So, am I ready to ROCK???

[SeanB.]

My thinking is that one of the reasons I am getting the SWCG is convenience. My unit will be over-sized so it should last the longer end of the range, say 4-5 years. If, by using it to shock, I shorten the duration by say 10%, then I will only have lost 4-5 months. If you calculate a cell's value as a dollar figure, that means that extra shocking and reduced life will cost me about $30-$40 (less if you subtract the cost of the bleach). To me, that is well worth not having to buy, store and handle bleach over a four to five year period. Even if you double the figure to 20% loss of cell life, I still think it's a reasonable trade off.

[waterbear]

From my understanding the biggest problem with superchlorinating with the cell is not the shortening of the cell life (although that IS a concern) but that the FC levels rise slowly over the period and during this certain types of complex chloramines can form when FC levels are high but below breakpoint that make it more difficult to reach breakpoint than if the proper amount of chlorine is added all at once to reach a desired ppm of FC. (Richard or anyone else please correct me if I am wrong on this, my research indicates this is true).

[SeanB.]

From my understanding the biggest problem with superchlorinating with the cell is not the shortening of the cell life (although that IS a concern) but that the FC levels rise slowly over the period and during this certain types of complex chloramines can form when FC levels are high but below breakpoint that make it more difficult to reach breakpoint than if the proper amount of chlorine is added all at once to reach a desired ppm of FC. (Richard or anyone else please correct me if I am wrong on this, my research indicates this is true).

I wasn't aware of that. I was under the assumption that shocking with the SWCG was as effective. If that is not the case, then I will have to reconsider my plan. Has PoolSean ever given an opinion on this?

[Pool_Mike]

PB went out and called me to let me know that he back washed and it was very dirty. He said in less then 1 hour, now the lower end of the pool , now that you can see the floor.

yah!!

[chem geek]

From my understanding the biggest problem with superchlorinating with the cell is not the shortening of the cell life (although that IS a concern) but that the FC levels rise slowly over the period and during this certain types of complex chloramines can form when FC levels are high but below breakpoint that make it more difficult to reach breakpoint than if the proper amount of chlorine is added all at once to reach a desired ppm of FC. (Richard or anyone else please correct me if I am wrong on this, my research indicates this is true).

I don't have a salt cell, but even at full power and running 24/7 you are right that the increase in chlorine levels attained in the pool will be much slower than dumping liquid chlorine into the pool. However, if one is starting from an existing free chlorine level then whatever chloramines that would have been formed have already done so. Free chlorine combines with ammonia very, very quickly to form chloramines. It is the next steps to breakpoint that are slow (and free chlorine oxidizing organics can also be slow). So from a chemistry point of view, I don't think the slow increase in FC from a salt cell would be an issue unless the FC were at or near zero for some reason -- perhaps if there were a major ammonia (i.e. urine) accident that overwhelmed much of the free chlorine in the pool, remembering that you need about 10 times as much FC to achieve breakpoint as there is ammonia.

Urine is about 2.5% urea by weight and urea is about 50% nitrogen (ammonia) so 2 cups of urine produces around 5 grams of ammonia and that requires about 50 grams of chlorine for breakpoint. A 10,000 gallon pool is about 38,000 liters fo 50 grams of chlorine in 38,000 liters is 1.3 ppm. So depending on size of pool, FC level, and size and number of accidents, one could use up all the FC in a pool. And certainly one uses up the FC in a local area rather quickly.

So if you need to superchlorinate because of an "accident", then I agree that quickly administering a large dose of chlorine is wise so that breakpoint is more easily achieved. If instead it's just a small amount of measured combined chlorine that has accumulated, possibly from slow combining with organics (not ammonia), then a slow rise in chlorine would probably be fine. Of course, the issue of the life of the salt cell is real and adding liquid chlorine is easy and relatively inexpensive.

[EDIT] In the presence of CYA, the breakpoint chlorination process is slowed down considerably since CYA reduces the disinfecting chlorine concentration. A normal breakpoint at an FC of 2.0 ppm with no CYA takes around 30 minutes to effectively complete so with 30 ppm CYA this would take about 11 hours if there was no sunlight. I don't know how much faster breakpoint goes with sunlight. [END-EDIT]

Richard

[waterbear]

I don't have a salt cell, but even at full power and running 24/7 you are right that the increase in chlorine levels attained in the pool will be much slower than dumping liquid chlorine into the pool. However, if one is starting from an existing free chlorine level then whatever chloramines that would have been formed have already done so.
If the organic load is higher than the chlorine available then more chloramine would form as the chlorine was increased, correct? From my understanding it is only after all the ammonia and organics present have combined that breakpoint can be reached.
Free chlorine combines with ammonia very, very quickly to form chloramines. It is the next steps to breakpoint that are slow (and free chlorine oxidizing organics can also be slow). So from a chemistry point of view, I don't think the slow increase in FC from a salt cell would be an issue unless the FC were at or near zero for some reason -- perhaps if there were a major ammonia (i.e. urine) accident that overwhelmed much of the free chlorine in the pool, remembering that you need about 10 times as much FC to achieve breakpoint as there is ammonia.

My understanding of the chemistry is that incomplete breakpoint (from a slow rise in chlorine) would favor the formation nitrogen trichloride (even at higher pH) and if the organics are from complex sources such as algae and not just ammonia (which is often the real world case when we 'shock') then it would cause the formation of other disinfection by products such as organochloramines, which are difficult to break down. Also reaction would not go to endpoint, which would be the release of nitrogen gas as the ammonia is broken down but cause the formation of nitrates in the water, which are a souce of food for algae.



[EDIT] In the presence of CYA, the breakpoint chlorination process is slowed down considerably since CYA reduces the disinfecting chlorine concentration. A normal breakpoint at an FC of 2.0 ppm with no CYA takes around 30 minutes to effectively complete so with 30 ppm CYA this would take about 11 hours if there was no sunlight. I don't know how much faster breakpoint goes with sunlight. [END-EDIT]
Once again this seems to indicate that the slow rise in FC caused by superchlorinating with the cell is not an effective way to reach breakpoint.
Richard

Your thoughts on this would be appreciated....perhaps in the China shop?

[chem geek]

I've continued this discussion on the pre-existing Breakpoint Chlorination thread starting at this post. Please meet me there.