Re: How quick does chlorine dissipate?
You said: the rate of killing pathogens and algae and oxidizing bather waste remains constant, and the ORP remains the same.
comment: If you fix the free chlorine and let CYA levels climb (by using stabilized chlorine), you run into the case where there is too little active chlorine to kill germs and they overwhelm the chlorine. You then get plummeting chlorine levels and soon after you get poor water and algae. Also, kill times do increase with rising CYA levels.
I don't disagree, but what I wrote before what you quoted was "If one proportionally raises the FC to keep the FC/CYA ratio constant" which means that one CAN keep consistent disinfection, algae prevention, and oxidation by managing the FC/CYA ratio. In other words, looking at either FC or CYA alone is the wrong thing to do. Yes, for those using stabilized chlorine sources having the CYA rise then not increasing FC proportionally is a problem, but I don't want anyone thinking the pool industry mantra "CYA doesn't matter; only FC matters" or the opposite "CYA is bad" are true because neither are.
You said: Your third point of "Add as much chlorine as you like and beyond about 70ppm CYA the active chlorine tops out at about 0.18-0.2 ppm." is absolutely positively not true. That is not how chemical equilibrium works. 3 ppm FC with 30 ppm CYA has the same active chlorine level as 10 ppm FC with 100 ppm CYA or 50 ppm FC with 500 ppm CYA.
comment: at ever increasing CYA levels, more and more of the chlorine is bound. At 4 ppm chlorine and 20 ppm CYA the active chlorine is ballpark 0.1 ppm. Raise the CYA to 70 ppm and it takes about 20 ppm chlorine to get the same amount of active chlorine so, within reason of what most people will ever use, even 20 ppm chlorine will only get you to about 0.1 ppm active chlorine. It would take 40ppm free chlorine to get 0.2 ppm and within reason, most people won't do this so I stand by my comment.
You are missing the point that it takes a VERY low active chlorine level to kill pathogens and a somewhat low active chlorine level to prevent algae growth. See the chart in
this post that shows the times for 3-log reductions (99.9% kill) when the active chlorine level is the same as 0.1 ppm FC with no CYA at pH 7.5 so roughly a 10% FC/CYA ratio. The fact is that commercial/public pools in the U.S. with no CYA in them are by state law required to have at least 1 ppm FC and therefore are OVER-CHLORINATED because their active chlorine level is much higher than needed for disinfection and algae prevention. It means that swimsuits, skin, and hair are getting oxidized much faster and disinfection by-products are created faster. My wife has personal experience with this effect where she used to swim in a community center indoor pool with 1-2 ppm FC and no CYA over each winter season and we'd have to replace the swimsuits used (elasticity got shot) each season whereas in our private outdoor pool with 3-6 ppm FC and 30-40 ppm CYA the swimsuits would last for 7 years and the differences in her skin and hair between the two pools were very noticeable. It is such a big difference that we are now spending lots of money gas heating our pool through the winter so that she can use it.
Now if you are buying into the Kent Williams PPOA school of thought then you will say that you have to have higher chlorine levels for oxidation of bather waste but there are two problems with that. First is that for residential pools the bather load is so low that one does not need higher chlorine levels to handle bather waste, particularly for outdoor pools exposed to sunlight because the UV that breaks down chlorine produces hydroxyl radicals that are very powerful though short-lived oxidizers. Second is that when chlorine reacts with bather waste it produces disinfection by-products and higher chlorine levels not only produce them faster but can produce more of the most irritating and volatile nitrogen trichloride. The better approach is to use supplemental oxidation systems (such as ozone) or coagulants (such as SeaKlear PRS Stage 1 and 2) to remove the organic precursors without using chlorine to do so.
Europe understands this and in DIN 19643 specified only 0.3 to 0.6 ppm FC with no CYA with no ozone or 0.2 to 0.5 ppm FC with no CYA with ozone. They can't use CYA because their system uses coagulants (iron or alum) and activated carbon filters to remove organics and disinfection by-products from the pool (and such methods would tend to remove CYA as well). In the U.S., commercial/public pools could operate with a 20% FC/CYA ratio for a rough equivalent of 0.2 ppm FC with no CYA and have plenty of disinfection for everything except Crypto which isn't handled well even when there is no CYA in the water. The SeaKlear PRS I mentioned will coagulate Crypto and the CDC Model Aquatic Health Code (MAHC) requires secondary disinfection systems such as UV or ozone in high-risk venues to deal with Crypto. Instead of superchlorinating, one could get rid of Crypto overnight using chlorine dioxide generated on-site by adding sodium chlorite to chlorinated water (ideally with CYA in it since the lower active chlorine level minimizes generation of chlorate).
Also, why would you subject bathers to a higher level of CYA than is necessary just for marginal increase in chlorine retention? 10-20 ppm CYA gives good chlorine protection, reduces bather exposure to unnecessary levels of chemicals (CYA), lowers the exposure to free chlorine you must add (4ppm chlorine for 20 ppm CYA vs 20 ppm chlorine required a 70 ppm CYA to get the same active chlorine levels) and so on. The less chemicals in a pool the better off bathers are.
