Abstract
This study investigated the effects of novel membrane-osmotic agent systems on forward osmosis process efficacy. Glucose, sucrose and NaCl were investigated as osmotic agents, and nano-filtration, reverse osmosis and hollow fibre flat sheet as membrane types. The effects of draw solution concentration, flow rate and temperature as well as feed water flow rate and temperature were investigated for the aforementioned membrane types. The efficacy of forward osmosis process was measured on the basis of water flux, water recovery rate, water permeability, specific energy consumption and solute flux where applicable. Single, binary and ternary systems were considered. Experimental results showed the NF membrane and the RO membrane performed better at low and high concentrations of osmotic agent. Higher water flux rates were achieved by using NF membrane for both types of osmotic agents with changing osmotic agent concentration rates. The best result was obtained through the combination of NF membrane and glucose at lower concentrations and with sucrose at higher concentrations. The NF membrane-sucrose system showed better results for all parameters when changing feed water flow rate at lower temperatures while the NF-glucose system showed better performance when increasing temperature of the solution. Specific energy consumption increased in all the combinations of membrane-osmotic agent with increasing osmotic agent concentration rates. However, the lowest energy requirements were noted for the combination of NF membrane - glucose as an osmotic agent. Overall, both NF and RO membranes showed better results at different osmotic agent flow rates, but glucose proved to be the superior osmotic agent. For the binary systems, higher FO process efficacy across almost all parameters was noted for the systems with deionised water (DW). However, systems also used more energy; because such systems also used more energy they are not necessarily superior to other systems. Water recovery rate and water flux were considerably higher in a ternary system involving sucrose + NaCl + DW compared to the ternary system using brackish water as feed solution in FO process. Overall, FO efficiency for a ternary mixture of glucose osmotic agent + NaCl salt + BW was higher in water recovery and water flux than the other ternary mixtures with DW as feed solution. Based on the study results, a number of recommendations for future work are provided. Manipulated Osmosis Desalination (MOD) is a promising desalination approach that should be further investigated by analysing the factors affecting the process’ efficacy. Such factors are likely to include various membrane parameters (thickness, porosity and different pore diameters) and draw solutions as well as the process parameters including temperature, flow rates and osmotic agent concentrations. Further investigations should be conducted for binary and especially ternary systems involving different types of membranes and solutes for refining and optimising the process of selection. The Close loop circulation system needs to be replaced by the no-circulation systems for better FO performance in future studies. Importantly, the experimental results should be compared with the developed mathematical models to further validate the results. Finally, future MOD studies should focus on identifying optimal regeneration approaches. The selection of osmotic agents based on their regeneration economics along with other important FO parameters could be undertaken in future research.