[Your Choice w/c]

hi guys,what is the approximate cost of this membrane?

[dai]

If using tandem Di's make sure you test the TDS of the first one regularly, you gain nothing once the TDS goes above the level of the water going in.
If you are getting 22 ppm out of the RO then you would gain nothing once the TDS of the water coming out of vessel 1 exceeds this, in fact you would be losing.