Passed Tests But Algae Still Growing

[Eilum]

Mustard Algae. Passed OCLT, CC = 0, Crystal clear water. But yellow algae seen in clumps on floor.


Listened to pool store and added 2 lb of Sodium Bromate followed by 3 bags of shock. Although they "guaranteed" that this would clear the pool within 24 hours, it did not.

Added a total of 18 bottles of liquid bleach over the next 3 days with filter running and the pool cleared somewhat (generally kept the level at mustard algae shock and was above shock level the whole time) and last night finally passed OCLT and CC test. But this morning the algae was visible.

I brushed it up and retested and found significant chlorine loss. So added three more bottles to bring it up to FC = 24 and brushed like mad. I am out of chlorine and out of patience. What is going on here?

 

[Donldson]

Well, there is no easy way to say this: you turned your pool in to a bromine pool by adding sodium bromide. When you add chlorine it is being converted instantly to bromine. Bromine is not stabilized against sun light so it burns off quickly during the day.

You are going to need to do a pretty significant water change to get enough bromide out of your pool to stop causing problems. Don't drain lower than 1 foot in the shallow end or your liner can shift and wrinkle, so you will have to do several partial drain and refills to get things back in order.

For reference: you raised your bromide bank to 6.4 ppm. As little as 1 ppm can cause problems, so you want to lower your bromide level to less than 1 ppm.

 

[Eilum]

I just went to a different pool store and bought chlorine (alot of it). I told the expert there about the sodium bromide and he said it is not a problem. He said that the bromide burns out just like chlorine and it is no longer in my water. Is he lying to me?

What if I operate my pool as a bromine pool for now? Am I wasting my time and money by continuing to shock the pool? The test is showing high FC levels that are relatively stable. Is adding chlorine a waste of my time?

Lastly, I am closing the pool for the winter soon, Will the bromine still be in the water when I open next year?

 

[jseyfert3]

Ouch. TFP's unofficial motto is "don't listen to what pool stores tell you." Through wanting to sell you lots of expensive things you don't need, combined with a general lack of knowledge on part of most of the workers, you run into issues like this.

I doubt the worker is lying to you. He just doesn't understand bromine chemistry. With chlorine, the primary disinfectant is hypochlorous acid, or HOCl. When this oxidizes waste, it breaks down and you end up with a chloride ion, Cl-. Cl- is now just salt, and doesn't do anything anymore. Sunlight can also break down HOCl and you end up with Cl-, except now it didn't do anything for you, you just lost it to the sun. In short, you add chlorine, it does its thing, becomes salt afterwards, and that's it. It's done.

Bromine is pretty similar, with one big difference. The primary disinfectant is hypobromous acid, or HOBr. When this breaks down waste, like chlorine, you end up with Br-, which, like Cl-, can no longer do anything. HOWEVER, here's the kicker. If you have Br- floating around (as you now do in your pool), and add chlorine (HOCl), the bromine comes up, taps the chlorine on the shoulder, and dances away happily as HOBr, leaving the Cl- sitting around uselessly. The bromine doesn't go anywhere, so every time you add chlorine, the chlorine reactivates the bromine that's in the pool. It does this immediately and forever. Because of this, once a bromine pool, always a bromine pool.

You can operate a pool as a bromine pool, but as Donldson pointed out, it's not stabilized and the sun will quickly burn it off. (Note that burning off just means HOBr turns into Br- without doing any good, you don't loose the bromine) This is why bromine is typically only used for spas (which stay covered) or indoor pools, and outdoor pools almost exclusively use chlorine. My guess is it's not a good idea to try and run an outdoor pool as a bromine pool.

And yes, after winter the bromine will still be there. You will have to drain your pool to remove it, there is no other way to get rid of it. :-(