I have just started servicing a indoor commercial facility that reports heavy use of acid to control PH.
This is a Indoor facility about 8 months old with a plaster finish so I would expect heavy use of acid for the first 60 days but they report it has not decreaded at all.
They use liquid chlorine and have a UV system.
I can not remember exact volume but it is around 800,000 gallons.
They report using about 15 gallons of acid daily.
They also use large amounts of bicarb to try to keep the alk. up above 80 or 90.
They hav taken samples to local labs but have no answers or recomendations.
I plan to start getting in to it next week but thought I would see if I could get some new ideas.
Thanks for any help.
Hank
Replies
Thanks for the update. If there is any way for you to measure the amount of acid that is added per unit time (to maintain the pH) at different TA levels, then it would be interesting to see if there is a difference in required acid at the TA of 80 ppm vs. the TA of 40 ppm. The other way you could track this is by measuring the TA level over time where I suspect you'll see the TA dropping faster when it's at 80 ppm than at 40 ppm, assuming your acid controller is measuring pH to determine how much or when to add acid (if it's set for a fixed output amount of acid, then the TA drop will be proportionally constant to that acid addition rate so would be the same at all TA levels).
If you do end up having a lower average TA level, then you'll need to compensate with a higher CH to protect the plaster surfaces (i.e. to have a near-zero saturation index). It's too bad there isn't a relatively inexpensive pH buffer you could use that wasn't carbonates -- boric acid is very effective, but it's also rather expensive.
Thanks for the updates and sorry it has been a while since my last post.
Things are getting better. Let me provide some updates-
We use 12.5% liquid chlorine
pool use is down due to warmer weather folks not usuing pool much. Less arieation
We had a issue with UV system that has been corrected
We turned the Acid feeder down to slow the rate we put acid in
We add bi carb to ballancing tank and now add a little each day ( 30 to 50 lds every other day)
We changed auto fill that ran off and on daily 1.5 inch fresh water line blasting in to ballancing tank , now fills under water. Much less spray and bubbleing as filling.
We are using less chlorine and less acid. Alk is up and down between 40 and 80.
PH set point is 7.6
I do not have actual amount of acid used over last week but will get it this week.
Not planning to change to co2 or other chlorine at this time.
I feel like the staff is getting pretty focused on chemical ballance now mayn small items are helping.
Thanks
Richard A. Falk said:
Higher TA means more carbonates in the water including aqueous carbon dioxide. There is more of that in the water than there would naturally be when exposed to the air so it outgasses. When that happens, the pH rises (with no change in TA). This process happens more quickly with aeration so a waterpark or heavy swimming will increase the rate of this process.
So the question is what happens if one lowers the TA level. There are two effects. One is that there is less pH buffering so any acid or base added to the water will cause a greater swing in pH. However, the other effect is that there is less carbon dioxide in the water so less outgassing so less acid needs to be added to restore the pH from this effect. So lowering the TA level will lower the amount of acid that needs to be added over time if the pH rise is primarily due to carbon dioxide outgassing. The only question is whether it also lowers the rate of pH rise.
I can't give examples for commercial/public pools, but I can tell you that for residential pools and for most residential spas there is a slower rate of pH rise when the TA is lower. This is assuming that a hypochlorite source of chlorine is being used -- obviously, if acidic sources of chlorine are used (Trichlor, Dichlor, chlorine gas) then a higher TA is needed. I list below the real-world experience of a small sub-sample of pool and spa users where the lower TA led to greater pH stability -- that is, the pH rise slowed down or got close to stopping completely.
POOLS
link: TA from 80 to 50-60 made pH stable
link: A TA of 80 (lowered from 100) gave a slower rate of pH rise
link: log of reduced acid usage from TA 100 to 60 (SWG pool)
link: follow-up to log above when TA was 50 ppm with lower acid usage
link: pH stabilized when TA lowered to 65 ppm from 100-120 ppm (Trichlor usually needs higher TA)
SPAS
link: TA of 50-60 ppm slows down pH rise; 50 ppm Borates makes even more stable
link: pH stabilized at TA of 30-40 ppm; Borates recommended as TA is very low
link: pH under control (rise is much slower and much less acid needs to be added)
For spas, it was the use of the Dichlor-then-bleach method that prompted the need to have a lower TA to slow down the pH rise. This is because using Dichlor-only is net acidic while using bleach exposed the rise in pH from carbon dioxide outgassing. Dichlor-then-bleach is a method to initially raise the CYA level using Dichlor, but then to stop when it is at 30 ppm (after around 33 ppm FC is cumulatively added from Dichlor) and then switch to bleach. It has been very effective at letting people go about twice as long between water changes, keeping the water more clear during that time, and reducing the number of rash/itch incidents that were previously seen. However, lowering the TA alone wasn't always sufficient, especially with spas with ozonators, but using 50 ppm Borates (Proteam Gentle Spa, for example -- this is just primarily boric acid) as an additional pH buffer has worked very well to stabilize pH without being a source of rising pH itself. Because of lots of aeration from spa jets and many spas with ozonators adding to that aeration, most spas need a combination of low TA and the use of 50 ppm Borates to get the pH to stop or significantly slow down from rising, but it does work.
