In the water treatment process, at the chlorination of water, we usually have two kinds of residual chlorine (free+combined) after the chlorine demand met. But at the time of chlorination of reclaimed water, I am confused that what kinds of chlorine do I expect after the demand met. I have found somewhere that there should be one residual chlorine that may found either free or combined at the chlorinated reclaimed water. Could you please tell me which one do I get after chlorination of reclaimed water? Thank you.
You will probably have chloramines (resulting from the reaction of chlorine with nitrogen containing compounds), haloforms (resulting from the reaction of chlorine with carbon containing compounds), etc.
You can have an idea of the multiple possible reactions having a look at the introduction section of these articles:
Article Exposure to chloramines in the atmosphere of indoor swiming pools
Article Effects of nitrogen trichloride stripping on air quality in ...
Article Exposition aux chloramines dans les atmosphères des halls de piscine
You got very good and comprehensive answers. I want to add something, though. Keep in mind that haloforms, by definition, are neither free nor combined chlorine when we talk about breakpoint chemistry. So it is true that some of the chlorine as an atom will exist in bond with carbon, but it will not contribute to your residual chlorine measurement and have no effect (in practice) in disinfection. They are characterized by AOX and are important regarding toxicity.
I think reading about chlorine breakpoint chemistry might be a good help here.
This discussion is very useful. While the original question has not been directly answered, the responses seem to be suggesting that the bigger issue is that chlorination is a dangerous way to disinfect reclaimed waters. Is there some safer, similarly affordable alternative? Maybe ozonation or UV irradiation?
Patrick McLoughlin For ozonation, one always has to deal with bromate formation as well. So although ozonation might be a better option in some cases (low bromide), it is not an answer to all problems. On the other hand, UV directly affects RNA or DNA, and it potentially leads to a low amount of disinfection by products. Yet UV is very expensive to apply. Overall, alternatives in a direct comparison with chlorine can be safer but not affordable. That's why chlorine is still one the most used disinfectants (if not the most), although it is known to produce high levels of undesired DBPs. The disinfectant choice very much depends on the usage of reclaimed water. Surly, the standard for irrigation differs from drinking water, so do the DBPs, technology, and cost.
Thank you. Is it useful and practical to run the water through something like a GAC to reduce the organics the chlorine interacts with to form DBPs?
Granular Activated Carbon treatment prior to chlorination does indeed remove DBP precursors which could potentially reduce overall water toxicity post chlorination. Nevertheless, it is important to consider how this precursor removal will influence the formation of DBPs during chlorination as there may be a shift towards the formation of more toxic DBPs. It is also possible to add a prechlorination step, so that the GAC will remove many preformed DBPs.
The paper below gives a nice overview of this topic:
Article Does GAC with Chlorination Produce Safer Drinking Water? Fro...
Chorite, Chlorate and Perchlorate are also problematic residuals of chlorinated waters.
Chorite, Chlorate and Perchlorate are also problematic residuals of chlorinated waters.
Chorite, Chlorate and Perchlorate are also problematic residuals of chlorinated waters.
Chorite, Chlorate and Perchlorate are also problematic residuals of chlorinated waters.
Chorite, Chlorate and Perchlorate are also problematic residuals of chlorinated waters.
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