HOT TUBS: Calcium (and Bromine)

[chem geek]

Thank you so much for the thorough information. You think like a scientist even if you don't have the full background! Being observant is an important attribute.

I initially looked at using Bromine for our pool instead of Chlorine, but found that it was about 3 times as expensive and that you had to use about twice as much of it so it netted out to being way more expensive overall. I can see that this doesn't matter much for the much smaller volume of water in a spa, but it just doesn't make sense to use bromine in a pool. As for loss from degradation including the higher temperatures, the chemistry doesn't back that up. Yes, chlorine degrades from sunlight, but with a small amount of CYA you can avoid that. And yes, some have said you shouldn't use CYA in hot tubs and that is probably because you want more disinfection in hot tubs due to the higher "bather load to pool water volume" relationship, but I'm talking about very small amounts of perhaps 10 ppm CYA that would go a long way in preserving chlorine while cutting down your 5 ppm chlorine to 0.5 ppm HOCl which is still quite high (close to Ben's "shock" level, in fact).

Well, I suppose given how hard it might be to try and maintain low CYA levels in a spa (mostly the difficulty of measuring such low levels since current test kits tend to start at 30 ppm) and given the presumed higher risks of "spa itch" and other problems requiring disinfection, I guess using bromine in the spa makes some sense. If anyone out there is using CYA and chlorine in their spa sort of "like a pool", it would be interesting to know if they run into any problems. I've seen other threads (search "bromine" and under Advanced search "hot tubs") report users using BBB (with chlorine, not bromine) in their hot tubs without a problem.

As for the temperature effect, we keep our pool at 88F and there are many others on the forum who have their pool at 90F or above (though they may not want to -- they live in very hot climates) and I don't believe anyone is reporting unusual chlorine loss beyond that expected from the intense sunlight. It is true that chlorine breaks down from higher temperature and a 10C (18F) rise roughly doubles this breakdown rate (until you get to much higher temperatures), but at 77F a 10% chlorine (that is, VERY concentrated compared to your spa) has a half-life of 220 days while the very high 140F temperature shows a half-life of chlorine of 3.5 days, but for the concentrations in your spa the breakdown of chlorine due to temperature is neglible. So I can understand the concerns from the breakdown from sunlight (when no CYA is used), but not from temperature.

As for mixing bromine and chlorine -- DON'T! If you were to mix the actual concentrated chemicals (as opposed to pool and spa water), you can produce an explosive or certainly very, very hot mixture that is very dangerous! Chlorine and Bromine are chemically related, but chlorine is a more powerful oxidant. Chlorine (hypochlorous acid HOCl) will oxidize bromide ion (Br-) to produce bromine (hypobromous acid HOBr) and in fact when you add a non-chlorine "shock" to your spa water (such as MPS) it will regenerate HOBr from Br- in the same way. By adding spa water to your pool water you have added some HOBr hypobromous acid and/or Br- bromide ion which you will never get rid of. Unfortunately, your chlorine will waste its time converting bromide ion to HOBr instead of disinfecting and oxidizing other chemicals. It is true that if some HOBr got converted to Br- then this meant it actually did something useful (probably killing some "weak" bugs or oxidizing "easy" substances), but the point is that you want your chlorine to be maximally available in your pool and not wasting time creating the weaker sanitizer HOBr. Ironically, some brominating tablets contain both chlorine and bromine (since chlorine is a more effective sanitizer) though most contain just bromine.

Richard

[waterbear]

The main reason that calcium is needed in hot tubs is that most portable hot tub "skid packs" include electric heaters with copper heat exchanger tubes. Also gas hot tub heaters ususally use copper heat exchangers also. Some hardness to the water is necessary to help prevent leaching copper into the water...same as in a pool.
The reason bromine is ususally suggested in hot tubs (It IS more expensive) is because it is supposed to be more stable at the higher temps and is supposed to be more effective over a higher pH range. I suspect it is less affected by the constant aeration of the water from the bubblers and air injectors but am not really sure. I do know it is easer to maintain the bromine levels with less work than chlorine levels. I have kept hot tubs on both bromine and chlorine.
Most people, including many dealers do NOT know how to use bromine properly. It is different than chlorine. You need to add sodium bromide at each fill to create the bromine bank in the water. An oxidizer is used to convert the bromine ions to hypobromous acid. Plain old bleach is an excellent oxizider for this purpose and will not cause the pH drop that MPS tends to cause. Bromine itself seems to cause the pH and Alk to drop so you need to keep closer tabs on this than when using chlorine.
IF you are doing a 2 step bromine you add the sodium bromide to the water and then 'shock' (add oxidizer) to create the hypobromous acid with either chlorine, MPS, or ozone. The tub should be shocked with each use. When shocking no longer raises the bromine levels you need to add more sodium bromide or drain and refill and start over.
IF you are using a 3 step bromine you add bromine tablets in a floater to the above 2 step system. The tablets contain both bromine and chlorine or bromine and MPS. They help to maintain a more constant bromine level. You still need to add the sodium bromide and shock or it can take weeks for enough bromine reserve to be in the water since the bromine tablets are very slow dissolving. It is still recommended to shock once a week which will raise the bromine levels very high and help sanitize the water. I always kept my bromine levels 6-8 ppm.

