OK Richard, here comes some more questions ...
After reading your response it has dawned on me that the microbiologist may have been measuring bacteria and not specific bacteria. Since the dreaded "hot tub rash" is not a bacteria that is formed by the bacteria in the spa but from a out side source - how is it always associated with a person's tub going bad? Do you know if that bacteria dies when the suit dries? I ask since this seems to be the most feared thing in the hot tub world. I always assumed it was formed by bacteria in the tub that grew to epic proportions.
The bacteria is an issue when it gets into the water and grows to large numbers, just as you say. So only a relatively small number may survive in a suit and yes, if the suit dries out long enough the bacteria may eventually die but if there's any moisture (and there usually is), they can live for a while. Some bacteria live on relatively dry surfaces, but Pseudomonas found in hot tubs actually thrive in hotter water (ideal growth is body temp of 98.6F -- and they survive up to 108F) which is why they are usually found in hot tubs. Different bacteria are cultured using different media, but I suspect he/she measured generally and might have checked for Pseudeomonas as well -- it's hard to say since I don't know the kind of test he/she used. At any rate, if you wash your suits in soapy water, that usually kills the bacteria.
Also, the bacteria live on skin as well (again, washed off with soapy water) and most people don't get a reaction to it. So it takes a combination of a hot tub with the bacteria plus a person who is sensitive (usually those with an open sore of some sort or sensitive skin) to get the itch.
So if you shock your hot tub with high chlorine levels or if you maintain the 4 ppm FC with 20 ppm CYA or equivalent FC level that is about 20% of the CYA level for a while, then the tub should be clear until the bacteria gets reintroduced (from whatever source).
As for bleach - I read what happens and I guess in layman's terms - the 6% chlorine forms the acid equivilent of the 94% PH 11.4 inert material - is this correct? Is it due to the fact that an acid of (say) 3 is logarithmically more potent than a base of 11.4? Does heat have the same affect on bleach as sunlight does? Is there any other "garbage" in bleach to worry about the TDS going up and causing problems? I know it was said no but is bleach just chlorine and salt water? edit: After more thinking I realized that a PH of 3 (my number) is the mathimatically the same as a PH of 11 from neutral PH of 7 - so how can 6% chlorine neutralize 94% - of course I don't know what the PH of the acid that was formed is and that may answer it.
Though it's a nice conception to think of the bleach portion as having a separate pH than the water portion, that's not the way it works. You've simply got water with sodium ions and hypochlorite ions and a net of hydroxyl ions and this latter net results in a smaller amount of hydrogen ions and that is why the pH is high (the pH is the negative logarithm of the amount of hydrogen ions -- so 11.4 has 10^(-11.4) amount). The pH of bleach could readily be much lower, but this produces more chlorine gas so the reason the pH is kept low is to prevent that from happening (the process of making bleach also lowers the pH, but pH can be adjusted readily and is simply kept low for the reasons indicated).
To calculate what happens with pH when mixing two sources of water at different pH, you have to first convert from pH to actual concentration so need to take 10 and raise it to the negative power of the pH. Do that for both water sources, add the result together, then take the negative logarithm to get the resulting pH. This works when adding two sources that are both acidic or both alkaline or where one is neutral. If one is acidic and one is alkaline, then the net pH will be the difference in the two pH's relative to 7.0 so as you point out a pH of 3 mixed with a pH of 11 would produce a pH of 7 (neutral). However, a pH of 5 mixed with a pH of 11 would produce a pH of 7-((11-7)-(7-5))=7-(4-2)=7-2=5. [EDIT] NO! I screwed up on that one. A pH of 5 has 10^(-5) concentration of H+ and 10^(-9) concentration of OH- while a pH of 11 has 10^(-11) of H+ and 10^(-3) of OH-. The dominant H+ combined with the OH- to form water so the OH- is 10^(-3) - 10^(-5) so has virtually no effect so the result which is nearly a pH of 11 (it's 10.996). In other words, unless the acid and base are very close to opposite pH from 7, the dominant one remains. This is why unbuffered water is so very, very sensitive to small amounts of acid or base producing large changes in pH. [END-EDIT] These are approximations (it doesn't work in buffered solutions, for example), but reasonable ones. Interestingly, diluting an acidic or alkaline substance in half with neutral water just moves the pH closer to 7.0 by only 0.3. That's because of the rule I gave at the start of this paragraph where you convert to concentration. Diluting with neutral water essentially just cuts down the hydrogen ion concentration in an acid (or hydroxyl ion concentration in a base) in half and -log(0.5) = 0.301
Heat does breakdown bleach just as sunlight does, but there is a big difference. Sunlight breaks down chlorine even at low chlorine levels such as those found in pools. Heat only breaks down chlorine when the chlorine is concentrated. The chlorine is spas goes away faster than in pools not so much because of the higher temperature directly, but rather that all reactions are faster so it combines more rapidly with organics and in a spa the bather load is much, much higher -- one bather is in a far smaller volume of water than in a pool. There is probably more outgassing of chlorine as well due to the aeration from the jets, but I think the organic load is the real rapid consumer of the chlorine.
