Why is my combined chlorine gradually increasing?

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M

MPurcell

0
Feb 17, 2017
105
Dahlonega, Georgia
Current chemistry:
FC 6.5
CC 2.0
pH 7.5
TA 90
CH 150
CYA 30
borates 50

Due to many posts and testimonials here, we became convinced to switch from bromine to chlorine. I purged with ahh-some, cleaned, drained and refilled the tub at the end of July, so it's been about a month and a half now. Overall, we are very happy with the chlorine tub.

I made a mistake with the chlorine dosing after the refill, and wound up with approx. 23 ppm of chlorine in the tub. At that time, I tested my chlorine demand and got approximately 20% demand in 24 hours with the tub at temperature. We had to wait about a week for the chlorine to come down into a range that we could use the tub. During that time when I tested each day, the combined chlorine was 0.

My wife and I use the tub usually 20 to 30 minutes every day at about 100F. We don't miss very often. Once we started using the tub, on 8/4, there was initially a trace of combined chlorine. The pink was very faint, but definitely there. The combined chlorine began to gradually get darker pink. The first time it took 2 drops to remove the pink was 8/17, and it continued to get gradually darker. The first time it took 3 drops was 8/30. Today it took 4 drops for the first time so I'm now somewhere between 1.5 and 2.0 ppm on the combined chlorine.

My 24 hour chlorine demand has increased as well. On 8/13, which is the only time we have skipped soaking so I could test it, the chlorine demand was at approximately 50%, which is a little higher than what others report as normal.

The only things I have put into the water are calcium chloride, sodium bicarbonate, boric acid, dichlor, bleach and muriatic acid, and of course, bather waste. My reading and study have led me to believe that it should not be necessary to periodically shock the tub if the water is kept balanced, the chlorine is kept within the recommended range for the CYA level, and enough oxidizer (in the case of the dichlor then bleach method, this would be bleach) is added to neutralize bather waste. I have some MPS but have not used it since the refill. I'm puzzled why I have this issue at all. There is no problem with the water causing a chlorine smell so far, but it looks like I am going to have to shock to get the combined chlorine down. Can anyone shed any light on this for me? Thanks
 
Are you leaving the tub open at all with the jets running?

Your tub use would be characterized as "high bather load" therefore you need to elevate chlorine between uses and allow the CCs to outgas.

Is the tub in the sun at all? Leaving it open to the sun will allow UV to destroy some of the CCs.
 
Thanks, Matt. No, as a matter of fact, we generally use it just before bedtime, so I dose, run the air for a few minutes only, and then close the lid. If I try to leave it open, I often forget to put the lid back on and it stays off all night, so I stopped doing that. The tub is in the garage, so no direct sunlight. I've tried to keep the FC near or at the top of the recommended range, though.

I could take the lid off in the morning for a time. Perhaps that would be an acceptable pattern. I know it is probably best to do that right after soaking, but that doesn't seem practical in my situation. This inflatable tub has either a lot of air circulation, or a very weak 120 v pump with one outlet. 30 minutes of the air running would raise pH significantly, so I probably wouldn't do that daily. How long do you think I would need to leave it open to the air?

I have had the lid off this afternoon for about 3 hours with the pump running. Guess we'll see if that produces an improvement in CC.

Edit: Previous FC and CC readings above were done earlier this afternoon. After 4 hours with the lid off and the pump and heater running, FC 6.0 and CC 1.5. Based on color, the reduction in CC was probably just a little bit less than 0.5.
 
I'd try leaving it off for an hour or two a few times per week to see if that helps. The fact is, you're trapping the CCs in the tub and when there's nowhere for them to go, they'll stay put. The only other way to get rid of them, as you accidentally discovered, is to raise your chlorine to well above shock levels and oxidize them. You could shock your tub for 12-24 hours to lower the CC levels and then use peroxide to neutralize some of the FC. The equivalence goes something like this - the volume of 3% hydrogen peroxide needed to neutralize X amount of FC is equivalent to the volume of 6% bleach that would raise the FC by X. For example, it would take roughly 7 fl oz of 6% bleach to raise the FC 10ppm in 350 gallon hot tub. That means that 7 fl oz of 3% hydrogen peroxide would lower the FC by about 10ppm. The neutralization of chlorine by peroxide is acidic so you could expect the pH to drop when you do that. Otherwise, you could use a supplemental oxidizer like MPS, but just realize that the MPS will show up as extra FC in the FAS-DPD test unless you buy the specialized MPS interference removal reagent from Taylor.

Any chance you want to spend some money on more spa equipment? This is exactly where a UV sanitizer would be very helpful. Circulating the water through a UV system, like a Paramount UltraUV system, would help to breakdown the bather waste and reduce CCs. Since this tub has no chance of seeing daylight, a supplemental UV system would help out a lot.
 
Thanks for mentioning the hydrogen peroxide method for reducing chlorine. I had read about it before, but didn't think about it at all when I refilled the tub, or since. I could oxidize off the combined chlorine and be back to using the tub without losing a bunch of days. The equivalence you mentioned makes it pretty simple. Is the byproduct of the chemical reaction just sodium chloride and hydrochloric acid?
 
