black algae treatment

[newbbber]

Hello all.

I'm a recent convert to the BBB method, having lost far too much money to the pool store. Water was finally replaced in March due to very high CYA. I think I have my water reasonably well balanced now but I think I have some black algae left over from before the refill. It's scattered around the pool in very small black spots that turn dark green if I scratch them.

I have been maintaining FC at about 8ppm and brushing periodically, however the plaster is really old and seems to be scratching off a little as I brush it (Stainless steel brush) so I don't want to brush too hard. Also the plaster is really rough so i think the algae can hide from the brush.

Is there anything else I can do to help kill it besides brushing and maintaining FC?

I have various chemicals left over from the previous owner and my pool store days (60% polyquat, black algaecide, copper algaecide, yellow out, phos free) but after reading TFP and pool school have been reluctant to use any of them.

So, since I have them anyway, would any of them help with the black algae or should I stick to bleach/brushing only? Is the black algaecide just a more concentrated version of copper algaecide or does it have something special that would help here, and if I use it what problems can it cause to my water balance, if any?

FC 8.0
CC <0.5
PH 7.7
CYA 60 or 70
TA 100
CH 330

Daily FC drop is around 3 or 4 ppm. I am using the Taylor k2006 kit

Thanks!

 

[msgtdan]

Welcome

Perform the SLAM process, for your CYA level of 70 you need to bring your chlorine level up to 28 and keep it there until the algae is gone. Brushing will help let the chlorine get to it to kill it, nylon bristles are fine for that no need to damage your plaster

 

[JasonLion]

Rubbing each spot carefully with a trichlor tablet often works, though that is really only practical if there isn't that much of it.

 

[Suziqzer]

I would avoid the algaecides as many introduce metals into your water that cause staining.

Read through the SLAM process and get started there. That is the way to go, without adding unknowns to your pool.

 

[newbbber]

Ok, thanks for the quick response. I will probably try the trichlor tablet first... The black spots are scattered but there's not too many of them so I think it's manageable. I ran out of the R0870 powder this last weekend so won't be able to do the OCLT/SLAM until the coming weekend.

I'll hold off on the algaecide, and maybe save them in case I have to leave town for a few days.

 

[newbbber]

An update...

I tried rubbing the black algae spots/specs with the tablet and it worked reasonably well. It's almost gone but it'll need a little more work, on closer inspection there were more spots than I thought(or it spread) and with all the pitting in my plaster they have lots of places to hide.

I finally got around to running the OCLT once my DPD powder and cya reagent refills arrived. I had an overnight loss of 1.5ppm FC, so I guess not terrible but something is obviously there. I also redid the cya test which I had doubts about. New cya estimate is 75ppm, a little higher than I'd previously thought, so I guess I'll be keeping my FC a little higher than before.

Then, after reading other posts on this forum I decided to look inside the light fixture, and found big gobs of green/white algae which quickly spilled out into the deep end. Mental note for next time: have suction hose in hand before removing light.

Needless to say, I have started the SLAM process.

Question about shock level: pool calculator suggested 21fc for my cya level, but the pool school chart suggested something like 30. I have read that both are fine but the higher level will obviously be quicker. My question is, which is more efficient in terms of total chlorine usage, considering the fact that higher FC levels will have higher UV breakdown and I'm in sunny Arizona, but higher FC will also work faster?

Thanks!

 

[duraleigh]

Go with 30 ppm. Just thinking of that calculation makes my head hurt. :lol:

 

[newbbber]

Okay, different topic but I will keep it with my original post.

I have recently been noticing that my pH has been dropping faster now than when I first switched from pucks to bleach (4 or 5 months ago), but I can't figure out why. I'm trying to minimize chemical usage(cost) so wondering if I can do anything about it. My current chemical levels are:

FC - 8.5
CC - 0
pH - 7.3 (after adding 100oz of 14.5% muriatic yesterday)
TA - 90
CH - 370
CYA - 55 or 60

My TA has historically always been about 100 or 110, I'm guessing the current measurement is due to my very recent acid addition? I have to add a little over half a gallon of muriatic a week now to keep the pH in check. Since adding acid is supposed to lower both TA and pH, shouldn't my TA have gone down more by now (with weekly acid?)? What would be keeping the TA up?

My CH started at about 200 when I last filled/drained in March, and has been rising ever since due to evaporation. Can the higher CH cause pH to rise due to aeration?

My CYA used to be about a 100 and has been coming down slowly in the last 5 months or so with backwashing every couple of weeks. Does a lower CYA affect pH rise in any way?

I understand it's normal with BBB method for pH to rise slowly, but what causes this? From what I've read here, bleach should be pH "neutral" when accounting for the high initial pH cancelled by the lower pH when oxidized by organics or broken down by UV. My plaster is very old, but does this contribute?

I really shouldn't complain too much, I'm spending way less now than when I was being pool-stored, but with all this testing I do now, I'm becoming a pool-perfectionist!

 

[chem geek]

Your pH drops faster from acid addition now than before probably because your TA is now lower. Half a gallon of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in 18,000 gallons lowers the TA by 14 ppm.

With Trichlor, it is net acidic so will tend to make the pH drop, so you keep the TA level higher to have more carbon dioxide outgassing to help counteract that. Even so, the pH usually still tends to drop and the TA will slowly drop as well. You probably used pH Up (sodium carbonate) to restore the pH and TA. You say that your TA was historically 100-110, but when you were using Trichlor I would have expected the TA to possibly get higher unless you had a lot of carbon dioxide outgassing, say from aerating water features (waterfalls, spillovers, fountains, etc.).

With bleach, it is closer to pH neutral when accounting for chlorine usage/consumption, but there is some amount of "excess lye" in bleach and chlorinating liquid so this will tend to make the pH rise a little and adding acid will lower that back down. However, most of the pH rise is usually due to carbon dioxide because pools are intentionally over-carbonated in order to provide (ironically) some pH buffering and to protect plaster surfaces by saturating the water with calcium carbonate (the carbonate comes from the TA). You are correct that regular acid addition should slowly lower the TA if the pH is rising from carbon dioxide outgassing. However, evaporation and refill will raise the TA (and CH) from the fill water.

The rate of carbon dioxide outgassing, and therefore the rate of pH rise, if faster at higher TA and lower pH (see this chart to see how over-carbonated the water is with respect to air). Your TA of 90 ppm is on the higher side so you can see if things improve if you lower it to 70 ppm. Also, once your TA is lowered, you should not try and lower your pH below 7.5 and instead if the pH likes to settle around 7.7 or 7.8, leave it there. You may have difficulty keeping your TA low if you have a lot of evaporation and refill, especially if the water is high in TA. Also, if you have water aeration features you can turn off, try doing that and see if it makes a difference in the rate of pH rise.

 

[newbbber]

Thanks Chem Geek. I see now I made a critical typo... I had meant to say that my pH was *rising* faster than it used to, not dropping. But I think you answered my questions anyway. So the pH rise is due mainly to carbon dioxide out-gassing from the higher-than-suggested TA, rather than my chosen source of chlorine (bleach), though the bleach may have a small effect. So, I will try to lower my TA to your suggested levels to see if that helps. I think my fill water TA is around 110, so as you say that would tend to raise it or keep it high.

I don't have any real aeration features, however I recently noticed some very small bubbles in the return, which I narrowed down to a leaky suction-side valve. I replaced and refilled the broken grease caps on the valves, and now don't see any bubbles any more. There really weren't many bubbles, but I wonder if this was aerating at least a little.

Thanks!