Small Algae Outbreak FC dropped to 0 - Course of action?

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D

Docker_rob

0
Feb 7, 2015
43
Perth - Australia
Hi Folks,

Hoping to get some advise on a course of action for a small Algae outbreak. First a background so you can hopefully point me in the right direction on what to do.

I have been testing my pool regularly with my new TFTestKit as we head into summer. I had to get my CYA levels up a while ago and in the process as that got sorted my FC shot up. On the 18th October FC was 16.0. To let that settle down I turned my SWG down to 1 and kept an eye on it. Was slowly dropping a few points per week and by the 8th November I was at 9.5 FC level. 1 week later though 15th November and I checked the pool to see it was looking murky and a brush showed algae was starting to take hold. Ran a test and FC reading was 0!

Anyway took to the pool, gave it a good vacuum and brush. I added Chlorine as per the Pool Calc. To aim to get back to 5ppm. Let the pump run for 24 hours and within a day it was visually looking Ok.

So one week on my FC levels are OK but Algae is visually still around in spots (Corners and areas I see it starting to take hold) So below are my readings from the last couple of weeks so you can see what has happened.

8th November
FC 9.5
ph 7.6
TA 110
CYA 60


16th November
FC 0
Ph 7.5
TA 110
CYA 75

(Added Chlorine as per Pool Calc to get towards 5ppm) Tested 24 hours later and was reading approx 6ppm (Daily Test not drop test)

20th November
FC 3.5
Ph 7.8
TA 110
CYA 75

I also tested for CC today which gave a reading of 2.5. Visually the pool is not sparkling clear and I can see green algae in corners / steps in areas.

I know I need some acid but Before I go any further with Chlorine, can I get some advice on what I need to do? Do I need to shock the pool to knock this Algae on the head? Although I have all the above readings its still visible (not like a green swamp but there's something lurking)

Thanks all
 
You need the Shock Level And Maintain Process. Keep the chlorine at shock level and brush until your arms and shoulders are screaming in agony and kill it all for keeps. You're just at the start of your season, so kill it now and keep it dead so you won;t see green again.

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Thanks Richard

I've just added liquid Chlorine about 20% less than the Pool Math Calculator has shown based on my CYA level. I was worried my Bleach % may be higher than the Calculator assumes so figure easier to bring it up slowly. Which leads me to my next question

How soon after adding Liquid Chlorine can I retest the FC levels? Does it need a certain amount of time to mix in properly?

Cheers
 
The SWG is on and the pump has been running for last 24 hours (and still is)

I just took a FC Reading and overshot my mark a bit. Was aiming around 30 ppm FC and after 90 minutes post adding chlorine I am at 40 ppm (Hope that wont cause me other problems?)

In a short time the Pool already looks noticeably better - water is "sparkling" actually. I also tested for CC when taking this reading and that is at 0 were it was 2.5 this morning. I think I caught this potential problem pretty early so hopefully it gets knocked out just as quickly. I can still see a few nastys hiding behind my Pool light so will pull that out and give a scrub but otherwise it looks great.

Based on the Pool School Slam Guide is my only real next test to pass the Overnight FC test?

You are done when:​

CC is 0.5 or lower; - Checked at 0
You pass an OCLT (ie overnight FC loss test shows a loss of 1.0 ppm or less); Yet to be Tested
And the water is clear. Check​
-
 
You MUST have the SWG off to perform an overnight test.

The SWG will produce FC overnight which will skew your results. Your last FC test of the evening needs to be about an hour after you turn off the SWG.

It is unlikely that one shot at slam levels will take care of visible algae, but good luck to you!
 
Yes understand SWG must be off overnight. I will shut it down tonight and test FC levels again and first thing in the morning. See how we go.

Just had a swim and was a bit worried about FC being so high but didn't really notice it too much.

Thanks everyone for the advice, its been invaluable.
 
Pleased to report the pool passed the OCLT test so looks like I am done.

FC sits at 30ppm so I've left the SWG off and I will monitor over coming days as the FC levels drop.

I am curious though being this is the first time I have ever SLAM'd my pool and being I have a relatively small pool. On a busy weekend with lots of kids in the pool I suspect my FC levels may drop a lot more readily than a larger pool just based on the water volume and high use with the neighborhood kids in and out all day.
(Less water but just us much dirt / skin / sunscreen oils etc as a pool twice its size, makes me think overall less FC to do its job)

I'll be honest I don't check FC every day but I'm thinking maybe I should be more vigilant especially over the weekends and if I see any sign of FC dropping add a cup or 2 of Liquid Chlorine, just so this type of thing doesn't happen again.

