Confused About Chlorine

[Beefcake]

Hi all, I'm new to the hot tub scene as this is my first. That said, I've had a pool for a while and I know they are different, but jsut thought I'd through in that I know to adjust alkalinity and hardness, then pH, and finally chlorine levels. Here's my setup....

Spa56

Renew before each use

Nature H2 mineral sanitizer in filter

Sundance 250mg/hr ozonator

As of last night, the water was cloudy, so I tested it with the strips I bought. Alkalinity, hardness, and pH were in the ideal range while chlorine (total and free) was nonexistent. So, as per the spa56 directions, I added 1oz. to super chlorinate. I ran the jets on low speed for 40 minutes total. I tested again with the strips and everything looks good while the chlorine levels were both a little above ideal. The filter cycle runs from 1am -3am and 1pm-3pm. I know I am need of a new filter cartridge as the current one is starting to fall apart in some "fins". Fast forward to this morning. The water is clear (at idle) for the first time since the initial filling (5 days ago). So, I test with the strips again and the chlorine level is again non-existent. Do I need to keep adding spa56 to super chlorinate until the levels stick or is there something else at work here "using up" all the chlorine?

What should I be doing here? I want to enjoy my tub again, but I want to get the chemistry right before doing so.

[chem geek]

Chlorine is a consumable and will get used up and needs to be added regularly. Having an ozonator is both a plus and a minus. If you don't use the spa regularly, say every day or two, then the ozonator will usually increase the chlorine demand such that instead of losing around 25% of the Free Chlorine (FC) level each day you may lose 50% instead. However, if you use the tub every day, then the ozonator will reduce the amount of chlorine you need to add.

The rough rule-of-thumb for how much oxidizer is needed to be added after a soak for every person-hour of soaking is 5 fluid ounces of 6% bleach, 3-1/2 teaspoons of Dichlor, or 7 teaspoons of non-chlorine shock (43% MPS). With an ozonator, the needed amounts may be half, but you just experiment by adding enough such that you get a measurable residual FC of 1-2 ppm by the start of your next soak the next day (if you don't soak every day, then add enough to measure 3-4 ppm FC the next day and then add more every day or two to maintain that level).

It would be better for you to get an accurate test kit so that you can accurately measure your chlorine demand and get to "know your tub" better. The Taylor K-2006 kit is able to measure Free Chlorine (FC) and Combined Chlorine (CC) to within 0.2 ppm and also tests for pH, Total Alkalinty (TA), Calcium Hardness (CH) and Cyanuric Acid (CYA).

Read the following links for more info about the Dichlor-then-bleach method if you want your hot tub water to last longer between changes.

Nitro's Approach To Water Maintenance

Dichlor/bleach Method In A Nutshell

Chlorine Demand

Note in particular the superchlorination/disinfection procedure that you should always do with a new tub since after wet-testing there is often water remaining in the pipes and bacterial biofilm can form. Also, your water will likely need to be changed sooner for your first fill -- new tubs have all sorts of residual chemicals in them.

[Beelzy]

Keep up with the chlorine until you get a new filter cartridge. The degrading one is putting fine particles back into the spa and is causing the

excessive sanitizer consumption. IMO

[Hillbilly Hot Tub]

I agree, you need new filters or you will be fighting a losing battle.

[quantumchromodynamics]

The rate at which chlorine is depleted by ozone is dependent on the pH and the concentrations of chlorine and ozone. Ozone reacts with the hypochlorite ion, but not hypochlorous acid. The pH determines how much of the chlorine is in the form of hypochlorite and how much is in the form of hypochlorous acid.

pH........HOCl.......OCl^-.......HOBr.........OBr^-

7.2........66 %........34 %.......96 %..........4 %

7.5........48 %........52 %.......94 %..........6 %

7.8........33 %........67 %.......87 %........13%

8.0....... 22 %........78 %.......83 %.........17 %

There are two different reactions of ozone with hypochlorite ions.

O₃ + OCl^- --> 2O₂ + Cl^-

2 O₃ + OCl^- --> 2O₂ + ClO₃^-

The first reaction accounts for 77 % of the reactions and the second reaction accounts for 23 % of the reactions. The overall net is:

1.23O₃ + OCl^- --> .77Cl^- + .23ClO₃^- + 2O₂

As you can see from the chart above, the rate of chlorine loss will be 2.3 times higher at a pH of 8.0 than at a pH of 7.2. Most ozone used in a chlorine tub goes into destroying chlorine. CD (corona discharge) ozonators will destroy much more chlorine than UV ozone generators.

If we assume that your all of the ozone (0.250 grams per hour) goes into reacting with the chlorine, then every hour your ozonator will remove 0.3 grams of chlorine (measured as Cl₂). This would be a maximum amount of chlorine loss due to ozone. The actual loss would typically be less, and would depend on multiple factors, such as ozonator run time, pH, temperature, contact time, how well the ozone is mixed into the water etc. Adding 20 ppm of cyanuric acid should help reduce the rate chlorine loss because most of the chlorine will be bound to cyanuric acid. I don't think that ozone has much of a reaction with chlorine that is bound to cyanuric acid.

