Misc Clarification Pls

[dlleno]

Chem Geek: In my efforts to understand the differences between chlorine and bromine I came across some comments of yours -- from various threads, and from TFP as well. my apologies I did not collect the original reference. I would be grateful for clarification in the context of starting up a bromine spa:

You normally superchlorinate starting with new fill water or water that doesn't have a bromide bank in it.

Certainly, during the startup procedure there is an opportunity to do this quite conveniently, i.e. superchlorinate prior to adding the bromide bank. moreover, one could easily wait until FC decays to some level (say 4 ppm FC) before adding the bromide bank (presumably to delay the onset of bromine and prolong the benefits of chlorine) , at which time the spa would be instantly converted to 9ppm bromine, right? My curiosity is around why would the shock be more effective this way, if the below is also true:

Bromine isn't quite as thorough an oxidizer as chlorine. So bromine does disinfect and keep the spa from growing bacteria and it does oxidize some bather waste, but not all of it. Chlorine does a better job so shocking with chlorine every now and then helps to keep the water clear. The chlorine will first attempt to oxidize some of the chemicals that bromine hasn't handled and then it will oxidize bromide to bromine since that takes somewhat longer (depending on what's in the tub).

This is significant, and a clear argument in favor of using chlorine bleach (or even dichlor) to oxidize the bromide bank on a regular basis for normal usage, right? or choosing tablets that contain chlorine? what about the MPS behavior: does it also attack waste residuals before becoming available to oxidize the bank?

as for the CYA contributions of dichlor: is this truly a don't care in a bromine spa or would a growing accumulation of CYA (due to regular oxidizing with dichlor) have an impact?

[chem geek]

As you referenced, chlorine is generally a stronger oxidizer than bromine so if one will superchlorinate, then it is better to do that before adding sodium bromide to create a bromide bank. So that way you superchlorinate with chlorine rather than generating bromine.

As for what to do with normal usage, yes using chlorine to oxidize the bromide bank does give chlorine some chance to oxidize other chemicals that bromine might not have handled well. If you use MPS you get different behavior because MPS is slower to convert bromide to bromine so will hang around longer to oxidize other bather waste. MPS will oxidize some things that chlorine won't, but generally people find that using chlorine keeps the water clear at least as well if not better than MPS, though MPS is not a bad choice (it is more expensive, however). The main advantage to MPS is that whatever it oxidizes it does not create chlorinated or brominated disinfection by-products.

The accumulation of CYA in a bromine spa really doesn't matter since it does not affect bromine. It may slow down chlorine, but chlorine oxidizes bromide to bromine very quickly so that probably doesn't matter. As for slowing down chlorine oxidation of other bather waste that bromine isn't handling well, that is a possibility but we don't have enough reports to know if using Dichlor regularly has a problem after a while in a bromine spa. My guess is that you really don't use enough Dichlor to build up a lot of CYA in a bromine spa. It's Dichlor-only users in a chlorine spa where you see issues after 100-200 ppm CYA has built up.

[dlleno]

ok, good; that makes sense -- If I may play this back in my own words to insure I have understood: Chlorine has a chance of oxidizing some wastes before the conversion to bromine completes, but this time window is very short. Therefore:

1. cost factors aside, regular oxidation via dichlor is fine and dandy.

2. the initial chlorine shock is advantaged by waiting for some time before adding the bromide salt.

MPS is an interesting twist on the story. looks like there may be some tertiary benefit in using MPS occasionally for a shock. Empirically I can point to a few historical experiences where an MPS shock did produce a noticeable difference compared to a straight dichlor shock though -- especially near the end of the drain interval when using 85/15 dichlor/bromide granules regularly. in fact, that experience is what started me thinking (years ago) about a "brominating concentrate then switch to dichlor" method - presuming that after some period of time the addition of more bromide would do little more than contribute to TDS.

[chem geek]

If you were comparing the use of MPS at the end of a Dichlor-only spa usage (even when the Dichlor is used for creating bromine), that's not a fair comparison since after a lot of Dichlor-only dosing the CYA is high so the chlorine isn't very effective and oxidizes bather waste too slowly as a result. So in that environment, MPS will look like it works better. The better way to compare is with the Dichlor-then-bleach method for chlorine (or using this method for creating more bromine) which has a consistent CYA level once you switch to using bleach. Usually the water lasts a lot longer (at least twice as long, compared to Dichlor-only) and use of MPS at the end doesn't have a noticeable difference.

[dlleno]

yea thats what my results seemed to indicate -- that high CYA may in fact impact the bromide conversion, in which case MPS would yield this result. that is, the conditions under which MPS gave a good result is when the CYA was high in my bromine spa. I was noting this because I was trying to rationalize my eperience with your earlier statement.

The accumulation of CYA in a bromine spa really doesn't matter since it does not affect bromine. It may slow down chlorine, but chlorine oxidizes bromide to bromine very quickly so that probably doesn't matter.

I was attemptiong to suggest that that CYA accumulation in a bromine spa in fact might be a factor after all, and that for one-step users it might be benefitial to stop using the mixed granules at some point and use bleach for the oxidier. Moreover, to me is strong evidence that a good two-step program would involve bleach, not dichlor, for the oxidizer to avoid the same thing.

