Can my SWG keep up with Mustard Algae Min FC level at CYA=70ppm?

[elkind]

I'm sorry if this is a dumb question or if it's already been covered. I looked around for a bit, but did not find my answer.

I have a 16,000 gal pool in SoCal with an IC-40 SWG. The IC-40 is rated for up to 40,000 gal and is supposed to produce 1.4 lb/day of FC. I assume that for 100% output running 24 hr.

If my math is correct, a SWG producing 1.4 lb of FC in a 16,000gal pool would be adding 10.5ppm per day, if run 24 hr at 100%.

From the various TFP charts/discussions that I have seen, I gather the following:

SWG should use ~70 ppm CYA

At 70ppm CYA, the mustard algae min FC level is 10.5ppm (Ref: Chlorine/CYA Chart)

At CYA=70ppm and FC=10ppm I should expect an HOCl concentration of ~0.06 ppm Cl2 per (Ref: Pool Water Chemistry)

And at CYA=70ppm, FC=10ppm and HOCl=0.06ppm my total chlorine loss would be 0.7ppm/hr. (same as the last reference)

At 10 hr of sun per day, that would be 7ppm of FC that my SWG would need to compensate for. So, it would need to be on (with the pump running) for 7/10.5 x 24 hr/day= 16 hr/day. And I certainly don;t want to run the pump that long each day as the power rates here are really high.

I'm sure I am missing something, making this a dumb question. So, if someone could please help me understand where I have gone astray, I would be most appreciative!!

Thx!!

Pentair IC40 SWG, 16,000gal in-ground w/Spa, Nautilus FNS60 DE filter, pebbletec...

 

[MITBeta]

If you have mustard algae, you need to follow the SLAM Process followed by the Mustard Algae process.

Then after that, you follow the standard FC/CYA Levels.

 

[Jimrahbe]

E,

SWCG's can't make chlorine faster than algae can grow.

You need to turn the SWCG off, and then SLAM with Liquid Chlorine until the algae is dead. Then turn the cell back on.

Thanks,

Jim R.

 

[elkind]

Thank you to Jim R. and MITBeta for responding. I sure appreciate you both.

Yes, I am on top of that. First thing I did was thoroughly clean the filter. Then I did the full SLAM process with FC= 28ppm since my CYA=70ppm, SWG=OFF. During the slam I cleaned out the light niches (they weren't too bad) and soaked the hose sections in a bucket of bleach (they had some yellow in the corrugated areas and I also used a toothbrush on those areas) and then I brushed/vacuumed the pool paying special attention to the places that are hard to reach (which is where the yellow has been appearing). And the SLAM actually went fast with the FC level holding at 30ppm overnight that 1st night. So then I bumped the FC to 42ppm and basically added a gal of bleach every hr till sunset (I picked a sunny day to do this and so I was losing about 6ppm an hr). Then I added 1 more gal and FC sat at 48 ppm overnight. So, I think that the SLAM and Mustard SLAM are now done. I will leave the SWG off until FC drops to ~15ppm and then see what it can maintain.

It may be that I have had a simmering algae situation that my SWG was eventually unable to manage. I do get that.

And I must be doing the math wrong. It HAS to be that the IC40 can maintain FC=10ppm in a 16,000 pool, if there is no algae to deal with.

But let me ask one more dumb question: The FC/CYA chart that MITBeta referenced says that for CYA=70ppm that the FC min =3ppm and FC target =5ppm. That's simple enough. BUT the chart at Chlorine/CYA Chart mentions that for CYA=70ppm FC min=5.1, Target=8, Yellow Mustard MINIMUM=10.5 (15% of CYA level). Maybe that is outdated?

At CYA=70ppm, what FC do you think I should TARGET, given that I have just dealt with mustard algae in this pool? 5ppm? 10ppm? Higher?

Thanks!!!
Jerry

 

[mknauss]

Maybe that is outdated?

One chart is for SWCG pools, the other is for adding liquid chlorine daily pools.

For your situation, I would maintain right at 10 ppm for awhile. That will allow you to test pH accurately and keep the FC elevated to combat the mustard algae.

 

[Jimrahbe]

Jerry,

An IC40 should be able to maintain an FC of 10 ppm in a 16K pool with no issues... as long as your CYA is 60 or 70 and you do not have algae.

The CYA chart does show a minimum and a target FC level.. But, I never recommend trying to keep your FC between the min and the target. I suggest that you keep your FC at your target or higher and that you treat the minimum as a large cliff that you never want to fall off.

Keep in mind that you have to run the pump long enough, and the cell's output high enough, to generate the amount of chlorine that your pool needs.

Thanks,

Jim R.

 

[elkind]

Jim R. and mknause, Thanks again for your guidance! I think I understand. Even though the chart Chlorine / CYA Chart says to use an FC target of 5ppm for a SWG at CYA=70, I will aim for 10ppm due to my mustard history, at least for a while. And now that the algae is presumably gone, I am hoping that my IC40 can do the job at 8hr/day.

pH has been tricky on this pool. I have gradually been working TA down from 140 to below 100. I'm trying to get the pool to be happy at a pH closer to 7.5 (instead of 8.0). That's a work in progress.