Well, I found that raising the chlorine level for 36 hours did not significantly reduce the combined chlorine.

Current chemistry after neutralizing chlorine and rebalancing:
FC 5.0
CC 1.5
pH 7.5
TA 80
CH 125
CYA 30

Saturday 10:00 pm, I took the chlorine up to 25 ppm and left the cover off. Sunday 4:00 pm FC was 18 ppm so bumped it up to 26 ppm. Monday 10:00 am chlorine was 16 ppm so I bumped it up to 25 ppm again. I had the cover off and the circulation pump running the whole time, except I ran the air for about 2 hours on Sunday and another 2 hours on Monday. 12:00 noon Monday I neutralized the high chlorine and balanced the water. Using that much air had the pH up to above 8.0 even with the acid nature of neutralizing the chlorine.

The net result was a slight but almost unmeasurable reduction in combined chlorine.
 
I have experienced a similar issue. After about two months I start to get CC readings in the 1-2 PPM range. I read a post somewhere that stated there are types of compounds in bather waste that can not be oxidized by chlorine and that this is not unusual for smaller tubs after a couple of months of heavy use. I've noticed that I can keep the water clear but need to add more post soak oxidizer than during the first 6 to 8 weeks. Shocking didn't affect the CC levels much for me either. I think it's an indication that the water may be reaching the end of it's life. That being said I was able to maintain it under these conditions for over 4 months with clear water and no CC smell. I'm going to start draining every 3 months instead of 4 though.
 
Try adding the MPS. Just note that you have to wait 24-36hrs to test so you limit the interference with the FAS part of the test.
 
@Arobbert, thanks for sharing your results. The water is actually still beautifully clear, and there is no additional chlorine odor at this time. We'll be back to soaking tonight, so I guess we'll just watch to see what happens over time. It's pretty easy to drain and refill with this tub. And now that I know I can take the chlorine up really high and neutralize it without losing a day's soak, I may try that again in a few weeks.

@JoyfulNoise, I think I will try MPS. How do I calculate a reasonable dose?
 
In doing some additional online research, I ran across this Aqua Magazine article. A quick and simplified synopsis is that chlorine breaks down inorganic chloramines but is mostly ineffective in breaking down organic chloramines. The article summarizes research that indicates that even super chlorination to breakpoint does not break down organic chloramines. If this is (mostly) true, then it would explain the experience I am seeing of gradually increasing combined chlorine, and it would also explain why my super chlorination attempt produced essentially no result. Apparently, the primary research was carried out by a Dupont chemist, but I don't see that that invalidates the results. The general implication seems to be that the use of MPS is superior to breakpoint chlorination for reducing chloramines. The article also mentions that ozone and UV light are effective.

I'm not willing to try UV light, at least not at this juncture. However, I do own a commercial ozone generator that puts out a significant amount of ozone per hour; I believe the rating is 20,000 mg/hour or 20 g/hour. It's primary purpose is to do shock level ozone treatment in dwellings. So, I think I will try an ozone treatment of the hot tub water, and if that is not productive, then I will do a treatment with MPS to see if that will reduce the chloramines.

My portable inflatable hot tub is designed with a ring around the interior base that has several hundred holes for air flow. The pump and blower assembly which is external, is designed to be either/or for pumping water or air circulation. The unit has an air intake area that is approximately 2" high by 10" wide right at the base of the unit. I'm not sure of the air flow rating, but it is quite significant. If I put both garage doors up for ventilation, and tape the output hose immediately in front of the air intake, I think the majority of the ozone will be pulled into the blower and thus channeled into the tub water. I'm thinking I will run this configuration for about an hour and then test for combined chlorine to see what result I get. Will probably do this tomorrow morning.

Stay tuned.
 

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Yes, creatinine and urea are hard to oxidize with chlorine alone. In an outdoor swimming pool, UV is in ample supply and bather loads are extremely low. Your tub sees a lot of use and so a secondary oxidizer is needed.

I would simply use MPS or do more frequent water exchanges to dilute the bather waste. Messing around with a commercial grade ozone generator is not really a safe practice and ozone will react with and neutralize FC. So, if you intend to use the ozonator, don't be surprised if you see a drop in FC as well.
 
I found using MPS lowers my PH to much. I got tired of constantly testing and adjusting the PH every time I added MPS. Now I just monitor the CC and know it's not a problem unless it gets above 2. I do notice that you have to pay more attention to the chlorine level though. Things can go South quick if it gets much below .5 PPM. As long as you maintain an adequate free chlorine level it's not a problem. If CC gets above 2 and doesn't respond to a shock of chlorine I know it's about time to change the water. Like I said the water stays clear and there is no odor but who wants to soak in water with elevated creatinine and urea levels?
 
Well, I ran ozone through the water while running the air for one hour, and the result was no measurable change in combined chlorine. Surprisingly, the free chlorine did not change either. The pH raised from about 7.6 to about 8.0. I did channel the ozone into the air intake, so I'm reasonably sure a significant portion of the ozone was being pulled into the air inlet and through the water. I'm going to try the MPS next.
 
