Extreme Chlorine Depletion After Heavy Usage...

[mraybone]

Hi guys, I was hoping to find some help here if possible.

I recently acquired a hot tub with a house. I got some advice from a friend who is a spa dealer, I cleaned the tub, changed the water, superchlorinated and all was good.

I had a hot tub party (7 bathers, a few hours) a few days ago and afterwards, due to some water loss, I topped up and tested levels only to find, to my surprise, that there was 0 chlorine. I usually have 2x chlorine tablets in a floating dispenser and this keeps FC and TC around 5ppm usually, sometimes slightly higher than I'd like. I added a few more chlorine tablets, with no apparent effect and then an extra one, and only now the FC has just started to register at nearly 0.5ppm.

I'm worried about low chlorine levels and bug growth, so I'm about to pile in a load of extra tablets. After reading some posts regarding Chlorine Demand, noting "100% CD is Bad. This could mean you just had a Hot Tub Party...", is this chlorine behaviour normal after heavy use, and will extreme superchlorination help get things back to normal, or do you think there could be underlying issues?

Bear in mind the tub was cleaned and refilled (twice) only a month ago, levels have been rock solid (usually too much chlorine) and it has only been used heavily this one recent time. Thanks!

[chem geek]

There should be no surprise. The amount of chlorine that is needed/used is proportional to the bather load and you had a very high bather load. If there is no ozonator, a rough rule-of-thumb is that every person-hour of soaking in a hot tub takes around 3-1/2 teaspoons of Dichlor or 3-1/2 fluid ounces of 8.25% bleach or 7 teaspoons of non-chlorine shock (MPS) to oxidize the bather waste. With an ozonator the amounts might be in half or less.

Now I doubt the tub was very hot (i.e. 104ºF) if it were used for "a few hours" but even 7 person-hours would need a heck of a lot of oxidizer.

Instead of adding tablets (are you talking about Trichlor? that's not normally used in spas since it is very acidic and dissolves too quickly in hot water), you should add chlorinating liquid or bleach.

[Spanky]

During "Hot tub / Pool " parties, I always add about 2oz of chlorine bleach into the filter box every hour or so. Adding to the filter box, with the pumps running, disperses the chlorine so that your guests are not exposed to high level.

I also make a point of shocking the pool before I turn in for the night and shock the tub as soon as I get up in the morning.

Pool/ Spa parties are great fun but they're a lot of work for the host... Food, bar etc.. AND maintaining a safe sanitizer level in the spa at all times!!!

[mraybone]

Thanks for the replies! You're right - it should be no surprise now, but at the time I wasn't expecting it. The tablets are trichloroisocyanuric acid, my spa eats about 2 of them in about 5 or 6 days (96ºF, little to no use). As it happens, after adding a couple more tablets today, the chlorine levels are rising, thankfully.

For info, during the heavy 'session', the spa dropped from 96ºF to 85ºF. I will definitely shock after use now and perhaps a tablet in the filter box during heavy use would be useful. Due to the slow nature of shocking with tablets, what other chlorine shock product - you mentioned dichlor or bleach? - would you recommend?

And yes, the spa is a constant worry for the host - especially now! Do you think that the couple of days of it being virtually chlorine free can be balanced with a big chlorine shock, or might I have to consider a clean+refill?

[chem geek]

If you get a chlorine demand even with no bather load, then that can indicate biofilms that formed during the zero chlorine period. If that happens, you can consider using Ahh-Some and then drain and refill.

[mraybone]

Right, thanks, I'll see how this chlorine splurge goes. If, after shocking, it depletes rapidly, I'll look at an Ahh-Some clean.

[mraybone]

Also, I'm in the UK, is this the right stuff? http://www.hottubdiy.co.uk/ahh-some-hot-tub-cleaner-6oz.html

[carl333]

 

There should be no surprise. The amount of chlorine that is needed/used is proportional to the bather load and you had a very high bather load. If there is no ozonator, a rough rule-of-thumb is that every person-hour of soaking in a hot tub takes around 3-1/2 teaspoons of Dichlor or 3-1/2 fluid ounces of 8.25% bleach or 7 teaspoons of non-chlorine shock (MPS) to oxidize the bather waste. With an ozonator the amounts might be in half or less.Now I doubt the tub was very hot (i.e. 104ºF) if it were used for "a few hours" but even 7 person-hours would need a heck of a lot of oxidizer.Instead of adding tablets (are you talking about Trichlor? that's not normally used in spas since it is very acidic and dissolves too quickly in hot water), you should add chlorinating liquid or bleach.

I'm doing some quick math. 6 people in for 1 hour. I will need to add 21 oz. of bleach at 8% to consume the waste?Am I figuring this out correctly? If so, after adding 21 oz, how much time should elapse before (with jets running on full) it is safe to re-enter the water?

 

[chem geek]

Also, I'm in the UK, is this the right stuff? http://www.hottubdiy.co.uk/ahh-some-hot-tub-cleaner-6oz.html

Yes, that's the right stuff.

