Chlorine Dissipation Rates

[Walter White]

So I just finished decontaminating my spa and I'm trying to verify success by monitoring chlorine dissipation.

Chlorine PPM
2:30p Fri: 5
10p Sat: 2
then I added some dichlor and went to bed.  Next measurements:
6:30p Sun: 4
10:30a Mon: 3

Are these the kind of dissipation rates one should expect from a clean spa?  It's a covered/above-ground spa so I think CYA doesn't matter.  I ran the jets maybe 15 minutes each day as I added some dry acid, other than that the circ system runs for 2 hours twice daily.  Spa temperature is at 100 degrees.

[Tman122]

Seems right to me. Your source water can have a lot to do with it also.

[chem geek]

Do you have an ozonator?  The usual chlorine loss rate without an ozonator is around 25% over 24 hours.  Your loss is higher than that, but if you have an ozonator then that could explain it since the loss usually 50% or more in that case.  As for CYA, that does make a difference.  If there is little or no CYA in the water, then the loss rate is often higher since outgassing is faster (as are chlorine reaction rates).

[Walter White]

I do have an ozonator, and now it sounds like I have a method to figure out if it's working, too.  I unplugged it and took a chlorine reading just now so we'll see what happens after 24 hours of no ozonator.

CYA levels are at 0, at least as best as I can tell from the junkie test strips.  I haven't added any CYA.

Thanks for the input, guys.

[Chas]

I haven't added any CYA.

If you are using Dichlor, you are adding CYA.

 

[Walter White]

Chas, thanks for setting me straight on CYA.  If I use dichlor to keep my spa chlorinated but otherwise don't add any CYA, will CYA be increased by a noticeable amount?

After some technical difficulties, I ended up with the following data samples:

Tue 11:30p: 3.5
Thu 11:20a: 2.7
Thu 11:15p: 2.2
Fri 9:30a: 2.2

I attribute the 2.2s to my own color-matching error.  But it seems like I'm in the neighborhood of a 25% dissipation rate per day so I went ahead and put in new filters (spa had been running without filters since I began decontamination process).

Three months after I first developed Hot Tub Rash, I still have the rash.  And, it worsened after I went back in the spa a few times (after completing the first decontamination in late December).  The rash isn't itchy or painful, but it seems to last forever.  Haven't been in the spa in 3 weeks now.  Can anyone confirm whether I would create a risk of re-contamination by reentering the spa before the rash goes away?

My plan going forward is to use scumballs, wipe down walls above waterline with bleach maybe once a week, maintain pH, keep Chlorine levels between 2 and 5 at all times, and use a little more than 1 teaspoon dichlor per 20 person-minutes in the tub to shock after each soak, and keep tabs on chlorine dissipation rates to see if they stay at 25%/day when spa is not in use.  I'd like to use bromine again but I think it's better I don't, on the off chance that the rash was caused by a bromine allergy.

Any other recommendations?  I got a bottle of Natural Chemistry Spa Foundation but haven't used it yet, any opinions on that stuff?

[Walter White]

Or rather, now that I've been using dichlor for a week, I'll now switch to using bleach, after finding one of Richard's posts recommending the "dichlor then bleach" method.

[chem geek]

Natural Chemistry Spa Foundation is mostly boric acid with some citric acid and lanthanum sulfate.  We usually recommend 50 ppm borates in the water and using boric acid is an easy way to do that without changing the pH very much.  The lanthanum sulfate is used to remove phosphates which really isn't necessary to do (and it may temporarily cloud the spa until the precipitate gets filtered out).  You can use this product if you want to, but you can also just use boric acid directly or a combination of 20 Mule Team Borax and acid.

You won't recontaminate the spa by having a rash.  If your rash is bacterial, any bacteria exposed to the chlorinated water will be killed quickly.

As you have learned from other posts, for every 10 ppm Free Chlorine (FC) from Dichlor, it also increases Cyanuric Acid (CYA) by 9 ppm.  Though it's possible that your continued use of Dichlor making the chlorine less effective over months, but it's more likely you weren't adding enough disinfectant after each use or in between uses and the chlorine got to zero which allowed bacteria to grow.  That is, if your rash is bacterial and not a chemical reaction or something else.  You rash might not even be related to the spa.  Does it get worse after an equally hot bath?  You might be sensitive to hot water temperature.