Volume......................loss per hour.......... loss per day

300 gallons.................0.264 ppm.................6.3 ppm

350 gallons.................0.227 ppm.................5.4 ppm

400 gallons.................0.200 ppm.................4.8 ppm

450 gallons.................0.176 ppm.................4.2 ppm

500 gallons.................0.159 ppm.................3.8 ppm

The same thing happens with bromine. However, as shown in the above chart, there is substantially less hypobromite ion concentration for any given pH. DMH (The carrier base in bromine tabs) might provide some protection for bromine in a similar way to how cyanuric acid provides protection for chlorine. However, that's less certain. Some bromine is attached to the dimethylhydantoin. However, I do not know the equilibrium constants for bromine with DMH. Multiple sources indicate that the level of dimethylhydantoin in the water must be limited and should not exceed 200 ppm.

1.23O₃ + OBr^- --> .77Br^- + .23BrO₃^- + 2O₂

Ozone + hypobromite ion --> bromide ion + bromate + oxygen gas

Ozone will also combine directly with bromide to form bromate:

Br^- + O₃ --> BrO₃^-

The formation of bromate will result in the loss of bromide ions over time. The use of tabs should replenish the bromide ions faster than they are removed.

In addition, the ozone will oxidize bromide ions into bromine again.

Br^- + O₃ --> O₂ + OBr^-

Which quickly combines with water to become hypobromous acid.

OBr^- + H₂O --> HOBr + OH^-

Therefore, for bromine, you have loss of bromine as well as gain. As long as there is a sufficient bromide ion concentration, there should be more bromine created than lost. When the water contains 50 ppm of bromide ions, most of the ozone will be used oxidizing bromide to bromine.

Reference

[GB315]

New member here, but I wanted to add a data point to the ozone+chlorine question.

Recently I bought a new 2010 spa with an ozonator. My intent was to use the dichlor/bleach method. For the first few weeks, with light use, everything seemed fine, although I did notice that the chlorine demand was somewhat high, >50%. After my kids had a hot tub party, I had a hard time getting the CC down, but since it had been about a month since the initial fill, I figured it was time to drain and refill anyway. I first did a 45 minute bleach decontamination to ~100ppm Cl, drained, then refilled, which left about 10 ppm. In 12 hours, there was no Cl. I figured that was because there was no CYA. I shocked up to 13 ppm with dichlor. That disappeared in 12 hours too. Suspecting the ozone was eating the Cl, I unplugged the ozonator and shocked to 13 ppm with dichlor. Voila! 12 hours later, the FC was 11 and continued to disappear at the rate of about 33%/day. With the ozonator off, I have not had any problem maintaining the chlorine level.

So, my empiric test confirms QCD's explanation. I don't know if the Cl would be more compatible with ozone now that I have more CYA in the tub. I've now added a total of 3 oz of dichlor to the 500 gallon tub, so I should have 20-30 ppm CYA by now. When I bought the tub, I thought the ozonator was a great idea. Now, I'm not convinced. By adding another reaction to the chemistry, it would seem to make the Cl levels less predictable.

Any thoughts?

[Razorhog]

New member here, but I wanted to add a data point to the ozone+chlorine question.

Recently I bought a new 2010 spa with an ozonator. My intent was to use the dichlor/bleach method. For the first few weeks, with light use, everything seemed fine, although I did notice that the chlorine demand was somewhat high, >50%. After my kids had a hot tub party, I had a hard time getting the CC down, but since it had been about a month since the initial fill, I figured it was time to drain and refill anyway. I first did a 45 minute bleach decontamination to ~100ppm Cl, drained, then refilled, which left about 10 ppm. In 12 hours, there was no Cl. I figured that was because there was no CYA. I shocked up to 13 ppm with dichlor. That disappeared in 12 hours too. Suspecting the ozone was eating the Cl, I unplugged the ozonator and shocked to 13 ppm with dichlor. Voila! 12 hours later, the FC was 11 and continued to disappear at the rate of about 33%/day. With the ozonator off, I have not had any problem maintaining the chlorine level.

So, my empiric test confirms QCD's explanation. I don't know if the Cl would be more compatible with ozone now that I have more CYA in the tub. I've now added a total of 3 oz of dichlor to the 500 gallon tub, so I should have 20-30 ppm CYA by now. When I bought the tub, I thought the ozonator was a great idea. Now, I'm not convinced. By adding another reaction to the chemistry, it would seem to make the Cl levels less predictable.

Any thoughts?

Sounds like taking the ozone out of the equation for now is the right thing to do.

[quantumchromodynamics]

I think that in many cases ozone is counterproductive. It is only helpful in a limited number of cases. And, when it is done, it needs to be done properly. Pumping it directly into the return line is not the proper way to do it. The correct way to use ozone is fairly complicated and involves contact tanks, off-gassing, etc.

Here is a paper about ozone.

[GB315]

Thanks, QCD. I read that too and came to the conclusion that ozone should only be used with a continuous supply of chlorine.