[chem geek]

OK, I see your point and yes I should have qualified that the buildup of CYA will eventually matter in a bromine spa in the sense that bromine alone doesn't oxidize everything that well so using chlorine helps keep the water clear. When the CYA builds up, that chlorine oxidation slows down so when that happens the use of another oxidizer such as MPS can be helpful.

And yes, the 2-step method should generally include the use of bleach, but having at least some CYA in the water is helpful to minimize disinfection by-products and to lessen any risk of the chlorine being too strong for too long or from outgassing too quickly. In theory, the chlorine should just oxidize the bromide to bromine quickly, but if you were to deplete your bromide bank by accident, then you don't want the chlorine bleach to not be moderated in strength by CYA. Hence, the best compromise is to use Dichlor initially and then switch to bleach, just as with the Dichlor-then-bleach method. The only difference is that you have a bromide bank to produce bromine.

[dlleno]

good that makes a lot of sense: Just use the std dichlor-then-bleach (or perhaps MPS occasionally if you have it) method after building the bromide bank.

on a similar note: you've said before that, at least emperically, folks are getting longer (2x) drain intervals from the dichlor-then-bleach method. What is the primary reason for this? is it (primarily) because CYA no longer grows without bound with each dose of dichlor? or is it (also) because people using this method are more likely to be more careful about water balance, and more likely to employ more precise measurements of adjustment chemicals?

on a related subject -- the startup. I've seen different comments, no doubt aiming at different things, regarding what adjustments to make first. In particular, when to shock and when to heat. I have to admit these have never been an issue for me, as my source water is dern close to perfect out of the tap so I can shock and heat right away. But to help me understand the underlying principles, here are some thoughts I would like to check:

- I can see that one should be motivated to introduce chlorine shock to the spa asap after filling, to control bacteria growth. However, if there are metals in the water it would seem best to add a sequestrant first (if it is needed) perhaps even as the spa is filling. How long should one wait to shock after adding the sequestrant? After the fill, then shock, unless of course your water is too wacked out...

- If the source water is really wacked out, one might not want to shock until things are within reasonable limits. for example, if TA is very low, you risk a precipitous drop in pH (baking soda easy/quick fix). On the other hand, if TA is very high then you shouldn't wait until you have achieved balance perfection before shocking -- as that would give the bad guys too long to grow. It appears that Superchlorinating with a high pH and high TA shouldn't be an issue because you will be bringing those parms down over the next few hours as you fine tune.

- It seems like the same precautions as above would apply to turning on the heater. So, for example, if you were in a hurry to start the heating process (mine takes 8-9 hours to heat), then you should at least wait until the water is safe enough to shock before heating. Then with heater on, you can continue to fine tune the balance.

So:

1. fill/metal sequestrant if needed

2. quick balance if needed

3. shock.

4. turn on heater if you're in a hurry

5. fine tune balance while FC is decaying

3 and 4 would happen nearly at the same time. i suspect many (including me) can skip 2. Any holes in my thinking?

[chem geek]

For chlorine users, the Dichlor-then-bleach method gives them roughly 2x time between water changes compared to Dichlor-only mostly because the CYA isn't building up making chlorine less effective. I don't think it's because people are being more careful since some people still used Dichlor-only with appropriate dosage after their soak to handle their bather load and their water would need to be changed more often. For some, they didn't want to handle bleach and didn't mind the water change since they didn't use the spa frequently.

It is true, though, that a lot of people were having issues with their spas because they weren't dosing enough -- their water tended to go bad and need shocking when that happened -- and that this under-dosing was independent of disinfection type (i.e. chlorine or bromine). People who followed standard spa store recommendations didn't properly scale their dosing relative to their bather load -- the number of people in the spa times how long they soak. So for those with heavier loads, especially longer soaks, they often weren't adding enough.

If there are metals in the water, you can always lower the pH first before shocking if you are concerned with staining. Note also that with iron, conversion to precipitate often catches iron oxides in the filter rather than staining. Nevertheless, if you want to add a metal sequestrant first, you can, but realize that high chlorine levels tend to break them down so you'll need to add more sooner than when you have lower chlorine levels.

If your TA is very very low, then yes, you can raise it some easily before adding a bunch of Dichlor that will tend to make the pH drop as the chlorine gets used/consumed. Note that the pH doesn't drop initially because Dichlor is close to pH neutral upon addition. Also, you may not need to shock at all if your fill water is decent and you balance and disinfect the water in a reasonable period of time after filling. Bacteria take 15-60 minutes to double in population under the best of conditions so the water won't turn into a cesspool in a couple of hours. It's waiting for 8 hours or more where you start to take risks.

As for heating, I don't think it matters much unless you have high TA and CH fill water such that there is risk of calcium carbonate scaling since that occurs more at higher temperatures including the 30ºF higher temp typical in the gas heat exchanger. In this case, you'd want to lower the TA first before heating the spa.

So generally your sequence list is fine and as I wrote you might not even need to shock -- you might be able to add a normal chlorine level and not have it decline quickly -- depends on the quality of your fill water.