Semi batch dual-pass nanofiltration as scaling-controlled pretreatment for seawater purification and concentration with high recovery rate

Abstract

In this paper, a novel dual-pass nanofiltration (NF) as scaling-controlled pretreatment and concentration system for reverse osmosis (RO) seawater separation is reported, and a total reflux model of brine was employed to obtain more permeate. Two different NF membranes (after choosing experiments) were employed in the novel dual-pass NF, which could depart SO42− and Ca2+ into different spaces to reduce the collision chance of these ions. Thus the scaling risk was reduced obviously, and the recovery rate of the dual-pass NF could reach 90% which was higher than the reported value. The permeate of the dual-pass NF was pumped into low-pressure RO (LPRO) and high-pressure RO (HPRO) system to produce fresh water the recovery rate of RO system was (82.5%) and obtain NaCl (87.7 g/L) solution. As a consequence, high RO recovery rate was reached, and an alternative choice to solve the discharge problem of SWRO brine was provided. By combining the dual-pass NF with RO system, the total recovery rate could reach 65.7% without adding any antiscalant. The specific energy consumption (SEC) of dual-pass NF was 0.04–0.06 kWh/m3, which is economical. The SEC of LPRO and HPRO was 4.41 kWh/m3 and 7.08 kWh/m3 separately. If a EDR was employed, the SEC of RO system could be reduced from 5.55 kWh/m3 to 4.34 kWh/m3.