Precipitation risks of carbonate species due to stripping of dissolved CO2 in aquifer water

Greetings to all!

I wanted to know the risks of scaling in ground water/aquifer pretreatment process when we have a deaerator or air stripper column in the treatment scheme.  For cases wherein ground water/produced water needs to stripped of dissolve hydrocarbon gases, we need to employ an air stripper column followed by liquid phase GAC media.

My concern is what would be the effect on CaCO3/MgCO3 species due to the addition of the stripper column.  The end water quality only requires reduction in dissolved hydrocarbon gases, as the water will be injected in the well and some of it will be in used for floor washing, gardening.

Please enlighten me on trouble shooting aspects for scaling issues in the air stripper column.

Best Regards

Anvar Mansuri

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4 Answers

1. During  water degassing, the CO2 in the water will be released which will increase the pH to exceed the value 8 and thereafter the bicarbonates (HCO3-) begin to be transformed into carbonates  (CO3--) which will react quickly with calcium ions to form CaCO3.
   There is no risk of  precipitation of MgCO3 which generally takes place at pH > 9
   Cordially

2. Adjusting pH to a lower value can help a bit the issues of Saturation along with Scheduled CIP plan & Scale control additives (may not be needed, but good to have).

3. It is likely, as localized concentrations of minerals may exceed their solubilities.  It is a common problem in both cooling towers and strippers.  The following references indicate pH control may help, or one of the operational cleaning methods.

   https://www.environmental-expert.com/articles/co2-degasification-air-stripping-tower-design-case-study-784732

   https://frtr.gov/matrix2/section4/4-46.html  (You know this information already)

   https://www.geothermal-energy.org/pdf/IGAstandard/AGEC/2010/Ngothai_et_al_2010.pdf

   https://blueearthproducts.com/municipal-water-treatment/air-strippers/

   1 Comment

   1. thanks Steven, these are interesting reads, definately.

4. You will need to look at the water chemistry to determine if you will reach scaling conditions or not.

   CO2 (carbonic acid), HCO3 (Bicarbonate) and CO3 (Carbonate) exist in some form of equilibrium in water. Above pH 8.3 effectively all of the carbon is either HCO3 or CO3. Between pH 4.3 and 8.3 you will have CO2 and HCO3.

   Assuming your starting pH is less than 8.3, you will have CO2 that will be removed by the stripper. This will change the ratio of HCO3:CO2. Since dissolved CO2 is carbonic acid, reducing the amount of  acid present will increase the pH, but not above 8.3.  So if your Ca and Mg are hot precipitating at pH 8.3 you should be fine with the stripper. Depending on your TDS, something like the Stiff and Davis Solubility Index can be used to determine Ca precipitation.

   If you starting pH is over 8.3, you effectively have no CO2 and the small amount removed by the stripper will effectively not change the pH or the scaling probability.

    

    

   1 Comment

   1. thanks Pat Stanford, this is very insightful.