Mathematical model of the volume fluxes through neutral membrane with concentration boundary layers on both sides of this membrane is presented. This model, based on the Kedem-Katchalsky equations, describes the volume flux generated by osmotic and hydrostatic forces for non-homogeneous and non-electrolyte solutions. Nonlinear equation for volume flux was used for numerical calculation in linear regime of hydrodynamic stability. In the steady state of non-homogeneous solutions the dependence of volume flux on pressure difference is shifted with regard to this dependence for homogeneous solution, while the volume flux as a function of osmotic pressure between chambers is characterized by different angle of inclination for homogeneous and non-homogeneous solutions.