Combined Chlorine

[LilRed]

I am getting this chemistry better after getting my taylor kit. My question is how to lower the combined chloroine? I am using bleach like Chem Geek suggests. I read in the Taylor book to lower the CC you need to supershlorinate or shock. It says to raise the FC to 10X the CC. I have a 200 gallon tub. Would this mean adding enough bleach to bring up the FC to 10 if the CC is 1ppm? Can I do this wiht non-chlorine shock as well? Thanks so much for all the great advice.

[chem geek]

First, I assume you used Dichlor initially for about a week before switching to bleach. If you didn't, then you should do that -- otherwise, you are overdosing with disinfecting chlorine.

Second, I wouldn't worry about Combined Chlorine unless you get a level above 0.5 ppm. That is, it is quite normal and perfectly OK to measure 0.2 or 0.4 ppm Combined Chlorine, especially in a spa.

Third, though you can shock with chlorine (bleach) to get rid of the CC, you are right that you can also use a non-chlorine shock (potassium monopersulfate aka MPS) to help prevent its formation in the first place. Non-chlorine shock is not as great for getting rid of already formed CC, but it will help -- it's mostly for prevention (as it oxidizes organics and ammonia before chlorine gets a chance to combine with them). However, some people report sensitivity to MPS.

Fourth, if you keep your spa uncovered longer and maintain a reasonable chlorine level close to 4 ppm FC, then you may find that there are less CCs that form. This is most especially true just after you've used the spa. Keeping it uncovered for up to an hour helps it "breath" and release the nitrogen gas that comes from the chemical reaction of ammonia with chlorine (the "breakpoint" of typical Combined Chlorine or monochloramine with more chlorine). So adding some bleach after you get out of the spa and leaving it uncovered for a short time should help.

Fifth, if there is any way to expose your spa to direct sunlight (not through a window), then the UV rays from the sun (or a UV lamp, for that matter) will break down combined chlorine very effectively. This is why most outdoor pools exposed to sunlight do not get significant combined chlorine (assuming proper chlorine levels are maintained).

As for the 10x rule of chlorine to get rid of combined chlorine -- it's not really correct. For technical reasons I won't get into here, you really only need a minimum Free Chlorine that is 50% of the Combined Chlorine as measured in the Taylor test (i.e. measured a ppm chlorine gas equivalent) in order to get rid of it. Nevertheless, the 10x rule makes the process go faster and is reasonable given that there is CYA in the water since the disinfecting chlorine concentration is fairly low. As mentioned above, it is better to prevent the formation of Combined Chlorine in the first place -- either through use of MPS or through "continual breakpoint" by maintaining the FC level and letting it air out sufficiently (or exposing to UV).

Richard

[wireman]

How about putting some UV lamps in around your tub? Wonder if that would lower the CC's?

[LilRed]

Yes I did use dichlor. I am gtting 1.5ppp of combined chlorine. I have only been using the non-chlorine shock once a week. I will start using it more ofter. I will leave it uncovered after use and will let the sun "have at it today". This tub really "eat chlorine. I know it is the ozinator. I will have the chlorine up on Friday and with no use by Sat night it is down. I am wondering what is the endpoint tolet you know you need to change water? My tub water always look clear. I keep everything balanced. Any info there? Thanks so much for all your advice. It sure helps. Jeanie

[chem geek]

Jeanie,

It's a bit strange to have Combined Chlorine with the ozonator as the ozone should break down the combined chlorine (and as you've noticed, it can oxidize chlorine as well). At any rate, try the uncovering and the sun exposure and let us know what happens. Are you seeing a usage of more than 4 ppm FC per day and are you using the tub every day?

Also, be sure to measure Combined Chlorine before you get into the tub/spa. If you measure it soon after, you will measure CCs, but they will be breaking down over time. At 20 ppm CYA and 4 ppm FC, it takes about 20-30 minutes to break down half of the 1.5 ppm CCs (if they are monochloramine) and about 80-100 minutes to break down 90% (at least in theory). If you find that the CCs seem to still persist, then you can certainly shock with bleach raising the FC to 10-15 ppm. The FC should drop fairly quickly and you can use the tub/spa even if it's at 10 ppm or lower if you don't want to wait (it's still far less disinfecting chlorine than in most indoor pools that don't use CYA).

The typical standard is to change the water every 3 months if you use the tub frequently. That's reasonable even if it looks clear. You don't have to change it if everything is in balance, the water is crystal clear, water feels OK, etc. but eventually the salt will build up though that will take some time. I wouldn't let it go for more than 6 months though (again, if you use it frequently). Most people comment on how fresh a new fill of water feels, but if your water seems that way after several months, there is no problem. If you were using only Dichlor, then changing the water more frequently would be a must given the buildup of CYA, but using bleach just has a rise in the salt level of around 576 ppm in 3 months if you add 4 ppm FC every day. That's not too much salt -- remember that SWG pools have 3000 ppm salt. Since your tub is only 200 gallons, I'd just change the water every 3 months since it's not that much water.

Richard

[LilRed]

I left the tub uncovered today. It isn't in direct sun and I uncovered it for about an hour. tonoight the FC ws 1.8 and the CC was 0.6. So it did come down. I would have to say it uses about 4 PPM of FF/day on average. I get init 4 times a week on days off. I work 12 hr on Fr Sat and Sun so I just add bleach to keep the chlorine up in case I want to do a quick soak.

I will empty the tub the end of the month. I will see if I feel a difference.

Time to get in. It is dark and the stars are out. I live in W North Carolina mountains. The Air Force is training now and I can watch the F15 jets zoom about while I am in the tub. Pretty cool. Thanks again Richard. Jeanie

[wireman]

Chlorine dissapates fast. mine will go from 2 ppm to .5 or so in a day. Just part of the fun!

[chem geek]

Jeanie,

With only 200 gallons, 4 ppm FC per day isn't too bad. This would be equivalent to a loss of only half that in a larger 400 gallon tub (in terms of total chlorine quantity). Though some of the chlorine outgasses due to the warmer spa temperature, most of it is oxidizing organics and ammonia (urea) mostly from sweat. If you used more non-chlorine shock (MPS) then you may find that the chlorine consumption is lower. You might want to use more and do so on the day you don't use the tub so you give it a chance to drop a bit in concentration by the time you next get in.

Dupont recommends dosing with MPS at a rate of one pound per 10,000 gallons in a pool one or two times per week depending on bather load. For spas and hot tubs, they recommend 1-2 ounces per 250 gallons after each use or at least once a week if not used.

I think these amounts are on the high side of what is needed, but nevertheless you can see that a single small dose weekly is probably not enough. Why don't you increase the dose on the "off" days you aren't using the tub and see if that helps reduce chlorine consumption, especially on the days that immediately follow. Everyone's routine is a little different. Obviously, if you get any skin irritation or sensitivity from the MPS, then you can cut back.

Richard