For pools, it is saltwater chlorine generators (SWG) that really got us down the path of looking at lower TA since those pools tended to have more problems with rising pH. Though a lower TA (usually around 70 ppm) would help, we found that having a higher CYA level (and proportionately higher FC level) also helped reduce absolute chlorine usage which let people turn down their SWG on-time. Also, use of 50 ppm Borates not only increased stability as an additional pH buffer, but was also a mild algaecide that usually lowered chlorine demand and let people turn down their SWG on-time even more. The lower SWG on-time reduced both aeration from the SWG and possible outgassing of undissolved chlorine gas (we don't know for sure the exact factor involved here).
For commercial/public pools, there are other concerns. Without supplemental pH buffering, say from 50 ppm Borates (which can get expensive), the pH will swing more in both directions from acid/base loads in the pool and it's not clear if high bather load would cause that. Clearly, the higher bather load requires more chlorine and if that is from sodium hypochlorite sources, then the excess lye would require acid to compensate and nothing can reduce that factor except getting a higher quality sodium hypochlorite that has lower pH (12.5 or lower). The acid injection controllers that test for pH may be "tuned" for specific TA levels so might overshoot in both directions (too little and too much acid) at lower TA levels. Ideally, they would have some sort of adjustment for this or, better yet, would dynamically adjust to account for actual pH buffering (i.e. they would "learn" from the effects of their dosing).
I figured it wouldn't hurt to at least try to see what a lower TA did, at least with respect to a lowering of acid addition (and corresponding TA addition). It would very likely lower the amounts of chemicals needed -- the only question is whether the pH controllers that do acid injection would be "smart" enough not to overreact at the lower TA level. If it didn't work, one could easily raise the TA back up and just grumble and complain about the pH controllers not being smart enough.
I have managed the water for two large water parks, The hardest thing to maintain is the ALK in most situations. We would always shoot for a higher value than normal. An alkalinity level of 120-140 will allow better control of PH. The way we applied the bicarb is essential. We would add the bicarb and disable the acid pumps for a few hours to allow the amendment to be thoroughly mixed in. To get more out of the chlorine we would shoot for a PH of 7.2-7.4 to maximize the amount of chlorine changing into hypochlorous acid.
If you leave your acid pump system active when adding bicarb sometimes the pumps will run nonstop reacting to the amendment of bicarb. Another thing to check would be the delivery rate of your acid pumps. Sometimes reducing the volume of acid pumped at one time can resolve the issue you are challenged with.
I hope this helps
Neal
If you tell me the amount of baking soda you add each day (or how much you add and how frequently) I can distinguish between how much the pH is rising from carbon dioxide outgassing vs. how much it is rising from the excess lye in the chlorinating liquid plus the curing of plaster (if any). I had assumed that the acid was full-strength Muriatic Acid (31.45% Hydrochloric Acid) so please correct me if I am wrong in that assumption.
Also, are you using 12.5% chlorinating liquid? If not, what strength? How much are you adding daily?
So what is your TA level now? Has the required acid amount gone down and if so, how much are you adding now? You indicated progress, but didn't give specifics.
Just as the outgassing of CO2 raises the pH with no change in TA, injecting CO2 into the water lowers the pH with no change in TA. Any change in TA over time when injecting CO2 comes from other basic/alkaline sources including the "excess lye" in chlorinating liquid or from the curing of plaster. There is a difference between total carbonates and carbon dioxide in the water vs. TA. When one has CO2 either leave the pool from outgassing or get added to the pool by injection, it mostly results in a change in aqueous carbon dioxide concentration with very little change in bicarbonate and carbonate ions which are the primary components of TA (and such changes cancel each other out along with hydroxyl and hydrogen ions). Let's look at specific examples where for simplification I assume there is no CYA in the water and no chlorine (since it has a minor pH buffering effect as well) and 77F standard temperature and I ignore ion pairs (a very minor factor in this case). Remember that carbonate ions count twice towards TA (they can accept two hydrogen ions before reaching the 4.5 pH endpoint in the TA test). The following does use ionic strength calculations assuming around 525 ppm TDS, but that doesn't change the basic principle of what is happening.
