Dear bimr,
Thanks for the valuable comments. The water contains a total alkalinity of 215 ppm as CaCO3. Calcium and Magnesium is 195 and 95 ppm as CaCO3 respectively.
Cation is(Ca-3.9, Mg-1.9, Na-0.61, K-0.03, Ba-0.001) and anion (HCO3-4.3, Cl-0.77, F-0.01, NO3-0.1, PO4-0.01, SO4 0.78) meq/l respectively. With this high level of alkalinity an hardness, the probability of scaling the RO is very high.
Of course using a two pass RO with acid injection to pH 5.6 in the first pass (at this pH, alkalinity is in the form of CO2 thus will not react with the harness to form carbonate scaling) and NaOH injection to 8.2 prior to second pass (alkalinity in form of carbonate/bicarbonate which then can be removed by RO) might seem to work, but antiscalant is required because hardness is not fully removed. Using this method, permeate quality is not consistent as it depends on the accuracy of the dosing pumps and pH analyser. The permeate still contains CO2.
Thus you are right, degasifer should be used after the RO unit, but you are right too that this will contaminate the RO permeate if not properly designed.
Thus, I tried to put the degasifier upfront instead. Purpose is to remove the alkalinity in the form of CO2 at low pH (5.6). The water is then pumped straight through a RO prefilter and a two pass RO system. When alkalinity is removed, the scaling of RO membrane will not occur.
The RO permeate tank must be blanketted with Nitrogen. Theorectically this seems to work.
On the other side, as explained earlier, if I use a softener, it will remove the Ca and Mg. Thus, with such high alkalinity, scaling will not occur with the absence of of harness. Running the RO unit at higher pH (8.3) will get rid of the alkalinity and in a way removing the CO2 from the water.
For your information, in a conventional demineralisation plant which consists of Cation, Anion and Mixed Bed ion exchanger, which is commonly used in Power Plant for their boiler make up water, a degasifier is commonly used. It is located in between the cation and anion exchanger. The theory is to benefit from the acidic water produced after the cation exchanger whereby the alkalinity is in the form of CO2 to remove by degasification. This will reduce the size of the anion exchanger and thus save on the regeneration chemicals.
I hope my theory is correct as I have difficulties in understanding it till now. Please correct me if I am wrong as I have designed demineralisation plant for power plant using degasifier. I would like to adopt this theory in my ultrapure water treatment system as well. However, as mentioned, the contamination is a problem and thus, I am considering the use of vacuum degasifier.
To conclude, see the following flow system (this is applicable only to the above mentioned water quality)
option 1
degasifier ==> 1st Pass RO ==> +NaOH ==> 2nd Pass RO
option 2
softener ==> 1st Pass RO ==> +NaOH ==> 2nd Pass RO
option 3
+Acid ==> 1st Pass RO ==> +NaOH + antiscalant ==> 2nd Pass RO ( permeate not constant and still contains high CO2)
Note:
1. We need to remove the alkalinity or hardness before the 1st Pass RO
2. We cannot run the 1st Pass RO at high pH without removing either alkanility or hardness. Instead, we need to run the 1st Pass RO at low pH where all alkalinity is in the form of CO2.
3. After the 1st Pass RO, most of the hardness is removed.
4. In the 2nd Pass RO, pH is raised to remove the alkalinity
Using Filmtec ROSA program, with option 1 and 2, I am able to get good results of the permeae with CO2 less than 0.09 ppm.