Drinking Water compliance

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I received 100% of drinking water supply from water treatment works with free chlorine of 0.89mg/l and total chlorine of 1.78mg/l at the reservoir, while from water treatment works free chlorine is 1mg/l and total is 2mg/l.

 Currently they reduce supply of drinking water from 100% to 20% from the water treatment works to reservoir ,  free chlorine and total chlorine now is higher at the reservoir free is 2.2mg/l and total is 2.2mg/l as well.

While free chlorine and total chlorine from the water treatment works still the same: free chlorine is 1mg/l and total is 2mg/l.

What is the cause of high level of chlorine while supply reduce from 100% to 20%?

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13 Answers

  1. How are you measuring the free and total chlorine residuals ? Amperometric or dpd? Pressure and pH will affect how accurate amperometric probes read and will affect free and total probes differently. 

    2 Comments

    1. HACH makes a few different varieties.  The cl10 is their amperometric probes (which don't use reagents) . The cl17 is their dpd flow through monitor and it uses quite a bit of reagent. 

  2. I expect from the free and total amounts that the water leaving the works is chloramminated by adding ammonia to convert to monochloramine.  I assume that the free chlorine level means that the breakpoint has been exceeded.

    It appears to me that the chlorammination  has been suspended hence the residual now being all as free chlorine. I'm not sure if you are saying that the quality is different at each end of the supply line, with a lower free chlorine at the works which would be hard to  achieve. If this is the case you would need to examine trends to make sure that you are not looking at a transient.

    When dropping flows substantially the dosing equipment may not be able to turn down as readily and may  lead to overdosing. Even if it can the process time will change and may throw out control function.

    1 Comment

  5. It is simply that the dosing method is pure manual, no stairer to mix the cholorine with the water . so, the content went down the bottom of the mix point. If you further check you will find out that the cholorine will have caked on the bottom of the point. only the water that passes through the surface as contact with the cholorine.

    1 Comment

  6. What is interesting here is the change from 50% combined chlorine to 0% combined chlorine (all free chlorine) by the time it reaches the reservoir.  This would suggest that chlorine dosing at the WTW is not under control as others have suggested, and that there is over-dosing such that break-point chlorination is achieved.  If you are using calcium hypochlorite  as the chlorine source, there may be a possibility that it has not fully dissolved where the sample is taken at the WTW, hence the higher readings downstream.

    1 Comment

  8. Is it an open reservoir or is it covered?  They might be overdosing or it could be a seasonal effect of solar and temperature. It  is good that there is residual at the tap (I hope). More sun would decompose chlorine in an open reservoir, and lower water temperatures would slow down the chemistry. More TOC would consume more chlorine. Is it dry (less runoff) or wet weather?

    1 Comment

  10. If the chlorine levels are varying so much I would suspect a problem with the dosing control at the wtw. Ideally the control should be flow paced with a feedback loop to ensure that the control remains within a much tighter control band. To give a more complete answer trends of the chlorine levels and water flow would be required along with the general arrangement between the wtw the reservoir and your connection point

    1 Comment

  11. Check the intake of chlorine dose. When the flow is reduced the chlorine does has also to be reduced proportionately. Please let me whether the there is a closed vessel in the process. Need more information on the complete process from intake to outflow to answer your question. Send me my email complete process description to answer your query.

     

    1 Comment

  13. Dear PHETLA MANGENA have you taken into account the water holding time into the reservoir? 

    2 Comments

    1. Can you please explain dosing mechansim  to the system?

      1 Comment reply

      1. Chlorine gas with ammonia, from treatment to booster pump station (i.e. Distance from WTW to booster pump station is 550km)  . Again from Booster pump station to community we disinfect with liquid chlorine (i.e. distance from booster pump station to community is 15km) .