SWRO Pre and post treatment methods

Question

Good Afternoon all, 
I was called out to a private Caribbean Island to do a plant assessment on a 90,000 gpd SWRO plant. Their issues were high permeate Ph (8.6) which of course was not interacting with the Calcium Carbonate reactors. So they installed sulfuric acid feed to the permeate water to drop the Ph before the CaC03 reactor tank which added alkalinity, but also of course raised their overall conductivity due to the added sulfates from the sulfuric acid injection. I'd never seen this post treatment method on SWRO before. Have any of you? 
To remedy the issue, I suggested installing the Sulfuric acid pre-treatment with their current Vitec 5200 pre-treatment. This should significantly lower the permeate Ph so that it can properly react with the CaC03. Does anyone have any hands on experience with this approach that can give me an estimate of the necessary dosage of 93% acid into the feed water to lower the permeate from the current 8.6 to 6.5-6.8? Feed water Conductivity is 37,000. Feed Ph is 8.5. Post Vitec 5200 injection feed water Ph is 7.9. Thanks in advance..

Ian Pearson · Answer

Hi Patrick,
Handling acid is always a nasty business.  I would propose rather using soda ash to increase alkalinity after RO as this would be simpler to handle and may work out at a similar cost to adding acid + limestone.  However if you wish to add acid pre-membranes, you would need to determine the alkalinity as well as the pH and conductivity.  You can download a software programme called "STASOFT" for free from the South African Water Research Commission website which will calculate acid dosing or soda ash dosing for you based on the water quality parameters. Just note that STASOFT may not be 100% accurate for the seawater feed conditions with high TDS, but should at least put you in the right ballpark.

Taxonomy