I need to treat water with following chemical component with high permeate recovery using reverse osmosis process:
Na: 13890.8 ppm
SiO2: 139.4 ppm
Ca: 676.4 ppm
Mg: 558.6 ppm
Cl: 20431.8 ppm
SO4: 5131 ppm
HCO3: 39.8 ppm
TDS: 40868 ppm
As mentioned above SiO2 level of the sample is too high. So I think RO process at a high pH level (pH >9) following of a chemical softening process (Ca and Mg removal) may be suitable for treating of this water. However, I am not sure about chemical softening of the mentioned high TDS water. Does anyone have any experience in this field? Which chemical (lime, soda, NaOH) is more effective for this process?
Thank you in advance.
Dear Hossein,
Sodium hydroxide (NaOH)(caustic soda) may be utilized for chemical precipitation. due to its advantages: produces less sludge than lime, or lime-soda ash processes. However its disadvantage are higher total chemical cost.
Lime, or lime and soda-ash is the preferred method of most facilities due to its low cost.
Therefore, your selection will be based on the money that you have. If cost is NOT an issue it is preferred that you use NaOH, if not you can use the lime or lime soda-ash.
In our facility we use NaOH.
Hoping this will be helpful,
Rafik
Dear Rafik
Thank you so much.
Have you ever treated such high TDS water?
Would you please introduce me a reference for design of this process?
HCO3 is low. NaOH alone won't work well for this chemistry. Best is to use NaOH or Lime to get rid of Mg(OH)2 and SiO2 and then Na2CO3 to get rid of Ca. TDS should not be a problem for chemical softening.
Determination of type of hardness of raw water is a must prior to selection of softening method. In your case, Sodium hydroxide treatment seems more advantageous than lime or lime - soda treatment because it can be used for both carbonate and non-carbonate hardness, due to a high concentration of sulfate ions, also you almost need to make effective flocculation after that to rapid settle-down of formed precipitate.
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