Aluminium Carry Over

Published on by in Business

I have aluminium carry over through the secondary filters. I'm trying to explain the importance of keeping the coagulated pH at 5.8 but the plant operator insists on a pH 0f 5.4.

Can someone send me an explanation so he will understand the importance of maintaining a good dosed raw water pH?

My coagulant is feric aluminium sulphate.

Taxonomy

7 Answers

  1. The attached graph gives a reasonable indication of aluminium precipitation against pH. It's not my work but at the moment I can't remember where it came from. In any case its one I've used a lot in the past to explain why either high or low coagulation pH is bad in terms of aluminium solubilisation or resolubilisation.

    I've also found that the exact pH limits depend entirely on water chemistry - so 6.4 might be optimum for one water but not for another. We dose acid at one of our plants to optimise the coagulation pH and maximise organics removal and have found that, in that water, any pH below 6.0 results in elevated soluble aluminium levels. Therefore we aim for a pH of about 6.2. Similarly a pH prior to filtration of 7.5 or higher can result in resolubilisation of aluminium from flocs.

    As mentioned in previous answers undertaking a series of jar tests at different pHs can be used to work out where the limits are for your particular water chemistry.

  2. Hi Roger Mcburney, 

    When doing my master thesis in 1990 using Alumimum Sulphate for fluoride removal in Tanzania I took samples for test at the Technical University in Denmark for residual Aluminium, showing the optimum pH of 6.5 (i.e. lowest level of residual aluminium). 

    Thus a simple jar test with a matrix of different concentration Alumimum Sulphate and pH (from Lime adjustment) will give you a range of samples to measure the residual aluminium versus pH in treated water sample using e.g. flame Spectrophotometry. 

    Regards, 

    Erik Thorbjorn Norremark

    Low-cost WASH

    https://www.linkedin.com/in/erik-thorbjorn-norremark/

     

     

     

  3. If there is sampling line after pH adjustment and coagulant dosing, just take a sample and analyze the water quality such as SDI. Then, you will know the difference. Difference between pH 5.4 and 5.8 is small, you may not be able to read and compare the result through Jar Test as it is a very rough checking.

  5. For Aluminium based coagulant the optimum pH is 6.0 to 7.0, where as for Iron based coagulant the pH range is 5.0 to 11.0.

    The optimum pH and dose rate for your system has to be derived from Jar test. 

    1 Comment

    1. Optimum pH for precipitation of zinc as zinc hydroxide is 9.45 from my experience. Above this pH zinc effectively comes 'back' into solution as a zincate Zn(OH)4 - - ...thus causing a rise in zinc concentration in your effluent outfall.

      Ferric chloride alone is not the best way for metal precipitation, I would use an anionic polymer after you add the ferric chloride and adjust pH.

      And most important:

      1 - First add ferric chloride
      2 - Adjust pH
      3 - Add polymer

      Ferric chloride is always acid, so if you adjust pH before adding it, pH values will be not consistent and you will have problems again..
       

  6. Look for information for removing zinc from waste water using coagulants.

  7. An optimum pH is of course very important, but an optimum coagulant dose is also required as coagulant underdosing can result in high residual coagulant consentrations as well.