Combined Chlorine.

[robquick]

Hi,

After a recent water change I changed to the Dichlor/bleach method and have since had real success with exceptionally clean looking water however, I'm really struggling to obtain an acceptable CC level. I had the same issue before when I converted to the Dichlor/bleach method half way through a water cycle, I put this down to the high CYA (90 ppm) at the time I converted.

I can't understand what I'm doing wrong as the fresh water was 0.20ppm CC and since then its slowly risen, after about four weeks it now sits at 1.75ppm CC. I'm *** with checking the levels and I never let the water go under 1ppm FC, the tub is used for about 3 bather hours per week.

The levels are as follows.

FC - 3ppm

CC - 1.75ppm

TA - 50ppm

CYA - 35ppm

PH - 7.54ppm

CH - 110ppm

Any help would be greatly appreciated.

Thanks.

[robquick]

Sorry ....... and 50ppm Borates.

[waterbear]

Do you have ozone? Do you shock with MPS? How are you testing?

[robquick]

Yes, I have Ozone.

I don't shock with MPS as my belief was that there was no need with the Dichlor/bleach method provided you always maintained good FC levels however, I did use MPS earlier today to see if it made a difference, I haven't re-tested yet.

I'm testing with a Lovibond professional Photometer.

Thanks.

[waterbear]

Lovibond uses DPD to test chlorine (if I am not mistaken) and ozone WILL test as CC with DPD testing if there is any residual in the water (There is not supposed to be but with the way many ozone systems are installed there is.) MPS will also test as CC.

[robquick]

Waterbear,

Thanks for the reply.

Are you saying that this is a short fall of DPD testing, some Ozone systems or both ?

Is there a testing method that doesn't suffer the same problems when measuring CC ?

What's the problem with how some Ozone systems are installed ?

If I was to disable the Ozone and not use MPS for a week or so are you suggesting my CC would measure considerably lower ? My water is crystal clear and smell free which suggests your theory is correct (ie something is generating inaccurate CC readings).

Apologies for the mass of questions however, I'm generally interested in understanding what's happening in my tub !!

Once again, thanks for your reply.

[chem geek]

Actually, I thought that ozone shows up as FC since it directly reacts with DPD similar to bromine and chlorine dioxide -- it's MPS that doesn't react directly but will oxidize iodide to iodine so shows up as CC. Also, normally the ozone level even with a residual in the water is on the order of 0.05 or 0.1 ppm, not usually high enough to register as CC. A lot of people have ozonators and don't have this CC problem -- then again, maybe this ozonator is on steroids.

Rob, if you are able to turn down or disconnect the ozonator and if that resolves the problem, let us know so at least I can be better informed about this (waterbear would be ahead of me on this; not the first time).

[waterbear]

Ozone is tested in the same was as CC if you check out Taylor's instructions for their ozone test! As you can see DPD #1, DPD #2, and DPD #3 are all added to the sample, just as if you were simply testing for CC.

[waterbear]

Waterbear,

Thanks for the reply.

Are you saying that this is a short fall of DPD testing, some Ozone systems or both ?

DPD testing.

Is there a testing method that doesn't suffer the same problems when measuring CC ?

Not really for a simple, readily available test. (at least not one I am aware of)

What's the problem with how some Ozone systems are installed ?

Where do I begin . Here is the very condensed version. Ozone is an irritating and toxic gas and there is supposed to be NO residual in the body of water in the tub.Ideally it should be added in a reaction chamber and the water returned to the tub should have no ozone present. However this adds to the cost so it is not often implemented. ozone is often just bubbled into the main body of water in the tub. If the tub is covered this can cause a buildup of ozone gas under the cover which can damage the cover and would not be good to inhale if you did not let the tub 'air out' a bit before going in.

If I was to disable the Ozone and not use MPS for a week or so are you suggesting my CC would measure considerably lower ?

IF the CC is from interference from Ozone or MPS, yes. If you really do have a CC problem then no. Realize that many tubs will always have CC because of the high bather to water ratio. However, ideally this should not be higher than .5 ppm.

My water is crystal clear and smell free which suggests your theory is correct (ie something is generating inaccurate CC readings).

Apologies for the mass of questions however, I'm generally interested in understanding what's happening in my tub !!

No apologies needed. This is how we all learn! One day you will be able to pass the knowlege on to someone else.

Once again, thanks for your reply.

[chem geek]

OK, I was going from Current Technology of Chlorine Analysis for Water and Wastewater where they wrote the following:

In general, all of the common chlorine methods will

detect other oxidants used as disinfectants — such as

chlorine dioxide (ClO2), ozone (O3), bromine (Br2),

hydrogen peroxide (H2O2), and disinfectant by-products

such as chlorite and chlorate — if present in large

amounts. In the free chlorine determinations, these

oxidants, in sufficient concentration, can react directly

with the colorimetric indicator or will be reduced with

thiosulfate or PAO in the titration method. Each of these

oxidants will oxidize iodide to iodine to a certain

degree, thereby interfering in the total chlorine

determination.

So it looks like ozone might show up some as FC if it's high enough and that it definitely shows up as total chlorine which is why R-0003 is used in the Taylor test. Good to know. Thanks.

[waterbear]

It also affects ORP readings

[robquick]

If I was to disable the Ozone how long would it take before I could take an accurate CC reading ?

Thanks.

[waterbear]

It really depends on the current ozone level, the amount of organics in the water, and the amount of aeration to help it gas off. However, I would certainly think a week should do it, particularly if you keep the tub uncovered so any residual can gas off.

[robquick]

Both,

I'm going away on holiday for 10 days as of next Tuesday, my intention is to turn the tub off and then dose with a high level of Chlorine and leave until I return. I'm guessing this would be an ideal time to see what impact having the Ozone switched off has on CC?

If its not the Ozone impacting my CC levels what would you try next ?

On a separate note what level would you dose the FC to prior to my 10 day holiday to ensure I maintain an acceptable FC level for the duration of my holiday ?

I will update you on the Ozone/CC levels when I return.

Thank again.

[chem geek]

If you let the water temperature cool down and you have the spa covered so that the chlorine is not in sunlight and with the ozonator off, then figure a 15% chlorine loss per day to be safe. (0.85)^10 = 0.197 so around 20%. If you shock to 10 ppm FC, you may have 2 ppm FC when you return. That's probably OK, but you could shock to 15 ppm FC for a little more margin in case the loss is greater than 15%. Note that a 20% loss per day would result after 10 days in around 11% while a 25% loss per day would result in around 6%.

[robquick]

Hi both,

Prior to my holiday I switched off the tub and after it had cooled I raised FC to 15ppm. Having returned 11 days later my FC was reading 'Hi' on my photometer indicating the FC was greater than 6ppm which I was pleased with. A target of 10-12ppm would have possibly been better for next time however, either way the water is safe which was my main objective.

With respect to my CC I took a water sample when I returned, once the FC has dropped to a lower level I will be able to get and accurate CC reading which should then indicate the impact that the Ozone has been having on the CC levels (water sample was with Ozone off for 11 days).

I will update you over the next few days as requested.

Cheers.

[chem geek]

So that was only an 8% FC loss per day. That's really good and probably because the temperature dropped fairly low (80ºF or lower perhaps?). I guess I was too conservative, but I wanted to make sure you didn't run into any problems. Looking forward to the additional reports.