Arobbert said:
I found using MPS lowers my PH to much. I got tired of constantly testing and adjusting the PH every time I added MPS. Now I just monitor the CC and know it's not a problem unless it gets above 2. I do notice that you have to pay more attention to the chlorine level though. Things can go South quick if it gets much below .5 PPM. As long as you maintain an adequate free chlorine level it's not a problem. If CC gets above 2 and doesn't respond to a shock of chlorine I know it's about time to change the water. Like I said the water stays clear and there is no odor but who wants to soak in water with elevated creatinine and urea levels?
Click to expand...

Usually with MPS use, one wants to have higher TA to help to buffer the acidity. But I agree, it definitely adds more work to the chemical balancing.

MPurcell said:
Well, I ran ozone through the water while running the air for one hour, and the result was no measurable change in combined chlorine. Surprisingly, the free chlorine did not change either. The pH raised from about 7.6 to about 8.0. I did channel the ozone into the air intake, so I'm reasonably sure a significant portion of the ozone was being pulled into the air inlet and through the water. I'm going to try the MPS next.
Click to expand...

Not totally surprising. Ozone has sparingly low solubility in water and so most of it probably stayed in the air bubbles and just floated away on the breeze. In order to get ozone to work as a water treatment process, water suppliers will use a contact tank where large volumes of water sit and the ozone is constantly recirculated through it. This allows a small residual of ozone to build up in the water and the holding time in the tank gives the ozone the time it needs to react with and destroy organics. Once the water is opened back up to the atmosphere, most of the ozone simply outgasses.

Dilution and MPS may be your best bet at this point...
 
Arobbert said:
Matt,
What TA level do you recommend for MPS? I have a lot of it and would like to use it, especially if it would help with this issue. I'm currently at about 60.
Click to expand...

If you plan to use the MPS regularly, try to take the TA up to 70-80ppm and see if that helps to offset the acidity. One thing to do would be to adjust the TA up a little, add the MPS and then run the air blowers. The natural aeration will help to raise the pH and offset the acidity of the MPS. It really depends on how often you use it. SOme people use MPS very regularly, like once or twice per week, and so keeping a higher baseline TA makes sense. If you only plan to use it occasionally, then I would keep a lower baseline TA and only raise it a little just prior to adding the MPS.
 
I added MPS. I couldn't find a definitive answer about how much to use in my situation, so I settled on a first dose of 1 oz by weight for 230 gallons. Before the first addition my chemistry numbers were:

FC 7.5
CC 1.5
pH 8.0

I ran the air for a few minutes while I added the MPS and then let the pump circulate for an hour, and my chemistry numbers were:

FC 7.5
CC 1.5
pH 7.6

So I then ran the air for about 15 minutes after adding a second ounce of MPS, and then let the pump circulate for the rest of an hour, and my chemistry numbers were:

FC 10.0
CC 2.0
pH 7.5

So clearly, the MPS registers as FC as well as perhaps CC. I'm not sure how to interpret the numbers. But I have evidently put in enough MPS to still have an MPS residual in the water after an hour, so I guess we'll see how the numbers react in the following days.

@Arobbert, how much MPS did you use as a weekly shock to reduce CC's? Or was it daily?
 
MPS will show up as FC on the DPD-FAS test. Only if FC is truly zero does it show up as CCs.

With MPS, you could be seeing higher CCs at first as the urea and creatinine breaks down and starts reacting more with FC.

Give the MPS 24 hrs to completely get used up.

I was wrong here, see this article by Taylor Technologies -

Interference
 
Last edited:
JoyfulNoise said:
If you plan to use the MPS regularly, try to take the TA up to 70-80ppm and see if that helps to offset the acidity. One thing to do would be to adjust the TA up a little, add the MPS and then run the air blowers. The natural aeration will help to raise the pH and offset the acidity of the MPS. It really depends on how often you use it. SOme people use MPS very regularly, like once or twice per week, and so keeping a higher baseline TA makes sense. If you only plan to use it occasionally, then I would keep a lower baseline TA and only raise it a little just prior to adding the MPS.
Click to expand...


Thank you Matt. I'm going to try using it occasionally to combat the build up of the CC that chlorine won't oxidize.
 
MPurcell said:
@Arobbert, how much MPS did you use as a weekly shock to reduce CC's? Or was it daily?
Click to expand...

I used 1 to 2 Tsp. daily post soak for 200 Gal. This was when I was using a mineral system and shocking with dichlor once a week or so. Since I switched to the dichlor/bleach method over the past year I haven't been using it much.

Checked my CC level today. It was 1 after 2 months. I use the tub almost daily and sometimes have a guest. I'm not concerned about this level and will check it again in another month unless I notice a high chlorine demand. If it's up to 2 or more I may shock it with a Tbl. of MSP and see if it affects the CC level.

I found the thread below that you may find of interest if you haven't seen it.
Non-chlorine Shock (mps) Dosing For Hot Tubs / Spas - Hot Tub Water Chemistry - Pool and Spa Forum
 

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