Chlorine Usage Increase.

[robquick]

Hi,

Ever since changing to Dichlor/bleach I use about 20ml of 14/15% bleach (between bathing) daily to maintain a good FC (2-4ppm). Over the last four days I've had to start using about 5x this quantity to even get a FC reading 24hrs later. Do you think this is purely down to a need to change my water ?

Water is clear.

Bleach is about 3 months old.

Water is nearly four months old.

CYA about 80ppm.

CC is high at about 2.0ppm and always has been.

TDS is 1900.

Filters 2months old and cleaned every two weeks.

Thanks.

[chem geek]

That's a huge change. Something is off. Either the chlorine isn't as strong as expected or there's something in the tub consuming a lot of chlorine. If you are going to change the water, I suggest using a product such as Spa System Flush or Ahh-Some to make sure you don't have biofilms or organic buildup. Note that the CYA of 80 ppm is higher than recommended, but it sounds like its been like that for quite some time. The higher CYA makes the active chlorine level lower so will oxidize organics more slowly, possibly the reason for the CC you have measured.

How much are you using the spa? In a 350 gallon spa, 4 months of Dichlor-only would work with up to around 20 person-minutes of soaking per day while with Dichlor-then-bleach one should be able to have at least double that bather load or 40 person-minutes per day.

[robquick]

Chemgeek,

We use the tub for about 3 hours equivilant bathing per week.

I've just changed back to Dichlor and I'm getting much better readings of FC which suggests the bleach is off I guess. Ive listed a couple of points below which I would be interested to hear your opinion on.

I store the bleach in a cabinet in the summerhouse which is in direct contact with the sun, over the last few days we've had good weather with much warmer temperatures. Could this have killed the bleach over such a short period (3 days) I read in another link you sent out earlier how damaging heat is to bleach?

What temperature is bleach best stored at?

What's the life expectancy of 14/15% bleach, I purchased the smallest volume I could (5 litres), would you expect it to be used up before it goes out of date ?

Is there a granular version of bleach that has the same characteristics?

Thanks.

[chem geek]

Yes, chlorinating liquid and bleach (both are sodium hypochlorite) are sensitive to breakdown from higher temperatures. Lower quality bleach that has some metals in it will break down faster. Higher concentration chlorinating liquid also breaks down faster. See the table at the bottom of this link for the half-life of high-quality chlorinating liquid.

The only granular equivalent to bleach in that it doesn't increase CYA like Trichlor or Dichlor and doesn't increase CH like Cal-Hypo is Lithium Hypochlorite, but that is very expensive. I suggest you just store the chlorinating liquid in a cooler environment, if possible -- room temperature (72ºF, 22ºC) would be fine. You could also dilute it 50/50 in clean (distilled or deionized) water as that will have it last longer, though you'll obviously have to add twice as much when you dose.

[robquick]

Could the strength of the bleach have been effected that quickly with the hot weather ?

[chem geek]

If in direct sunlight, then certainly, but if just hot from temperature, I would think it would take longer than a few days to lose that much strength.