How much sodium bromide to use after refill when using 24x7 ozone generation?

[dteirney]

I've just finished decontaminating our spa after not being used for about 6 months. So much useful information here at TFP.

I'm looking to use the 3-step bromine system in the spa since it will only be used several times a week. I'm somewhat hopeful that the ozone generator will convert enough of the bromide to bromine to keep it at around 4-6ppm so I don't need to use bromine tablets in the skimmer door.

It's an old Sundance 850 Series Optima that has a 24x7 circulation pump. The ozone generator runs for pretty much all of that time that the circulation pump runs (as best I can remember). I've just ordered a new ozone generator after finding out that the existing ozone generator was no longer working (UV bulb was blown). I've ordered a Del Ozone Spa Eclipse that should generate between 50 and 70mg/h.

Anyway, onto the question... There are only a few places to buy sodium bromide here in Auckland, New Zealand, either in 250g or 500g containers. I've no idea how much I will need to create the necessary bromide bank in the 1800 liter spa (475 gallons). How do I find out - there was nothing that I could see in the Pool Math calculator to help. And I didn't find any numbers from any of the other similar threads regarding sodium bromide.

 

[chem geek]

The normal recommended dose for sodium bromide is 1/2 ounce per 100 gallons as that will get you roughly 30 ppm of bromide. That would be 67 grams in 1800 liters. That's the easy part. What is harder is to know how much you have to continue to add to maintain that bank since ozone will oxidize some of the bromine it produces into bromate. How fast that occurs depends on the strength of the ozonator.

 

[dteirney]

Thanks so much chem geek. I've ordered a 500g container of sodium bromide and will add the ~70g necessary when it arrives. The new ozone generator might not arrive for several weeks so I'll also look to add a couple of 96% pure BCDMH tablets into the skimmer wall. Each tablet is ~20g.

Per instructions in another thread, once the ozone generator is running I can check the bromine level in the spa ~12 hours after a soak in the tub to see if the ozone generator is converting enough of the bromide into bromine to keep up with bather load. If not then either supplement by adding a BCDMH tablet to the skimmer wall or look to put a small amount of liquid chlorine into the spa after each soak.

The amount of liquid chlorine to add to the spa is quite small (just 50ml of the 12.5% liquid chlorine I'm using for the pool will increase FC by 3.5ppm in the spa) so I'll need to find another smaller container to hold some of that just for the spa.

 

[chem geek]

The difficulty will be in knowing the size of your bromide bank. What you could do is see if the ozonator when on is able to create bromine especially when there is no bather load (i.e. when it's easiest to create bromine without it going away too quickly). If it can, then you probably have enough bromide. If it can't, then you probably need to add more.

The problem is that the chlorine/bromine tests do not distinguish between chlorine and bromine so you can't tell which is which and there's also no simple test for checking bromide by itself.

 

[dteirney]

The Del Ozone Spa Eclipse unit turned up last night. I installed it very easily and it seems to be working great. There is new smell when lifting the cover (I assume ozone). The bubbles from the circulation pump return line are larger than you'd get from a can of lemonade but smaller than pearls so I've probably got a little bit of off-gassing that I might look into at the weekend by restricting the air flow a little. Maybe I'll look into one of the ozone mixers that you can put immediately after the Mazzei jet.

I also put 70g of sodium bromide into the spa last night.

I measured the Cl/Br level this morning (approximately 12 hours after turning on the ozone with the NaBr in the spa) and the reading was 1ppm Cl (2ppm Br). I don't know what it was prior to the ozone generator being switched on. The 2 bromine tablets I had in the skimmer are mostly gone.

I also measured the Cl/Br level this evening after mixing in 300g of boric acid to take the borates level to ~30ppm. The Cl reading was 2ppm (4ppm Br).

I'm going to use the spa this evening and then measure the Cl/Br level again in the morning to see if it has dropped below 2ppm. If the level stays at 2ppm for the Cl reading does that mean I've got enough sodium bromide in the water?

 

[dteirney]

The bromine level this morning was 2ppm. Last night it was 4ppm before I soaked in the spa for half an hour.

So, at this point in time, it doesn't look like ozonation only is going to keep the Br reading between the necessary 4-6ppm with the current bromide bank. I'm not really sure what that means in terms of whether I should increase the bromide bank or whether I should continue with 2 bromine tablets in the skimmer door.

@chem geek, what's your recommendation?

Here are the current readings for the spa measured using a TF-100 kit.
CYA - 20 (not sure why I put CYA in since any Cl added will be converted to Br - perhaps too much reading of articles here at TFP re use of Chlorine only in spa).
FB - 2 (free bromine - measured as 1ppm Cl with the test kit)
PH - 7.5
TA - 50
CH - 150
BR - 30 (bromide bank, guess based on addition of 70g of NaBr).

Following writing this reply, I'm going to add 50ml of 12.5% sodium hypochlorite to increase Br level by 8ppm (after bromide is converted to bromine) for the weekly shock.

I've ordered a Mazzei Liquid Gas Mixer (LGM) to help ensure all the ozone that's generated does it's job in the water rather than leaking out via the larger than champagne bubbles from the return line. http://www.loveyourhottub.com/mazzei-liquid-gas-mixer-lgm-for-spas/

 

[chem geek]

Usually an ozonator is enough to handle maintaining a background bromine level with no bather load, but usually isn't enough to handle bather load as well unless it is fairly low. Of course, it depends on the strength of the ozonator and how long it is on.

Increasing the bromide bank will only help a little and it will increase over time with spent bromine from the tabs. I wouldn't change it at this point. Just add an oxidizer after soak to help oxidize your bather waste. You can use bleach, Dichlor, or non-chlorine shock MPS for creating more bromine and oxidizing bather waste.