An Integrated Low Energy Comsumption HRNF-LPRO Membrane Process For Seawater Desalination

Integrating nanofiltration process in the sea water desalination system is the trend for reducing energy comsumption. However, the low salt rejection rate of the nano filtration stage in the current nanofiltration-reverse osmosis(NF-SWRO) seawater desalination system tend to cause high operation pressure for second RO stage, resulting in higher overall energy consumption. In this study a novel low energy comsuption sea water desalination process using high rejection nanofiltration and low pressure reverse osmosis(HRNF-LPRO) membranes, e.g. NF90 and BW30 LPRO membranes, is proposed, whose performance was investigated both by experimentation using an small scale bench-top setup, and simulations using Dow Filmtec Reverse Osmosis System Analysis(ROSA) software.The inpacts on permeate flux, rejection of NF process from feedwater operation pressure; TDS concentration and temperature were studied. It was observed that the flux of NF90 increased with the rising of operation pressure, and decreased with the rising of feedwater TDS concentration. The rejection of NF90 for TDS and monovalent ions declined steadily with the increase in feedwater TDS concentration while its rejections for bivalent ions showed very little decline. NF90 acheived a TDS rejection rate and permeate flux of 82.73% and 9.61L/m2·h, respectively, at the operation pressure of 2.6MPa, feedwater TDS concentration of 34998 ppm and temperature of 20°C in.The effects of operation pressure, feedwater TDS concentration and temperature on the flux and rejection of BW30 were also studied. The results showed that, the flux of BW30 increased linearly with the rising of operation pressure, but steadily decreased with the rising of feedwater TDS concentration. The salt rejection of BW30 increased with the rising of operation pressure feedwater, and decreased with the increase in feedwater salt concentration.In the small scale benchtop studies, the integrated HRNF-LPRO syatem could reject 99.84% of the salt conent from the artificial seawater at much lower energy consumption by dramically decreasing the send stage RO operational pressure from 6.6 to 3.0 MPa at the same first stage operating pressure. About 1.0 kwh/m3 of the operating energy may be saved from conventional NF-SWRO systems. Based on the experimental parameters, the ROSA simulation predict that an optimized scale up HRNF-LPRO system using 1280 of HRNF membrane elements and 480 of LPRO membrane elements, with no frequency floating pump or pressure transition energy saving device, may achieve an overall salt rejection of 99.345% and permeate flux of 44.65m3/h at the energy consumption of only 4.527 k Wh/m3. The energy consumption of the process would reduced to 3.264 k Wh/m3 considering the potential energy saving from energy saveing devices with 90% of energy recovery in the NF and RO passes in scale-up desalination systems. The energy consumption of HRNF-LPRO system could be much lower than that of the current NF-SWRO system, 3.7-5.8 k Wh/m3. If the feedwater were standard Pacific seawater with a TDS of 34967.39 ppm at 20°C, ROSA simulation showed that the TDS of permeate should be as low as 231.17 ppm. All the experimental and simulation results indicated that HRNF-LPRO integrated system may be scaled up for fresh water production from sea water desalination with much lower costs.