Reading an old thread from Chem Geek

gregsfc

Bronze Supporter
May 27, 2014
132
Cookeville,TN
I was researching my mustard algae problem that's hopefully gone for good by completing mustard algae SLAM and noticed that ChemGeek was discussing a recommended FC level for mustard algae on an old thread; not a shock level, but a regular maintenance level FC level. It was said to be 15% of the cya level. This was way back in 2007. Does anyone remember this concept or technique? Does anyone know if any of that sort of recommendation is still a valid technique for those who have or had seen mustard algae in one's pool? If so, for how long? Pre SLAM, post SLAM or from hence ever more? Thanks for being here for all of us!
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,058
Tucson, AZ
Usually with mustard algae it is advisable to use a higher FC/CYA ratio as a daily target due to the nature of how that algae grows and it’s ability to neutralize chlorine by producing and excreting beta carotene. So if you have mustard algae and you defeat it, you need to use a higher FC/CYA ratio. You must maintain that higher ratio post SLAM. There’s no hard and fast rule for how long but you can slowly reduce it a week at a time and see if/when the mustard algae returns.
 
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red-beard

Gold Supporter
May 27, 2019
222
Houston, TX
I've had a lot of issues with mustard algae over the years. It has been all about the extremely high CYA in my pool. I was using the stupid trichlor pucks as my company suggested. I drained my pool last year, but over ran the CYA level (70). I drained again this spring and was extremely careful to get to only 30 ppm CYA.

This year, I've been running my FC up to the top of the recommended level every day. I sat down with the charts from the CYA and found a suggested higher HOCL.

The HOCL line for bacteria is about 0.011 ppm. For Algae, it is 0.050 ppm. For Mustard, the number recommended was 0.070. With 30 CYA, this is a minimum 4.3 FC. For 40 CYA, this is abut 5.7! This is one of the reasons I'm running only a 30 CYA instead of 40. And about 7.2 for 50 ppm CYA!

I lose about 2.5-3 ppm per day in my pool. I was running the FC up to 8 to keep it above 5. I have switched to raising it to 6 PPM FC each evening. Next morning, it is still 6 FC. It degrades to 3-3.5 FC during the day and then I raise it back to 6 PPM. I have not had a mustard outbreak since I switched to this. I think part of the reason is the pool stays above the mustard killing level for at least 18 hours per day, if not more.

I couple more things, I am running my VS pump at low speed 24 hours per day (about 20 hours at 1050 RPM/40 GPM). I am also now running about 27 PPM Borates, which will be raised as soon as my boric acid arrives. Using CalHypo 73% for chlorine, to also raise my CH level (320 ppm). As soon as I reach 350, I'll switch to liquid bleach 10%/12%.
 
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mgtfp

Bronze Supporter
Mar 5, 2020
276
Melbourne, Australia
I like reading those old ChemGeek threads, good information in there.

Nice field report, red-beard. Curious if you'll be able to move back towards the 0.05 ppm target over time. Will you try, or are you happy to just keep the 0.07ppm target?
 

red-beard

Gold Supporter
May 27, 2019
222
Houston, TX
Well, I started by adding enough to take my FC up to 8 each night. Now I'm just going to the top of the range (about 6), and letting it drop throughout the day to 3-3.5. This is definitely working. I can try to take it a bit lower and see if it blooms. I think fundamentally, the lower CYA allows me to keep it in check, without higher FC numbers. SOMEPLACE I read that even 10-15 ppm stabilizer makes a huge difference. And you can see it in the charts, the half-life of FC at 20 is around 5 hours and 30 only improves it to 6. At 20 CYA 0.07 is FC 3.0. 30 you need 4.3. But 40 you're at 5.8!!!

I think the highest FC numbers overnight help as well, since it is at maximum for at least 12 hours and above 4.5 for at least 18 hours per day.

Sometime this summer, I'll try taking it lower. But it is working for now.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,058
Tucson, AZ
You need to understand that those UV extinction charts are not “Gospel truth” ... they simply show the variation of chlorine loss (from OCl- photolysis into oxygen and chloride) with CYA concentration. There are other UV absorption mechanism at play with higher levels of CYA and depth/shielding effects that have been demonstrated (but not theoretically modeled) that produce an “anomalous” component to the UV loss that is not shown in that data.

For instance, in my own pool, I absolutely can not go below 70ppm CYA without seeing far greater daily loses than what is accounted for by that graph. As long as I keep my CYA above 70ppm, my FC losses will be around 1 to 2ppm per day. Below 70ppm and my loss rates go way up. This effect of high CYA driving down loss rates is often used in the pool service industry as a method of maintaining pools - chlorine gas injectors will increase the CYA to 100-120ppm level and then inject enough chlorine gas to get the FC up to 14–16ppm. That will give them about 14 days worth of FC before they come back again.

TFP often recommend that in high UV climates like the south that a pool owner uses a higher level of CYA to get better FC retention. This is why the range is 30-60ppm - these numbers reflect very manageable FC levels to use throughout the day and the FC loss does improve with higher CYA beyond what that charts predicts.
 

mgtfp

Bronze Supporter
Mar 5, 2020
276
Melbourne, Australia
Guess that shows two things: Mustard algae needs perseverance, and each pool is different and you have to find what works for yours.

Good luck, red-beard. Will be interesting to see what happens over winter. Do you keep your pool open all year, or do you close it in winter?
 

red-beard

Gold Supporter
May 27, 2019
222
Houston, TX
All very good, but I can't control mustard algae in my pool at 70 CYA.
Guess that shows two things: Mustard algae needs perseverance, and each pool is different and you have to find what works for yours.

Good luck, red-beard. Will be interesting to see what happens over winter. Do you keep your pool open all year, or do you close it in winter?
Open all year. I stop swimming in early November and start again late March/April.
 

mgtfp

Bronze Supporter
Mar 5, 2020
276
Melbourne, Australia
All very good, but I can't control mustard algae in my pool at 70 CYA
Exactly. That's what I was thinking of when I said that everyone needs to find out what's best in their pool. And the perseverance stretches well beyond the slam, maintaining higher FC at least for a while after that, as you are doing :)