So without an ozonator, remember the rough rule-of-thumb that every person-hour of soaking in a hot (104ºF) spa needs around 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach (3.6 fluid ounces of 8.25% bleach) or 7 teaspoons of non-chlorine shock (43% MPS).  The MPS is only a disinfectant when using the Nature2 system with silver ions.  The real rule, however, is to add whatever is needed so that you still measure at least 1-2 ppm FC for the start of your next soak and that it doesn't get to zero anytime between soaks.

[Walter White]

As you have learned from other posts, for every 10 ppm Free Chlorine (FC) from Dichlor, it also increases Cyanuric Acid (CYA) by 9 ppm.  Though it's possible that your continued use of Dichlor making the chlorine less effective over months, but it's more likely you weren't adding enough disinfectant after each use or in between uses and the chlorine got to zero which allowed bacteria to grow.  That is, if your rash is bacterial and not a chemical reaction or something else.  You rash might not even be related to the spa.  Does it get worse after an equally hot bath?  You might be sensitive to hot water temperature.

If that were the case, it would have to be a new development.  I have gone in many spas at different resorts, and the hotter the better.  I have never had any problems until now.

I just got back to the dermatologist.  She says everything about my condition -- the u-shaped pattern that matches the jets, the pink splotches with red dots in the middle, the lingering "stains" on my skin, the fact that the bumps were weeping puss soon after they first appeared, and the (initial) responsiveness to antibiotics -- points to Hot Tub Folliculitis.  But, she said she can't say with 100% certainty that it wasn't a bromine allergy made worse by the jets pounding bromine into my skin, or even something totally unrelated to the hot tub.

Hearing that I won't recontaminate the spa lowered my anxiety tremendously.  Thankyou, and thanks for the comments on Spa Foundation.

Can a combined chlorine measurement be used as another indicator as to whether a spa is contaminated?  Nobody has gone in it since I refilled and balanced the water, so I would think the cc level should be low if the spa is clean, right?

[chem geek]

If you measure CC (hopefully using an accurate FAS-DPD test kit so you can get 0.2 ppm resolution using a 25 ml water sample), then you should do so before your soak.  If you measure soon after your soak, the CC will be higher because chlorine will combine with sweat and urine to form monochloramine almost immediately and it takes some time for it to get fully oxidized by chlorine.  There's also chlorourea that gets formed and also measures as CC.  Usually at normal FC/CYA levels and hotter spa temperatures, the monochloramine is taken care of in an hour or two while the chlorourea and other CC may take up to 12 hours, sometimes longer.  If the CYA builds up from Dichlor-only use, then these times get longer which is one reason why in high-use spas the water starts to get dull/cloudy and a water change is needed sooner than with Dichlor-then-bleach.

A CC reading doesn't tell you that your spa is contaminated.  It just tells you that there are chemical compounds combined with chlorine -- mostly monochloramine and chlorourea.  To prevent bacterial growth in the spa, you need to maintain the Free Chlorine (FC) levels at all times to NEVER get close to zero except optionally during the hour of soaking.  The monochloramine that forms during soaking will still inhibit bacterial growth, albeit more slowly.  You should add chlorine right after your soak in quantities sufficient to handle the bather load and to still have a residual for in-between soaks.  You may need to add chlorine in-between soaks if you do not soak every day or two (and as you've seen, an ozonator will use up chlorine faster so is useful when you soak frequently but not if you soak only once a week or so).

One way to see if your spa has significant bacterial growth or unoxidized organics is to do a 24-hour chlorine loss test, preferably with the ozonator turned off.  If the loss rate at hot spa temperatures (100-104ºF) is roughly 25%, then that's normal (at lower temps closer to 80-90ºF it's around 15%).  If your loss rate is significantly higher than this, then that indicates something consuming chlorine in your spa.  Unfortunately, you can't tell whether it's a buildup of unoxidized organics if you got behind in chlorine dosing or if it's bacterial growth or other pathogens chlorine is trying to kill.  However, you can have enough bacterial growth to be a problem and yet not be enough to show up as significantly increased chlorine demand.  It's far easier to prevent a problem by just making sure there's always chlorine in the spa.  That also means periodically running the jets or other areas where water might linger so that the chlorine circulates to those areas to kill bacteria.

