UV Disinfection, Residual Chlorine Dose and the Contact Tank

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In my organization there are some treatment plants with UV disinfection + residual chlorine dose + contact tank.

Lately it has been questioned whether the presence of the contact tank is really needed.

In terms of being compliant with UK legislation, there is no requirement to have a contact tank after UV, as the UV and residual dose are sufficient.

However, the risk of pathogenic contamination would effectively increase by removing the contact tank after. 

Is a contact tank after UV and residual chlorine dose necessary?

What is common practice in other organization and is it usual or not to have this arrangement of UV + Residual with contact tank after?

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21 Answers

  1. The Communities of villages are dependent on the income from farming which is received in peasantry work. The receivable items include the seeds which are only utilized in food intake and fodder. These communities have no asset except the small number of livestock and hens. Every household of villages has income of 200-300 in the form agriculture labor and formal activities while going to the city of Mirpurkhas. It is analyzed that almost all the families of the villages are living below the poverty line and therefore deprived of the basin human development and sustainable development facilities.

  2. Most of the population of the aforementioned villages is dependent of the daily basis earning in the form of doing labor work in the district otherwise they will have no survival option. Averagely, 200-300 per day is earned by the each family head depending on the availability work. Therefore, if the family head receives no work his family will have to suffer in starvation. The primary skill of the communities is of agri-based labor but this depends on the season. However in the off seasons the family members start moving to the cities for the odd labor on the daily wages.

  3. Yes contact tank is very important, to control reaction. In order for organisation to take responsibility of any incident after effluent. Now you change from good to bad, chlorine has no retention time, just dose and discharge. What if chlorine is less or high? How are you going to measure.

    Thank you,

    Mangena P

  4. Hello everyone,
    The use of chlorine is the most indicated and mandatory in most countries because it is proven efficient, mainly as a residual effect, none mentioned an important factor for the effectiveness of the dosed chlorine, which is the pH, with values higher than 8.0 the acid Hypochlorous agent most effective in disinfection has its very small dissociation, attached a presentation in Portuguese, but the tables are in English, I hope it is useful.

  5. Dear Gloria Fernadez,

    UV technology offers an added sense of assurance that the surface water is microbially free and clear without the unpleasant taste and odor of chemical disinfectants or potential chemical disinfection by-products.UV is a proven and safe alternative for primary disinfection as it is free of the harmful by-products associated with chemical disinfection. UV also has the added benefit of not compromising the taste, color or odor of water.V systems are being integrated with existing chemical disinfection treatments. UV works by irradiating or disrupting the DNA of microorganisms, rendering them unable to reproduce. It’s generally accepted that microbial DNA absorbs UV most effectively at 265 nanometers (nm), a wavelength that medium pressure lamps produce in abundance. In addition to emitting UV at this optimum peak wavelength, medium pressure lamps also emit UV over a very broad range of wavelengths (about 185-400 nm). Independent research has shown that, with proper pretreatment and a well-maintained UV disinfection system, this broad output causes permanent inactivation of both pathogenic and non-pathogenic microorganisms such as  Escherichia coli,  also known as  E. coli .Low pressure UV lamps, the common alternative to medium pressure lamps, produce just a single peak of UV output at 254 nm. While also having a strong germicidal effect, it has been shown that some microorganisms are able to repair or reactivate themselves after exposure to UV from low pressure lamps, especially if they’re subsequently exposed to sunlight. This is known as photo reactivation.

     

    Regards,

    Prem Baboo

     

     

  6. Hi

    I assume we are talking potable systems here.

    we have numerous systems installed and we do not use a contact tank. The uv should be installed before the chlorine dosing. We dose chlorine or chlorine dioxide up to potable standard. Depending on the pipe work you may need a static mixer inline to help mixing. 

    This can then either go straight to service or into a storage tank 

    hope this helps 

    Kevin 

  7. njour 

    Ce qui serait une bonne information à votre post: A quoi sert ce réservoir de 'contact'

    Il a une appellation qui ne veut rien dire mais certainement qu'il sert ni plus ni moins de réservoir tampon entre le tratiemetn qui a eu lieu en amont et la distribution.

    Il sert de décanteur pour retenir certaines charges d'adjuvants  

    Hello, what would be a good information in your post: what is this reservoir of 'contact'

    He has a name that means nothing but certainly it serves neither more nor less of reservoir buffer between the tratiemetn which was held upstream and distribution.

    It serves as a decanter to hold back certain expenses of adjuvants  

  8. Hi,

    I think that you may have a final tank or stock tank to stock the treated water.

    Before entering the stock tank, chlorine or NaOCl at a low level is injected in-line after the last stage of the water treatment chain. Another option, before leaving the water treatment plant a small dose of chlorine is injected.

    Best regards,

    Djamel Ghernaout

  9. Hi,

    Chlorine and all the remaining disinfectants (chloramine, ozone, UV, etc.) are killing agents. However, these chemical products are highly toxic by their selves. Moreover, they interact with natural organic matter (NOM), microorganisms, and algae to produce Disinfection by-products (DBPs) which are as well poisonous. The use of disinfectants must be avoided or at least reduced as possible at the lowest level.

