Ozone and hot tubs - some basics. Part 2 - Long!

[Vermonter]

Continued from Part 1:

Ozone as a Sanitizer:

If the oxidative demand of the water in the tub or in the "contact chamber" is met, then, theoretically, ozone can function as a sanitizer.  However, simply "injecting" ozone will not make this happen.  To have ozone act as a sanitizer, you have to have ozone in the dissolved, or aqueous form, vs. the gaseous form that we all equate as "ozone" (i.e. the bubbles that you see and smell).  All hot tub CD ozonators use air as the makeup gas - the ratio of air to ozone coming out of the ozonator is typically well over 95% and well less than 5% ozone.  To get ozone into the water the best approach is to use a venture type of injector - such as the Mazzei (a brand) used in many hot tubs.  This makes use of a pressure drop within the injector to "suck" in the air/ozone gas from the ozonator and to then diffuse it into the water flowing through the infector in an effort to transfer the gas into the liquid.  Ideally, you want very, very small bubbles - so that for a given volume of gas you will have a maximum ratio of surface area of the bubbles to allow the ozone to enter the aqueous form.  To do this effectively requires a long contact chamber and sufficient time for the phase transition to occur.

As indicated, once the ozone / air bubble is produced, it has a very limited time to transfer the ozone to the water.  By the time you see bubbles rising in your tub, it is too late.  Anyone who says that you can put a spa blanket on the water and "hold" in the ozone is simply wrong (my opinion).

If you succeed in getting ozone into the dissolved or aqueous phase, it doesn't stay there long.  Ozone, in pure water, at a temp of 20'C (68'F) has a half-life of about 22 minutes (estimates vary a bit); meaning that after 22 minutes you have 50% of what you started with, after another 22 minutes you have half of that, etc.  Hot tub water is not "pure water" and it is not at 68'F.  The half-life of ozone in typical hot tub water is likely to be in the single minutes - perhaps even seconds (I have not seen studies on this, but comparable ones for other water types would indicate these numbers to be in the right ball park).

Heat is a big enemy of ozone half-life - so under the best of conditions (ozone generator, injector...) you are fighting an uphill battle in hot tubs.  In my HotSpring Grandee (2001), the ozone is "injected" on the heater line - meaning that, if the heater is on, the ozone is being injected into water that is about 120'F - not good at all for efficient introduction of ozone.  There are other problems associated with ozone - including such factors as pH and general water quality.

OK, but can ozone work as a sanitizer in a hot tub?  Sure, IF you can achieve a dissolved ozone concentration and if that can be maintained for a long enough time to meet the CT (Concentration x Time)  "kill" factor required for the target microbes.  Ozone is one of the most powerful sanitizers (actually, in this sense, disinfectant and ozone is second only to fluorine) known and is effective, given enough CT, against essentially all microorganisms.  BUT, to get a CT of above "0" you have to have both the C and the T above "0" or the product is "0" and you won't sanitize.  

In the past I have challenged anyone to provide defensible results showing that dissolved ozone concentrations are present in EITHER the contact chamber of, more importantly, the main hot tub water.  To do so, the method used must be an ozone-specific method.  Many of the kits sold and "used" are DPD based - and will pick up any type of oxidizer present (chlorine, oxygen, etc.).  There are ozone-specific methods such as indigo trisulfonate.  I have tried measuring ozone myself - in my hot tub, in my contact chamber, in a few other hot tubs and have been unable to get any measurable (my detection level is 0.03 ppm) levels of dissolved ozone.

If you don't have a dissolved ozone residual - you will not get any "kill".

Best,

Vermonter

[Chas]

Heat is a big enemy of ozone half-life ...In my HotSpring Grandee (2001), the ozone is "injected" on the heater line - meaning that, if the heater is on, the ozone is being injected into water that is about 120'F - not good at all for efficient introduction of ozone.  

Yes, but that heater is not on very much of the time, and yet the ozone goes into the water 'round the clock.

I know that the difference between 102 and 120 is still not that much, and that both of these temps are far above the optimum for ozone half-life, but since it is injected constantly, you have a better chance of a noticable result.

I know that testing for ozone in the body of most any hot tub is going to net a tiny number, if any. But I also know that there is a very real difference in water quality with vs. without ozone - at least on the tubs I have owned, sold and serviced. So there might be more to the action of the ozone in the chamber - directly downstream of the injector - than you are thinking?