Since it's PH neutral then as the spa aerates isn't the spa going to climb in PH - opposite of what happens with dichlor? Eventually acid will be needed to bring down the PH which in turn will affect the alkalinity also - is this correct?
Yup. That's exactly correct. The aeration from the jets in the spa causes carbon dioxide to outgas and causes the pH to rise. If you are using Dichlor, then it acts essentially as an acid since its addition AND chlorine consumption combined is acidic so the net result is a lower in TA (because acid lowers both pH and TA). If the aeration is strong enough and dominates over the Dichlor net acidity, then you may need to add extra acid separately.
My original post was based on what I believe was some misconceptions - on both sides. Given what you quoted on PPM for bacteria & pseudomonas aeruginosa using dichlor doesn't seem that bad IF you realize that you can't run a tub with 1 PPM FC and need to shock weekly or every 2 weeks with extra chlorine, if one filters out the pseudomonas aeruginosa end of the equation. If the tub is used daily as suggested, more frequent water changes are needed. If hot tub itch is suspected then a shock of high FC is required, then a drain of the water - which is something that is suggested on hot tub forums.
The problem is that most people only change the water every 3 months while many use the hot tub frequently -- if not every day, then at least several times a week. If they use only Dichlor, then after about a month they are likely to have conditions where Pseudonomas can form. If they shock WITH CHLORINE weekly, then they are less likely to have a problem, but if they shock using Dichlor as their chlorine source then they increase CYA more rapidly needing even more chlorine for shocking -- getting to the point where they simply don't have enough since they'd need to shock with 30 ppm FC at 150 ppm CYA and that's not typical. Also, some hot tub owners shock with a non-chlorine shock, potassium monopersulfate, which is great for oxidizing organics, but it won't kill bacteria.
I'm just being conservative and most people don't get hot tub itch. Also, on another forum, the 4 people who had hot tub itch (some diagnosed from physicians) all were Dichlor users with very high CYA levels. When they tried doing the 4 ppm FC and 20 ppm CYA I suggested, one said they still had hot tub itch after a while, one did not, and the other two never reported back. So I can't say definitively anything other than what I know about the chemistry of chlorine and CYA and the studies measuring the CT (chlorine concentration times time) values for bacteria (including the higher CT for Pseudomonas).
The original poster had problem with his tub and the problem was something that I have never heard of. The musty smell is usually associated with poor sanitizer or mold on the cover but the staining I have never heard of before. I personally don't believe that it is caused by dichlor and based on your explanations most people won't have poor water quality by using dichlor properly. Being in both a pool and spa, I have measured the CYA of my tub and after 3 months it was at 100 and yes it needs more chlorine to be effective. I tend not to shock with chlorine all the time but I do every 2nd or 3rd shock basing it on how much chlorine I have added since the last shock. I also add chlorine based on a per person formula( minimum 3 PPM FC to max of 6) and maybe that's why I don't have problems with bacteria unless I miss dosing - but it has no baring on which chlorine you used.
You may also not be sensitive to Pseudomonas or may not have it in your hot tub. All the other common bacteria will be readily killed with regular chlorine levels even with high CYA. Smells can come from various sources including bacteria (underarm smell is usually bacterial in nature), but can also come from molds and fungus as you indicated.
If it wasn't for Pseudomonas, I wouldn't worry about using Dichlor all the time in a hot tub. In a pool, one needs to worry more about CYA due to its direct affects on plaster and due to algae which requires far higher disinfecting chlorine levels than it does for killing the common bacteria in pools (Pseudomonas being less common since the temperatures tend to be lower). A spa generally doesn't get algae since it's not usually exposed to sunlight (it's usually covered) and the algae doesn't usually like the high 104F temperature.
IMO, to out and out state that dichlor is wrong to use is a little harsh. Yes, I might not have had my "facts" 100% correct but I don't think the "facts" stated were 100% correct on this side either. I also think that the way the tub is being used has to factored in as well - a family tub is going to see a different bacteria load than the party tub. Also, I think that some people are unaware that water will get old and needs to be changed, your example of using the tub every day would only be good for 90 days if a tub of 300 gallons was used by 1 person and I think that's where people get in trouble as well.
I don't think I ever said using Dichlor was wrong. Perhaps you were referring to other posters. I was only saying that IF you use Dichor, then you need to realize that CYA increases over time and this reduces chlorine's effectiveness proportinately and can become an issue for Pseudomonas. I mostly picked up on this issue from a hot tub forum where some people reported hot tub itch and I noticed that they were all Dichlor-only users and also read about the Pennsylvania recommendation against Dichlor -- that's how I got focussed on it since I had already figured out the chlorine/CYA for pools earlier.
Based on your answers to my questions you might be seeing more questions from me.
Thanks!
Vinny