Other things equal, a low P_permeate would be best - because raising it means you're operating at greater depths, which generally costs more. The brine water i

Other things equal, a low P_permeate would be best - because raising it means you're operating at greater depths, which generally costs more. The brine water is denser than the surrounding feed water - with a bit of clever design, gravity-driven circulation could remove the need for pumps (aka expensive points of failure) on that side of things.