[Walter White]

So my spa's CYA is at 40 and the Chlorine/CYA chart says min 3, target 5, shock 16.

How does an ozonator change those numbers?  I have the little blue box ozonator with a yellow-green light, and it runs for 2 hours twice a day, plus any time the heat comes on.  Also, is the same level of chlorine less irritating to the skin at higher CYA levels than low ones?

My plan is to just use bleach, for both chlorination and as a shock.  When I bought the spa the dealer gave me MPS, but from reading here I haven't found any need for it.  Since bleach seems to have no downsides and everything else does, I'm afraid to use anything else.  Although the guy at Leslie's said they recommend against bleach as a chlorination source for acrylic spas... what do you guys think about that?

[chem geek]

If you are looking at the chlorine/CYA chart at Trouble Free Pool (or The PoolForum), that is for pools and has minimum levels for preventing algae growth.  That isn't the required minimum for spas, though most of the time it will be close.  1-2 ppm FC at the start of a soak is fine and will minimize chlorine/chloramine smell during the soak.  If you are soaking with strangers and are concerned with person-to-person transmission of disease, you can increase your starting FC so that it won't run out during the soak, but that's up to you.  Since you are worried about contamination given your previous hot tub rash, you can err on the side of a higher FC level (3-4), but you may notice that more when you soak.

An ozonator doesn't change your target FC at the start of your soak, but it will change the amount of chlorine dosed after the soak -- probably to about half -- and it changes the amount of chlorine needed to be added in between soaks, probably about double.  Again, you really need to just measure the FC level and adjust your dosing accordingly.  The good news, is that the amounts will be relatively constant in terms of per person-hour amounts and per-24-hour losses.  The bad news is that if you don't soak every day or two, an ozonator increases the frequency of your maintenance since it uses up chlorine more quickly in between soaks (roughly 50% FC loss per day compared to 25% loss).

If you properly maintain your spa with chlorine and don't let it get to zero, there should be no need for you to shock the spa.  Of course, if you want to do that given your rash experience, that's up to you.

Bleach is fine to use in spas IF you have CYA in the water first.  Also, as with any source of chlorine, you want to add it slowly to the water with the circulation pump running so that concentrated chlorine (from any source) does not settle in one place.  Once dissolved and mixed, chlorine stays mixed.  Also note that with bleach, the pH will tend to rise from carbon dioxide outgassing so you'd need to keep the TA lower (to 50 ppm, if needed) and use an additional pH buffer such as 50 ppm Borates (usually from boric acid).  You'll want to use Dichlor for a day once a month to bring the CYA back up (most people find it drops 5 ppm per month, but you can test it once a month or so to be sure).

As for other oxidizers such as MPS, it's fine to use for supplemental oxidation (say, after a soak), but it's not a disinfectant unless used with silver ions (as with Nature2).  It's only main downside is its expense.  It doesn't oxidize all the same things that chlorine does so when used alone as with the Nature2 system, one may need to use chlorine on occasion to keep the water clear.

[Walter White]

thankyou yet again, chem geek!

does an ozonator serve any useful purpose in between soaks?  would it be better to turn it off if the spa won't be in use for more than a day?

[chem geek]

Some claim that an ozonator keeps the spa clear and safe even if the chlorine runs out, but we've had too many reports of spas going south if only an ozonator is being used, though that may at least in part be that the ozonator isn't on all the time.  Also, unless enough ozone remains in the bulk water, then bacteria can still grow on spa surfaces and not get circulated.  So chlorine or other disinfectant really is needed in the bulk water and no system that works via circulation, such as ozone or UV, is sufficient on its own.  These are supplemental oxidation systems only.

So you are correct that it would probably be better to turn off the ozonator if you aren't going to soak for a couple of days or longer.  It will then be easier to maintain a chlorine level in between soaks.  You'll still have to add chlorine, but not as frequently.  Having the ozonator run for 24 hours after your soak would be useful to lessen the amount of chlorine you need to handle your bather waste, especially for heavy soaks (high person-hours).