Your thoughts please...

[loop_pea]

I run a well used school pool and have posted on here before. We use a mixture of trichlor pucks and liquid chlorine. Last year the pool ran pretty well. I had to increase the water replacement when we got to higher CYA levels but that worked out okay all things considered.

This year, things went well for the first five weeks or so, but then went a bit haywire. The first sign of trouble was the filter pressure going up 3-4 days after the last backwash. (I've never seen the filter pressure go up before.) Other signs were increasing CC and the pH going up with it. In this pool the two things always seem to go hand in hand. Strangely, the chlorine consumption didn't seem to be affected.

When this happened, I closed the pool and raised the chlorine level to 25PPM for two nights (CYA=50). I didn't have a FAS-DPD kit at that point, so I used a dilution method with a photometer, so my testing wasn't super accurate, but I don't think it was terrible either. The CC went up the first night, but then came right down over the next day, virtually to zero. (I've never seen zero CC with a photometer).

We reopened and everything seemed okay for several days and then the CC and pH started to go up again. So this time, I purchased a FAS-DPD kit and took the pool up to 30PPM (CYA now about 45 because I'd added water and vacuumed to waste). The CC was never more than 0.5, even on the first evening of shocking. I kept the pool at this level for 48 hours and brushed and vaccumed when I could. We don't have any ladders or lights to worry about, and I left the vacuum and brush in the pool to clean them. The CC went to zero pretty quickly. I can't be 100% sure about the overnight test because there was daylight on the pool for four or five hours before I could get to test it in the morning, but it only fell by 3.5PPM with those hours of daylight included.

We reopened when the chlorine dropped below 10. But by the end of the first day of use, the pH has gone up, the CC is up, and now the pool is starting to look ever so slightly milky. I don't think it's calcium clouding as the water is fairly balanced (TA is 90, CH is 230, and pH 7.6). There was still 8PPM of FC in the water by the end of that day, which seems like plenty.

I'm getting a bit fed up now. I thought I'd reached the right kind of shock levels to kill even mustard algae and maintained them high enough to prevent it coming back. Have I overlooked something in the shocking process? The only other thing I can think of is that the chlorine isn't keeping up with the demands made on it due to the number of bathers, and the filter isn't keeping up either. I think most people on this forum run residential pools, but your thoughts would really be appreciated.

 

[RobbieH]

You aren't finished shocking until CC is .5 or less, the pool is clear, AND you pass the overnight chlorine loss test (less than 1ppm loss overnight). If you did not hold shock level until you passed all three, you didn't finish the process.

 

[duraleigh]

RobbieH is essentially correct, I think. What sort of swimmer load did you put in the pool once you let chlorine go back down? I think it might be a combination of stopping the FC process prematurely and then immediately introducing a heavy swimmer load.

 

[chem geek]

With a commercial/public pool with higher bather load, you would be better off with a lower CYA closer to 30 ppm and a higher FC/CYA ratio so an FC perhaps 20% of the CYA level, so 6 ppm FC. It sounds like the bather load is too high for the chlorine to handle. Is this an outdoor pool exposed to sunlight? Is there anything different this year compared to last such as a difference in the weather or the amount of bather load?

As for the filter, I presume it's sand. What did the water look like when you backwashed after the pressure went up?

 

[loop_pea]

With a commercial/public pool with higher bather load, you would be better off with a lower CYA closer to 30 ppm and a higher FC/CYA ratio so an FC perhaps 20% of the CYA level, so 6 ppm FC. It sounds like the bather load is too high for the chlorine to handle. Is this an outdoor pool exposed to sunlight? Is there anything different this year compared to last such as a difference in the weather or the amount of bather load?

As for the filter, I presume it's sand. What did the water look like when you backwashed after the pressure went up?

Hi chemgeek. Yes, it's an outdoor pool. It has about 90 children in it per day, which is the same as usual. The big difference between this year and last is the weather - it has rained and rained and then rained some more this year. They swim in all weathers though - the kids get wet anyway, and the staff stand at the pool with brollies!

