Chlorine lockup

[Larry]

We usually use monopersulfate shock when we need to keep the chlorine level down due to the pool being open to bathers. The MPS is also great because you can swim so soon after use.

With our communal pools we insist on having the pool closed for one day a week and shock the pool with cal hypo the evening before. If the CH starts to rise we switch to sodium hypo (12.5% - pH 13) and compensate with extra pH decreaser. Any other oxidiser shock treatments must be done with MPS or we get bleached bathing suits, dry hair and skin, and bloodshot eyes (for the kids who spend all day in the water).

We use (a token) algaecide but do not count on it to prevent algae. We make sure that FC never falls below 1ppm (at <0.3ppm CC) and we test and dose the chlorine manually 3 times a day.

[chem geek]

If you maintain an appropriate chlorine level relative to the CYA level and make sure it is stable under varying bather load (which means adding more chlorine when the load is high to keep the same FC), then you should hardly ever need to shock the pool. Shocking the pool is usually necessary if you aren't keeping up with oxidizing bather waste or preventing algae growth. If your pool communal pools have CYA in them, then 1 ppm FC is not nearly enough to prevent algae growth unless you are using a supplemental algaecide (e.g. weekly PolyQuat 60) or a phosphate remover.

Though I only have a residential pool, I never had to shock even once this past season and only once the season before (not counting opening when I usually shock just for the heck of it). The 16,000 gallon pool is in use nearly every day during a 7-month swim season, but has a light load of 1 person for an hour during the week and 2 on the weekends. I only add 12.5% chloirinating liquid to the pool and the pH is rock solid. The pH will only rise over time if your TA level is too high. Hypochlorite sources of chlorine do NOT make the pH rise permanently. The pH rises when you add the chlorine, but then it drops as the chlorine gets used up. The chemistry is described in this post. My pool would have a significant rise in pH every week if this were not true. Of course, I also have an electric opaque safety cover so outgassing of carbon dioxide is minimized, but if the TA is lower then that reduces that rate of outgassing. It's one of the most counter-intuitive things in pool water chemistry that unless you are using acidic sources of chlorine or other chemicals, then a lower TA leads to greater pH stability over time.

If one wants more stable pH even in the short-run, say for chlorine addition for shocking or from MPS shocking, then one can use 50 ppm Borates in the pool. This also acts as a mild algaecide.

By the way, I'm not saying that MPS isn't useful, and in a commercial/public pool with higher bather load it can be useful even if you are using appropriate levels of chlorine since it does oxidize some chemicals faster than chlorine. It's just that for most residential pools, shocking and algaecides aren't necessary if one maintains an FC that is at least 7.5% of the CYA level in a manually dosed pool.

Richard

[pool jockey1f7a317r]

Pool Geek you helped me a lot with your 2009 discussion of chlorine lockup produced over the winter with a complete loss of CYA and a high total chlorine level but no free chlorine. I have a 15K gallon in-ground pool and last year it had chlorine lockup at startup. It took about 8 gallons of liquid pool chlorine to finally release some gasses and give me FC. I shared my good experience with my local pool supply company who found the info helpful too. This year I've not been so lucky. My ammonia tab is again reading 3.0 ppm. My liquid pool chlorine label says that 1 gal of it will raise the chlorine levels in 10K gal of pool water about 6ppm. I then estimated that 9-10 gal would do it for a 15K gal at your formula of 10x the ammonia level (8.5 gal of chlorine) and even added 1.5 gal more. It didn't happen -- ammonia levels still at 3.0 -- no FC -- total chlorine still high at about 5ppm. No gasses released. Using new pool & ammonia test strips. Any ideas/suggestions? Hate to lower and refill but may have to.

[chem geek]

The ammonia test just gives you a minimum chlorine level that would be needed (10x the ammonia reading), but you could need a lot more because there can also be partially oxidized CYA that doesn't get mesured in either the CYA test nor the ammonia or CC tests. If you look at a log of my experience with this problem, you can see that from the time I measured ammonia at 2 ppm, I had to add about 50 ppm FC instead of the 20 ppm FC that would be needed just for the ammonia reading alone.

Also, in your case, note that when you are measuring FC and CC, that the ammonia test will read CC as well if it is monochloramine. Basically, the ammonia test reads ammonia plus CC/5. So in your case you may just have CC that needs to get oxidized and that can take hours -- exposure to sunlight usually helps as the chlorine breaks down to create hydroxyl radicals.

You can always do a bucket test with pool water to estimate how much chlorine will be needed where 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC.

You can also avoid having to deal with this problem if you close your pool later when the water is colder and shock it with chlorine and then add PolyQuat 60 algaecide and then open the pool earlier before the water warms up, adding chlorine to it right away. Maintaining chlorine levels throughout the winter would be even better, but is not practical in a winterized pool.