[chem geek]

I'm doing some quick math. 6 people in for 1 hour. I will need to add 21 oz. of bleach at 8% to consume the waste?Am I figuring this out correctly? If so, after adding 21 oz, how much time should elapse before (with jets running on full) it is safe to re-enter the water?

 

That's the amount that may be needed without an ozonator, but that's also for a hot (104ºF) spa and your temperature was probably lower so people would be sweating less. Since you were clearly using too little, you could try adding 10-15 ounces for that bather load and see if it is enough. You'll know you've added enough if you still have some chlorine residual 24 hours later.

You probably should not use the spa again for at least 8-12 hours, though with that high a bather load 24 hours would be better. If you plan on having such high bather loads regularly, then an ozonator would be useful or you could use some of another oxidizer such as non-chlorine shock (MPS) though chlorine is the least expensive and most effective.

[mraybone]

Thanks! Well with 4x as many chlorine tabs in the spa, the levels have only reached as far as you'd expect for day to day use, it never shocked, so something is eating chlorine. I'll be getting some Ahh-Some and doing a clean.

[Spanky]

Ozonators can help but the're "after the fact" since most set ups do not allow the ozone to function while the spa is in use. They are programmed to start 1/2hr after the last panel input an therefore not much help during a Spa Party. Even if the pumps are not running , the ozone will not start until 30mins after the lighting is turnrd off. If you don't add chlorine during the party the sanitizer will reach 0!!! Unless you start with a dangerously high level. During these situations there is no substitute for hourly dosing. This is similar to "Public" spas, which are required by law to have constant sanitizer injection, usually a peristaltic pump. Under normal use private spas never need this but during Spa Parties the situation is diferent

[mraybone]

I was wondering - what is more likely... A high level of contaminant in the spa after a big pool party preventing my chlorine tablets from being effective at shocking due to their slow release, or a biofilm?

Also, my tabs are trichlor, if I were to add some dichlor (it's the powdered stuff, there was some left over from the previous owner), would that be bad due to mixing chemical types?

I have Ahh-Some on the way, but was considering a last ditch attempt at super-chlorination using the dichlor powder, just to see if the chlorine depletion remained high after a successful shock.

[Spanky]

Be careful with trichlor!! It also boosts CYA which reduces the effectiveness of the chlorine. If you suspect biofilm then your only option would be to flush & decontaminate. When you refill, use dichlor then bleach, NOT trichlor!! You'll find it a very effective system an extremely economical.

The next time you have a Spa Party don't forget to hourly dose and shock immediately the next morning. At 102deg F and 0 sanitizer bacteria will grow exponentially!! Biofilm can develop in mere hours.

[mraybone]

When in use, what's the best way to add chlorine? The granulated dichlor that I have says to add it directly to the tub. If I'm adding it hourly during bathing, is it best then to put it into the filter chamber? And with a full tub of 7 bathers - that's a lot of chlorine required...

[chem geek]

You want to add concentrated chemicals over a return flow with the circulation pump running to ensure thorough mixing. Don't dump a lot in one place since it could settle and possibly stain/bleach a spa surface.

As for adding chlorine during a soak, most people just start out with 1-2 ppm FC so that they don't notice the chlorine and they don't soak long enough (usually not more than an hour) for bacteria to grow enough to be a problem or to form biofilms. They then add the required dose of chlorine right after the soak to kill off any bacteria and to oxidize bather waste.

Commercial/public spas can't operate this way since there is the much higher risk of person-to-person transmission of disease where one sick person could infect many others. In a residential spa this is less of a concern, especially if you are soaking with family members. On the other hand, the fecal-to-oral route isn't one you normally get exposed to with your family except changing diapers or doing laundry. So it's up to you whether you think you want to keep adding chlorine during the soak. If you do, then yes, adding it to the filter compartment would be easier to distribute it more evenly and quickly rather than adding it in the bulk water while people are in the spa.

You say you would be adding it "hourly" so it sounds like the spa will be used for long periods of time. I presume that if the spa is hot that people are going in and out of it. So yes in that situation it's more like how commercial/public spas are used so regular chlorine dosing would be needed. However, note that this will likely smell a lot as the chlorine oxidizes bather waste. To minimize that burst when the chlorine is added, you could add smaller doses of chlorine more frequently to more continually oxidize the bather waste. It will still smell though -- no way around that if you use chlorine to oxidize bather waste (or bromine, for that matter -- ozone would not produce the same volatiles).

[mraybone]

Ok thanks. I'll probably go for the small amounts more frequently approach. I really don't want to encounter another 0 chlorine incident. I did say 'hours' but that's on the basis the last time the tub was used, it was for about 2 hours, with 4-7 people always in.

Anyway, I'm about to crack open the Ahh-Some...

[mraybone]

Currently rocking the tub with some ahh-some, there's definitely a white coloured deposit on the water line, nowhere near as much foam as I expected though...