Stubborn pools not holding Chlorine

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T

TreeFiter

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In The Industry
Jul 2, 2012
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Saugerties, NY
In the past few weeks, I've seen a handful of pools that won't hold chlorine at all. Usually when I see this, it is a pool with phosphates, and we either treat phosphates and/or shock the pool. Lately several pools have done this without a phosphate problem. Shocking them doesn't seem to solve the problem like I would expect. Usually 4lbs of trichlor will bring a pool back even after it has gone green and cloudy. These pools are usually still clear, a few have been slightly cloudy, or have had small amounts of algae growing on the liner. What seems confusing is that I've seen a few get quite cloudy and come right back with 4 lbs of trichlor, but others will be crystal clear, and just test low for FC and don't seem to be phased by it. I would think the cloudy pool has a bigger algae problem and would eat up chlorine faster. So why am I having trouble getting these pools to respond. In all of these pools, chemistry is all where it should be except for Chlorine levels. Most of them are using SWGs. Most of my pools are running somewhere around 50% this time of the year. The pools in question typically start showing lower than usual readings (around 1 or 2 ppm FC, sometimes lower) and even after turning the SWG up to 60 or 70% for a week, we don't see it rise.

This all sounds like algae trying to bloom, but why does it take so much to stop it? One of these pools had been shocked several different times, and finally started holding chlorine once I put in 12lbs of trichlor. We tried 6lbs a few days before, and the next day there was no FC in the pool. Any thoughts? Is it normal to have to shock this hard? Am I wrong to feel like 12lbs of trichlor is excessive? Or am I on the right track with hitting these pools hard with trichlor?
 
TreeFiter said:
Am I wrong to feel like 12lbs of trichlor is excessive?
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Nope, I personally feel the appropriate amount of trichlor to use is 0lbs.

Also, I know you are in the industry so you may not believe me and if you do your co-workers will tell you otherwise, but phosphates are meaningless. If you maintain proper chlorine levels for the CYA level then algae will not grow no matter how much phosphates are in the water.
 
I'm aware that treating phosphates can be a waste. I usually try to keep things in check by maintaining FC levels, but once things start to slide, my boss wants us to test for phosphates, and if they are there in high enough concentrations, we treat them. Since we only do maintenance on these pools once a week, it can be difficult to keep them under control with high phosphates. Once FC dips, the pool is green almost instantly. If we get rid of phosphates, it gives us a little bit of leeway.

As for not using trichlor...I don't have much say in what chemicals we use. I have to work with what I have, and what I have it trichlor.
 
I can't argue any of that. It is why I am not a fan of weekly pool services. That's not a personal attack on you, a job is a job and you obviously care about the work you do. I just feel the system is not a good setup and the use of trichlor powder does not help change that opinion.
 
We use test strips for everything. So I'm not very confident about CYA levels, but its really all I have to go on. I'm reluctant to think that what I'm seeing is caused by CYA because for the pools I'm seeing problems in are testing the same as the pools that are fine. I try to keep FC at 5 or higher in all of my pools because I don't trust CYA levels.

Is the concern that CYA is too low, and FC is dissipating, or is it a concern that CYA is too high, and preventing FC from being effective, resulting in algae growth which in turn consumes chlorine?
 
When the CYA is low in those pools, the SWG is unable to keep up from the chlorine loss from sunlight. Most SWG systems are not sized properly for a pool -- the manufacturers recommendations on pool size are woefully inadequate and some are only able to keep up when the CYA is at the high level the manufacturers recommend which is 60-80 ppm (70-80 ppm preferred).

So if you keep the CYA lower, you need a higher runtime from the SWG while if you keep the CYA higher you have less loss of chlorine from sunlight. However, in both cases, you must have an absolute minimum FC that never, and I mean never, gets below the minimum FC that is 5% of the CYA level.

Maintenance of an SWG pool where the FC is at least 5% of the CYA level only works if you have killed off nascent algae growth. At that point, the SWG is able to maintain the chlorine level assuming there isn't something else that uses up chlorine in the pool such as a pool party with urinating kids.

Nascent algae growth can sometimes not be visible at all and the water is clear. This leads some in the industry to swear that phosphates somehow directly consume chlorine, but that is simply not true. We have thousands of SWG pool owners on this forum who don't have the problems you are seeing -- some did have such problems when they first came here, but no longer. So what FC level do you target as a percentage of the CYA level for these pools? Since you only visit once a week, you should probably target higher than the 5% we recommend to allow for things that could go wrong during the week. An FC that is 7.5% of the CYA level or 10% of the CYA level should provide an ample buffer for you, especially if the CYA level is lower.

See the table in this post (in a thread where we have talked to you before about this very same problem last year) that shows that a lower SWG on-time is needed when the CYA is higher even when the FC is proportionately higher. Also note that this table has on-time percentages based on 0.5 ppm FC per hour which is higher than the 0.3 to 0.4 ppm FC per hour that SWG manufacturers typically recommend.

Since you are using test strips, anything is possible. Your FC may be lower than you think and the CYA may be higher than you think so that algae is able to grow (i.e. the minimum FC/CYA ratio is dropping below 5%) or it may be possible that your CYA is lower than you think and the FC is dropping faster due to sunlight. The only way to know for sure is to do an overnight chlorine loss test since the chlorine level will drop from algae growth (and chlorine reacting with algae) overnight, but there is no sunlight loss overnight. Even if you don't use a proper test kit for every pool, do yourself a favor and get one so that you can at least check on the problem pools so you can have at least some idea how to fix them.
 

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Thanks again chem geek. As always, you have been a big help.

Assuming the test strips are reasonably accurate, the CYA levels tend to be around 30. I try to keep my FC at around 5ppm.

After looking at the talbe, I should bring the CYA up quite a bit. It sounds like I need to bring the CYA up to around 50, and if I keep FC around 5ppm, I should have a good cushion since I only need 2.5ppm. I should also be able to run the SWG at about 30%.

I think I am going to have to bite the bullet and invest in a good test kit. It would really make my life easier whenever there is uncertainty.
 
Understanding that an accurate test kit is the best tool for deciding what you need to do is the most important step in getting a better pool. If you know the CYA is low, you can safely treat the pool with a week's worth of trichlor and come back next week but if the CYA is too high, the trichlor only going to mask or aggravate the problem. It would also help you to dial in SWG better so they may not need any trichlor.

Can CalHypo tablets be used in a weekly maintenance program?
 
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