An outdoor hot tub using chlorine does need CYA and this is why dichlor is ususally used as the chlorinating chemical. For indoor hot tubs Cal Hypo and liquid chlorine are ususally used.
Most hot tubs are covered when not in use (in a perfect world!) so, in theory, degredation of the chlorine from sunlight is not supposed to be an issue! However, I know many people leave their tubs uncovered for periods of time and also use them during the day so the effects of UV light do become an issue.
Maintaining chlorine levels in a hot tub do require more work and daily testing is a good idea. Play close attention to combined chlorine and shock when necessary. (Or maintain a level of MPS in the water at all times for an indoor tub on chlorine and remember to keep tabs on your pH since MPS will tend to lower it) My current hot tub is not covered but is chlorinated by my SWG and shares the filtration equipment with my pool so I just have my SWG chlorinating my hot tub to a higher level than my pool (I keep it at 6 ppm) and have not had any problems.

TDS might play a more important role in hot tubs than pools since it is a much smaller volume of water and so many chems are used if you follow the dealers advice. Then again I have seen hot tubs that were recently filled and all the chems added that had TDS that indicated draining was in order even though the tubs were clear, not foaming, and sanitizer levels and all other parameters were perfect so I still think it is a bogus measurement! I do not think all of the defoamers, seqesterants, hardness increasers, clarifies,etc. the dealers try to sell are necessary. If you balance the water with baking soda, calcium if needed (my feeling is anything in the neighborhood of 100 ppm or higher is fine to protect the heater), borax to raise pH and dry acid to lower it (muriatic acid is hard to use in most hot tubs because of the small volume of water....It is usually needed in teaspoons and tablespoons and I personally don't want to measure that small amount of it out of a gallon jug!), then adjust your santizer and keep tabs on your water chemistry you will not need all the other stuff.
IF you do have metals in your fill water then a sequesteant is needed.
Final thing, Hot tubs should be drained and refilled every 1-3 months depending on bather load. Foaming is usually a good indicator of when this is necessary.

I have done a lot of research on hot tub chemistry and the above is what I have been able to find that seems to make sense based on my understanding of the chemistry involved and my own experiences (and that of my customers with hot tubs)

[chem geek]

Thanks Evan. The information you provided is great. It all makes sense though it would be interesting to see if chlorine depletes faster than bromine for indoor spas not exposed to sunlight at all. Perhaps the depletion is a "good" thing in that it is oxidizing more organics or killing more pathogens since chlorine is a more powerful oxidant than bromine.

Are you sure about the calcium protecting the copper in heaters? Please refer to this post I wrote on another thread. I know that the conventional wisdom in heat exchangers is to use water balance to slightly scale calcium carbonate to form a protective layer, but that is typically for cooling towers where the water is already quite saturated and where parts of the system are acidic (pH < 7). It is not clear to me that in basic waters (pH > 7) that the protection from calcium is needed or works. If a thin film of scale is required, then the water balance would need to have lots more calcium than 100 ppm (with typical TA of 80-120). If the mere presence of calcium ion somehow inhibits corrosion, then that's new to me.

I think this issue of whether or not non-plaster/grout pools and spas need calcium is critically important to figure out correctly. There has been a lot of advice on this forum that vinyl and fiberglass pools do not need any calcium. If it is needed for spa heaters as well as pool heaters (which typically use copper pipes for heat transfer since copper is very efficient at transferring heat), then we'd better get this sorted out.

Richard

[chem geek]

In fact, I don't believe that hot water in the copper pipes in a home corrode very quickly. Even a hot water heater that has a sacrficial anode to prevent rust does so to prevent the steel tank from rusting -- it's not so much to prevent the copper pipes. In fact, steel has a greater tendency to rust when it is near copper or brass.

So it is possible that if steel is used in spa or pool heaters then rusting could occur, but I think that in these heaters only copper is exposed to the water, not steel.

I certainly want to know if I'm wrong about this or if I'm missing something.

