Use of Quasi Alkanility in drinking water quality

Dear All

I wanted to know the importance of reporting & testing of quasi alkalinity for SWRO potable testing regime.  Why it is done and how to intrepret the results?

Best Regards

Anvar Mansuri

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3 Answers

1. Sorry, I see that I should have referred to Dallas's comments and not Anvar's.

2. As Anvar says, not sure why you refer to "quasi" alkalinity, perhaps because on the feed side of the membrane alkalinity may vary both with distance from the membrane, and as the water flows along the membrane length.  Alkalinity governs to a large extent the stability of your water and the tendency to be corrosive or scale forming.  Alkalinity, together with Ca and Mg, TDS, temperature, Cl can be inputs to your stability calculations.  For RO feed water, you want to avoid scale formation on the membrane which is often not possible unless you add scale inhibitors as you will reach scale forming conditions at the membrane surface, so using alkalinity and the other determinants above you can determine the amount of scale inhibitor that is needed.  For RO permeate you will need to re-stabilize the water so that it is not corrosive as Anvar has noted, usually by adding e.g. CaCO3.  This is very important for the protection of your downstream infrastructure.  Good wishes for your endeavours to get a good quality permeate and a long membrane life.

3. @ Anvar,

   What do you mean by "Quasi" alkalinity?  Alkalinity, as determined by titration, is a real measurement that should apply to SWRO.  However, the Ca and Mg are near zero with higher removals than Li, Na, K.   The alkalinity, CO2 partial pressure, and pH are all related so if you measure 2 you can calculate the third.  For drinking water you need to add Ca and Mg back in.

   The CO2 should go through the membrane and have the same partial pressure on both sides (maybe kinetic issues to this equilibrium)