Wouldn't pumping from 230 meters be exactly equivalent to just pumping through the membrane at sea level?
If you have a pump 230 meters below sea level trying to push water up a pipe, to get water to the surface that pump must be exerting the same amount of pressure as you'd be experiencing 230 meters below sea level (since otherwise it wouldn't be strong enough to push up the water column). So you might as well just take the pump up to sea level and have it pump water through the membrane, since it would be exerting the same amount of pressure.
Or alternatively, you could just find a hill that is 230 meters high, pump sea water to the top of it, and then have the sea water come down in a pipe and go through a membrane in the bottom. Should be totally equivalent without requiring anything underwater.
If you have a pump 230 meters below sea level trying to push water up a pipe, to get water to the surface that pump must be exerting the same amount of pressure as you'd be experiencing 230 meters below sea level (since otherwise it wouldn't be strong enough to push up the water column). So you might as well just take the pump up to sea level and have it pump water through the membrane, since it would be exerting the same amount of pressure.
Or alternatively, you could just find a hill that is 230 meters high, pump sea water to the top of it, and then have the sea water come down in a pipe and go through a membrane in the bottom. Should be totally equivalent without requiring anything underwater.