CYA over 100 with yellow/mustard algae

[MikeM]

Hi Ben. Thanks so much for the detailed response.

1. I did the CYA dilution test, as you suggested, and it did come in a little lower at 80. I also re-ran the regular non-diluted test and it was still over 100, but a little less than before. As I said, the pool store said I'm at 91; and the strips, while hard to read, seem to be dead-on at 100. In any case, my CYA seems to be somewhere in the range of 80-100.

2. Regarding borates, I have the LaMotte strips, which seem to be right between 30 and 50. Also, the pool store said I was at 29, and that was right before I added 2 boxes of borax, which should have taken me above 40. Are you sure I need 7 boxes of borax to go from 40 to 60ppm in 18kgal? According to the bleachcalc.exe, 7 boxes of 76oz borax (532 oz) would increase borate ppm by 90. I do recall reading here that the bleachcalc app might be inaccurate for borax. Just want to make sure before I dump that much borax in the pool.

3. I'll take the chlorine to 20ppm tomorrow after a trip to Wal-Mart to restock. Are you sure 20-30ppm is going to be high enough to kill this beast? From what I've read here, others have gone much higher (35-40 for 7+ days), and they had much lower CYA than me.

4. I'll get the OTO kit tomorrow also. I wasn't looking forward to counting 100+ drops in the DPD test.

I'll brush the pool regularly and update back here with any improvement. One question: because I have a cartridge filter, I can't vacuum this algae to waste. My pool store rents a portable pump for this purpose. Is that something I should consider also? Losing some water might also help drop the CYA a bit. Or should a cartridge filter trap this stuff? Do I need to clean the filters during this process?

Thanks again for the detailed response.

[PoolDoc]

You've already added 2 boxes, so deduct those, but yes, 7 boxes was what I intended. Bleachcalc has an error in the borate calculator. Feel free to work up to 60 ppm in 2 box doses, if you prefer.

FC = 20 won't necessarily kill it, but it will likely stop it. Mustard algae doesn't take over the pool suddenly, so if you get to 20 ppm, and brush, and still see it recurring -- add more.

When testing, use the 10ml sample size, rather than 25ml sample, At 10 ml, 20 ppm = 40 drops, not 100.

A cartridge filter will normally trap dead - but not live -- algae.

[MikeM]

Update... I took the borates up somewhere north of 50 (not sure exact level), and I held the FC at 20ppm for last 10 days. I've been brushing almost every day and also set the Polaris to run 6 hours per day. The filter pump is running 24/7.

No sign of algae anywhere. But I'm guessing that could be due to the excessive brushing and Polaris run-time. So, as of yesterday, I'm letting the FC come back down to 8-10 and I'll stop brushing for a couple days to see what happens. I also reset the filter pump and Polaris to normal run times (12 and 2 hours respectively).

So far, so good. We've been swimming with no ill-effects at 20ppm, and the water looks fantastic.

Thanks for all the help. If the algae returns, I'll update again.

By the way, any suggestions on how to get the CYA down to a more normal level? I think I'd like to keep it around 50-60.

Mike

[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.