Remineralized Potable Water Turbidity Control

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For desalination plant the use of lime or calcite along with carbon dioxide is inevitable.  This comes with a potential risk of control the turbidity spike at the outlet of lime treatment point.  

My question to all is, what should be limiting turbidity in finished water product using only lime or calcite application?  What I understand is during slaking Ca(OH)2 thus generated is the entity which creates the turbidity spike. 

Can we achieve turbidity less than 0.5 NTU and at the same time reach our minimum Ca2+ target in potable water?

What is the consequence of supplying 1 NTU to 5 NTU finished potable water in the distribution system, both for human beings and the safety of pipe network?

Your help much appreciated!

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6 Answers

  2. Appreciated Mr. Mansuri, turbidity in post-treatment comes from a few different sources. For lime dosing systems, this is a major drawback and often results in water qualities that are out of specification. The main sources are the impurities that exist in the raw material that do not dissolve, and the formation of calcium carbonate due to excess calcium and carbonate ions. With calcite contactors, the turbidity comes from impurities also that are not correctly backwashed, but also from the fines that are added during refills.

    Currently and based on the experience of DPC (Dr.-Ing. Peters consulting for membrane technology and environmental engineering) of now 43 years as international independent consultant with one emphasis on seawater desalination with reverse osmosis as well as on the results of our continuous independent international product evaluation methodology (CIIPEM) there is only one remineralization process that avoids turbidity through the use of an innovative Membrane Calcite Reactor. This is the Omya Advanced Remineralization Process. You can find information about this process on the internet. Or feel free to contact us at dr.peters.consulting@t-online.de.

    Best regards

     

  4. Introduce a flash/static mixer near the Lime/CO2 injection site and relocate the Turbidity meter a sufficient distance downstream to allow Lime dissolution. Add prescribed dose of CO2 then Lime into acidic water for faster dissolution and hence lower Turbidity. Also use a bubble trap for Turbidity sample line in case turbulence/air entrainment from the static mixer has not achieved a laminar flow prior to Turbidity sample intake.

  5. The use of Ca(OH)2 for remineralization of desalinated fresh water is not inevitable. Actually I prefer other methods because the use of Ca(OH)2 gives problems in keeping the solution agitated before injection

    a) injection of remineralizing salts (typically NaHCO3 and CaCl2 (both inexpensive and soluble in  water to keep the water clear without turbidity)

    b) addition of CO2 to the water and then filtration through a deep bed of CaCO3

    The most diffused use of marble scraps is the filling of filtration drums for distillate remineralization (export from Italian marble mines).

    Your problem of turbidity can be solved by either improving the mixing performance of the agitation before injection (no concentration peaks) or adding more CO2 to the water before processing (to dissolve the concentration peaks)

  6. We use our filter media KARBOFILT (>99% CaCO3) or SEMIDOL (approx. 69% CaCO3 + approx. 25% MgO) to remineralize water after a desalination plant. You can use CO2 or alternatively acids such as H2SO4 or HCL to remineralize the water. In case of using acid, you will increase SO4 or CL balance of the water. The remineralization is done in pressure vessels (filter) and you shouldn't see an increase in turbity. Should you require more information, just let me know under kiel@fluidtechnologie.com and I can support you with further details on this issue.