Mustard/brown Algae - Chemicals Perfect

[mntn-biker]

This is our 2nd year with an above ground pool. This is what we have and how my chemicals check:

21' round ~10,000 gal pool

Sand filter

Aqua smart inline chlorinator

AquaChek Select Test strips to test water (and an occasional trip to Leslie Pools to check for phosphates and doublecheck my readings)

Total Hardness = ~150

Total Chlorine = ~3

Free Chlorine = ~3

ph = ~7.4

Ttl Alkalinity = ~120

Cya Acid = ~50

Phosphates = 0

Water temp about 80 deg. F

2 weeks ago, I learned that even though all my levels that I was checking on my test strips were fine (I cannot check for phosphates), I can get mustard algae. I found out my missing key with chemicals. I was supposed to use a phosphate remover - nobody told me about that part of the water recipe... We had mustard/brown algae and the water was cloudy. This is what we did to kill off the mustard/brown algae (the brown sediment looking stuff that was settling in the imperfections on the bottom of the pool):

I bumped up the ph to ~8.2 and dumped in the appropriate amount of yellow out and superchlorinated the pool.

We threw everything in the pool - vacuum, hose, skimmer, pool toys, goggles, solar cover, etc.

I superchl. the pool again after 12 hours and again after 24 hours all this time keeping the pump running

The brown stuff turned to white almost immediately.

I dumped in 2 bottles (68 ounces) of "Pool Magic and PhosFree" to remove the phosphates

After 24 hours, I vacuumed the white stuff on the bottom of the pool to waste and backwashed the filter

The water was clearing up little by little.

I vacuumed to waste and backwashed the filter again the next 2 days to get rid of the dead white residue left on the bottom.

I brought my ph down, had Leslie Pools check for phosphates (now down to 0), and balanced my water chemistry

I began using "Poolperfect and PhosFree" on a maintenance basis.

Everything looked good, we left for a 1 week vacation (left our solar cover off the entire week), and when we returned, I started noticing the brown residue on the bottom again. Chemicals checked fine, free chlorine was a little low at ~1ppm. I bumped the chlorine to get it up to 3ppm right away with some granulated chlorine. I brought a water sample in to Leslie Pools - they confirmed my chemicals are ok and the phosphates are still at 0.

Finally, the question - Is it possible for Mustard/brown algae to grow/return given the information above?

Thanks - Mike

[chem geek]

Mike,

First of all, test strips are not accurate, especially for the CYA reading. Even pool stores do not do this test accurately and their results vary a lot. Please do yourself a favor and get a proper test kit -- either the Taylor K-2006 kit you can get at a good online price here or the TF100 kit from tftestkits.net here with the latter kit having 36% more volume of reagents so is less expensive per test.

Since you are using an inline chlorinator, that implies use of Trichlor tabs/pucks as your regular source of chlorine. For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm. Even at a low 1 ppm FC per day chlorine usage, after 6 months of Trichlor use the CYA will climb by over 100 ppm if there is no water dilution. With your sand filter you are probably regularly backwashing so getting some dilution, but probably not enough to keep the CYA as low as 50 ppm so I suspect that number is wrong. In any event, even if it were 50 ppm FC, your minimum FC at that CYA level would be 3.8 ppm so your 3 ppm (or lower) would be too low and algae can grow (yellow/mustard algae is more resistant to chlorine so can require an FC that is 15% of the CYA level to keep away, but if you completely get rid of it and do not reintroduce it then the normal FC minimum that is 7.5% of the CYA level is OK).

The amount of active chlorine (hypochlorous acid) is proportional to the ratio of FC to CYA so as the CYA level climbs you would need to increase the FC level to have consistent sanitation/disinfection, algae prevention, and oxidation rates. If your CYA level is high, then your FC is too low and algae can grow.

It is possible (but unlikely in your case) that you have pollen or dirt in your pool, not yellow/mustard algae. This algae does not like direct sunlight so will tend to grow on the shady side of the pool. The fact that it turned to white after superchlorination (and Yellow Out, which is sodium bromide so with chlorine produces bromine) treatment makes it more likely that this is indeed algae. By the way, yellow/mustard algae usually does not make the water cloudy. In your case, the cloudy water could have been green algae starting to form as well or it could be poor circulation/filtration.

As for the phosphate remover and the return of algae, this is possible because phosphate removers only remove one form of phosphate known as orthophosphate. There are also small organic phosphates that algae can continue to utilize for growth, though such utilization is slower. It's also possible that the phosphate test at the pool store is wrong. Taylor makes a phosphate test kit, the K-1106 but you don't need to worry about phosphates and can keep a pool clear of algae using chlorine alone so long as you maintain the appropriate FC/CYA level. I have 2000-3000 ppb phosphates in my pool and 300-500 ppb phosphates in my fill water yet keep my 16,000 gallon pool (shown here) free of algae using chlorinating liquid alone plus a small amount of acid every month or two. That's it -- no phosphate removers, no algicides, no clarifiers, no shocking. $15 per month due to my relatively low 1 ppm FC per day chlorine usage because of the opaque pool cover (the pool is used 1-2 hours every day and more on weekends).

So first thing you should do is get one of the good test kits I recommended. That way you will know your true water chemistry levels. I suspect the CYA level is higher than 50 ppm. If it is, you will probably need to do a partial drain/refill to lower it. After that, you can go through shocking the pool again using either chlorine alone at elevated levels or using the Yellow Out product -- same procedure putting everything into the pool (don't forget to wash your swimsuits as well). After that, you would maintain an FC that was no lower than 7.5% of the CYA level. You could also consider adding 50 ppm Borates in your pool as "insurance" in case you accidentally let the FC drop too low for a short time. You would use primarily chlorinating liquid or 6% unscented bleach as your primary source of chlorine, though would need to add it every day or two unless you had a pool cover in which case you might be able to add it twice a week (that's what I do). Note that when adding concentrated chemicals to the pool, including chlorine, that you want to do so by slowly pouring it over a return flow (at the deep end, in pools with a deep end) and then for extra safety in a vinyl pool you can lightly brush the side and bottom where you add the chemical to thoroughly mix it.

Read the Pool School for more info.

Richard

[mntn-biker]

Is it possible to test the residue/sediment to see if it is mustard/brown algae? Was thinking if I could carefully scoop some into a vial and cap it, maybe I could bring it in someplace to test. FYI - I just vacuumed the pool last night to waste and backwashed right after vacuuming. 18 hours later, the brown is showing up again in the same spots in the middle of the pool and various other locations shady and sunny spots. It has been cloudy, overcast, and rainy since I vacuumed it last night around 6pm.

[chem geek]

Yes, if you can scoop some up carefully, possibly with a turkey baster, then you can do one or more of three things: 1) mostly dry some out and feel it -- if it's gritty, it's dirt; if it's squishy it might be pollen; if it's slimy it's probably algae, 2) put some in a jar with water and see if it grows; if it does then it's algae, if not then it might not be algae, 3) put some on a slide and look at it under a microscope -- dirt is oblong and solid, pollen is usually round sometimes spiky and is solid, algae is sometimes round and sometimes rectangular or filamented but is transluscent.

If it is yellow/mustard algae, then it's like a very fine dust that disappears when you sweep it (i.e. it seems as if it dissolves in water). With dirt, it will resettle rather quickly compared to yellow/mustard algae.