Hi Gloria, speaking about ...

As you can see you have lots of different answers and I am going to give yet another.  When I have been involved in private water supplies, then there has been a contact tank because there is a need for storage and an inbalance between usage and supply.  In the case of water recovery from a waste treatment plant, followed by UF & RO, then the water was UV treated then a residual chlorine dosed.  There was virtually no chlorine demand and because of the distance between the plant and the factory, no contact tank was used. That was the exception.  In almost all plants, there is a need to allow the chlorine to react with any material in the water.  If the water supply is mains, then the chlorine demand will have been overcome, but a tank will be required to balance flows.  Where there are other water sources, the chlorine will need to overcome the chlorine demand and provide a residual chlorine.  UV is used for several reasons.  Firstly, filtration followed by UV is the only realistic way of controlling parasites such as Cryptosporidium because protozoa are resistant to chorine, chlorine dioxide etc at normal dose levels.  UV does reduce viruses and a combination of UV and chlorine will prevent them from being a problem.  If there is a microbial burden in the water, UV will reduce the bugs by 99.99% and chlorine dosed afterwards will prevent growth on the cell contents and further kill not only good cells, but damaged ones.  The use of in line mixers can be of use but only to extend a reaction time.  One of the comments provided a table of different materials and their residual effect.  Two of them, chloramine and chlorine dioxide are highly volatile so in-line mixing can de-gas them rendering them ineffective as killing agents.  There is another reason, which is that often the reaction tank is also a class 1 break tank separating the mains and private water supplies.  If the water supply is mains and the water is treated with UV and chlorine then there must be a class 1 air break to prevent any feed from the factory supply back into the mains.

As far as the microbiology is concerned, there is a need for the reaction to take place between chlorine and the any bacteria and the contact time will vary depending on the bacteria.  Whereas the impact of UV is immediate causing the bacteria to burst, chlorine works in a different manner.  The presence of the chlorine will tie up any organic cellular material from bacteria and killing others.  Whereas the reaction with the organic contents is very quick, the reaction to kill further bacteria is slower.  I hope the answer helps.

Published July 29, 2017 15:14 by Don Sharpe

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