Richard

[waterbear]

In fact, I don't believe that hot water in the copper pipes in a home corrode very quickly. Even a hot water heater that has a sacrficial anode to prevent rust does so to prevent the steel tank from rusting -- it's not so much to prevent the copper pipes. In fact, steel has a greater tendency to rust when it is near copper or brass.

So it is possible that if steel is used in spa or pool heaters then rusting could occur, but I think that in these heaters only copper is exposed to the water, not steel.

I certainly want to know if I'm wrong about this or if I'm missing something.

Richard

My understanding is that heaters for hot tubs do not have a holding tank. They use a series of tubes as a heat exhanger that the water flows through from the tub and back....much like 'tankless water heaters' used in homes.

[waterbear]

I personally think that pH is the most important parameter to protect the copper heat exchangers but until there is definitive answers one way or the other I see no harm in maintianing a calcium level in the water. Also most portable spas are acrylic and fiberglass and there seems to be some conflicting info on the need for calcium to protect the gelcoat finsish and help prevent staining. Bottom line is that many manufacturers recommend a certain level for their warrenty and THAT should be followed!
Calcium certanly IS needed for plaster or tiled spas and for now I guess we should follow the recommeded guidelines of 200-400 ppm.
For reasons I don't fully understand bromine santized spas have a tendendy to become acidic. Don't know if this is from the bromine itself or the widespread use of MPS in spas. Richard, perhaps you can shed some light on this.

My only real experinece with indoor spas on chlorine was the one my brother had in his old house in Miami. He needed to check his chlorine levels daily since they did not hold. He kept the spa at 104 degrees and used both liquid chorine and dichlor. His other water chemistry parameters tended to stay fairly stable except pH when he used dichlor. It would drop a bit and he would add sodium bicarbonate to raise it. His TA would raise somewhat but the spa had a waterfall that was always on when the pump was running so (I am guessing) the constant aeration would cause his TA to drop over time. I do know that the spa was in a very large bathroom of his house (there was also a sauna in this bathroom!) and there was a constant chlorine smell in the air in this room. Smelled like an indoor pool!

[chem geek]

For reasons I don't fully understand bromine santized spas have a tendendy to become acidic. Don't know if this is from the bromine itself or the widespread use of MPS in spas. Richard, perhaps you can shed some light on this.

My only real experinece with indoor spas on chlorine was the one my brother had in his old house in Miami. He needed to check his chlorine levels daily since they did not hold. ... I do know that the spa was in a very large bathroom of his house (there was also a sauna in this bathroom!) and there was a constant chlorine smell in the air in this room. Smelled like an indoor pool!

Certainly MPS is quite acidic so that would explain the situation when MPS is used. If MPS is not used, then the acidity is probably from outside sources, probably bather sweat (that's a guess).

Well, the chlorine smell (I assume you mean the "clean" smell of chlorine gas and not the "smelly" smell of chloramines) would indicate an outgassing of chlorine. If your brother don't use CYA, and with his indoor spa I would assume that he didn't, the outgassing is much faster (due to the higher HOCl concentration) and the typically higher TDS found in spas also contributes to the problem (higher Cl- chloride ion concentration). At a 77F temperature, I calculate the half-life of a 1000 ppm TDS to be 34 hours and if I boldly assume that the outgassing rate doubles with each 10C (18F) of temperature, then the half-life would be around 12 hours so each day you would lose 1-0.5*0.5 = 75% of your chlorine. This half-life rate is limited by the reaction rate of conversion of hypochlorous acid to chlorine gas and would normally be limited by aeration, but in a spa aeration is high so the net rate is somewhat closer to the reaction rate. I doubt that your brother actually lost 75% of his chlorine every day, but at least the calculations show that this theoretical maximum (with perfect aeration) explains his situation.

Richard

[waterbear]

Should have clairifed better....by indoor pool smell I meant choramines! He sold that house a few years back and now has an outdoor portable spa on bromine and a pool with a SWG.

[chem geek]

Boy, you sure know how to blow a theory right out of the water! Well, if it's chloramines then that means not achieving breakpoint and that's more like the problem Ben says that indoor pools have in general. Without sunlight and possibly good aeration, breakpoint doesn't happen as readily. Either that or there is something else going on we haven't yet figured out. If indoor pools and spas truly have a serious problem with chloramines, then the use of a maintenance dose of MPS would make sense since this oxidizes organics and ammonia really well and does so before chlorine gets a chance to form chloramines (and even some, but not all, chloramines get broken down from MPS according to my discussions with DuPont).

I just find it surprising that an indoor pool or spa without CYA has trouble achieving breakpoint as long as chlorine levels are sufficiently high. This is where the "real-world" butts heads with theory.

Richard