Design and optimization of a unique pilot scale forward osmosis integrated membrane distillation system for seawater desalination

The integration of forward osmosis (FO) and membrane distillation (MD) technologies for seawater desalination applications is recently gaining more importance since the system can be easily integrated with renewable energy resources. In this study a pilot scale FO integrated air gap membrane distillation (FO-AGMD) system is designed, commissioned, and tested for the desalination of the Arabian Gulf seawater (AGS) using 70,000 ppm solution of NaCl as the draw solution (DS). During the FO-AGMD operation, the FO feed-in conductivity decreased from 66.45 mS/cm to 63.31 mS/cm while the FO feed-out conductivity decreased from 62.92 mS/cm to 60.17 mS/cm. The DI water dosing system connected to the feed loop of the FO module decreased the feed-in and feed-out conductivities to maintain a uniform Δπ across the FO membrane. As the MD flux increased, to maintain the FO-AGMD system equilibrium the FO flux was also increased by raising the DS concentration using the dosing tank containing the stock solution of NaCl. Under the optimized operating conditions, the average system water recovery stabilized at 33% at the operating temperature of the MD at 85 °C. The water flux was stable between 6.3 and 7.3 L/m²h for the FO and 3.75 and 4 L/m²h for the MD. The major power consumption by the FO-AGMD system is the heating and cooling powers for heating the feed to ∼85 ºC and cooling the permeate to ∼10 °C in the MD section which varied in the similar range of 10–12 kW/h.