Mustard algae despite FC 4.5 and CYA of 30?

[camvy02]

Thanks to reading a lot on this board and the Pool School, I am now using the BBB method. Two Sundays ago, my pool was totally green. I used liquid bleach, the TF 100 kit and cleared everything up (2 nights of < 1ppm FC change) by Saturday. My husband was in awe at how great the pool looked. We put on a solar cover.

Then, 3 days later, Tuesday night, I added borates to the pool (according to waterbear's method). I noticed on Tuesday night a few spots that looked like dirt, but brushed it away when I was mixing the pool chemicals. Well tonight, when I went to check on the pool, there were more of these yellow/brown (dare I say mustard) colored spots (20-30 spots spread out to 1/3 of my 15x30 above ground pool). The water is sparkling clear and warm otherwise. I'm sure it was algae and not dirt or pollen, esp since the solar cover was over it. Now, it hasn't been 48 hours since I added the borates. The sand filter has been running 24/7 since I started this all 11 days ago. (Water glass in sand filter is clear)

I checked my CYA and FC and shocked it to 19 (according to the pool calculator). CYA is 30, FC was 4.5 (which has been maintained daily since the pool cleared.)

I seek your advice as to how to proceed in time for our Sunday pool party! Here are my questions:

1. Do you think this one shock level should be enough to get rid of this algae or will I have to maintain this through several more nights? Now, I don't understand how this algae grew to begin with as I've had a FC of 4.5 every night when checked and a constant CYA of 30. (I'm no longer using any Trichlor tabs - just liquid bleach). Could adding the borates have anything to do with this?

2. The FC 19 is quite high, so will I need to remove the solar cover in order to get the FC evaporated more quickly by Sunday should I just cross my fingers tomorrow and not add any more liquid chlorine into the pool? (It's been cloudy daily and I've been bleaching at night w/a target of 8 which means I've lost about FC 3.5 during the day for the last 3 days)

3. What is a safe FC to swim in?

Any other recommendations or insights?

Thanks so much in advance for your help. This forum has been a blessing and I no longer feel helpless as I did before with the pool (I was at the mercy of the pool supply store who were useless).

[chem geek]

I'd let the chlorine level get lower for your party and deal with the algae/dirt after that. Just vacuum/sweep up what you can prior to the party.

If it's yellow/mustard algae then it should tend to grow on the shady side of the pool (the cover would tend to make the algae happier as well -- it doesn't like direct sunlight very much). Once infested, it takes a higher chlorine level to keep away -- around an FC that is 15% of the CYA level, but that's the 4.5 ppm FC at 30 ppm CYA you are using. If this is this kind of algae, then the shock level to kill it is higher at around 60% of the CYA level, or 18 ppm FC as you are doing, but it must be maintained until there are no more visible signs of the algae and there is no more than 1 ppm FC drop overnight and no more than 0.5 ppm Combined Chlorine (CC) and I'd give it two more days at the shock level even after this occurs. Also, put in any poles or equipment you use in the pool to shock it as well. Finally, if there are removable areas or light niches you can expose, then do that since this algae will hide out in those areas. It's a difficult algae to get rid of, but it can be killed.

As for swimming safety, a pool with a shock level FC that is 60% of the CYA level is technically equivalent to a pool with 1.4 ppm FC and no CYA so roughly the same as some indoor pools. Not great, but not a disaster either. You don't want people drinking lots of pool water, but that's not normal and we're talking quarts here. The EPA drinking water limit of 4 ppm FC is for 6-8 quarts per day every day. As I mentioned above, I'd just let the chlorine drop a bit -- if it gets down to 10-15 ppm FC, it'll be fine and more like a pool with 0.6 ppm and no CYA.

The Borates were unlikely to have anything to do with this. I suspect that you had multiple algae species in your initially green pool and that after you killed off the green algae, the yellow/mustard algae (which is more chlorine resistant) had a wonderful time filling in no longer competing with the green algae for nutrients. If you can carefully collect a sample of this algae, then you can feel it to see if it's slimy. If it's squishy instead, then it's more likely pollen; if it's gritty, then more likely dirt. As you say, with a pool cover it's less likely to be pollen or dirt unless it just settled.

In the worst case if you find recurrence and don't mind spending more money, you can use a chemical alternative such as a phosphate remover. There are also other products like Yellow Out, but that just creates bromine and is most useful when the CYA level is high when you can't practically shock with a high enough level of chlorine. That's not your situation since your CYA is reasonable. Keep us posted on your progress.

[camvy02]

Richard, I believe your hypothesis of other types of algae lurking in my pool after it apparently seemed to clear is correct. I kept wondering why I was losing 2.5 FC daily when it's been cloudy, the pool has been covered with a solar cover, and no one has been swimming in it. I read in your posts that you use 1 ppm daily and that's with an hour of bather load. So, was I losing this 2.5 FC primarily due to algae that was not initially visible? Just asking to increase my knowledge.

I decided to buy Polyquat 60 and put it into the pool yesterday afternoon. I still saw a few mustard spots today, but I'm hoping they won't pop back up tomorrow. How long does it take for the algaecide to get rid of all the algae since I used it in the very early stages of visible signs of mustard algae? Water is beautifully clear.

My stats tonight were FC of 3, CC of 0.5, ph of 7.5, T/A of 100, and CYA of 20.

My final question is with regards to the CYA. Two weeks ago it was 40. I haven't backwashed this week (when CYA was 30). It hasn't rained. I don't understand why my CYA appears to be dropping. I'm using the same TF test throughout, so I don't think I've changed the way I see the black dot.

Should I get stabilizer to try to increase my CYA to 40 or 50? The pool store told my husband that we didn't need to change the CYA at 20. I know this is an acceptable minimum, but is it better for me to try increase it to more in the middle of the range of acceptability?

Thanks a million. I think we're ready for our pool party on Sunday. Now, if I can only get my 2 yr old to get over his fever....

[chem geek]

Algaecide is better at preventing algae than killing an existing outbreak, but combined with chlorine the two can work together. Usually chlorine alone is sufficient, but you can look at algaecide as an additional helper and insurance if the chlorine level gets too low. It won't be a fast kill of the algae -- shocking with chlorine is faster. Just keep in mind that PolyQuat 60 breaks down from chlorine so you will need to add it every week. It is also a clarifier so some will get caught in the filter if there are particles in the water you will also lose some from backwashing your sand filter -- just add the maintenance dose every week if you want to supplement your chlorine to prevent alage (if you fought hard to completely get rid of the yellow/mustard algae, then an algaecide would not be needed, but using an algaecide is an alternative).

As for the CYA level, it shouldn't be dropping so fast, but at high shock levels of chlorine some of it can slowly oxidize CYA, but that usually takes months to notice. In your covered pool, I'd shoot for around 30 ppm since that's reasonable for an hour or two of sun exposure per day as with my own pool. At 20 ppm it will be harder to test and won't provide quite enough protection against sunlight. Why don't you wait, though, to get rid of the algae first.

As for your covered pool chlorine loss, it is higher and that is likely to be the chlorine fighting the algae. You should find it dropping as the algae goes away. You may find some initial loss the day you add PolyQuat 60 each week.