would probably lose something comparable to what we recommend for residential pools, while the higher 4 ppm FC with 20 ppm CYA (or 6 ppm FC with 30 ppm CYA) would lose more, BUT in a commercial/public pool such extra loss is minor compared to the much larger losses from bather load.

When you say it is hard to maintain a consistent level when the temperature changes, I presume you mean this is because you are trying to mostly maintain a level by predicting an average loss rate overnight vs. when the sun is on the pool during the day and use a sensor more for longer-period validation since shorter-term checks could fluctuate and over shoot (zig-zag) too much. The problem, as you point out, is not just variation of sunlight during the day, but even more importantly the temperature of the water. I have found that chlorine consumption in a low bather load pool varies a lot based on temperature and the difference between 80ºF and 90ºF is quite significant -- perhaps going from 0.3 ppm FC per day to 0.7 ppm FC per day between these temps (with no bather load and no sunlight), though obviously the actual amount depends a lot on the specifics of the pool and the variation can be even larger if there is differing organic load such as blown-in pollen. In commercial/public pools where bather load is the dominant factor, one pretty much has to either have outstanding measurement and control or have enough of a chlorine buffer as "insurance" to protect oneself. CYA, of course, performs this latter function.

As for the chlorine demand itself from bathers, I don't have experience with that in residential pools since the bather load is too low to make such changes obvious (though there is "peeing in the pool" that I described in this thread), but I have seen on another forum focused more on spas that one person-hour of soaking in a hot (104ºF) spa creates a chlorine demand of around 7 ppm FC in 350 gallons. About 1 ppm FC of this is 24-hour normal chlorine loss (at an average 4 ppm FC level), but that still means the bather load portion is around 6 ppm FC in 350 gallons. This is probably a lot from the additional sweat rather than mostly from urine as it is in pools (except for swimming competitions where sweat may again dominate). My rough guess is that in a swimming pool the net chlorine demand from swimmers is roughly half this amount so approximately 3 ppm FC in 350 gallons (i.e. scale accordingly based on pool volume). It would be interesting to collect actual statistics from real commercial/public pools looking at the daily chlorine demand as a function of bather load. Obviously, it depends on the nature of the bathers -- a bunch of young kids might have some who urinate and would vastly increase the chlorine demand.
 
Thanks Richard,

I don't believe my figures were off, the results were taken over several days of cold weather 60deg and then when it warmed up 95deg. I belive after a short email with Dr Dryden that his ACO works in bright sunlight protecting the chlorine but is much less effective in cooler weather with daylight so it isn't a complete replacement for CYA yet but it is early days in the experiment so please don't judge too deeply.

I am considering changing to amperometric probes to enable much finer tuning of the system and then produce more accurate results.

Regarding batherload, for this pool it is always light 4 persons in 11,000 US gallons but could not take readings as it was too cold to venture into the water!
 
I just want to chime in on the zero CYA algae issue. From 1946 to 1970 my family ran our pool with no CYA based on daily additions of 16oz. Cal-Hypo into 40k gal. A one-day skip did NOT result in algae. Ever. Neither did a one-day skip when we did 30ppm CYA and 1-3 ppm FC using the same 16oz. of Cal-Hypo every OTHER day, that we used from 1970 to about 1980, when we switched to pucks. (That worked fine, too, until 2004, when we stopped dumping the pool every year).

I consider myself a BBB'er, but I use the old 30ppm CYA and 1-3ppm FC recipe today, substituting the equivalent of bleach or LC for the Cal-hypo, now that I don't dump every year due to new pool. This is less FC than the TFP chart calls for, but it works for our low-use, mostly adult pool. Some may consider my sanitizer level inadequate, but we are a remarkably healthy crew since 1946 and our water sparkles. I don't use algaecide or anything besides Chlorine to sanitize and I almost never shock (except start-up). In my opinion, for our profile, there is some real overkill in that FC chart, but it is designed to be error tolerant and universal, so I don't criticize it, I just do things my way in my pool based on my 40 years experience.

I would never go back to zero CYA, however. You could feel all that unbound FC and suits (admittedly cotton at the time) would bleach and rot. Plus it requires continuous or at least daily dosing.
 
Thanks for the input, Durk. Just to be clear, the comments about possibly developing algae when there was no CYA weren't about the no CYA but the low 0.2 ppm FC level. In your case, you had more typical FC level above 1 ppm and with no CYA that is a very high active chlorine so it is no surprise you didn't get algae. Also, one day without chlorine won't have visible algae unless your pool is fairly rich in algal nutrients. It takes 3 to 8 hours for algae to double in population so they don't get very far in just one day, but the chlorine demand can be seen to go up once you start adding chlorine again until you kill off what has grown.
 
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