Why Change The Water

[oldparr]

Hello everyone,

I have a 550 gallon SPA. The water in it is now 6 months old. I'm using Dichlor than bleach method. I use it twice a week, two people for about 1.5 hour each time. The water is perfect and my question is: do I have to change it now? My concern is the concentration of salt (Na-Cl) which I do not have a test for. How much salt will be bad for the hot tub?

Thanks!

[waterbear]

It is not so much the salt that is a problem but rather the build up of non oxidizable matter in the water given the small water to bather ratio found in hot tubs. This is why water changes about every 4 months is recommended.

Every time you soak you are adding feces, urine, and sweat (which is chemically very similar to urine) to your water and you are keeping that water hot so it is literally a soup of all the stuff you are adding. You are trying to keep that water clean by adding chlorine but after several months of twice a week extended soaks the water has a lot of stuff in it that you really can only get rid of by replacing the water. You would not keep the same water in your bathtub for 6 months, keep it heated, and put some chlorine in it every day but that is essentially what your hot tub is, a heated bathtub for two!

[oldparr]

Thank you for the response Waterbear! I'll change it this week.

Regards,

oldparr

[mak]

I am a new member, although I have been learning from this site since I bought my spa in the spring. I have been using the dichlor/bleach method and have been able to maintain a sparkling tub, thanks to the expert advice on this site, so I want to thank all of you. I am, however, considering a switch to 3-step bromine, mostly because my wife complains about the smell of our chlorine tub.

The question I have was posed in this thread, so I am resurrecting it. Waterbear, what is the non-oxidizable matter you refer to? And why should I be concerned about it? The only things I can think of that build up are salts and minerals. Do they affect the ability of sanitizers and oxidizers?

Sure, feces, urine, and sweat sound disgusting, but they are oxidizable and are kept at bay with normal treatment. So what happens that requires the changing of the water?

[chem geek]

Are you starting with 1-2 ppm FC at the start of your soak? If you smell chlorine or chloramine you are probably starting out with more chlorine and that has the water smell more when it reacts with your bather waste. Bromine will do the same, though it will smell differently. Some people like it better; some don't like the smell as much. Tap water is now often disinfected with monochloramine at around 1 ppm or so, so if you start with a low chlorine level during your soak it shouldn't be much different than taking a bath using chloraminated water (except if you use jets in the spa then that will aerate the chlorine and chloramines). The idea is to start out with a low level of chlorine and then add a large enough dose after your soak to handle your bather waste and end up with a low (but not zero) chlorine level for the start of your next soak.

As for the buildup of slow-to-oxidize organics, with the Dichlor-then-bleach method you can usually go twice as long between water changes compared to the Dichlor-only method. The mineral buildup is just salt so isn't much of an issue. For every 10 ppm FC added by bleach, the salt increases by 17 ppm. For a 350 gallon spa, one should be able to have the water last through 544 ppm FC cumulatively added assuming one is soaking every day or two. So that's a buildup of 925 ppm salt. The reason to change the water isn't the salt but that some organics are slow-to-oxidize or don't get oxidized by chlorine at all so build up. It makes the water start to turn dull-looking. This happens when using Dichlor much faster because the buildup of CYA lowers the active chlorine level so slows down oxidation so that even things that chlorine does oxidize get built up faster.

[mak]

Regarding the chlorine levels, I have contributed to the smell because I'm usually up around 5ppm at soak time. I haven't gotten good enough with maintenance to have it around 1ppm at soak time.

Are there organics that are not oxidized? If so, what are they? Do they present a health concern?

If it's just a matter of replacing water when it is dull-looking, I'll just wait till then. Right now I am happy that I can maintain a sparkling tub without adding the crap the dealer sold me when I got the tub -- bleach is all I need. (Aside from a little pH Down every two weeks or so.)

[chem geek]

OK then, that explains it. 5 ppm FC will certainly smell a lot more after it combines with your sweat during soaking. Unless you want to switch to bromine for less maintenance between soaks due to the bromine tabs continually feeding bromine, then I'd just lower your dosing level so you start your soak with 1-2 ppm FC and see if that's better for your wife. Also, don't forget that you initially use Dichlor to get to around 30 ppm CYA and that around once a month you need to use Dichlor for a day or two since CYA slowly breaks down at a rate of around 5 ppm per month.

The types of organics that don't get oxidized very quickly include things like lotions and sunscreen, some types of body oils, and portions of dead skin cells, but usually most of these get caught in the filter, but not all do. They are not a health concern. Technically, if you had finer filtration, you could probably remove much of what causes the hazy water, but spas generally use cartridge filters and they don't filter down as low as DE. If you are really trying to avoid changing your water, you could use The Slime Bag, but usually for a spa it's just easier to change the water. However, none of this will remove the salt buildup so at some point you would end up with higher salt levels so again, changing the water makes sense to do.

[mak]

Thanks, Chem Geek.