filtration of iron (precipitate?)

[singingpond]

I'm not sure if this is the best place for this question -- please move, if necessary.

We have a seasonal pool, filled every year with well water. The well water has iron in it (don't remember levels, as it's been years since we had a water test done). We've never done anything about the iron, and never have trouble with discolored water in the pool. What happens, by dumb luck apparently, is that the iron does come out of the new fill water when we chlorinate, but the precipitate (if that's what it is) is big enough to be caught by the cartridge filters we've been using. That's been true even of the el cheapo filters we picked up at Walmart last year. So, for two weeks or so, at the start of the season, the cartridges are totally rust brown whenever I pull them out and hose them off. After that, no more rust color for the rest of the summer.

So, on to my question -
we're thinking of switching to a sand filter next year (existing cartridge filter is very much undersized for the pool, as is common knowledge here) -- since the sand is a coarser filtration medium, is it less likely to catch the iron compound and take it out of the water than the current cartridge filter? Or will the overall better filtration (larger filter) somehow make up for this?

Does anyone know what the iron precipitate is likely to be, chemically, seeing that it is large enough to be captured by the cartridge filter?

 

[duraleigh]

Capturing the iron precipitate is sorta' like a turkey shoot. If you could get it to behave predictably, you would never have to work another day in your life.

My temptation would be to go to a larger cartridge not knowing if the sand would capture the particles you are precipitating.

 

[SunnyOptimism]

I'm not sure if this is the best place for this question -- please move, if necessary.

We have a seasonal pool, filled every year with well water. The well water has iron in it (don't remember levels, as it's been years since we had a water test done). We've never done anything about the iron, and never have trouble with discolored water in the pool. What happens, by dumb luck apparently, is that the iron does come out of the new fill water when we chlorinate, but the precipitate (if that's what it is) is big enough to be caught by the cartridge filters we've been using. That's been true even of the el cheapo filters we picked up at Walmart last year. So, for two weeks or so, at the start of the season, the cartridges are totally rust brown whenever I pull them out and hose them off. After that, no more rust color for the rest of the summer.

So, on to my question -
we're thinking of switching to a sand filter next year (existing cartridge filter is very much undersized for the pool, as is common knowledge here) -- since the sand is a coarser filtration medium, is it less likely to catch the iron compound and take it out of the water than the current cartridge filter? Or will the overall better filtration (larger filter) somehow make up for this?

Does anyone know what the iron precipitate is likely to be, chemically, seeing that it is large enough to be captured by the cartridge filter?

Precipitating any element out of pool water is a questionable methodology at best and is highly dependent on many factors. Calcium precipitates out as calcium carbonate and calcium phosphate; we know those as white/grey calcium scale on our nice pool tiles. Phosphates are removed from water by using lanthanum chloride to intentionally precipitate lanthanum phosphate out in your filter medium. Iron and copper precipitate out as iron oxide (rust) and copper oxide. Sadly, those last two often lead to discoloration of our nice pool surfaces. All of these precipitates, their particle size and where they precipitate are a function of water chemistry imbalance and surface nucleation sites.

I think the fact that iron has always precipitated out in your filter is more a matter of blind luck than anything you can control with chemistry or equipment.

As for filtration, a sand filter will definitely let finer particle get through but there are methods describe in these forums for adding DE or cellulose fiber to sand filters to improve fine particulate filtration performance. If fine particulates are what you are after, the best option is the more expensive DE filter. Cartridge filter size will not directly matter to your particulate filtration or creation, it will simply lengthen the time intervals between filter cleaning.

As for your iron problem, I think you'd be best to try to filter your water at the source and, if you can afford it, look into a water softener installation. You can't fill a pool with a water softener, but adding it to your autofill line would reduce the continuous dosing of iron you are getting from the well.

 

[singingpond]

Capturing the iron precipitate is sorta' like a turkey shoot. If you could get it to behave predictably, you would never have to work another day in your life.

My temptation would be to go to a larger cartridge not knowing if the sand would capture the particles you are precipitating.

An interesting suggestion -- I was sick of cleaning cartridges, and I also like the idea of a very cheap replacement medium (sand) on the next filter we get...

