CYA over 100 with yellow/mustard algae

[MikeM]

OP here with another update. Well, I still had the yellow algae all winter, despite maintaining FC in the 6-10 range with CYA around 30. I did buy a second pint of Orenda PR10000. I've now used all but about 3oz over the last several months, but phosphates are still hovering around 300-400ppb. Over the last 12 months, I've used 2 quarts of a Clorox product and now 2 pints of Orenda. According to the labels, this should have removed several thousand ppb. There is no phosphate in my source water and no other obvious source. I read PoolDoc's explanation of "attached" phosphate with much interest, so I'll keep trying.

Another oddity, same as last year, my CYA mysteriously dropped like a rock to zero in the Spring. I've added some CYA to get back to about 30-40. I have a cartridge filter and don't lose much water normally, so I don't understand this loss of CYA. The pool was running all winter and water looked fine except for the recurring yellow algae.

In any case, I'm now holding FC in the 10-15 range since April and sweeping several times each week. I'm also running the polaris about 3 hours per day instead of my normal 1 hour. So far, no yellow algae, but this could be because the polaris and I are sweeping it up every day. I'll cut back sweeping and see what happens this week. I'll also use the last 3oz of Orenda, but based on my experience, I'm quite certain that will have no effect on phosphates in the water.

If any other suggestions, please let me know. Here are my numbers from this morning:

FC=13.5
CC=0
pH=7.6
TA=90
CYA=35
CH=280
Borates=50
PO4=400

[chem geek]

That's too bad that the Orenda product isn't cutting down the phosphates below 300 ppb. In my pool it got down to 100 ppb when it was used years ago. Since then, my phosphates have crept up to around 400 ppb (after around 4 years). I have 300-500 ppb phosphates in my fill water, but I use a pool cover so have minimal evaporation/refill from about 1 hour of pool use most days.

As for CYA disappearing over the winter, that is possible if you let your pool go over the winter (i.e. if you don't continually disinfect it) where bacteria can consume CYA turning it into ammonia (which creates a HUGE chlorine demand upon opening) or to nitrogen gas (which doesn't have any chlorine demand). Sounds like you may have had the latter happen.

With your low CYA you shouldn't need to keep the FC so high. Even to keep away yellow/mustard algae using chlorine alone that would take an FC that is around 15% of the CYA level instead of the more usual minimum for green/black algae prevention of 7.5%. At 35 ppm CYA, an FC of 5-6 ppm should prevent yellow/mustard algae growth if you have decent circulation in the pool. HOWEVER, this is at normal pool temperatures above 80ºF. In colder water, there is a lower active chlorine level so at 50ºF, for example, you'd need closer to 17 ppm to be equivalent (it might not be that extreme -- I'm using the CYA temperature dependence from Wojtowicz but those numbers may be off a bit). This seems consistent with what you've been seeing, though usually at colder temperatures the algae grows more slowly.

Keep us posted with how it goes.

[MikeM]

Thanks chem geek. Regarding disappearing CYA, I thought that would only happen if you close the pool down and let it turn into a green sludge pond. I've never done that. As I noted in my prior post, mine was circulating all winter and was well chlorinated at all times. Under those conditions, is there any other mechanism that would consume CYA? The recurring yellow algae perhaps?

I'll reduce the FC as you suggested, but at the first sign of the yellow beast, I'm taking it back up again. This is the lowest my CYA has been in a long time, so I want to take advantage of it and rid my pool of this hard-to-kill algae.

Regarding phosphates, did you read what PoolDoc said in post #51 about "attached" phosphates? Do you agree? Should I just keep dosing with Orenda until I see a breakthrough below 300?

Thanks,
Mike

[chem geek]

I don't know why the CYA would disappear (other than water dilution from rain overflow) over the winter when it was disinfected. If the chlorine and pH levels were high, then the oxidation of CYA by chlorine is faster, but in cold water that shouldn't be very fast. In a pool during the summer the loss is usually around 3 ppm per month while in a hot spa it's around 5 ppm per month.

I don't know what to do about the phosphates. It should get lowered more and maybe if they are released as PoolDoc says then doing another treatment might work, but I'd expect to see it drop soon after treatment and only slowly rise up if in fact the phosphates are "attached" elsewhere. If you had HEDP metal sequestrant in the pool, then chlorine oxidation of it will produce more orthophosphate -- do you have HEDP in the water?