    You may find some useful papers about disinfection process and DBPs in my personal page in academia.edu (search Djamel Ghernaout).

    Best regards,  

    Djamel Ghernaout

  10. Here in Saskatchewan, Canada, contact time is an absolute requirement in order to achieve the proper log removal of viruses and bacteria.  And we must submit our contact time calculations to the regulatory authorities before we can distribute to our customers.  And the contact chambers are to be baffled to obtain the ideal serpentine flow. 

    And even if we hit the water with UV as the post treatment, the contact time must be satisfied first   The UV is really just a backup for us  

     

    Kurt

    1 Comment

    1. To improve the responsiveness and accountability of local government, corporations, and other powerful interests on a wide array of issues impacting the livelihoods. In this regard, SALBWS has been organizing district and national level events to raise awareness of issues. SALBWS also aim to strengthen the organizational and management capacity of local communities in coastal Sindh and to empower them work to protect their livelihoods and rights. For this, SALBWS has been conducting community meetings in villages in taluka keti bunder, district thatta, and organize training and capacity-building workshops.

  11. Thank you very much everyone for your time and helpful answers. I've learned quite a few things from this so thanks again to all. 

    All the best,

    Gloria

  12. The requirement for a contact tank depends on the disinfection requirement of the water. UV is not very effective at killing viruses and to get a significant kill requires a much higher 'dose' than is generally applied (40 mJ/cm2) . Different virus species have different levels. However viruses are killed rapidly by a low dose of chlorine and so the residual chlorine dose will be effective for this. Generally the contact time will be much lower than required for a chlorine full disinfection where the CT is typically 15 and the contact time is 30 mins. I have used systems with a 10 min contact time after UV. 

    Another point is that the DWI are now reinforcing the requirement for full disinfection within the site boundary. This means that the old contact main or 'it will be sorted out on the way to the res' won't wash and so if there is a virus potential then there needs to be a contact tank.

    Also depending on your view of risk there is the issue of glass fragments and mercury from possible broken lamps. Personally I see this as a theoretical rather than an actual risk but it is often easier to compromise. A contact tank will help to capture fragments.

    2 Comments

    1. A further point is that there is not really a need for the contact tank to be a free surface atmospheric tank. For smaller volumes and where the system is pressurised pressure vessels can be used, thereby avoiding repumping.

    2. Thank you very much for your reply.

      We run the CT tool and indeed the required contact time was below 5 minutes. The sources are quite ok, although there's a present risk for crypto, hence the UV. 

      Good point about the DWI, we'll need to consider their requirements in a future.

      Very interesting point about the glass fragments, I'll check how often this happens with the Ops teams.

       

      Thanks very much again for your feedback. 

      Gloria

  13. Sorry to rain on this parade of very nice and caring professionals. Their knowledge of ancient waste treatment is admirable.  Currently in the industry one by one waste treatment facilities are phasing out chlorine in favor of microbial degradation.  This modern microbiological process has 40 plus years of ZERO pathogens. In addition the O2 content of the clean potable water is much higher. Not more O2, just more available.   Most old and really outdated facilities are not cost effective to change over.  Usually the new facilities are built at a greatly reduced price and the discharge can be directed to any of 3 main ways. 1. Put back into the drinking water system 2. use this water for agriculture. (free to farmers, they pick up). 3. Never need to dump in any river, lake or ocean ever again. But if you decide on this option no more killing the microbes in nature that provide life to all things.  The microbial species is Archaea. The process is called Bioremediation.

    To learn more and ask all the questions you want go to Archaea Bioremediation Group.   

    1 Comment

    1. Whilst I admire your persistence in pushing your products your post here is completely unrelated to the original question.

  14. The contact tank is post UV disinfection and should be there to allow for the chlorine to reach its potential prior to going directly into a distribution system. If the water is going to be pumped to ground or elevated storage, then it wouldn't necessary. Where is the chlorine residual being measured? After the contact chamber?

    1 Comment

    1. Thanks very much for your answer, very appreciated. 

      The water is pumped to a reservoir or storage, and the chlorine residual is currently being measured after the contact tank, yes.

      Sorry to ask, but could you please clarify why if it's pumped the Ct is not necessary? 

      Thank you very much again.

      Gloria

  15. USP Technologies, which is owned by Trojan UV, recommends PAA versus chlorine.  PAA breaks down to acrtc acid and water with great biocidal properties.  USP Technologies supplies Turn-Key PAA solutions to supplement UV disinfection.  Please let me know if you would like to learn more about this approach.  I am the Business Development Manager for USP, and would welcome the discussion.  Please write me at sroop@usptevhnologies.co or contact me directly at (239) 989-3581.  Hope this helps.  Sincerely, Sean L. Roop

  16. Hi Gloria,

    Technically, it would be better if you 1st use chlorine for killing most of microbes keeping at least 1 hour retention time with minimum free residual chlorine 0.2 ppm (preferably 0.5 ppm) and then use UV if legislation requires! Usually if you use residual chlorine for 2 hours retention, there is no need for other disinfection. However, one may prefer it to make it doubly sure!

