Application of sodium bicarbonate as draw solution in forward osmosis desalination: influence of temperature and linear flow velocity

Forward osmosis (FO) has been recently drawn the attention as an emerging sustainable desalination technology. This article focuses on the effect of temperature and linear flow velocity of feed and draw solution on permeation of water through forward osmosis membrane, using FO process. A bench-scale FO setup was used, in which sodium bicarbonate (NaHCO3) and sodium chloride (NaCl) were used as draw solutions. The draw solutions were initially compared for their permeation flux at the same osmotic pressure, using distilled water as feed solution. However, for FO desalination of a model saline feed solution, just, NaHCO3 draw solution was employed. Increasing the temperature from 25 to 45 degrees C led to 33 and 30.64% water flux increase for NaCl and NaHCO3 systems, respectively. Higher permeability dependency of NaHCO3 on the temperature would be attributed to the larger effect of temperature on its molecular diffusion coefficient in the membrane. By equally increasing the linear velocity of flow in the feed and draw solution sides of the membrane, from 0.088 to 0.231ms(-1), the water flux was enhanced. This would be due to the reduction of external concentration polarization effect on both interfaces of the membrane. As expected, for the saline feed water, the water flux had a 1.7-fold reduction, as compared to the distilled water.