I would like to know when the presence of bacteria affects the breakpoint chlorination curve. I know it takes about 10 times the dose of ammonia to reach the breakpoint. Does this mean that the presence of bacteria does not influence the chlorine demand?
2nd question: when calculating the CT value, at no time is the number of bacteria present in the water taken into account?
Thank you for your help
The breakpoint curve represents the evolution of free and total chlorine based on the injected dose of chlorine. This corresponds to the elimination of ammonia but also to all elements likely to react with the chlorine like reducing agents (Fe2 +, H2S, etc.) or organic matter.
In reality, the resulting curve is never as elegant as in the books.
Bactericidal action is mainly obtained once free chlorine appears, although chloramines have a slight disinfectant effect.
Indeed, when calculating CT values, we don't take into account the number of bacteria and one can imagine that even with a 6-log reduction, there may still be bacteria in water containing 107. The reason for this is straightforward. We never disinfect polluted water. It’s not for nothing that disinfection is the last step in our treatment plants. In general, 90% of bacterial elimination is done during coagulation/sedimentation/filtration.
So, either we have clean water and simple chlorination suffices—which is the case for a lot of deep water—or the water is polluted and requires a whole series of steps to disinfect it, which is the case for surface waters.
In other words, we don't chlorinate water until only a very limited number of bacteria are left.
Thank you very much for your explanation; it's much clearer now.
Is it fair to assume then that the dose of chlorine to be injected in order to achieve a residual amount at tank outlet, in the order of 0.3 ppm, is approximately 10 times the dose in ammonia + 0.3 ppm?
So, to obtain a residual of 0.3 ppm at tank outlet you need:
10 times the dose of ammonia + the dose of chlorine to oxidize the iron (which I know how to calculate) + the dose of chlorine to oxidize the organic matter (which I don't know how to calculate) + 0.3 ppm?
The best approach is to perform a lab test that allows for adjustment of the dose in a practical way, because if the formula is fairly easy to reproduce for ammonia, it is more difficult to model the immediate demand for organic matter.
For iron oxidation, there is indeed a demand for chlorine, but iron is oxidized as Fe(OH)3, which will settle in the pipes if there is no additional filtration.
Caution: the breakpoint treatment is often accompanied by de-chlorination/re-chlorination because, in many cases, it is difficult to maintain a consistent residual.
I would advise you to simply measure chlorine demand and that way you'll know what your water consumes. Caution: You will have to increase these numbers to offset the demand for chlorine in your tank.
Thanks for your answers. I think I will have the lab do a chlorine demand test.
-Do you know the protocol for this test?
-At tank outlet, the residual chlorine concentration has to be between 0.3 and 5 ppm, maximum. But do you know if there is a maximum dose of chlorine to inject to avoid any smell of chlorine from the tap?
Chlorine demand test:
Measure out the concentrated bleach that will be used for the test (favor iodometric method over colorimetric)
Prepare a diluted solution according to the dose to be applied (based on ammonia content) and the type of pipet. Two successive dilutions are often necessary. For example, bleach at 25° (chlorometric degrees) is diluted to 1/250. This is then further diluted to 1/5 and, using a pipet ranging from 0.1 to 0.5 ml you can testing theoretical doses of 0.13 to 0.64 mg Cl2/l on a water sample of 50 ml. To specify the range, you can start with the ammonia concentration and base the maximum injection amount at 15 times its value, so as to be sure to frame the breakpoint.
After a contact time of 30' in the dark, you measure free and total chlorine using colorimetry and determine the demand curves.
If you're not too sure about bleach values, you can do a parallel test on distilled or demineralized water and take the total measurements of chlorine as theoretical injection values.
This is the general principle; there may be variations depending on the user.