     

    Regards,

    VM

    1 Comment

    1. Thanks for your reply. UV is required for the risk of cryptosporidium.

  17. Hi Gloria,

    All the techniques based on O3, Cl2, ClO2, Chloramine, and UV have a high bactericidal effect (Ability to reduce germs) however for the resudual effect, ie, a residuel effect after disinfection Is only for chlorine and its derivatives. From Dégrement, 1989 we have:

                                                   O3        Cl2         ClO2        Chloramine       UV

    Bactericidal Effect             +++        ++             ++                       +             ++

    Residuel Effect                      0           +              +                        ++            0

    So a need of the residual effect is there for any reuse  of the disinfected water, transport of water etc. 

    1 Comment

    1.  

      Chloramines aren't used  in Switzerland an Germany as additive for drinking water and in England rather seldom*. In Switzerland, they are seen as byproduct of amine/ammonium contamination and chlorination, and they aren't welcome at all. Chloramines aren’t listed on the positive list for allowed drinking water additives / treatments in Switzerland.

      Quelle: https://www.admin.ch/opc/de/official-compilation/2017/1023.pdf, Verordnung des EDI über Trinkwasser sowie Wasser in öffentlich zugänglichen Bädern und Duschanlagen (TBDV) vom 16. Dezember 2016, abgerufen am 28.07.2016)

       

      Also in Germany the chlorination as prevention tool is not allowed anymore, chlorination with it is only permitted as emergency tool in case of contamination.

      “Eine vorbeugende Desinfektion des Trinkwassers, die sogenannte „Schutz-Chlorung“ ist in Deutschland seit der Novellierung der Trinkwasserverordnung vom 1. Januar 1991 nicht mehr zulässig.”Quelle: https://de.wikipedia.org/wiki/Chlorung, abgerufen am 28.07.2016

      When you are using UV-C, in Switzerland the "state of the art" is to control the turbidity, which shall be below 0.2 FNU ==NTU (Formazin-NephrelometicUnits==Nephelometric Turbidity Unit). If the waters turbidity is above that, the water must be rejected or filtered. Additionally, the UV-C machine checks inherently the transmission together with the lamp power, so if the water would have much UV-C absorption, the sensor wouldn't get enough UV-C light and therefore the machine would go into "Alert-state". But I never seen that in real life, it is more common, that the lamp goes "dark" because of is use after several thousand working hours, or after very long still stand, a biofilm build-up on the quartz-tubes and sensor window can provoke the same result.  

      Seldom low chlorination levels(equal or lower 0.1 mg/l) may be used in Switzerland for protection of regrowth of germs in the drinking water net, especially in summer, but rather the net is planned to have a constant flow so that the water stays cool in all the tubes. That is why it doesn’t make sense to shut down all the public fountains for energy or water saving purposes…

      2 Comment replies

      1. Thanks very much for your answer and all the information attached. Very useful and interesting.

        Regards,

        Gloria

  18. There is UV & UV be careful what equipment you buy. It will work for most bugs if the correct "dose" is applied for the specific bugs you may have. As @Muna advises turbidity does not help & if you are not measuring the UV you don't know.. You don't mention what industry you work in but don't underestimate the hygienic design aspects of downstream distribution and maintaining its sanitary condition. Relying on a residual should be a last resort. If you need it its useless without contact time.. If you are producing food / beverage consider the use of ClO2 rather..

    1 Comment

    1. Thanks very much for your answer.

      I work in a utility, it's a water treatment plant for water supply. 

  19. Ultraviolet light will not kill all type of microorganisms (water turbidity will also affect negatively). Their is always possibility risk from fecal, and  pathogenic contamination. Or any Fault of UV light / Fault Shutting down. 

    So better to have an extra prevention measures which is residual ​chlorine dose + ​contact tank.

  20. This needs to be considered along with the microbial risks that are in the water. We have a lot of UV units, most of which have only residual disinfectant. However, we do have some sites where we are aware that there is a pathway for faecal contamination and therefore viruses. UV is not wholly effected against all viruses so we have chlorine contact to deal with the potential presence of these.

    In short - it's not a one size fits all approach and is all about the risk management of the raw water

    Hope this helps

  21. Hi Gloria,

    speaking about the law point of you I can't help you but from the technical point..... yes:

    After the UV treatment is necesary a chlorine residual. The contact tank could help you to do a schock dose if it was necesary and in general increase the retention time. 

    To be sure that everithing is didinfected you must find out a 0,1-0,2 ppm of chlorine residual after the contact tank.

     

    1 Comment

    1. 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.

      1 Comment reply

      1. Very clear. Thank you very much for your reply! 

        Regards,

        Gloria