Improvement of overall water recovery by increasing RNF with recirculation in a NF–RO integrated membrane process for seawater desalination

This study explored the effect of increasing nanofiltration (NF) permeate recovery (RNF) by recirculation and dosage of chemicals on NF separation performance, concentration polarization (CP) and accumulation of scalingprone ion concentration on NF membrane surface. The related coupling effect on seawater reverse osmosis (SWRO) desalination was also evaluated. The results show that when RNF for a single NF membrane element increased from 40% to 65%, pH of NF retentate and recycling ratio (Rr) increased from 5.02 and 0.75 to 5.10 and 3.58, respectively, with the dosage of both acid and antiscalant. In the same RNF range, the rejection of Ca2+, CO2−, SO2−, TDS and total hardness decreased from 35.1%, 78%, 96.3%, 18.2% and 56.1% to 28%, 76.9%, 96.1%, 3 4 10.9% and 33.9%, respectively, for NF in without chemical dosage. The CO2− and SO2− ion concentrations both 3 4 in SWRO brine and on SWRO membrane surface were only around 0.12, 98, 0.12 and 99 mg·L−1, respectively, 2− which were much lower than those in typical SWRO brine. Higher rejection for SO4 ion and higher selectivity of divalent ions over monovalent ions by the NF element were demonstrated in this study, which indicates that ESNA3 membrane yields a relatively higher efficiency of seawater pretreatment. © 2015 Published by Elsevier B.V.

Using an ultra-low-pressure and high selectivity NF membrane as pretreatment of SWRO and with one single element NF vessel on a semi-closed-loop operation, the effect of increasing RNF on the performance of NF and SWRO was evaluated in three different schemes. Interesting results were obtained for the performance of NF–SWRO process at the condition of high RNF. The following conclusions can be drawn based on the experimental results: (1) Compared with the Robs values, the Rreal for Ca2+, CO2− and SO2− ions shows a different increase degree under the same RNF condition. After considering the concentration polarization effect, only the Rreal for Ca2+ ion shows a significant difference for different schemes compared with the corresponding change trend in Robs during the 2 h period. (2) The pilot-scale test reveals that under the condition of adding acid and antiscalant, ESNA3 NF membrane yielded a larger RNF of 65% under the optimum operating conditions of the operating pressure 1.94 MPa, cross-flow velocity 0.05 m·s−1, Rr 3.58, feed pH 5.0 and antiscalant concentration at 20 mg·L−1. The rejection of Robs of Ca2+, Mg2+, CO2−, SO2−, TDS and total hardness rejec3 tion are 32.8%, 36.9%, 18.7%, 96.5%, 13.8% and 36.3%, respectively. (3) Under the same operating condition, the ki values of CO2 − are about 360 times higher than those of SO2− in the NF module, which leads to the much higher concentration polarization extent of SO2− than that of CO2−. (4) Higher rejection of SO2 − ion and higher selectivity of divalent ions over monovalent ions by NF element are obtained and demonstrated in this study, which indicates that the NF membrane yields a relatively higher efficiency of seawater pretreatment.