Estimating Bleach Volumes.

[robquick]

Hi,

I've recently switched to bleach and I'm trying to understand volumes of bleach needed as a comparison to the volumes of Dichlor I used to use.

Between bathing 20ml of bleach daily ensures a FC level of roughly 4-2ppm over a 24 hour window which is fine however, I'm really struggling to estimate the volume of bleach needed after bathing.

What would be the correct way to calculate the volume of 14% bleach needed after 4 people bathed for 2 hours. The FC when entering the tub was 2ppm and I'm guessing after bathing the FC would have been zero ?

I've calculated my CD to be around 30-35% and I do have Ozone.

Thanks.

[chem geek]

The usual rule-of-thumb is 5 fluid ounces of 6% bleach per bather-hour so that would be 5*(6/14) = 2.14 fluid ounces of 14% chlorinating liquid or about 60 ml. However, since you have an ozonator, it will takes less than that -- probably not half because your daily chlorine demand isn't that high (not 50% or higher which a more powerful ozonator would do) so I'd guess you need around 40-50 ml per bather-hour.

So in theory, 4*2 = 8 bather-hours would be 320-400 ml, but there's no way you could sit in a tub for 2 hours at 104ºF and the rough rule-of-thumb formula assumes a hot tub. So you'll likely need to use less, but how much you'll have to figure out by experimentation. Yes, the 2 ppm FC you start with during the soak will likely become mostly monochloramine from quickly combining with the ammonia from your sweat (and urine). So why don't you try something like 200 ml and see if that's enough. The goal is to add whatever is needed so that you have a chlorine residual for the next time you dose. You just don't want the chlorine to get to zero for an extended length of time.

By the way, 20 ml of 14% chlorinating liquid in 1325 liters is 2.2 ppm FC so that sounds about right when you wrote 4-2 (is this 4 to 2 ppm FC?), but that would be a 50% chlorine demand if it dropped from 4 to 2 and you wrote 30-35% so I'm confused.

[robquick]

Chem Geek,

After a recent bathing session of 2 hours with 4 people I put 100ml of 14/15% bleach. 72 hours later the FC was 0.3ppm, clearly I'm still using far to much bleach. I'm happy to play around with the bleach levels to ensure I have a very low FC level 24hrs later however, will there still be enough bleach to prevent the need for using MPS for shocking ?

Thanks.

[chem geek]

Chlorine usually does a better job of oxidizing bather waste than MPS, so yes if you use sufficient amounts you shouldn't need to use anything else as an oxidizer.

You wrote 0.3 ppm, is that what you meant? I'll bet you meant 3 ppm FC after 72 hours, right? Then yes, you are using too much and can adjust downward. Your ozonator is probably more efficient so is probably handling more than half of the bather waste that chlorine would otherwise have to oxidize.

[robquick]

Chem Geek,

I meant to say 1.3ppm 72 hrs later!

I may be missing something however, I thought the idea was to get the FC at an acceptable bathing level 24 hours later? Mine was at 7.0ppm 24 hrs later, clearly too high ?

My point was if I reduced the volume of bleach initially to obtain a safe bathing level 24 hrs later would it still be enough to oxidise the water so I can stick with bleach only rather than having to use MPS too ?

Thanks.

[chem geek]

You aren't missing anything. You are correct that 7 ppm 24 hours later is too high, at least if you plan to soak that day. If you aren't soaking right away then having it be higher so that it more slowly goes down will let you add chlorine less frequently. That's up to you. But if you want to be able to soak the next day then 2 ppm FC would be a better target assuming you don't want to smell more chlorine/monochloramine during your soak.

Yes, if you lower the bleach amount to get to the target 2 ppm FC it should still be enough that you won't need to use MPS. This is most especially true when you have an ozonator since it should be able to handle oxidation of some of your bather waste so makes it less likely for the bleach to get behind even with heavier bather loads.