Mustard Algae? How to get rid of it?

[chem geek]

A couple of points. An FC of 3 ppm with a CYA of 80 ppm is not sufficient to keep away algae -- not even green algae (the only exception being with SWG pools which perform more stable chlorine dosing and superchlorination in the SWG cell). With a CYA of 80 ppm, the FC should have been an absolute minimum of 6 ppm.

As Marie points out, it is very hard to kill mustard algae thoroughly and at a high CYA level it is much easier to dilute the water via a partial drain and refill and to then combat the algae at the lower CYA level. You would probably want a lower CYA level anyway unless you plan on maintaining the higher FC level (see Ben's Best Guess CYA chart). The chart shows a minimum of 5 ppm, but 80 ppm CYA is in the higher end of the 60-90 range which is why I said 6 ppm minimum.

If you dilute your CYA in half to 40 ppm, then 24 ppm FC should kill the mustard/yellow algae if maintained for several days after no more visible algae is present. A yellow/brown dust is what that algae looks like but pollen can sometimes look like algae. The algae will often start on the shady side of the pool and only algae will be on the walls -- the pollen will usually settle to the bottom. As Marie mentioned, when chlorinating at such a high FC level, even after the partial drain/refill to get to a lower CYA level, it is best to lower the pH (with acid) to 7.2 before adding the chlorine.

Richard

[CarlD]

In short, there is no short-cut to getting rid of algae. You have to kill it and kill it all, and the ONLY way you do this is by raising your FC level. Bleach is the same as Liquid Chlorine, but pool stores sometimes sell LC at 12-14% levels--so 1 gallon is the same as 2 gals of 6% Bleach. You can use LC as well. You can use the Bleach Calculator to figure how much you need.

But there's the rule of thumb: 1 gallon of bleach added to 10,000 gallons of water adds EXACTLY its concentration. So 1 gal of 6% adds 6ppm, 1 gal of 5.25% adds 5.25ppm and 1 gal of 12.5% adds 12.5 ppm to 10,000 gallons.

You need to add two gallons of any of these to 20,000 gallons to get the same increase in ppm.

So say you have 20,000 gallons in your pool and your ppm is 3. You need a minimum of 17ppm. So if you add 3 gallons of 12.5%, you'll get an increase of 19.5ppm. Or you'll need 6 gallons of 6% bleach for the same effect (increase of 18ppm).

It's not too hard to figure.