mustard algae - what's my problem?

[kazoo]

I can't figure this out. I noticed a few streaks of mustard colored algae in my pool two weeks ago. So I shocked the pool ( using salt water chlorinator ) and tossed in creepy crawly. Now my pool looks like mud. It's green and cloudy and the algae keeps coming back everywhere the pool water doesn't circulate I get mustard colored deposits. As far as I can tell my water is perfect.

PH 7.4
TA 110
CH 200
CYA 40
Salinity 3900
Free chorine 3.0
Total chlorine 3.3
Combined chlorine 0.3
Water temp 82F

The only thing I've done differently lately is leaving the solar cover on, and I just recently added some CYA because I lost a lot of water due to a pool repair.

As a last resort, I'm going to shock it again ( using salt water chorinator ) to get the chlorine up to 10.0 and put in a "clear-sok" to try to filter out the fine algae, but if this doesn't work, I'm at a loss. I've been running this pool for 10 years, and never had a problem like this.

Help!!!!!

- JD

[chem geek]

While a 3 ppm FC level at 40 ppm CYA (an FC level of 7.5% of the CYA level) is a minimum to keep away green algae in a pool that is already clear (i.e. not in an algae bloom), mustard/yellow algae takes a higher minimum of 15% of the CYA level to keep away or 6 ppm at 40 ppm CYA. It also takes a higher FC level for shocking to get rid of it -- 60% of the CYA level (or 24 ppm FC) vs. 40% of the CYA level (or 16 ppm FC) for green algae. When shocking at high FC levels, lower the pH to 7.2 first and keep the FC level high until the following three things all occur 1) the pool is clear of algae (not cloudy or dull, either), 2) there is minimal (< 1 ppm FC) drop in chlorine overnight and 3) there is minimal Combined Chlorine (CC < 0.5 ppm).

Also, if you measured 3 ppm FC, then unless that is when you normally would be adding chlorine, then the FC could be lower at other times. So I suggest shocking your pool hard and then maintaining a higher FC level. Another alternative (after shocking to get rid of the algae) is to use a weekly PolyQuat 60 algaecide or to use 30-50 ppm Borates (also an algaecide at that level -- but if you have dogs that drink water every day from the pool, don't use the Borates) or you can rid the pool of phosphates with a phosphate remover or other methods with their own issues (e.g. copper ionization).

Richard

[kazoo]

Thanks chem geek! I knew that CYA reduced the efficacy of chlorine, but I didn't realize that you could calculate required CL as a percentage of CYA. Very useful information.

One follow up question if I may. My local pool store recommends that I keep CYA at 30-50. My problems with algae did start after I brought the CYA up to 40, I think it was around 15-20 before. So what is the down-side to running the CYA at lower levels? With a salt-water generator continuously adding CL to the pool, as long as the generator can keep up with CL loss on a sunny day, and keep the CL above 2-3ppm shouldn't this be OK?

Thanks again,

- JD

[chem geek]

JD,

Thanks for reminding me that you have a saltwater chlorine generator (SWG). Most residential pools with an SWG are able to keep a lower chlorine level (FC) of around 4.5% of the CYA level and keep away green algae. But for mustard/yellow algae, a higher level is required and we thought it might be around 9% but that's not much higher than your 7.5%. The reason for the lower FC requirements with an SWG are twofold. First, there is better more consistent dosing so any conservatism in the recommendations isn't needed. Second, there is some superchlorination in the SWG cell since water passing near the chlorine generation plate is exposed to high chlorine levels (80+) at high acidity (so high disinfecting chlorine level).

However, the superchlorination effect only exposes part of the water going through the cell (otherwise you'd have 80 ppm FC coming out of the returns, and that doesn't happen) so it takes many turnovers to expose most of the water in the pool to this superchlorination. Also, any algae stuck in biofilms and not free floating isn't going to go through the SWG cell at all. Mustard/yellow algae tends to stay settled in one place, unlike green algae that is more free floating. Brushing the pool regularly to try and get the mustard/yellow algae to flow through the circulation system into the SWG cell might help.

