question regarding chlorine/CYA chart

[rhythm]

Regarding treating mustard/yellow algae, I wanted to make sure I'm clear on the length of time certain levels are to be held. First of all, after I shock with FC at the level shown in the far right column, how long do I hold the shock level? After that, I assume that I need to maintain FC for a certain period of time using the level shown in the "yellow/mustard minimum" column. So, how long do I maintain that level before I can go back down to the levels shown in the FC minimum or target FC level columns?

Thanks,
Greg

 

[JasonLion]

Mustard algae can be very persistent. You often need to hold the higher shock level for several days after the FC level starts holding overnight.

Are you sure you have mustard algae? Various kinds of pollen are easily confused with mustard algae. Mustard algae tends to grow on the walls of the pool that are more shaded, while pollen tends to spread across the entire pool. However, algae can sometimes be everywhere, so it isn't always easy to tell.

 

[rhythm]

JasonLion,

I'm pretty sure it's the mustard. I got this last year and it appeared in the skimmer mouth and on the walls on the shady side of the pool. I tried getting rid of it using chlorine alone, and I wasn't successful. But I may not have held the levels high enough for a long enough period of time. By January, the yellow had started to get a little out of control. Brushing wasn't completely getting it off in some spots anymore. (I was afraid I had let it go long enough that I had staining). So this time I used a product called "Yellow Treat". (This particular product is 88.8% Sodium Bromide and does not contain ammonia). I used a small 5 oz bag (my pool and spa combined are about 12,000 gallons I believe). With this and a chlorine shock and hold, I think I got it. After a few days, all the yellow was gone, and I didn't see any for over two months. I just noticed it again for the first time yesterday. Because of the time between, I'm thinking this is a new attack, but I guess I can't be positive. I do know that I was under the wrong impression about the required FC level to keep algae away. According the chart (I have a low CYA level right now, about 30), the bare minimum FC would be 2.2 ppm. I was keeping mine between 1.5 and 2. I guess I know better now!

Anyway, what I noticed yesterday looked very familiar, and in the same couple of spots as it started last year. So I think it's the mustard again. I guess I can shock at a level of 18.2 or more, and hold it there for several days. But after that, what should I do? There's the "yellow/mstrd min" column which in my case shows a level of 4.6 ppm. I take it that after the several days of 18.2 I need to hold it at 4.6 for a while to prevent it from coming right back? If so, how long should I hold it at that level before I can return to the bare minimum/target levels (2.2 - 3.5 in this case)?

Thanks,
Greg

 

[JasonLion]

Growing on shaded walls means mustard algae and given that you have seen it before you are very likely to have the identification right.

Fighting mustard algae can be complicated. If the high shock levels of chlorine really get everywhere and really kill all the algae then you can go back to the minimum levels. But what often happens is that mustard algae, which doesn't need all that much sunlight, can get established in hidden corners with bad circulation, say inside a light niche, and you don't ever really kill it all. At that point the higher daily chlorine level can be used to keep it from spreading and becoming a problem.

It is important to keep in mind that running with FC down near the minimum is dangerous. It is easy for the FC level to dip down below minimum, because of neglect or extra bather/debris load, and allow algae to get going. It is always good to keep FC above minimum so that even when it dips down for some reason you still have the minimum level.

Products like Yellow Treat are most useful when CYA levels are high. Yellow Treat basically converts your pool to a bromine pool for a short period of time. Bromine is not affected by CYA, so the effective levels can be much higher that chlorine would be with the CYA. It can get tricky because bromine is deactivated by sunlight fairly quickly and any chlorine you add just reactivates the bromine so the effective sanitizer level can fall very low for a while afterwards, until the bromine decays. At lower CYA levels it is simpler and cheaper to just use chlorine and not have to deal with all that.