Osmotically enhanced reverse osmosis using hollow fiber membranes

Abstract

Osmotically enhanced reverse osmosis (OERO) effectively increases the water recovery in desalination as it can reduce the transmembrane osmotic pressure by a sweep solution. Utilizing a model based on mass-transfer principles, we report the performance of a hollow-fiber RO membrane module in OERO as a function of the operating conditions, the fiber geometry, and the membrane properties. The hollow fiber system allows the feed and sweep solutions to flow on both sides of the membrane. To avoid energy-intensive solute/water separation, fertilizer can be employed as a “green” sweep solution as it can be directly used for fertigation (fertilized irrigation). Simulations indicated that the water recovery is significantly enhanced by increasing the water permeance and decreasing the structure parameter of the hollow fiber membranes. Further, an investigation into the role of feed salinity suggests that longer fibers can provide a higher water recovery in the case of low-salinity water reuse, while larger-diameter fibers achieve a more efficient seawater desalination. A single-stage OERO process facilitates the enhancement of water recovery from 35.5 % and 14 % to 36.5 % and 34 % in the case of low-salinity and high-salinity desalinations, respectively. This study provides theoretical perspectives into the design of hollow fiber modules for OERO processes.