Our iron issues aren't very severe in general -- haven't done anything inside the house to remediate, and we don't have trouble with laundry, discolored fixtures, or anything like that. We do get a little orange discoloration in shower stalls over time (if regular cleaning is neglected), but nothing major.

Before we bought the house we had a water test done, and some iron was reported. I was reminded of that when we started seeing the rusty dirt in the pool filter at the beginning of each swim season. I suppose we should do a bucket test (I remember reading about that somewhere) to see how bad the water looks on addition of chlorine.

Thank you for your comments!

 

[SunnyOptimism]

Our iron issues aren't very severe in general -- haven't done anything inside the house to remediate, and we don't have trouble with laundry, discolored fixtures, or anything like that. We do get a little orange discoloration in shower stalls over time (if regular cleaning is neglected), but nothing major.


Thank you for your comments!

If that is the case and if you are filling your pool by garden hose, then there are iron filtration devices of varying cost and complexity that you can hook up to your external spigot. They can't typically handle a lot of iron, but they might be good enough for your needs. Depending on how much fill you do each swim season, it could just be a matter of buying enough cartridges for each swim season.

Good luck with your filter upgrade.


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[singingpond]

Precipitating any element out of pool water is a questionable methodology at best and is highly dependent on many factors. Calcium precipitates out as calcium carbonate and calcium phosphate; we know those as white/grey calcium scale on our nice pool tiles. Phosphates are removed from water by using lanthanum chloride to intentionally precipitate lanthanum phosphate out in your filter medium. Iron and copper precipitate out as iron oxide (rust) and copper oxide. Sadly, those last two often lead to discoloration of our nice pool surfaces. All of these precipitates, their particle size and where they precipitate are a function of water chemistry imbalance and surface nucleation sites.

I think the fact that iron has always precipitated out in your filter is more a matter of blind luck than anything you can control with chemistry or equipment.

As for filtration, a sand filter will definitely let finer particle get through but there are methods describe in these forums for adding DE or cellulose fiber to sand filters to improve fine particulate filtration performance. If fine particulates are what you are after, the best option is the more expensive DE filter. Cartridge filter size will not directly matter to your particulate filtration or creation, it will simply lengthen the time intervals between filter cleaning.

As for your iron problem, I think you'd be best to try to filter your water at the source and, if you can afford it, look into a water softener installation. You can't fill a pool with a water softener, but adding it to your autofill line would reduce the continuous dosing of iron you are getting from the well.

Thank you for taking the time to explain some of the chemistry involved! It seems that the size of the rust particles depends on variables that are not very well known, or at least not readily measured in the environment of the home pool.

Given that we don't really have trouble with the iron in general (see my previous post), we probably won't be looking at whole-house solutions. Also, no autofill on this pool... unless rainwater counts -- rain has pretty much kept the pool level where it should be this year, come to think of it .

We've had a pool of this type (smaller volume in previous years) up for five seasons now, and have always had this convenient rust precipitation in the filter, so we've barely thought about the issue. It was only recently that it occurred to me that the behavior might change with a different filter medium.

- - - Updated - - -

If that is the case and if you are filling your pool by garden hose, then there are iron filtration devices of varying cost and complexity that you can hook up to your external spigot. They can't typically handle a lot of iron, but they might be good enough for your needs. Depending on how much fill you do each swim season, it could just be a matter of buying enough cartridges for each swim season.

Good luck with your filter upgrade.


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Thanks again for the helpful reply!

 

[SunnyOptimism]

Hard to tell where the rust will settle out on. A cartridge filter is a really good surface for particle nucleation given it's fine pores and large surface area. A sand medium may not be as good as that.

If you want to see if it is truly iron oxide, you can soak the dirty filter in a solution if ascorbic acid as that is the procedure used in large pools for dissolving iron stains. If mixing up the ascorbic acid is too big/expensive of a job, you can take a vitamin C tablet, wet it and the dirty filter a little bit and put the tablet on contact with the filter medium. If the rust stains disappear, then you have iron. If not, then it might be an organic compound of some kind.


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