Mold Growing Above The Water Line, But Crystal Clear Water, With Bromine Method

[Mech33]

I've been using the Bromine method for about a year now on a brand new tub. The bromine has been great at keeping the water crystal clear at all times.

But for the last 6 months or so, I have been battling a growing blackish-greenish mold problem. It started on the cover, but then spread to the edges of the control knobs and under the filter cover, to virtually all the inside exposed edges of the tub. But the water is still clear.

I then used a strong mix of chlorine and water and decontaminated all the surfaces. The mold quickly turned light brown and easily washed away with a pressure washer with no trace.

All was well for a few weeks, at which point the spots started coming back on the cover, followed by the control knobs.

So again I repeated the cleaning procedure, and again it looks clear for about 2 weeks at which point it slowly starts coming back.

This is not a battle I want to try and maintain indefinitely...

Is a few steady ppm of bromine insufficient to keep enough sanitizer up on the exposed tub areas? Should I switch to a chlorine method, assuming chlorine evaporates faster and will cost more of the inside surfaces and sanitize them more thoroughly on an ongoing basis?

I currently only use the tub every few weeks. I check the levels weekly, swapping in additional bromine tabs, adjusting pH (it's usually a little low each week), and dump on MPS shock (weekly even when I don't use it, and after every use when I do). Spa is 400 gallons and has a UV system (clear ray), but no ozone and no mineral system.

I am honestly at my wit's end with this issue, so any help or insight from the experts would be greatly appreciated! Thanks in advance.

[chem geek]

Because Cyanuric Acid (CYA) is used with chlorine, the chlorine doesn't actually outgas much faster than bromine. When people say chlorine outgases faster, that is when there is no CYA. With CYA, that changes the rate substantially.

It may be, however, that the outgassed bromine is not as strong a disinfectant against mold than chlorine would be. To be honest, I don't know, but I haven't seen this mold problem reported much so whether this is more likely with bromine vs. just a statistical fluke, I can't say.

Mold needs moisture, warmth, and organic substances (i.e. a food source). You may have greases/oils or biofilm acting as a food source on some spa surfaces. So after you've used a disinfectant like bleach to clean them, you should probably use a strong surfactant such as Ahh-Some not only for your spa water but also to use to thoroughly clean surfaces. This may help make mold growth take longer since it will take a while to buildup organics on the surfaces, at least if it's only evaporated water reaching those surfaces and not direct spa water which of course contains lots of organics leftover from bather waste.

[Mech33]

Thanks for the quick reply, I'll give the surfactant a shot!

What are your thoughts on s salt water based system to try and address such issues? Then I would have constant chlorine generation with some outgassing, and presumably the slight salt cockney ration would inhibit some growth of mold?

[chem geek]

That's an option that might work, but the higher salt content (usually 2000 ppm for a spa) will be more corrosive and might void a warranty if the spa is fairly new. If the spa components use higher quality stainless steel (316/316L, not 304) and have a corrosion-resistant heat exchanger in the gas heater (curpro-nickel alloy or titanium, not copper) then a saltwater chlorine generator will probably be OK, such as the ControlOMatic Technichlor.