Chlorine Breakpoint Curve

April 15 is upon us, which means it’s Tax Day in the USA. Make sure you’ve filed your returns! It sounds like we have a slight reprieve until Monday, due to some obscure holiday observed only in Washington D.C.
Another glimpse at the calendar tells us that we are only five weeks away from our next Treatment Operator certification exam here in California. This is a good week to begin your review studies. As we have suggested in the past, do a little bit at a time over a prolonged period for the best results on exam day. Don’t plan on The Cram in the last day or two before the exam — it just doesn’t work.
Probably the most important topic on the Treatment exam is disinfection. Look back through our Archives here for brief overviews of several disinfection topics. As I reviewed them, I found my discussion on today’s topic to be a bit lacking, so I thought I would add to it today.
As we look at the horizontal axis of the breakpoint curve, we are starting at zero chlorine added, and increasing the total chlorine dosage as we move to the right on the curve. The vertical axis measures the total chlorine residual. If you think about it, our “curve” should be a straight line: if we add 1.0 mg/L of chlorine, we would expect to see a residual of 1.0 mg/L. This is true for a short while, but then a most unusual thing happens: as we add more chlorine, the residual decreases!
The vertical — or “y” — axis measures total chlorine residual. Technically, that is the sum of the free chlorine and the combined chlorine — or chloramine — residuals. In reality, we will only have one or the other. On the left side of the “breakpoint,” we have only combined residual; on the right side, we have only free. In essence what we are witnessing as the residual drops while we continue to add more chlorine is the transition from combined to free residuals.
In this decreasing portion of the curve, the chloramines formed in the first “rise” of the curve are being destroyed: if we have a ratio of chlorine to ammonia in excess of 5:1, the excess free chlorine will actually destroy some of the chloramines. This is where we see the residual drop on the curve.
What happens to the chloramines? A number of different chemical reactions are going on here, but predominantly, the chloramines are becoming nitrogen gas and nitrate — not forms of chlorine residual. By the time we get to the breakpoint, essentially all the chloramines have been converted to these two substances, along with a little chloride. And the decrease in residual came about by an increase in dosage!
This is definitely the most difficult concept to master on the chlorine breakpoint curve. I have seen many veteran Operators make the disastrous mistake of adding more chlorine when they saw their residual drop to undesirable levels. That would be most folks’ natural reaction, wouldn’t it? But it’s exactly the opposite of what is needed!
Do you understand the chlorine breakpoint curve well enough to realize that you may need to decrease your chlorine feed in order to increase your residual?
That takes a deeper understanding of the chlorine breakpoint curve than the vast majority of Operators possess. I hope you are in the minority in this case!
And an important announcement: my web site programmer is having technical difficulties. He told me today that we probably won’t have our online continuing education classes available to you for another 6-8 weeks. Sorry about that. Stay tuned for the latest on our course availability.

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