HEAVY Chlorine Loss

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Poolsean

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TFP Expert
Apr 15, 2007
1,421
Ft Lauderdale, Florida
OK Pool Experts...just need your thoughts on this.

I'm dealing with a commercial customer in the Long Island area, 140,000 gallons, Sand Filter, CYA = 10 ppm, TA = 0 ppm, CH = 60, pH = 7.4. New York State does not allow stabilizer in commercial pools, except with a waiver.

They have the Pool Pilot Professional system, with an output of 30 lbs/day maximum, and we're running every ounce of that 30 lbs/day! Water looks great. They're just able to maintain a decent chlorine residual (5 ppm) without adding gobs of sodium hypochlorite in addition to the Pool Pilot system running full tilt!

So, I determine that they have a heavy chlorine load since we're not able to keep up with a 5 ppm FC level, and I suggest that they shock the pool and run an overnight chlorine drop test.
It dropped from 30 ppm to 12 ppm overnight. Ta Da (I thought). Let's shock it again....same results. On the third shock dosage, this weekend, we went from 40 ppm to 31 ppm (Friday to Saturday 4pm), and by Sunday morning, down to 25 ppm. Today, we're back down to 12 ppm.

What am I missing? We've dosed almost 75 gallons of sodium hypochlorite (pool store strength - 10 - 12%), 3 times.
Keep in mind that the pool water is rather cool, 68 degrees, and there has been NO swimmer loads.

I don't think the sand filter has been recharged with sand in many years, and I hear there is a daily heavy MICE KILL load in the skimmer, which I'm sure ends up with some level of organic load in the system.

I'm actually going to be on site tomorrow (Tuesday) and they are asking, "What now?"

I think we're almost overcoming the heavy chlorine demand condition and would like to see them shock it one more time. Do I recommend sand replacement too?
Other ideas?

Thanks,
 
Shocking is a process that needs to be maintained above the minimum amount required for your CYA level, it took a week of shock to kill the algae in my pool...been great ever since and I have not had to shock it again.

Your on the right track but with your CYA low (is this an outdoor pool?) you will need to keep on it, there will be others who will chime in with more advice.

P.S. with my CYA level at 100 I had to keep the pool shocked at around 50ppm for that whole week to kill all the nasties, make sure everything gets the treatment...brushes, slides, skimmer nets, diving board etc. You need to brush the pool while your in shock mode too to get the biofilm to break up and allow the chlorine to work, good luck with that size pool!
 
30 pounds of chlorine in 140,000 gallons is 25.7 ppm FC per day. With no bather load it does sound like an accumulation of organics. Did they have a high chlorine demand before you put in your system? This could just be carryover from earlier high bather loads, gunk in the sand filter, mice in the skimmer (yech!), etc.

Is the TA really 0 ppm? And with the CH of only 60 ppm I hope this isn't a plaster pool. If the CYA were 0, then you could lose half the chlorine, so around 2.5 ppm, every hour for perhaps 4-6 hours peak equivalent this time of year so maybe up to 10-15 ppm FC during the day. If the CYA were 10 ppm, the loss would be less.
 
How often are they required to test?
Has it stayed above 10ppm FC the entire time during the shock process?
If the answer is yes then I'm thinking it's time to open up the sand filters and take a look.
I don't know what they consider a "heavy" mice kill but I can't imagine it'd produce much of a organic load on a pool that size. It'd take a bunch of meeeces to do that.
 
My view is to pop open the filter as well or at least make sure it is properly backwashed. TA needs to come up for sure. Lastly, if CYA is really 10 then perhaps they can just shoot for a avg FC of 3 ppm rather than 5? FC of 5 seems rather high as a taret, given the low CYA level. Just my 2 cents :goodjob:
 
Nassau County.
The test results are correct. Low levels of TA, CH, Cya...Cya is intentionally low, as Commercial Guidelines do not allow the use of stabilizer in commercial pools. The 10 ppm is probably from granular trichlor shock.
This is a plaster pool and there is no bather load right now. They are just about to close the pool for winter, but we're letting it run right now so we can figure this condition out.
They are testing daily. Once in the morning around 10 am, then again around 4 pm.
Scott, I don't know what their circulation system set up is, but I'm heading there later this morning. I do know it is a 24 hr run cycle, skimmer pool (mice caught in the skimmer baskets), sand filter (last serviced 4-5 yrs ago but not replaced sand), 4" pvc return lines. Circulation is suppose to be around 400 gpm.

I think they've got a biofilm that's sucking up the chlorine. I think the shock dosages are doing its job and they have been keeping it up above 10ppm. Yesterday morning, it finally dropped down to 1.5 ppm, from 12.5 ppm the previous afternoon.

I just needed a fresh set of eyes on this situation.

Thanks everyone for your feedback.
 
With a pool that is not even opened (no chlorine demand from swimmers) but circulating, what should the chlorine loss for an unstabilized 140,000 gallon pool be overnight? Went from 30 ppm on sat morning to 6ppm this morning.
Richard, is there a calculator that factors this?
 
I know I'm more on the inexperienced homeowner side of the school of thought, but if we are talking about an overnight FC loss test with no sunlight, swg off and any UV/ozonator systems turned off as well, I'm thinking the loss should be 0-2ppm maximum. 20+ ppm drop almost certainly sounds like there is something organic or perhaps an amonia condition eating the FC up :scratch:
 

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That was over 2 whole days though...no way of knowing just based on those numbers. Someone really needs to test after dark and before the sun comes up; once the sunlight hits the water the results are inconclusive...

Regardless of the size of the pool or stabilizer levels, with no swimmers the overnight loss should be low...<1 ppm ideally, but that may not be practical.
 
Without sunlight and bather load, the chlorine loss should be very low -- less than even 1 ppm FC per day unless the water is very warm (well above 85ºF). However, if you have ever had high bather loads in the pool and didn't completely change the water, then there will probably be lots of leftover organics in the water built up over time and this is probably what the chlorine is trying to oxidize. This is one of several reasons why one should use supplemental oxidation in a high bather load pool.

In my own pool when it was covered with a mostly opaque cover most of the time (years ago; now the pool is used every day), the chlorine loss was around 0.7 to 0.8 ppm and that was at warmer temperatures of around 88ºF for the pool. Some of this loss, probably around 0.2 ppm or so, was likely due to chlorine oxidation of cyanuric acid. Some was due to oxidation of algae and other organisms trying to grow mostly above the water line. Some was due to oxidation of pollen and leaves that infrequently got into the pool (infrequent because it was mostly covered). Some loss was from chlorine outgassing and likely reacting with the cover. The loss is very temperature dependent.

In a pool without CYA, there will be higher losses from outgassing and any oxidation of organic matter in the pool since the active chlorine level will be much higher, but it still shouldn't be anywhere near as high as you are seeing. You've definitely got something in your pool water. If you took a clean bucket of tap water and added the same chemicals you've got in the pool and put it by the pool or sitting in the pool (to be at the same temp), I'll bet the chlorine won't be dropping quickly in that bucket. This is basically like the evaporation vs. leak test except for checking on chlorine losses.
 
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