If you read what I wrote, it's not marginal but significant. Having 1.5 ppm FC with 30 ppm CYA in a pool compared to 4 ppm FC with 80 ppm CYA has around 60% higher chlorine loss. You are again missing the fact that the protection of chlorine from sunlight is NOT just due to the chlorine bound to CYA but also from a non-linear shielding effect such that higher CYA levels even with proportionally higher FC levels uses less chlorine. You seem to be stuck on the PPOA charts when the reality is NOT those charts and this has been proven not only in observations of thousands of real pools but in bucket and spa experiments.
What are you talking about reducing bather exposure to CYA? CYA does not absorb through the skin (see
this paper), is not volatile, and is less toxic than ordinary table salt (see
this link).
You also are missing that how much chlorine you add is based on how much you lose, NOT on the absolute FC level. I am NOT proposing high CYA levels for commercial/public pools. Most of their chlorine usage is due to bather load, not loss from sunlight. So 20% FC/CYA levels with 4 ppm FC with 20 ppm CYA or 6 ppm FC with 30 ppm CYA is reasonable. Most CYA test kits only measure down to 20 or 30 ppm CYA anyway. However, in residential pools, most chorine loss is from sunlight, not bather load, so protecting chlorine from sunlight DOES reduce chlorine demand and does so significantly.
You said: Most people don't keep their FC/CYA level at double the minimum 7.5%. They add enough chlorine so as not to go below the minimum when one doses the next day.
comment: I think they do, if they're smart. At 30 ppm CYA, 7.5% yields about 2 ppm chlorine and it takes 4 ppm to yield 0.1 ppm active chlorine which is needed to ensure thorough killing. They claim the threshold required is only about 0.05 ppm or so but one should error on the higher side of 0.1 ppm. Also, don't some agencies such as the UN or the World Health Organization require more than this such as 0.1 to 0.15 ppm? Thanks.
Most people don't operate their pool at 30 ppm CYA unless its an indoor pool where 20-30 ppm CYA may be used. For outdoor pools exposed to sunlight, most on this forum operate in the 40-50 range and those with SWG pools are closer to 80 ppm CYA. My pool is lower in CYA because I have a mostly opaque pool cover but that means a lower chlorine loss of only 1 ppm FC per day.
Where are you getting that 0.1 ppm FC equivalent is needed "to ensure thorough killing"? That simply isn't true. Look at the chart I linked to above where you can see the kill times at 0.1 are very fast where for fecal bacteria it's a 3-log reduction (99.9% kill) in 1-1/2 minutes or less. Having half that rate of kill would still be fast. I'm not talking about commercial/public pools where a 20% FC/CYA ratio (e.g. 4 ppm FC with 20 ppm CYA for indoor pools or 6 ppm FC with 30 ppm CYA for outdoor pools) may be more reasonable (i.e. 0.2 ppm FC equivalent), but residential pools where the risks are far lower especially for person-to-person transmission of disease.
Where are you getting the U.N. or WHO recommendation of 0.1? Are you talking about the WHO document
Guidelines for safe recreational water environments - VOLUME 2 SWIMMING POOLS AND SIMILAR ENVIRONMENTS? Even that document has things in it that are simply not true such as the following:
High levels of cyanuric acid cause a situation known as ‘chlorine lock’, when even very high levels of chlorine become totally locked with the cyanuric acid (stabilizer) and unavailable as disinfectant; however, this does not occur below cyanuric acid levels of 200 mg/l.
So even these people at WHO don't understand chemical equilibrium. There is no such thing as chlorine lock. If you raise the FC proportionally with the CYA level you retain the same active chlorine level. Chemical equilibrium is defined by RATIOS of concentrations of chemicals. The ratio of (in simplistic terms) [HOCl]*[CYA]/[Cl-CYA] is a constant and since most of FC is bound to CYA (so is Cl-CYA in what I wrote) then increasing FC (so Cl-CYA) proportionally when CYA increases keeps HOCl (hypochlorous acid) constant.
The WHO document also has the incorrect 10x rule that was misapplied to Combined Chlorine (CC) when it is only correct for ammonia in ppm Nitrogen units. It is incorrect when applied to CC because 1) the units of measurement for CC are in the same units as FC, namely ppm Cl
2 which is a factor of FIVE larger then the units for ammonia which is ppm N, and 2) one chlorine is already attached to the ammonia (I already went over this with you in the thread
Breakpoint Chlorination).
Destroying chloramines requires free chlorine levels at least 10 times the level of combined chlorine.
This WHO document does not talk about 0.2 ppm -- it says instead 1 ppm FC but is silent about CYA level and doesn't understand that CYA is a hypochlorous acid (active chlorine) buffer and therefore one doesn't need 1 ppm FC levels of active chlorine. The distinction between having chlorine in reserve vs. the active level is lost on most in the pool industry.
For a conventional public or semi-public swimming pool with good hydraulics and filtration, operating within its design bathing load and turnover and providing frequent (or online) monitoring of chlorine and pH, experience has shown that adequate routine disinfection should be achieved with a free chlorine level of 1 mg/l throughout the pool.
Even DIN 19643 goes lower than this though as you might imagine it is almost impossible to adequately maintain 0.2 ppm FC throughout a pool without outstanding circulation. Pools in the U.S. tend to have much worse circulation so using CYA in moderation allows one to have an ample FC buffer while tuning the FC/CYA ratio for adequate disinfection.
