Unexpected Outcome Bleach vs. Algae

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TexasTwister

TexasTwister

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In The Industry
Feb 9, 2008
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Richmond, Texas
I have a customer with a pool FULL of algae.

Here are the specifics:

TC 0.0
FC 0.0
pH 7.5
TA 70
CH 500+ (tested twice)
CYA 0
6 year old plaster over gunite pool, 25,000 gallons

Because of the high CH number, I decided to try the bleach method of shocking the pool instead of Cal-Hypo. According to the pool calculator, I should add 518 ounces of 6% bleach. So I added 3 x 174 oz bottles of Walmart Ultra Bleach (6%) to the skimmer. This amount comes out to 522 oz. Close enough I thought. I also decided to add 10 ppm CYA to help the chlorine stay in the pool. For the next 3 1/2 hours I scrubbed all the steps, swimouts, spa, pool walls, pool floor etc. I also spent a lot of time fishing around the bottom for unseen organic debris. To give you an idea of water clarity, my pool brush disappears 12 inches under the surface. The filter pump ran the entire time.

At the end of the 3 1/2 hours, my TC was 0.5 ppm, not 10 ppm as anticipated. I tested this twice. Is this because my initial dosage was eaten up by the algae? Please tell me this is so :shock:

Before I left the house for the evening, I went to a nearby grocery store and bought 3 more bottles of Safeway regular bleach (6%) and added them to the skimmer. This came out to 546 oz, or a total of 1064 for the day.

Thanks for your replies. I'm kinda freaking out at this point! I can hear the Cal-Hypo calling to me from the garage!
 
Yes, algae does consume chlorine rather rapidly. It might well take many times as much chlorine as you have already used to clear out that intense of an algae outbreak. The key to shocking is to keep raising the FC level to shock level as frequently as possible. If you give the algae a break without adding more chlorine they just grow back to where they were before.

I would retest and add more chlorine as frequently as every half hour, you want to knock the algae right out of there without giving it a moment to bounce back. You might also want to add some more CYA. 30 ppm is the lowest I would ever aim for in an outdoor pool. Also, remember to keep an eye on the filter pressure and backwash/clean the filter as needed (which is a good reason not to add CYA directly to the skimmer as it sits in the filter and is partly lost if you backwash/clean the filter in the next several days).
 
Thanks Jason.

Actually, I mistyped earlier. Before leaving for the evening, I added 4 x 182 oz = 728 oz. of 6% bleach. I did this because I have to go to a funeral in the morning so I can't get back out there until after noon (I'm a one man operation). Hopefully this will hold it until I can get back out there. My wife suggested using a combination of bleach and Cal-Hypo. I wouldn't be adding as much calcium to an already bad situation, and it might provide the 1-2 punch that we need. What do you think?

With regard to the CYA, I got the 10 ppm number from other posts in this forum. I'll keep the 30 number in mind for later on after we backwash the filter, etc.
 
Chlorine is chlorine is chlorine, for killing algae it doesn't mater which form you use (other than the side effects of adding salt, CYA, calcium, and/or changing the PH). If you get sick of lugging bleach bottles around then there might be some motivation to use cal-hypo, but given your CH level it would be best to avoid it. This is a situation where you could use a controlled amount of dichlor to add chlorine and CYA at the same time, but that requires more attention to calculating amounts correctly for both chlorine and CYA target levels.

With a plaster pool you can go up to the mustard algae/high shock level with each chlorine application for the first few rounds. That will speed things up a bit. Assuming CYA is 10, high shock would be a FC of 12.

CYA around 10 is suggested for indoor pools. For outdoor pools without a SWG the recommended CYA level is 30-50.
 
soxlover said:
Granulated trichlor like AlgiBan work great for killing algae. Lugging all that liquid around is too much work for me. Even if trichlor is a little more expensive, it makes up for that in convenience factor.
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This is fine if the CYA level is low and you want to raise it. With Trichlor, for every 10 ppm FC that it adds, it also increases CYA by 6 ppm. If the CYA level is already where you want it or high, then adding a shock level of Trichlor will make things worse in the long run since the higher CYA level will need an even higher FC level to kill algae quickly and after the algae is gone one would need to maintain a higher FC level (because of the higher CYA level) to keep the algae from coming back.

If the Calcium Hardness (CH) level isn't too high, then one can use Cal-Hypo for shocking in which case for every 10 ppm FC Cal-Hypo also adds 7 ppm to CH. If one doesn't want to increase either CYA nor CH but wants a compact form of chlorine, then one can use Lithium Hypochlorite, but that is VERY expensive. Unfortunately, there is no compact (dense) form of chlorine that is both reasonably priced and does not add other chemicals with side effects (CYA or CH). So, the next best thing is to use the strongest chlorinating liquid, which is usually 12.5% from a local pool store which is relatively inexpensive and doesn't add to CYA or CH, but it is heavier to carry (since it's mostly water).

Richard
 
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