Sanitizer decay rates

dlleno

Well-known member
Sep 9, 2016
60
denver, CO
#1
I'd like to start a discussion about sanitizer decay rates. I'm getting 1.5pp. Per 24 hours. What are your numbers? Note that my spa has an ozone generator so my decay rate should be worse than normal .

My observation is that measuring decay rate as a percentage isn't very useful because the actual decay rate is highly linear, at least in my findings. In fact, I find it useless. I have attached my decay curve , which is taken from a covered spa , just purged with ahhsome (after 1 week to stabilize)

When I figure out how to attach an image You will see that the decay as a percentage depends on the nominal chlorine level , and that the more useful number is ppm per 24 hours. At 4ppm my decay rate is about 37% but at 1.6 ppm nominal chlorine my decay rate is 100 %. So I no longer care about decay rate as a percentage unless the nominal ppm level is specified. I care about it ecpressed as ppm per 24 hours

I expect those without ozone should get measurably better results than this. Otherwise , something is chowing down on your free chlorine!

(Fyi x axis is hours )
 

Attachments

JamesW

TFP Expert
Mar 2, 2011
12,819
#2
If chlorine is decaying from ozone, then the rate is proportional to the amount of ozone introduced. So, that's a ppm thing and not a percentage.

If chlorine decay is from bather waste, that's also proportional to introduced amounts.

Sunlight seems to work more on a percentage basis. Assuming a constant UV exposure, that would imply that only some uv makes contact with chlorine and some doesn't.

For ozone or bather waste, the ozone or bather waste all react. So, it's ppm.

I think that the uv is mostly a percentage but it might be more complicated than that.

There are two types of ozone generators. One uses uv light to generate ozone and the other uses corona discharge.

UV generators make very little ozone. Corona discharge makes more.

Your ozone unit should have an output rating in grams per hour. What is its output rating?

Not all ozone dissolves in the water, some outgasses.

Unless the spa will be very heavily used, ozone will increase the amount of chlorine needed, because ozone reacts with chlorine to form chloride (77 %) and chlorate (23 %).

(123)O3 + (100)OCl- --> (200)O2 + (77)Cl- + (23)ClO3-

Here's the reaction. So, you can convert grams of ozone to moles and then calculate the loss in ppm chlorine.

1.23 moles react with 1 mole chlorine. Use 70.9 grams per mole for chlorine because chlorine is measured as ppm Cl2

For example, 59 grams of ozone is 1.23 moles, which reacts with 1 mole of chlorine, which is 70.9 grams, which is 47 ppm in 400 gallons.

Every gram of dissolved ozone reacts with 1.2 grams of chlorine.
 

dlleno

Well-known member
Sep 9, 2016
60
denver, CO
#3
Sorry I should have clarified. The decay measurement is only meaningful under no load and eliminating all possible variables, so the measurements I took were spa covered and no bather load. I mean zero as in the ew water has never touched a body. I don't know what my Corona discharge ozone generator rating is, but my interest is in simply accommodating this as the dominant variable. It would be fascinating to know the expected consumption rate with my ozone generator so I will try to find out what its rating is!

Meanwhile, given the above what ozone output would be consistent with this decay performance? What do you think it is?
 

JamesW

TFP Expert
Mar 2, 2011
12,819
#4
3.5 ppm chlorine is 5.3 grams of chlorine gas in 400 gallons of water. That's 4.2 grams ozone reacting in 56 hours or 0.075 grams per hour.
 

dlleno

Well-known member
Sep 9, 2016
60
denver, CO
#5
Nice! Does that sound like a reasonable output from a hot springs grandee 500 gal spa with Corona discharge? How much bromine production can one expect from such ozone output?

Clarifying question; are you saying that the natural CL decay under true no load conditions in a covered spa (no ozone) is linear ppm per unit time? or will it asymtotically approach zero?
 

JamesW

TFP Expert
Mar 2, 2011
12,819
#6
75 mg/hr is in the range of spa ozonators.

Check to see what model of ozonator you have. See if the output is specified in the manual or written on the ozonator.

20 to 500 milligrams per hour ozone output is a typical range for a spa ozonator. UV generators tend to be at the lower range. CD (Corona Discharge) is typically used for higher output.

Chlorine can break down by disproportionating into chloride and chlorate
3OCl- = 2Cl- + ClO3-
or by forming chloride and oxygen.
2OCl- = 2Cl- + O2.

I'm not sure what the bromine reaction is. Ozone will convert bromide to bromine, but it might also have a similar reaction to bromine where it results in bromide and bromate.

Ozone can break down or outgas. So, the total number of reactions that can happen with ozone make it difficult to calculate the effect.
 

Certified106

Well-known member
Sep 10, 2018
86
Athens, Ohio
#7
Sorry I should have clarified. The decay measurement is only meaningful under no load and eliminating all possible variables, so the measurements I took were spa covered and no bather load. I mean zero as in the ew water has never touched a body. I don't know what my Corona discharge ozone generator rating is, but my interest is in simply accommodating this as the dominant variable. It would be fascinating to know the expected consumption rate with my ozone generator so I will try to find out what its rating is!

Meanwhile, given the above what ozone output would be consistent with this decay performance? What do you think it is?
I Probably shouldn't check mine then since I have been using twice a day with two people lol
 

wogster

In The Industry
Apr 30, 2018
33
Toronto, Ontario, Canada
#8
I wonder if there are too many factors to know for sure, every spa is different, even when they are the same make and model of spa, installed the same day, by the same crew, because the water level may be different, meaning the amount of water is different, testing may be different, temperature and UV exposure may be different, CH and CYA levels may be different, bather loads will be different, all affect sanitizer decay rates. pH can affect sanitizer efficiency, and TA affects pH.....
 

dlleno

Well-known member
Sep 9, 2016
60
denver, CO
#9
Ok nice info guys I think i have characterized my spa quite well with no load and what I have established is about 1.5ppm decay rate per 24 hours. It appears safe to say the decay is dominated by ozone. What this also suggests is that a similar decay rate without ozone in the picture would suggest an external contaminant such as critters or dust or biofilms


Its time to convert to bromine! Fyi I normally run bromine and have noticed in the past that the decay rate is not linear and the ozone is capable of maintaining some measurable level for several days. but I've never chsracterized it under new water conditions. I'm a big fan of bromine with ozone because the decay rate is so low that vacations become easy to manage.
 

dlleno

Well-known member
Sep 9, 2016
60
denver, CO
#10
Related to this. What is the influence of low cya in this examination? I'm using straight dichlor granules in new water so cya accumulation is very low . I'm raising this because I disconnected my ozone generator and my first data point atcv hour 14 shows the same decay rate. Something is going on that I don't understand. Brainstorming suggests;

Low cya (new water no bathers yet)
Ozone wasnt that strong in the first place
Residual ahhsome (week 2 after purge)
An unknown contaminant (after 2 purges)

Thoughts?