Desalination and Reverse Osmosis

Are you treating a gallon a day, or a gallon a second? If you supplied some usable information, you'd get a usable answer. What else do you know about the water quality? What are you doing now?

we need 1000gallons per hour.

now i have added cation and anion exchange resins, but it does not answer.

TDS 1086ppm. total hardeness is 400ppm as CaCO3

Yes, vacuum distillation, but it is costly. You can also use ion exchange; the reason why your present process is ineffective is because you did not soften the water first. Pass the water through softener first, then pass it through deionizing resins. Didn't I advise you on this before?

Reverse Osmosis is chosen partly for the economic attaction of it over other processes. Why the aversion?

Further, what is the target TDS/conductivity?

target TDS is 100-120ppm,

cant go for a RO

Okay, now why can't you go for RO ? Is it because of the hardness? If so, all you need to do is pass the water through softener first before passing it through RO . Also, your target is 100 - 120ppm. Since RO can easily achieve 90% TDS removal, you can get a TDS level of 70ppm. Isn't this below your target, or are you saying that you need to achieve 100 - 120ppm, not a maximum of 100 - 120ppm? If it's the latter, RO can still achieve it. Just analyze what your RO permeate TDS level is, and then add some brine concentrate reject back to the permeate to raise the TDS level to your desired concentration.

Any reason why this can't be done?