I am having to add far too much sequestrant

[mitch08]

I just found this thread through the search function. It is very interesting to me.
I am using Metal Magic and feel like I am having to add far too much to keep the metal sequestered. Close to 2 bottles a week. The CYA in my pool is extremely low. Prob under 20. I am curious after reading this if the reason my sequestering agent is breaking down so quickly is because of the low CYA. I am going to immediately add stabilizer to the pool, bring it to around 60 and see if it helps the agent last longer in any significant way...

 

[chem geek]

Re: Sequestrant HEDP Photodegradation, Iron Copper Scale Removal

Normally, it would be the FC/CYA ratio that would be the determining factor since chlorine unbound to CYA (probably hypochlorous acid, but could be hypochlorite ion) oxidizes HEDP. What is the FC level you usually keep with your low 20 ppm CYA? If it's more than 2 ppm, then that may explain the faster degradation rate. Keep us posted with what you do but remember to raise your FC target as you increase your CYA level so you can continue to prevent algae growth.

As for photodegradation, that occurs when there is metal such as iron bound to HEDP. As noted in this paper, HEDP has a half-life of 7-8 days when exposed to sunlight AND bound to iron. If not bound to iron, the sunlight degradation rate is too low to notice compared to the normal oxidation rate from chlorine. As to whether CYA will help with this is an open question -- it may if CYA can shield lower depths from UV.

 

[mitch08]

Re: Sequestrant HEDP Photodegradation, Iron Copper Scale Removal

Normally, it would be the FC/CYA ratio that would be the determining factor since chlorine unbound to CYA (probably hypochlorous acid, but could be hypochlorite ion) oxidizes HEDP. What is the FC level you usually keep with your low 20 ppm CYA? If it's more than 2 ppm, then that may explain the faster degradation rate. Keep us posted with what you do but remember to raise your FC target as you increase your CYA level so you can continue to prevent algae growth.

As for photodegradation, that occurs when there is metal such as iron bound to HEDP. As noted in this paper, HEDP has a half-life of 7-8 days when exposed to sunlight AND bound to iron. If not bound to iron, the sunlight degradation rate is too low to notice compared to the normal oxidation rate from chlorine. As to whether CYA will help with this is an open question -- it may if CYA can shield lower depths from UV.

Thanks very much. I have a basic problem (caused by my own ignorance) and it is causing my issues to go around in circles, constantly continuing.

For the last several years, I have always kept my cya very low. Kept my FC very low as well. The sun would obviously eat up my FC throughout the day and at night I would add a bit more to bring it back to 2ish... The pool would always be crystal clear.

But now that I have the metal issue, this is no longer working. Because (from everything Ive learned here), all the sequestering agent I am using is creating a situation where I can not let the FC drop to zero during peak sun. Because by the time I get home from work, the pool would start becoming very cloudy. And because the pool is cloudy, I can no longer just bring the FC to 2 at night. It wont clear up the cloudiness. So when this happens Im bringing the pool to 6-8FC. This clears the pool up within a few hours, but as you say, with my low CYA, I believe it is destroying my sequestering agent as well. And this has been a constant problem for me for about 3 to 4 weeks now.

So I believe one major answer to my problem is just to bring my CYA up to a reasonable level. If I bring it up to 60, I can hold my FC during the day. The pool shouldnt cloud up, requiring me to put in even more unbound FC. And the Sequestering agent should last significantly longer. On top of hopefully helping with any photodegradation as well.

Hope that makes sense. and I will supply results to this and the culators which I should be receiving within the next few days...

 

[chem geek]

Re: Sequestrant HEDP Photodegradation, Iron Copper Scale Removal

Yes, with what you describe you would be better off having a higher CYA level and a proportionally higher FC level. Not only should that reduce the rate of chlorine loss in an absolute sense, but it also provides a buffer of more chlorine available so that you don't run out and let algae grow. It should also prevent you from having to elevate the active chlorine level and that should help reduce the loss of metal sequestrant. This is why we normally recommend a higher CYA level than what you were doing -- not just because of a metal sequestrant, but for reducing chlorine loss generally.

 

[mitch08]

Re: Sequestrant HEDP Photodegradation, Iron Copper Scale Removal

Yes, with what you describe you would be better off having a higher CYA level and a proportionally higher FC level. Not only should that reduce the rate of chlorine loss in an absolute sense, but it also provides a buffer of more chlorine available so that you don't run out and let algae grow. It should also prevent you from having to elevate the active chlorine level and that should help reduce the loss of metal sequestrant. This is why we normally recommend a higher CYA level than what you were doing -- not just because of a metal sequestrant, but for reducing chlorine loss generally.

Thanks a million!!! I really appreciate all advice given and am in the process of bringing up CYA now.