[Vermonter]

Hi Chas,

I agree with what you say about the time the heater is on - and tried to indicate that the extreme twmp of 120'F or so only occurred while "on".   I failed to point out that it isn't on very much.  I also agree that, whether 100'f or 120'F, it is not a good temperature for optimizing ozone mass transfer or ozone half-life.

Regarding your comment about ozone having an effect...  I do think ozone has an effect on the oxidative side of things.   One evidence of this on my own tub is the discolored tubing just downstream of my injector - i.e. the ozone has oxidized the iron (or portion of it) in my tub makeup water and formed a reddish-brown oxide (rust).

Since ozone works first as an oxidizer, I believe it's benefit is seen there; with HS, you have 24/7 ozone addition.  Particularly for light tub use, that means that there is, at any one period of time, an essentially finite amount of oxidizble organic and inorganic material in the tub that, unlike microorganisms, is not replicating and growing in concentration (if not controlled).  Therefore, I think any ozone that may be immediately available in a dissolved form at the "head" of the contact chamber is continously working on oxidizing wastes and this will manifest itself (or, perhaps I should say, "can manifest itself" ) in the form of improved water quality.

However, seeing "clearer" water, etc. does not , in and of itself, equate to a microbially safe hot tub.

Vermonter

[Mendocino101]

I think the real bottom line with ozone in a spa is it is a tool and can aid in helping with water mangement....The problem I see is when it is oversold and its benefits are overstated ....I have found like Chas on our spas that people have found it easier to maintain their spas with the help of ozone but it is just a small piece of the puzzle to proper and safe water management and that much more important is to do regular water care....For most people it can be as simple as testing weekly and making the proper adjustments but many will neglect to do so.

[Chas]

I know this is very frustrating to folks who like to have a demonstrable ozone level, because you simply won't get one.

Didn't you test your water once? I know that you talked about testing your spa water for dissolved or aqueous phase ozone, and I know you have the lab and equipment to do so, and if I recall correctly you either got NO reading, or your equipment wouldn't go that low or something like that. Refresh my memeory please?

I know I can sound like I'm selling snake oil when I say this, but I do know that whatever the exact reason, a good ozone system does make a tub consume less chlorine or bromine, and it sure makes for better-looking water.

As you have said, and I think this is a very important point for spa owners to understand: sparkling water can still contain stuff you don't want to soak in. But even if the ozone is 'only' oxidizing organics out, then that is still leaving less 'work' for the sanitizer to do.

[Steve]

Mendo is 100% correct. I too believe there is a benefit to ozone to some degree but the fact remains that it is OFTEN oversold and therefore, the consumer relies far too much on it as the magic bullet!

Over the many years in this industry, I have heard savings of 10-90% of sanitizer usage with the addition of ozone. Is there any real data anywhere that will show the relationship of chlorine or bromine reduction and/or cost savings when ozone is introduced?

I have owned a spa with and without ozone. The reality is that my regular weekly maintenance stayed the same and water clarity (as well as quality) wasn't an issue with either spa. There may have ben a slight decrease in chlorine demand. That being said, taking into accout the added cost of an ozonator, can we accurately determine the "pay back" period based on any accurate data?

Nice to see ya here V!

Steve

[Chas]

can we accurately determine the "pay back" period based on any accurate data?

I don't think an ozone system would ever pay itself back. First: you most likely will substitute MPS for daily use, which costs more than Dichlor. Second: you will most likely add a Nature2 cartridge, which is an additional cost. Third: some ozone units require a 'chip' replacement about every year - that is extra money.

[Vermonter]

I know this is very frustrating to folks who like to have a demonstrable ozone level, because you simply won't get one.

Didn't you test your water once? I know that you talked about testing your spa water for dissolved or aqueous phase ozone, and I know you have the lab and equipment to do so, and if I recall correctly you either got NO reading, or your equipment wouldn't go that low or something like that. Refresh my memeory please?



Chas,

Yes, it is frustrating.  Do I take it (from your comment)  that this means that the industry is still unable to document a sustained ozone residual in a hot tub?

Regarding my testing, you are correct - actually, there were two facets to my tesing.  Keep in mind, that the information I'll be giving here is for my own tub; and may not be representative for the "universe" of tubs.

