Experimental study on reverse osmosis concentrate treatment based on Fertilizer-Drawn Forward Osmosis

Reverse Osmosis (RO) is one of the most advanced and effective membrane separation treatments in desalination. Forward Osmosis (FO) is also one type of membrane separation that can spontaneously draw the water from the RO concentrated water. However, the regeneration of draw solution requires much energy in the FO system. A Fertilizer-Drawn Forward Osmosis (FDFO) process is thus selected to reduce the energy consumption for the draw solution, where the fertilizer solution was used as the draw solution. The RO concentrated water is continuously concentrated, while the fertilizer draw solution is continuously diluted in the process of FDFO. The diluted fertilizer draw solution can widely be expected for agricultural irrigation without regeneration. In this study, the influence factors of the forward water and reverse salt flux in the FO process were determined under the different types, concentrations, and temperature of the draw solution, as well as the concentration of feed solution. KCl, KNO3, NaNO3, NH4HCO3, (NH4)2SO4, and NH4Cl were selected as the draw solutions for the single factor comparison tests of FDFO. The results show that the forward water fluxes and reverse salt fluxes differed greatly with different kinds of draw solutions. The forward water fluxes of draw solutions were ranked in the order of KCl > NH4Cl > NaNO3 > NH4HCO3 > (NH4) 2SO4 > KNO3. The reverse salt flux was NaNO3 > NH4HCO3 > KNO3 > KCl > NH4Cl > (NH4)2SO4. The forward water flux of KCl and NH4Cl was the largest, while the reverse salt flux was smaller. Therefore, KCl and NH4Cl were more suitable for a single fertilizer draw solution. A FO test was carried out under the different concentration and temperature of the KCl draw solution. The results showed that the forward water flux of the KCl draw solution at 2 mol/L was 3.56 times than that of 0.5 mol/L, and the forward water flux of KCl draw solution at 55 ℃ was about twice than that of 25 ℃. Therefore, the forward water flux increased significantly, with the increase of the concentration and temperature of the draw solution. However, there was no increase in the forward water flux and the reverse salt flux, when the concentration of draw solution increased exponentially. Additionally, the increased multiple of forwarding water and reverse salt flux was less than the draw solution concentration. Another FO test was also carried out under the various concentration and temperature of the NaNO3 draw solution. The results showed that the reverse salt flux increased by 2.94 times, when the concentration of the NaNO3 draw solution increased from 0.5 to 2 mol/L, while, the reverse salt flux increased by 1.64 times, when the temperature of the NaNO3 draw solution increased from 25 to 55 ℃. Therefore, the reverse salt flux increased, with the increase of the concentration and temperature of the draw solution. Furthermore, the temperature increased exponentially, so did the forward water flux. Since the RO concentrated and deionized water was selected as the feed solution in the FO experiment, it was found that the higher concentration of feed solution, but the smaller the forwarding water flux. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.