From Purple Reside to Blueish Green Water- Manganese?

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From Purple Reside to Blueish Green Water- Manganese?

Postby rlsmith » Fri 21 Sep, 2007 12:43

I'm in my 3rd day of a treatment process of reducing my TA and PH. I had my water tested and my TA was in the 400's and my PH in at 8.0. They are sending a water sample away to a lab to test for a metal called manganese. Apparently this metal causes purple residue and they seen a case a number of years ago from which this was determined. The pool owner lived in a city residential area........they say this metal is found in well water. In any event, has anyone ever heard of this?? My water has turned blueish green and is cloudy. My friend who lives in the same area had this same problem (purple residue) and ultimately had to replace his heater. His pool guy is saying that I'm "doomed" and will ultimately have to replace mine as well.

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Postby chem geek » Fri 21 Sep, 2007 13:39

You had posted about this in this thread. Yes, manganese can be associated with purple color, though over time with exposure to chlorine it should oxidize to brown-black color of manganese dioxide (manganese carbonate is also brownish in color). In fact, if you can take a sample of water with the purple dust into a separate container and then add chlorine (bleach) to it, then if it turns black (from oxidation by chlorine) then this is very likely to be manganese. A metal sequestrant can handle that just as with other metals (though there might be some sequestrants optimized to hold manganese).

That's why I was surprised to hear about the copper cyanurate or high CYA levels also producing a purple powder. So you might as well also test for copper while you are at it and definitely check your CYA level which you should know anyway in order to maintain proper chlorine levels. Higher levels of CYA require higher FC to keep away algae. In your case with well water, however, the manganese is the more likely culprit for the purple water -- to be sure, check the other parameters (copper, CYA) in your pool water. If you really had copper, then the high pH would more likely precipitate copper carbonate that is blue-green though can also form copper oxide which is black. So let us know what you find out.

With your very high TA levels, your pH will tend to rise a lot and it will take a LOT of acid to lower the TA. In fact, if your TA is high due to the fill water, then you should also check your Calcium Hardness (CH) level since that may also be very high. I'm surprised with the high pH and TA that your pool isn't very cloudy or that you don't see scaling -- perhaps the water isn't that high in CH after all. Get yourself a good test kit, such as the Taylor K-2006, so that you can test most of these things yourself.

In a 15,000 gallon pool, to lower the TA from 400 to 100 would take almost 9 gallons of acid (assuming no other source of acidity such as using Trichlor pucks) -- obviously NOT added all at once, but over time along with aeration of the pool. Be sure to check the TA and CH of your fill water which sounds like well water -- any evaporation and filling of more water will just add to the TA in your pool even if the well water was lower in TA. So in your situation investing in a pool cover to keep evaporation lower might be worth it (especially if you live in a hot area -- see this link to determine evaporation rates where you live -- typical summer rates are almost double the daily rate based on annual average).


Postby rlsmith » Fri 21 Sep, 2007 15:14

Thanks Richard,

A couple points to clear up here.... I'am not on well water but my water source is known to be high in TA. My CYA 3 days ago had a reading of 125 and my total hardness was 400. I have since replaced and have added water on 2 occasions already. I backwashed to waste to remove as much of the purple residue as possible before treating the TA problem. My pool is approx. 12000 gals. with a auto-cover. I've been told to add 3 lbs of dry acid (I have a 50 lb bucket of ph minus) every day for 9 days to bring down the TA, keeping an eye on the ph level. I'll test for copper as well. You say this might be the case since the water is blueish green now? Should I be looking for copper oxide? The purple residue is still a stumper......where else can manganese come from?
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Postby mr_clean » Fri 21 Sep, 2007 16:45

have you added any metal remover to pool, sense you say manganese is very high. I would recommend a double dose.
Have you had tap-water tested that you fill pool with?
Did you find out what CYA level is now at?

Postby rlsmith » Fri 21 Sep, 2007 18:11

No I haven't added any metal remover but will do so. I'll also check the CYA.

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