dosing calculation for potable water

Published on by in Technology

Hi All

 

Can anyone share an excel spreadsheet for estimating the amount of acid/caustic required to effect a unit change in pH of potable water from SWRO desalination plants.  Please note the finished water is not like river or well water wherein there is natural mix of minerals.  The desal plants mostly will have hardness, few Na & other ions.

Taxonomy

6 Answers

  1. The titration curve for very clean water near 7 pH is almost straight up, very hard to control. The addition of alkalinity, as Andre Visser mentions, would be helpful.

  2. I agree with Thomas Getz. Also keep in mind that the pH will be very unstable after RO (depending on specific membranes). I would advise that you also consider increasing the alkalinity to improve pH stability and limit corrosivity of the product water.

  4. The most practical approach is to collect a representative sample and titrate it from your starting pH to your target end point.   This will work for upward or downward pH adjustments.   This will be more accurate than any mathematical estimate.   The lab equipment for titration is inexpensive and an experience technician could deliver a result in 15 minutes.   After this determination of the ​amount of acid/​base ​required to ​effect a unit ​change in pH you can extrapolate to your full scale operation.   

  5. Dear Anwar,

     

    it's almost impractical to standardize this issue. The purpose of adding dosing acid to the feed is to neutralize the available alkalis in the feed water and reduce scale formation by carbonates.. there is no standard mathematics and it depends on your feed water analysis and the design of your plant. 

    I suggest you start with low acid concentrations and low dose on a side stream for a try before you put it in service and start your dosing. 

    Don't forget that acid for pretreatment to lower PH in feed while caustic for product PH readjustment in product line. You can use the membrane manufacturer program to assist in the initial estimation after you input your feed water analysis. 

    I. E. The more acid you will use in the feed, will require more caustic in the product to adjust PH later in the product. If you don't use acid in feed, you may not require caustic to readjust PH. 

    Tip: if you still follow me, the two amounts are almost the same after you count the mass balance, concentration of chemicals, acid valency/base and your recovery ratio (not all the acid you dose in feed will go to product, depending on your recovery) 

    N. B. The variation in PH after adding the acid is going to be high in case present alkalis are low and the TDS is low.  That's in the feed line, same for caustic in the product line. 

    Good luck 

     

    1 Comment

  6. Hi dear Anvar

    If I was faced to such situation, I woud chose a normal solution of caustic or acid , then test the amounts in real process and then check the results in a pilot vol. in 500 or 1000 lit system. Simply you can estimate the amount of material and then calculate the dosing pumps efficiency. Please note that you should use a propper pH meter with Hi/Low out-puts. Also paralel dosing systems just in case of dosing system crash.

    Calculation would be hard cuz some minerals can react like amphoters, complexants etc in some situations like temp, pressure and bio issues.

    Good luck.