80 ppm TA, pH 7.5 initially
Aqueous Carbon Dioxide (CO2(aq)): 101.49 µmole/liter
Carbonic Acid (H2CO3): 0.16 µmole/liter
Bicarbonate Ion (HCO3(-)): 1591.69 µmole/liter
Carbonate Ion (CO3(2-)): 3.28 µmole/liter
Hydroxyl Ion (OH(-)): 0.35 µmole/liter
Hydrogen Ion (H(+)): 0.03 µmole/liter
Total Alkalinity (TA) = 1591.69 + 2*3.28 + 0.35 - 0.03 = 1598.57
Add 0.73 pounds CO2 per 10,000 gallons which results in
80 ppm TA, pH 7.03
Aqueous Carbon Dioxide (CO2(aq)): 298.55 µmole/liter
Carbonic Acid (H2CO3): 0.46 µmole/liter
Bicarbonate Ion (HCO3(-)): 1596.3 µmole/liter
Carbonate Ion (CO3(2-)): 1.12 µmole/liter
Hydroxyl Ion (OH(-)): 0.12 µmole/liter
Hydrogen Ion (H(+)): 0.10 µmole/liter
Total Alkalinity (TA) = 1596.31 + 2*1.12 + 0.12 - 0.10 = 1598.57
As for using Trichlor tabs, since for every 10 ppm Free Chlorine (FC) added by Trichlor it will also increase Cyanuric Acid (CYA) by 6 ppm, significant water dilution will be required to keep CYA low. Since this is an indoor pool and does not have the benefit of UV from sunlight to help breakdown some of the combined chlorine faster, one doesn't want the active chlorine (hypochlorous acid) level to get to low which is what will happen as the FC/CYA ratio gets lower due to rising CYA from the Trichlor.
Also, as was noted in earlier posts in this thread, the fact that the TA is dropping over time proves that carbon dioxide outgassing is a significant source of the pH rise. If the source of pH rise were from the excess lye in chlorinating liquid (or from the curing of plaster or any other basic/alkaline source), then that would have both the pH and TA rise while acid would lower both pH and TA with the net result being no change in TA -- a strong acid and a strong base exactly counteract each other. So if one uses Trichlor, that is no different then using acid from a pH and TA perspective so would still result in a drop in TA over time. The key to getting out of this cycle is to realize that TA is a source of rising pH itself due to carbon dioxide outgassing so letting the TA drop and not increasing it through use of baking soda is the answer here (to a point -- you need some TA for pH buffering, but don't need 90 ppm since even 50 ppm would provide ample buffering in this case as I had illustrated earlier).
As shown in this chart, at a TA of 90 ppm (80 ppm in the chart if we assume there is no CYA in this pool) and pH of 7.5, there is 7.0 + 1 = 8 times as much carbon dioxide in the water as the equilibrium amount from being exposed to the air. At a TA of 50 ppm, there is 3.4 times as much and if we also let the pH rise to 7.7, then there is only 2.1 times as much so about a factor of 4 times less over-saturation of carbon dioxide. The result will be at least a factor of 4 lowering in the amount of acid needed, though in practice it will be more than this since the rate of CO2 outgassing is more proportional to the square of the TA, not linear as I just assumed from taking the numbers in this chart directly. Of course, this ignores the amount of acid needed to counteract the "excess lye" in the chlorinating liquid, but that would be required in any case -- at least the TA would become more stable and not as much baking soda and acid need to be added over time.
Most here probably have experienced something like 5ppm FC in clean water, hardly noticeable. Add some CC or some organics to help create CC and it stinks, your eyes start to feel like they are going to bleed. Especially in a Hot Tub or Kiddies Pool with aeration.
If you think of the NSPI handbook that used to state .2ppm CC as max, and accept the 10x rule for Superchlorination to remove CC you can see how 2ppm FC is theoretically always Superchlorinating the water.
Anyways, that forms part of my sermons when I get wound up. LOL
What about installing a trichlor tab feeder? If Cl2 demand is so high, combining trichlor tabs and liquid chlorine may balance out your pH drift, lowering need for huge quantities of acid which destroys TA and creates your greased pole.
Rick Larson said:
What about installing a trichlor tab feeder? If Cl2 demand is so high, combining trichlor tabs and liquid chlorine may balance out your pH drift, lowering need for huge quantities of acid which destroys TA and creates your greased pole.
Rick Larson said:
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