Yes it's a sand filter. The backwash water was really cloudy. That's usually how it looks, although it was more cloudy and stayed that way for longer than normal. I think the grey/white appearance is partly caused by the coagulant collecting stuff in the filter like its supposed to because when we don't add it, then the backwash water is much clearer.

What puzzles me was that I am running much lower CYA levels this year - roughly half of last year - as well as higher FC levels. Last year we ran the pool with the minimum FC set at just 5% of the CYA. This year, on the day that the water started to look less clear, the FC was well over 8 all day, which is 20% of the CYA, AND it had just come down from shock level.

 

[chem geek]

Well, then it sounds like it may just be high particulate load that would make the water cloudy and really just needs better coagulation/filtration to remove it. I don't think the change in active chlorine level is having any effect on it -- a higher active chlorine level would only keep it clearer if anything, but for suntan lotion you need physical removal since chlorine will not break it all down. I'm not sure why the extra rain is causing any difference unless it was "dirty" adding to particulate matter in the pool.

Regular sweat and urine won't generally cloud up a pool. Those components are fairly water soluble. It's mostly suntan lotion that is the biggest culprit. There is some skin oil that can also be an issue. Of course, having all bathers rinse off first (preferably with a little soap, but being sure to rinse the soap away) should help quite a bit not only with cloudiness but having a lower chlorine demand. The fact that you found higher CC but not a noticeably higher chlorine demand does seem to point more towards chemicals that may form some CC but aren't getting fully oxidized. Again, I'm guessing it's mostly oils including suntan lotion.

 

[loop_pea]

Yes, I think it must be the filtration/chorine not keeping up with the demand. Term ended, and the pool isn't open again until tomorrow, and now the water looks perfectly clear again although there is a fair bit of dust on the bottom. I don't think the main contaminant is sun cream though as we haven't had any sun!

I didn't have any issues at CYA levels of 30 or below, so your suggestion to run the pool at that level makes a lot of sense. It's taken me a while to get used to the idea of dosing with chlorinating liquid, but actually now I prefer it. Now that I've been using it for a while, I can see how this might work.

The only downside is that here in the UK trichlor tabs are about half the price of chlorinating liquid per ppm of chlorine added.

 

[chem geek]

The only downside is that here in the UK trichlor tabs are about half the price of chlorinating liquid per ppm of chlorine added.

Don't forget that you need to account for the pH/TA adjustment chemicals that are required when using Trichlor since it is so acidic. As described in this post (with older pricing), such chemicals can cost at least 60% the cost of Trichlor. Even so, it sounds like in your situation that Trichlor will still be less expensive, but of course has the side effect of increasing CYA.

Though you could technically just proportionately increase the FC level as the CYA level rises to keep the same active chlorine level, you'll probably run into some regulation that limits FC level by itself independent of CYA level.

 

[loop_pea]

[quote="loop_pea":2bogbgb1]The only downside is that here in the UK trichlor tabs are about half the price of chlorinating liquid per ppm of chlorine added.

Don't forget that you need to account for the pH/TA adjustment chemicals that are required when using Trichlor since it is so acidic. As described in this post (with older pricing), such chemicals can cost at least 60% the cost of Trichlor. Even so, it sounds like in your situation that Trichlor will still be less expensive, but of course has the side effect of increasing CYA.[/quote:2bogbgb1]

We start the season with a fresh fill of water that's got moderate calcium harness and high TA, so we need acid even with trichlor. With chlorinating liquid, we use even more acid.

Though you could technically just proportionately increase the FC level as the CYA level rises to keep the same active chlorine level, you'll probably run into some regulation that limits FC level by itself independent of CYA level.[/quote]

Yes, we run into trouble at 10ppm as we have to close the pool at that level. In fact, I have been using variable FC targets - it confuses the other chemical testers though.

In the meantime, I have worked out what the strange blobs in the pool are - rust! I found a lump on one of the handrail brackets that looked for all the world like a slug, but when I brushed it away, it broke up into small yellowy brown pieces that settled on the floor of the pool at the edges.