Anyway, the shock levels I told you are the same, even with an SWG. Obviously, you have to add chlorinating liquid or bleach to increase the FC levels -- you can't use your SWG for that (and it would wear out the cell faster if you tried).

As for the CYA level, that's an interesting question because most SWG users have found that increasing the CYA level to 60-80 ppm shows a drop in chlorine consumption and lets people operate their SWG at a lower power level or on-time. The reason is that the higher CYA not only reduces the disinfecting chlorine level (reducing its effectiveness, but also slowing down destruction from sunlight), but CYA itself shields chlorine at the lower depths from the UV. This latter effect seems to start to dominate somewhere in the 40-80 ppm CYA range. The downside to such a high CYA level is that shocking needs a LOT of chlorine so I don't think this would be good in your situation.

At the lower CYA levels of 30-50 ppm, it seems that if you maintain a constant FC/CYA ratio to keep the disinfecting chlorine (hypochlorous acid) level constant, that you end up using more chlorine at higher CYA due to the higher FC so more chlorine (in absolute terms) gets broken down from sunlight. The protection from CYA is not linear -- most of it comes with little CYA. I show graphs of this and other very technical info at this thread, but keep in mind that the FC half-life vs. CYA graph was done before this extra effect of CYA protection was figured out (so the curves probably start to shoot upwards somewhere around 50-70 ppm or so). Anyway, you can see that it may be better for you to operate at the lower CYA level and I assume you used to still have an FC of 3 so that would be an FC of 3 with a CYA of 30 so a 10% ratio. That is less than the 15% needed to keep away yellow/mustard algae in a manually dosed pool, but with the SWG maybe that explains the difference (could also be phosphate levels and other factors as well).

So I would say that after you get rid of this algae via shocking (and brushing the pool regularly while shocking), then you can either keep your FC level at a minimum of 3 ppm at 30 ppm CYA or at 4 ppm FC at your current 40 ppm CYA, but these would be absolute minimums as it seems your pool is "on the edge". Using the 15% ratios would be more guranteed -- so 4.5 ppm FC at 30 ppm CYA or 6 ppm FC at 40 ppm CYA, but you'll have to turn up your SWG somewhat to achieve these levels. No easy answers here and you can experiment a bit to see what works best for you. Unfortunately, it's hard to reduce CYA levels -- you have to dilute the water to do that (though over an entire swim season the CYA does drop a bit from a very slow breakdown by chlorine).

I should mention that there are some other alternatives to keeping away algae, but all cost more money (of varying degrees). You can add a weekly maintenance dose of PolyQuat 60 algaecide. You can add 30-50 ppm Borates to the pool (if you have a dog that drinks from the pool every day, then I wouldn't recommend the Borates). You can use a phosphate remover to remove phosphates from the pool. Or you can use an alternative algae prevention system with its own problems (copper ionization, etc.). Quite a few SWG users have gone the Borates route and you can read a long thread about this here where waterbear figured this out (it's great how various pool users work together to figure stuff out and help each other).

Richard

[kazoo]

Hi Richard,

Well I shocked it "hard" yesterday evening by adding 20L of liquid chlorine. Historically I have found that 1L Liquid CL raises my reading by about 1ppm, so given that I was already at 4, this should have pushed me up to 24ppm. CL is still off the charts this morning. My strips only measure up to 10ppm so all I can say is that I am well over that.

There is no more visible algae. The water is not perfectly clear, but I can see the bottom of the deep end quite clearly. It just sort of has a bit of a dull milkyness to it now. I suspect that is suspended dead algae. Anyway, I'm back in familiar territory. The milkiness always clears up by tossing a klear-sok in the skimmer basket and circulating for 24 hours.

I'll remember to keep my FC a bit higher to avoid repeat problems. The borate looks interesting ( I read the other threads ) and while I don't have a dog that drinks water from the pool, I do have two young children who I suspect do drink the water. Of course they deny it. They also deny peeing in the pool, but I was a kid once, so I'm onto them ...

Thanks again for the info.

- JD