Green or mustard? How to tell the difference?

[chem geek]

If you truly have yellow/mustard algae and not pollen or dirt, then it takes a higher level of chlorine to kill it. See this thread for additional info (and this thread for some pictures). The shock level required is an FC level that is approximately 55% of the CYA level if one lowers the pH to 7.2 before adding the large amounts of chlorine needed. The shock method of maintaining a high FC level is the same for this algae, just at a higher FC level (i.e. you must keep adding chlorine to maintain the high FC level and not let it drop). In areas where the yellow/mustard algae is regularly reintroduced (blown in), after the algae is gone, then it may be necessary to maintain the "Max" level of chlorine from Ben's chart or roughly an FC of 15% of the CYA level.

The results with pool owners using PolyQuat 60 to kill this algae have been mixed though it does seem to help prevent it (as it prevents all algae) IF lower chlorine levels are maintained. That is, chlorine alone can prevent and kill this algae, but higher levels are required as noted above. The use of 50 ppm Borates is another option for prevention.

Note that yellow/mustard algae is often reintroduced into a pool from bathing suits so be sure and thoroughly wash bathing suits (hot soapy water) after you've been in a pool with such algae.

As for determining for certain whether it is yellow/mustard algae vs. pollen, have an open bucket of water near the pool and see if it gets yellow dust on its surface. Also, use a skimmer sock and see if it collects a lot of material and see if it is slimy to the touch (algae) or not (pollen). Take a small sample and look under a microscope -- pollen will look like round balls (sometimes a bit rough or spiked) while algae has some transparency to it and is shaped more like cells with some distinguishable contents.

Richard