Would that be a sensible approach?

Or should I consider running at a slightly higher FC level?
 
That was a quick slam. With visible algae, it usually takes a bit longer. You may want to run a second OCLT just to be sure.

When you say a higher FC, higher than what?

I always raise my FC before a large bather-load, usually about 50% more. Eg. If target FC is 5, I will raise FC to 7.5-8.5
 

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If you allow lots of other people to use your pool then you should consider the bather load to be high. A rough rule of thumb is that it takes 0.1ppm FC to oxidize one person-hour of bather waste in a 10,000 gallon pool. That's just oxidation of bather waste and not pathogen control as well as sunlight degradation of FC. You can search the pool chemistry thread that Chem Geek wrote to figure out what your expected sunlight loss of FC would be.

So, if you add that all together, you should be able to come up with a reasonable expected loss for FC. If I were you and I were expecting the kids and friends to use the pool, I'd bump up the FC using plain old bleach by a few ppm just to add a little margin. The kids can safely swim in water that is chlorinated all the way up to shock level for your CYA but I think you'd only need to add a few extra ppm of FC to give you a good safety margin. You NEVER want your FC to fall below the minimum value in the FC/CYA Chart in pool school as that represents the point at which algae and pathogens can grow faster than the chlorine can kill it. Once algae growth rates exceed the chlorine kill rates, your SWG will not be able to keep up with the demand and you'll be back to a green swamp again.
 
Matt has hit the nail on the head, help the SWG out before an expected high bather load with a little liquid chlorine. I would say it's worse with kids, but adults after a few adult beverages can have accidents.......
 
Cheers Matt, I have around a 5000 Gallon pool and for example yesterday there were 9 kids in and out of the pool. I'd say they spent about 1.5 hours each in the water. It was a similar load on the day before but during the week it wasn't used at all as they are all at School.

So going back 2 weekends ago, pool was perfectly clear then a hot weekend lots of sun and high bather load and the week after is when the pool started to lose its sparkle and I got a few nasties growing. It didn't get to the point of a green swamp but I was seeing the start of something nasty coming. Hence the SLAM this weekend to knock it on the head before it got worse.

I've had this same issue over the last 3 summers since I first got the pool, As I was learning the ropes I didn't really know what was going on, but as I read and ask more questions it's sort of making sense.

The Chlorine / CYA chart says with a CYA of 70 I should target minimum FC of 5ppm

But based on above and if I read your comments correctly, rather than each weekend adding a cup or 2 of chlorine it may be acceptable to target a FC level of say 7 or 8 ppm and just ensure it doesn't drop below 5ppm based on my current CYA level. Then post weekend check the FC levels and if they are down below 5ppm then add a bit of chlorine to help the SWG out and stay on top of things.

Does that sound like a feasible plan?

From what I gather the effect of the higher FC might mean bathers wear out a little quicker is all, and maybe my Vinyl Liner might fade a bit quicker also?
 
It is doubtful the levels of chlorine you are talking about will fade the liner. Generally liners are safe up to and maybe even a little above shock level. Some have insisted that this may not be true for dark blue liners, but I have never read comments from anyone that has had problems themselves. It's always the unidentified "they".
 
Yes, 5ppm is the MINIMUM value that you should not go below. Therefore I would target the SWG output and pump run times to get your pool water to closer 7-8ppm.

The higher FC should have little affect on either your bathing suits or the liner. What causes the degradation of those things is the amount of active chlorine (hypochlorous acid) in the water and, at 70ppm CYA, the active chlorine level is high enough to kill pathogens and oxidize organic waste but not high enough to bleach a liner. Most of the dye colors in the liner are quite resistant to oxidation with exception of some of the blue dyes. But even those require high FC levels for extended periods of time to show any bleaching.

Since you have a vinyl pool, the best way to add chlorine is by pouring it slowly in front of a fast moving return (true the pump on high if it's a 2-speed pump). You can even use your pool brush after adding the chlorine to help agitate the water a bit.
 
Personally I think the liquid chlorine idea is better. The SWG adds chlorine relatively slowly compared to the relatively instant hit you get when you add liquid chlorine. Then by running the SWG when the kids are swimming after adding the chlorine, I feel better that my FC is holding up.

If you haven't got a solar cover I would highly recommend it for the Aussie summers. It significantly reduces evaporation and FC loss during the day but also keeps the water temp a perfect temperature.
 
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