Water Quality effluent

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I have question here regarding the effluent standard at one of the Wastewater Treatment Works, the chlorine levels are as follows: Free chlorine = 5mg/l and Total chlorine = 8mg/l, although E coli = >2044 per 100ml. The suspended solids = 20mg/l. 

We use chlorine gas as disinfection, the capacity of Treatment works is 12.5 md and it treat domestic influent only. And is Biological process. 

My question is:

1. Why E coli is so high although chlorine is too high?

2. Can anyone assist proper treatment of E coli.

NB: I will appreciated any responds.

 

Thanks 

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15 Answers

  1. Hi !

    What pH do you have before the desinfection ? Remember that chlorine works in a particular range of pH.

    Regards !

  2. Hi there, as most have indicated you seem to be over dosing and should definitely be achieving complete disinfection under these conditions...in fact you appear to be wasting chlorine that is costly!

    Some factors that could be influencing, retention time in the chlorine contact time - industry norm of around 15-20 mins at PDWF, WRC TT389 & WISA Design Guide referenced,

    Poor effluent quality, although TSS is stated as 20mg/L, presence of algae (or other organics) and ammonia can affect, so too can temperature and pH

    Of course one needs to remember that the sample bottle must be sterile and the sample collected under sterile conditions - very NB

    And the timing of reading the residual chlorine -must be as soon as the reagent has dissolved, the longer it remains the further it will react! And glass vials must be clean! I don't recommend utilising colour indicator strips if that is the method of testing you are utilising.

    Shout if you need to discuss to troubleshoot further

    Regards

  3. Hi,

    the efficiency of the disinfectant depends on its concentration but also on the CT (contact time X chlorine residual) and the pH (hypochloric acid or hypochloride anion)

  4. Hi,
    the action of chlorine varies according to the pH value. The more acidic the pH, the greater the effectiveness of the chlorine.

    In order to understand what the problem could be, you first need to know what the pH value of the treated water is.

  5. There are two main issues.  Firstly the suspended solids level.  This can harbour high levels of bacteria, which are then protected against chlorine treatment.  Secondly the nitrogen content especially any free ammonia.  If that occurs, the chlorine reacts very quickly to form a range of chloramines, which only kill bacteria very slowly.  In clean water, 1 mg/L free chlorine kills 99.9% of bacteria in minutes.  Suspended solids impact the kill rate of chlorine.  I used autoclaved yeast to simulate dirty conditions and the chlorine levels required multiplied by 50 times in some experiments.  The other question to ask is what method are you using to detect E coli.  There are certain Bacilli which are able to grow on the selective media for coliforms and the way to differentiate them is to take a colony and carry out a Gram's stain on it.  They tend to show Gram positive although older cells may look Gram negative but they will show spores present in their cells.  E coli is Gram negative and does not produce spores. 

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  6. Chlorine is very effective in killing E. Coli and injecting high (5ppm) concentration of chlorine is a common practice in Sewage or waste water treatment. Possible in your case the contact time is lesser than required. CT factor is important, so increase the contact time. Moreover the presence of nitrogen compounds and ammonia in in waste water cause interference in the bactericidal efficiency of Chlorine

     

  7. Chlorine is very effective in killing E. Coli and injecting high (5ppm) concentration of chlorine is a common practice in Sewage or waste water treatment. Possible in your case the contact time is lesser than required. CT factor is important, so increase the contact time. Moreover the presence of nitrogen compounds and ammonia in in waste water cause interference in the bactericidal efficiency of Chlorine

     

    1 Comment

  8. The chlorine level is exceptionally high and way beyond what we would recommend as a final residual. The concentrations will cause significant taste/odour issues in the receiving environment, in addition to corrosion of the system after the dosing point. Chlorine gas is typically used for potable water treatment due to the high quality of the water and relative sensitivity of chlorine gas dosing systems. When targeting wastewater, where you have solid particles, organics and the potential for biofilm, you need to make sure your setup allows for reduction in solids (which shield bacteria), elimination of biofilm (eg routine shock dosing of biocide) and then final chlorine disinfection using sodium hypochlorite (or similar). If your wastewater is variable, chlorine gas won't be sensitive enough. You need a system with downstream residual dosing monitoring and feedback to a dosing pump. Hypo is perfect for this, but only as one component of a multi-stage management routine.

    1 Comment

  9. Check for solids buildup in the contact chamber. Review your plans, have seen a plant with an in used line that was leaking into the discharge of the contact chamber. Dye check the contact chamber. Please describe your sampling and test method. 

    1 Comment

    1. Morning Mark,

       The sampling methods is raw, final, upstream and down stream and am using a clear sampling bottles (i.e. 2L for Physical and Chemical samples) 

      For Microbiological samples am using 500 ml sterilized  bottles for final, upstream and downstream.

      Chlorine i analysis it on-site with chloro meter,with 3 x 10 ml sample cell. 1st cell is for blank with sample, 2nd cell for total chlorine and the 3rd cell is for free chlorine samples.

      Thanks 

       

       

       

  10. Few things that come to mind that you need to confirm:

    1- Ensure that your sample point is sanitary. Typically stainless steel and it will need to be torched prior to taking the sample. It is very easy to contaminate a sample, so follow sampling guidelines strictly. 

    2- Having a residual is not enough information. CT is an important consideration ie. free residual x effective contact time. Make sure you are sampling after a reasonable effective contact time. The germicidal efficacy of chlorine is correlated with the product of the chlorine dose applied or residual (mg/L) and the contact time (min), also referred to as CT. Increasing either the
    chlorine residual or the contact time will increase the CT value and the log inactivation of target pathogens. Refer to your applicable guidelines for CT vs inactivation.

     

    1 Comment

    1. Good afternoon Mohammad,

       

      You are a start truly speaking, i was not aware of such information and will take it into consideration.

      Thanks 

  11. Chlorine is very effective in inactivating E. coli in a drinking water context, less so in a wastewater context.  The main issues affecting the effectiveness of chlorine in a wastewater context are the presence of particles, which can shield the E. coli from the biocidal effects of chlorine, and the presence of organic compounds that may preferentially use up the chlorine (i.e. exert a high chlorine demand).  It is a little bit unusual to be using chlorine gas in a wastewater context, normally hypochlorite solutions are used.  Anyway, if the free chlorine residual is 5 mg/L, then an E. coli result of >2000 orgs/100mL is very unusual, and, as other respondents have noted, you might like to look at sample contamination.  With that level of suspended solids and that much free chlorine you should be able to achieve a result of <100 orgs/100mL.  Best of luck with resolving the issue.

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  12. Efficiency of chlorine based disinfection against E. Colli is limited. Especially if biofilm protected forms exist. As TSS is relatively low I would go with UV treatment as the last phase of treatment after biology and final settling (sludge removal). Pls check dissolved Fe in water. If you use Fe-salts to enhence coagulations to obtain good settling sludge you may need more UV dose to combat E. Colli.

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  13.  The correct dosage must be evaluated in a test of treatability. In your case, if you want lower Ecoli values, you should perform the test in the laboratory. How to make? In a sample composed of effluent, separate into, for example, 5 equal vials. Apply different dosages to each sample. Shake vigorously. Wait at least 30 min. Measure results. The best result to apply in the treatment. Not satisfied to repeat with new dosages.

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