A study on the influence of feed and draw solution concentrations on the performance of the pilot-scale forward osmosis-membrane distillation system

Abstract

The increasing demand for freshwater and scarcity of natural sources of freshwater have led the world to turn to seawater as a main source to produce freshwater through conventional desalination processes. Multi-stage flash distillation (MSF) and reverse osmosis (RO) technologies are currently being utilized in the existing desalination plants. However, these technologies are energy-intensive and have their own limitations such as scaling and fouling. Therefore innovation in non-conventional desalination technologies has produced a number of promising systems including the newly developed integrated membrane system using forward osmosis (FO) and membrane distillation (MD). The scope of this study will assess the viability and efficiency of an FO-air gap MD (AGMD) integrated system of capacity 5 m3/day for desalination application using different concentrations of NaCl as feed and draw solutions. The FO-AGMD system performance was majorly evaluated in terms of water recovery. At a fixed DS concentration of NaCl at 70000 ppm, the highest water recoveries in the range of 50–60% were observed for the lowest FS concentration i.e. DI water attributed to the highest ∆Π difference across the FO membrane. The increased DS concentration at a fixed concentration of the FS decreased the average water recovery of the system attributed to the unique design of the FO-AGMD system. For the fixed NaCl concentration of FS at 5000 ppm, the FO-MD process with 35,000 ppm solution of NaCl as DS showed higher water recovery in the range of 46–51% compared to the DS concentration of 150,000 ppm NaCl, which showed water recovery in the range of 30–41% at the operating temperature of MD at 85 °C.