Generally, it firstly depends ...

Generally, it firstly depends on the physical and chemical characteristics of the membrane itself. Second, it depends on network design, i.e. water recovery rate, concentration release rate, and the degree to which the surface of the membrane is "polarized" by a chemical potential gradient profile.  Surface concentration of the ions may be considerably higher than in the bulk of water passing along the length of the membrane.  Generally speaking, salt flux is proportional to the concentration of the salt at the membrane surface, so the less well transported the salt is from the surface, the higher polarization of the salt gradient, and the higher chemical potential it will exert at the membrane surface, resulting in higher transport across the membrane.  Salt flux will therefore take place whether or not there is net driving pressure for water to flow across the membrane.