Evaluating the advantage of turbulent flow to diminish concentration polarization in Roto-dynamic RO system

Abstract

Concentration polarization (CP) is the major shortcoming of the reverse osmosis (RO) system for brackish/seawater desalination. The present study aims to understand the effect of induced shear stress, enhanced flow dynamics, and eddy diffusivity on CP, when the flow is turbulent. In roto-dynamic RO system turbulence can be easily generated by controlling the angular speed of the rotor without increasing the feed discharge. We have numerically simulated the salt rejection by the RO membrane and concentration polarization at the seawater salinity of 500 mol/m3 for two different configurations of roto-dynamic RO system when the flow is turbulent. We have used Reynolds stress model (RSM) and eddy viscosity-dependent eddy diffusivity models for salt transport. In turbulent flow regime, eddy diffusion enhances the mixing. In laminar flow regime, salt concentration over the membrane increases by 60%–110% for 70 bar feed pressure. In contrast, the increment of salt concentration is only about 3%–4% once the flow becomes turbulent, leading to 3–4 times higher permeate discharge for roto-dynamic RO system. Thus, it can increase the longevity of the membrane and improve overall performance of the RO module by delaying membrane scaling and fouling.