Present water supply treatment uses chlorine as primary disinfectant followed by the addition of ammonia to form chloramine as a secondary disinfection. When water temperature is above 23 -24 degrees centigrade nitrification is severe in downstream storage tanks. Decay of 1 -1.5 mg/L total chlorine is also observed in the bulk water mains before reaching the storage tanks. Treated water leaves the water treatment plant at pH 7.5 -7.8. Although not widely reported some water authorities had demonstrated that chloramine is much more stable at pH 8.5 - 9.0.
Chlorination efficiency is strongly dependent from pH. Also you can't forget about Langelier index. At high pH scale formation may take place.
Marta thank you for your quick response however I was asking about the stability of chloramine rather than about the efficiency of chlorine. Several papers refer to stable chloramine at pH = 9.0. Did you have any experience with this?
Thank you about reminding me about scale formation which I will need to check. Presently at pH 7.8 the L.S.I. is slightly negative which is ideal.
If your chloramine turns into nitrate it indicates you have a lot of biofilm in the storage tanks. They should be be drained and cleaned regularly.
If you have biofilm in pipes and other places that cannot be cleaned you can periodically dose chlorine dioxide which penetrated biofilm better than chlorine and chloramine.
Thanks Henrik
We undertake regular deep cycling and also clean sediment from our storage using divers. However we have identified that the divers do not clean the walls.
About chlorine dioxide - I have read that it is very effective for controlling nitrification provide it is dosed before nitrification sets. I understand that it is not effective once nitrification starts. I would appreciate your comment.
I have never tried to use chlorine dioxide specifically to kill of nitrification bacteria but generally to break down biofilm. Nitrifying bacteria are generally not very robust.
Once I helped a company to solve a problem with biofilm in the pipes causing a persistent count of heterotrops in a swimming pool that operated with 2.0 mg/L free chlorine. Here adding just 0.3 mg/L ClO2 apparently killed all the bacteria in the biofilm and the microbiological control samples got back to compliance and remained good for several weeks.
The following link may be helpful in answering your question Effect of Chlorine vs. Chloramine Treatment Techniques on ...
https://www.usbr.gov/research/projects/download_product.cfm?id...
Technical Memorandum No. MERL-2013-57
Effect of Chlorine vs. Chloramine Treatment Techniques on Materials Degradation in Reclamation Infrastructure U.S. Department of the Interior Bureau of Reclamation Technical Service Center Denver, Colorado
Dear Dr. Omran,
It seems your link doesn't function right now.
It is important to realise that chlorine dioxide has an unique ability to penetrate biofilms that free chlorine and chloramine lacks.
Isam
Please update the link to the technical memo as it is not functioning.
Thanks
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