Thank you again for the help!! You guys help make TFP is a truly great resource!

-Jerry

 

[MITBeta]

BUT the chart at Chlorine/CYA Chart mentions that for CYA=70ppm FC min=5.1, Target=8, Yellow Mustard MINIMUM=10.5 (15% of CYA level). Maybe that is outdated?

I think the chart you're referring to is outdated,the way I read it. I certainly haven't been around since 2007, so maybe one of the veterans will chime in. My reading around this suggests the mustard algae recommendation is to keep a chronic problem under control.

But, you have followed the steps to eradicate the mustard algae, so it no longer applies to you.

It HAS to be that the IC40 can maintain FC=10ppm in a 16,000 pool, if there is no algae to deal with.

It's easy to confuse quantity and rate. The IC40 can maintain at any rate you want AS LONG AS the consumption is less than production. Consumption is inversely proportional to CYA level, so yes, I agree that you should be able to hold FC at 10 with your setup.

 

[mknauss]

But, you have followed the steps to eradicate the mustard algae, so it no longer applies to you.

No. You eradicate and then keep a higher FC level for some time (typically several weeks) to be sure the MA is gone.

 

[MITBeta]

Ah, got it. How come this column isn't in the current version of the chart?

 

[Jimrahbe]

I am hoping that my IC40 can do the job at 8hr/day.

Jerry,

Hoping has nothing to do with it. It is a math problem.

Your cell will make 10 ppm of FC in your pool when run at 100% output for 24 hours. That is .42 ppm per hour.

8 hours at 100% gives you 3.3 ppm of FC.

Most pools use 2 to 4 ppm of FC per day.

So, as long as your pool does not use more than about 3 ppm of FC per day, and you run 8 hours at 100% SWCG output, you should be good to go.

Thanks,

Jim R,

 

[mknauss]

Ah, got it. How come this column isn't in the current version of the chart?

Mustard algae is extremely rare. So the chart in the thread in Pool Chemistry is left as the resource. No reason to further complicate the standard FC/CYA chart.

 

[elkind]

I was wondering the same things. Thanks for the additional clarifications!

 

[elkind]

JimR,

Thanks for commenting on the Math. I see what you are saying, but I am still a little worried that I will ultimately need to run the SWG (and pump$$) for more than 8 hr at 100%.

This is why: Based on the charts at Pool Water Chemistry I think that with CYA at 70 and FC at 10, my HOCl will be ~0.05 and then my FC loss rate will be about 0.7ppm/hr. Given how much sun we get here, I'd guess that at these chemical conditions my FC loss would be closer to 5ppm/day. But I'll try it and see.

I have to say, it is really GREAT to be able to get real-time feedback from TFP experts!!

-Jerry

 

[elkind]

Update: at 8 hr/day x 100% Output, my IC40 maintains FC at 8ppm. There's no overnight FC loss. I think that 70ppm CYA might be a bit much for my situation.

Question: For a SWG system with CYA =70ppm, what FC would you say I should use for a target to ensure no mustard algae. I know that the old chart showed FC=10.5ppm for a MA minimum, but it did not specify SWG vs. manual dosing, which I understand makes a significant difference. Would you say that FC=10.5 is what I actually NEED for my setup to completely avoid having MA come back? Or can I go confidently with 8ppm? Note: I dose 2X/wk to keep pH ~7.7.

If not I guess I have 2 choices: Run the pump/SWG 10-12 hr/day or lower CYA. Sound right?

Thx!!
Jerry

 

[Katodude]

I would not lower your CYA. Especially this time of year. Just maintain a higher FC level. If you have to run your pump a few more hours, do that.

 

[elkind]

Thanks! I will do that. But I'm not sure I understand why running, say, with CYA=50ppm would not also be a good option. I think that at 50ppm CYA I would not need to keep FC quite so high. What is the downside to running the SWG with CYA at 50ppm?

 

[mgtfp]

But with CYA 50, you'd have a higher relative loss to UV. Even though you could maintain a lower FC, you might in total still have a similar or even higher absolute loss to UV.

Not worth draining to lower CYA, just maintain the higher FC for a while.

If you don't want to run the pump longer, you could also top up with liquid chlorine as an alternative.

 

[elkind]

OK. I'll run 10 hr and add bleach as needed to maintain FC =10ppm. Thanks for all the great help!!!

 

[elkind]

I wanted to give a quick update and I am naturally very interested in any additional comments anyone would like to make.

The MA clean up went fine - I added bleach twice a day to make sure FC stayed where it needed to during the SLAM and MA SLAM.

Now I'm in normal operational mode and I find that with CYA=70ppm and SWG (IC40) "on" 8 hr/day @ 80% output I can maintain FC=6ppm. If I lower Output% to 60% then FC drops to about 4.5 or 5ppm.

My only question is whether FC=6ppm will for sure keep MA away. And I am guessing that it should, but also that I will have to wait and see for myself.

If anyone would like to comment on what to expect wrt MA under these conditions, I'd be very interested. Also, if anyone would like to comment as to whether I would be better off with a higher (or lower) CYA, I'd be very interested in that too.

Thanks again for the help!