Why in the world would you concentrate seawater to 100:1, and then dilute it, instead of just reducing it to 2:1? If it is a matter of heat efficiency, surely a long heat exchanger can recover it all on the way back to the sea, absorbing it into the incoming water.
It was a hypothetical to illustrate how much dilution is needed to get effleunt close to background level salinity. It isn't even possible to extract that much fresh, since seawater is 3.5% salt. I was not talking about heat at all.
To illustrate: the effluent dilution requirement in your case isn't actually much better. If you took in 100L of seawater, extracted 50L fresh, that leaves you with 50L brine at twice the background concentration. To get that effluent back to 1% above background, you have to dilute that 50L brine with 4950L of seawater.
