Precipitating iron with chlorine

[duraleigh]

This quote was taken from a thread in Chemistry 201 but there seemed like multiple (but similar) subject so I am creating a new topic....

bk406 said

I'm hoping that the HTH metal control i bought will work in a similar way. The active ingredient on the bottle is the same as Jack's, but the formulation could be different.

bk406,

I am tempted to try precipitating (with chlorine) the iron out of my vinyl pool but your sentence above confuses me. Why do you need the HTH metal control? Doesn't the precipitation of the iron eliminate the problem?

 

[JasonLion]

Briefly, that can work, but every time you try it there could just as well be staining as iron percipitating (or both).

 

[PaulR]

My dad (a chemical engineer) used to do this. Fill water went into a tank full of the 1" chlorine tabs we used, to precipitate out the iron. From there it went to a sand filter (not the main filter, a separate tank) to filter out the precipitate. From there to the pool. It was fairly dark concrete, but I don't recall any staining, so either the source water didn't have so much iron or his Rube Goldberg lash-up actually worked.
--paulr

 

[bk406]

I've found that you cant get it all out. There still can be some residual iron left. That why i used the HTH. Now, i will say that this year i didnt have as much luck. I believe there are a couple reasons for it.

1) This start up season, i didnt have as much iron, ppm wise, as I had at the end of last season. My fill water in July thru the first part of Septemeber has around 2ppm. In August, I was adding quite a bit of water each week. I had a leak, it turned out. So, everytime I raised the chlorine, it turned the pool emerald green. I raised the FC to around 20, and most of it filtered out. (The first time I did it was an accident, I shocked since I thought it was algea ). The pH was around 7.8 since my SWG was running.

2) This year I had the leak fixed when I opened. But, I drained out almost 8000 gallons in the process. I refilled. But..the iron level was around .2-.5 ppm in the fill water, AND my pH was around 7.2. My SWG isnt on yet, and my TA is set at 80, so my pH is a little lower than what it will be later on. So..I think the combination of the lower iron content and the lower pH resulted in less of the iron falling out when I raised the FC to 20.

If I were going to try it, I'd raise the pH to 7.8-7.9, then increase the FC to 20 ppm, and run the filter. As jason has said, it is kind of a catch as catch can thing. As of yet, I havent seen any iron plating back down on the pool. Normally I see it start to stain the return and suction fittings. But i have not seen that yet. As I said, the amount of iron i have right now is pretty low, so we'll see.

 

[JasonLion]

Fill water went into a tank full of the 1" chlorine tabs we used, to precipitate out the iron. From there it went to a sand filter (not the main filter, a separate tank) to filter out the precipitate. From there to the pool.

that makes sense to me, the staining would be inside the secondary filter, instead of in the pool.