CYA over 100 with yellow/mustard algae

[PoolDoc]

With a cartridge filter, you won't be losing much water on backwash. So, CYA levels won't drop on their own.

To go from CYA=100 to CYA=50, you'll need to dump 1/2 of your pool, and refill. The only other way is to allow the pool to get 'slimed' over the winter. CYA usually bio-degrades, to nitrogen gas (no problem) or nitrates (usually not a problem). But sometimes, it biodegrades to ammonia, which is a big problem. So, that's not really recommended.

If you do drain, be SURE to do so ONLY when the ground is reasonably dry. Even concrete pools will float out of the ground when emptied, if the ground water levels is high enough.

[MikeM]

Thanks Ben. I'm not at all inclined to drain half the pool due to risk of floating and cost of the water. I'm even less inclined to "slime" the pool over the winter.

So, I've begun to think maybe I'll just keep the CYA high and maintain FC levels accordingly high as well. I've read here about some of the benefits of that approach. The one significant downside seems to be that if you slip-up and get an algae bloom, it takes an enormous amount of chlorine to kill it.

One question: my daily chlorine loss is right at 2.2 to 2.3 ppm (after two weeks of careful tracking). That strikes me as a little high. Is that normal with CYA>100? What would my daily usage be at CYA=50-60? I'm in North Texas and my pool gets 7-8 hours of direct sun per day. Water temps range from 84-92 in the summer. Thanks again.

Mike

[Watermom]

I don't think losing 2ppm of chlorine per day is unreasonable at all.

[chem geek]

At 100 ppm CYA, typical FC loss is around 15%, but you are at 20 ppm so you're doing even better than the predicted 3 ppm loss. If you didn't have yellow/mustard algae, your minimum FC would be more like 7.5 ppm in which case you'd only be losing around 1.2 ppm FC per day which is lower than the typical pool (2 ppm is fairly typical; 3 ppm is not uncommon).

You should check behind light niches and under removable ladders. Yellow/mustard algae prefers shades and often hangs out in areas with poor water circulation. You need to completely kill it or else it will come back when you let the FC level drop below roughly 15% of the CYA level.

[MikeM]

Thanks to all. This is such great information. I've owned 3 different pools over the last 13 years, and for the first time, I actually feel like I'm in control, and know what I'm doing, and why.

For the last 4-5 days, I've been letting the FC come down to 8, then add 1 gal of 8.25% bleach to get back to 12, then 2 days later I'm at 7.5-8.0. At those FC levels and with CYA>=100, I should be losing around 1.5ppm per day, using chem geek's 15% typical figure.

Since I'm higher than that (2.2-2.3 ppm loss per day), I assume I'm still fighting something in the water, most likely the yellow/mustard algae. But I've checked everywhere and see no sign of algae anywhere. I'll keep checking. My filter pressure is up about 4 PSI over this 2-week timeperiod, so I hope that's the dead algae. I'll clean it out this weekend. Hope I haven't declared victory too soon.

I suppose it's also possible that my higher daily usage could result from the extreme heat and sun we've had over the last week (temps over 100 and no clouds in sight). Bathing load has been light. I'll keep monitoring. 1.2-1.5 sounds very good to me.

Bottom line, I'm starting to like the idea of high CYA... higher FC levels, lower daily consumption, and less frequent manual additions of bleach. Just need to make sure I've killed this yellow beast for good.

Thanks again for all the information and advice.

Mike

[chem geek]

It's more definitive to do an overnight chlorine loss test by seeing the chlorine loss rate overnight by testing after the sun if off the pool in the evening and before it hits the pool in the morning. If your loss is less than 1 ppm FC, especially if 0.5 ppm or lower, then you don't have unusual loss. It doesn't prove you don't have algae, but it's not enough to show up as chlorine demand.

The main downside to a higher CYA level is that IF you get algae or any other reason for needing to shock the pool, it takes a LOT of chlorine to do so.

[PoolDoc]

Chem_Geek is right about the very high chlorine levels required, if you do get mustard algae. But there are some ways to reduce that risk.

1. Using borax is probably the easiest and longest lasting, and has no undesirable side-effects. Like CYA, it's a permanent addition to the water. Unlike CYA, it is not subject to biodegradation. With a free form pool, your pool volume is only a guess, unless you actually measured it during filling, using the water meter. You'd need 20 - 25 boxes of borax + 5-7 gallons of muriatic acid, and some Lamotte borate strips (follow the link to the testkit page in my signature).

A borate level above 60 ppm reduces your pools susceptibility to algae quite a bit.

2. We don't normally recommend phosphate removal products, since if you keep your chlorine level adjusted, algae is usually not a problem regardless of phosphate levels. But, if you use them to lower your phosphates, and can avoid use of pool products containing phosphates, such as metal control products, then you can use phosphate removal products, during an algae episode, to lower levels so much that it virtually starves the algae. But, you'll need to test your fill water -- sometimes tap water has such high phosphates that this is not very practical.

3. Bromine. I'd consider this a sort of last resort. But adding sodium bromide to your pool, to produce a strong unstabilized halogen (bromine instead of chlorine) residual can also work. The downside is that your chlorine use will go WAY up during the period bromide is present, as it converts bromide to bromine. Eventually, things return to normal, since a small amount of the bromide is converted permanently to bromate on each conversion cycle, and the bromate is stable and doesn't interfere with pool chemistry.

If you choose to stay with high CYA, I'd recommend going ahead with borax. It doesn't cost that much, and gives you a margin of safety that's worthwhile. Waterbear, who has posted here for years, has found that it allows him to go on vacation for a week or so, without having to worry about algae on his return. He simply maintains chlorine levels, shocks before leaving, and then reports coming back to a clear pool. He reports that before borax, the pool was usually green on his return.

[chem geek]

I agree that the borates are probably the best insurance -- they are a mild algaecide so might not prevent algae completely if the FC gets to zero, but they'll certainly slow down the growth enough to prevent things getting really bad and along with regular chlorine should help against tougher types of algae.

One other algaecide approach to Ben's list is an ammonium product such as ammonium chloride. With chlorine in the water, this creates monochloramine which kills algae (at least green algae; I'm not so sure about yellow/mustard algae, not enough experience with that). The main advantage with the monochloramine approach, unlike the sodium bromide, is that you can get rid of the monochloramine by adding more chlorine. With sodium bromide, it can take some time to get rid of the bromine, though by not overdosing it might not take more than some number of weeks and having a bromine pool is not a disaster (mostly it's just a higher chlorine demand for a while).