1)  Ozone residual:  I modified my ozonator loop (contact chamber)  to give a sample port immediately downstream of the Mazzei injector.  I also rigged a sampling device to sample water coming out of the end of the contact chamber.  I turned off the heater so that the water temp being injected into was my normal tub temp at the time (about 102'F).  Using an accurate digital colorimeter and indigo trisfulfonate (a test specific for "dissolved" ozone - the form of ozone you want), I collected a "blank" sample to zero the colorimeter as well as replicate samples from each of the sample points.  No ozone was detected at either point - my  particular detection level was 0.03 ppm, so, if any dissolved ozone was present, it would be below that  level.  I also collectd random samples from various points in the tub - as you would expect, no dissolved ozone in any sample.

2)  Ozone and Nature 2 only - another test involved my operating my tub for a brief period of time without use of dichlor.  The tub was "seeded" by normal use - but then, instead of adding dichlor, I left the tub with only the HS ozone (24/7) and the Nature2 (silver ions - principally).  Within 48 - 72 hours, heterotrophic plate count bacteria levels went from my normal "0" or single count per mL to, as I recall (I can check rhtubs.com archived posts and find this) to counts of more than 1,000,000 per mL.  There are approximately 5 mLs per teaspoon and nearly 2,000,000 mLs in a 500 gallon hot tub.  So, if each mL had 1,000,000 bacteria in it, then you are taling of approximately 2 x 10 to the 12th power.   That's a lot of bugs.  The water was cloudy (although that isn't a good measure by itself).  I added a shock dose of chlorine (dichlor), took samples after one hour (or so) and levels were back down to detection levels.

Keep in mind, that heterotrophic plate count bacteria may not contain a single pathogenic bacteria - or it may contain a lot.  It is simply a very general grouping of bacteria that is used by some organizations (such as WHO) as a rough measure of microbiologicaly quality of drinking water.  Regardless of whether pathogens were involved or not, however, who (not I) wants to sit in water in which you have to wonder whether the bacteria you know to be present are harmful or not.

Perhaps subject of another post - use of ions and ozone as a secondary sanitizer, or perhaps as a bacteriostatic vs. bactericidal treatment...

Vermonter

[Steve]

I don't think an ozone system would ever pay itself back. First: you most likely will substitute MPS for daily use, which costs more than Dichlor. Second: you will most likely add a Nature2 cartridge, which is an additional cost. Third: some ozone units require a 'chip' replacement about every year - that is extra money.

This is something I struggle with Chas...

We know we can sanitize and oxidize the water without the need for ozone yet we promote this product to the general public as something they should have?

Understanding it's not 100% needed nor is there a direct payback in value, are we "squewing" the real benefit to the end user at the dealer level? If not, how are we really promoting this product and its value?

Is there more value in enzymes? Just thinking out loud...

Steve

[Spatech_tuo]

Understanding it's not 100% needed nor is there a direct payback in value,

Steve

I realize there is no direct payback to ozone but I've seen how well it can work and I've dealt with enough people who have water quality issues to realize that it makes life easier and the last thing a person needs with a spa is to have issues caring for their water. I've seen people who've struggled with water quality add a CD ozonator and a month later they're having a much easier time and they're the testimonials that will tell you it's worth it. I've also seen more than a couple people struggle over the past few years and we've been out to find that the ozone system wasn't working for some reason (pinched hose, bad check valve, not plugged in completely, bad injector, etc.) and after getting them going properly they always seem to report having a much easier time caring for their water.

At the same time, I think there are some low budget UV units out there that I don't think add enough to make it worth the add on cost and with those I'd say skip it and just use dichlor.

[Steve]

Sure there's testimonials like that tech.

If you took 100 of those people and taught them proper water chemistry and a weekly maintenance while making sure their spa is filtering at the proper rate based on usage, then I would hazard an experienced guess that the addition of ozone wouldn't make the difference between great water quality and poor water quality. Was that all one sentence? Whew!

After all, we know how people care for their water once we leave their house after instruction... That being said; I've seen how well a properly executed maitenance schedule for watercare can work also (with or without ozone).

If someone approached me with poor water quality conditions, this would be my personal suggestion for determining the cause(s):
- Sanatizer levels - Meeting the demand?
- Usage - More than usual?
- Filtration - clean filters- adequate filtration
- TDS
- pH
- Alk

There's a reason why people are having watercare issues and I believe it's more proactive to teach them proper care than relying too heavily on ozone.

Ozone doesn't even make it onto my list of "corrections" personally but that doesn't make it right or wrong I guess.

Steve