Tank disinfection with sodium hypochlorite

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Hello everybody,

I am trying to disinfect the water in a 500-L tank using sodium hypochlorite at 6 g/L concentration. I usually insert 125-150 ml of solution when the tank is full. My goal is to achieve a FRC concentration of 0.2-0.5 mg/L.

Anyway, by checking  the FRC I found out a layering of the disinfectant solution proving bad mixing: 0.8 mg/L at the output placed at the bottom of the tank and 0 mg/L at the surface water in the tank.

What could be a good way to have a homogeneous concentration of the solution? Inserting the solution at the water inlet or before filling the tank to increase turbulence?

Any other ideas?

Thank you for helping.

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12 Answers

  1. Hi Donato,

    Could you clarify if you are doing the disinfection as continuous or batch process?

    If it is a batch process I would recommend the following:

    1. Fill the tank to approximately 50-100L with water.
    2. Add the 125-150mL of sodium hypochlorite solution
    3. Make up to volume (500L) with water.

    The addition of approx 400-450L of water after the addition of sodium hypochlorite solution should promote mixing within the tank.

    If it is a continuous process I'll need some more details.

    As a side note, the addition of 150mL of 6 g/L sodium hypochlorite (as Cl2) to 500L should result in a final chlorine concentration of 1.8 mg/L (as Cl2), not accounting for the chlorine demand of the water.

    Regards,

    Daniel Couton

    1 Comment

    1. Hi Daniel,

      Thank you for your answer, Yes, it is a batch process and I will know test it as you said.

      Another question: Do you have experience with disinfection time of sodium hypochlorite? The producer suggests 30 minutes, but also after 1-2 hours the FRC keeps on decreasing indicating that the disinfection is still happening.

       

      Thank you.

      Donato

      1 Comment reply

      1. Hi Donato,

        Yes I do have experience with disinfection ​time of sodium ​hypochlorite!

        I'm not sure where/in which country your application is based, however,  the WHO recommends maintaining a free chlorine residual of 0.5 mg/L for 30 minutes within the tank.

        If you suspect cholera might be present you should increase this to 1.0 mg/L for 30 minutes.

        You might want to trial this in a beaker (using the water) before you do it at full scale to get an idea of what your initial chlorine concentration will need to be in order to still have at least 0.5 mg/L after 30 minutes (i.e. a chlorine decay test).

        Dose the tank and monitor the free chlorine within the tank over the 30 minute period. Try to measure relatively frequently (every 5 or 10 minutes) to make sure the free chlorine in the tank doesn't drop below 0.5 mg/L at any stage over the 30 minute period. If need be you can top up the chlorine during the process if it looks like it might drop below 0.5 mg/L.

        Note that the free chlorine will continue to decrease over time. This is not necessarily due to it killing harmful bugs (disinfecting) but is often due to it reacting with harmless organic matter present in the water. In addition chlorine will simply break down over time due to decomposition, particularly in the presence of light. But as long as you have maintained at least 0.5 mg/L of free chlorine in the tank for 30 minutes, that should be sufficient for disinfection purposes.

        Also let me know if the pH of your water is above 8.0. Sodium ​hypochlorite is less effective as a disinfectant in waters with pH greater than 8.0.

        Regards,

        Daniel

  2. It has all been said.  The issues are water or tank with a chlorine demand.  Circulation of the water once full taking water from the bottom and pumping it to the top of the opposite side.  Dosing the hypochlorite once there is a thin layer of water and allowing mixing as the tank fills.  This can also fail if the water filling is too gentle because the chlorine can still layer.  Mr Patel's is good advice.  I have too many clothes with holes in because of splashes of hypochlorite so certainly wear PPE.  

  3. It has all been said.  The issues are water or tank with a chlorine demand.  Circulation of the water once full taking water from the bottom and pumping it to the top of the opposite side.  Dosing the hypochlorite once there is a thin layer of water and allowing mixing as the tank fills.  This can also fail if the water filling is too gentle because the chlorine can still layer.  Mr Patel's is good advice.  I have too many clothes with holes in because of splashes of hypochlorite so certainly wear PPE.  

  4. Donato,

    One liter of 6g/L NaOCl into 500 liter water - will have 12 mg/L FRC, if there is no chlorine demand (RODI water).  My suggestion is to find out what is the chlorine demand for 15 min, add your residual FRC target on it. Make sure to have proper PPE when handling hypochlorite.   Take a five gallon bucket fill it with tank water and add half of the calculated amount of bleach mix it good.  Add the mixture into the 500 liter tank.  Give it 10 min to disperse.  Check free residual, there should not be any.  Now repeat the process with 3/4 of the remaining NaOCl and check residual.  based on the FRC reading, add only the required amount to get to your FRC target.  Check it after 10 min.  You should be within 2-5% of your target.  

    1 Comment

    1. Thank you Mr Patel for your detailed answer,

      I just want to go through what you said to see what I understood:

      1. "​Take a five ​gallon bucket ​fill it with ​tank water and ​add half of the ​calculated ​amount of ​bleach mix it ​good": In my case the calculated amount of NaOCl is 150 ml, therefore 75 ml in 20-liter bucket (more or less 5 gallon) gives 0.45 g of NaOCl.

      2. "Now ​repeat the ​process with 3/​4 of the ​remaining NaOCl ​and check ​residual": Again, 56.2 ml of NaOCl in 20 L gives 0.34 g of NaOCl as result.

       

      0.34+0.45=0.8 g of NaOCl in 500 L means 1.6 mg/L. Why do you say that I would be at 2-5% of my target?

       

      Thanks.

  5. Hi Don,

    From my experience in disinfecting small tanks, I normally dilute the Sodium Hypochlorite with water, pour into the empty tank to be disinfected, and then put a sprinkler or any object that makes the inlet water fill the tank with much turbulence. This gives a good mixing and I have been getting great results.

    Regards.

    Justin

  6. Dear Friend,

    Maintaining free residual chlorine to 0.2 - 0.5ppm in 500 lit tank filled daily (used daily 500 lit water) is possible using Chloritard available in India. Proper pouch is suspended in water tank will maintain chlorine to desired level for 1 month. For details contact +919422010900

  7. Hi.

    I advise you to set up the tank with a stirrer (manual or motorized) to be operated during the hypochlorite addition operations so as to homogenize the solution.
    The choice of a motorized agitator allows a remote control (eg timer) to be used periodically.

  8. Hi,

    You can dilute the concentrated solution in another tank of about 25L and then connect a pipe between the 25L tank to the flow line leading to the 500L tank.

    Mark. 

  9.  

    Dear,

    In order to work much more efficient, you could use EcoClearProx. Independent from pH and easy to use, odorless and no biocidal byproducts. 100% biodegradable.

    concentration is depending the load and use. normaly 12% of the amount of chlorine.

    Kind regards,

    Gino

  10. You can dilute the sodium hypochlorite first in a smaller recipient ( a 20 L tank for example) and then pour the chlorine solution into the 500-L tank.

  11. Sodium hypochlorite solution is denser than water so, if you pour a small volume into a 500-L tank of static water, it will sink to the bottom and mix only very slowly through diffusion. To mix it, add the hypochlorite as the tank is filling (assuming there is some turbulence), or stir the tank manually. Adding a pump or mixer would do the job too, but is probably excessively complicated for just a 500-L tank. It is best not to add the hypochlorite before filling the tank in case it corrodes the tank, splashes, or damages the tank through exothermic reaction when diluted.