Evaluation of scaling potential in a pilot-scale NF–SWRO integrated seawater desalination system

Abstract

Pilot-scale experiments were carried out on a nanofiltration (NF)–seawater reverse osmosis (SWRO) integrated membrane system (IMS) to evaluate scaling potential on NF and SWRO membrane surface using a kind of loosen NF membrane and costal seawater. The effect of NF permeate recovery (RNF) increasing on the occurrence of scaling in the NF and SWRO module was investigated in term of concentration polarization modulus (CP) of scalant ions, Stiff and Davis Stability Index (S&DSI), as well as Supersaturation Index (SI) of CaCO3 and CaSO4. The results show that the salt rejection by the loosen NF module is only about 10%, while the rejection of SO42− is higher than 95%; S&DSI is always less than zero, indicating that CaCO3 scaling could not form on the NF membrane surface when RNF was less than 35%. CPSO42− is much higher than CPCO32−; and at RNF of higher than 30%, SI of CaSO4 on NF membrane surface increased gradually higher than 1.0, which indicated that CaSO4 scale is more apt to form on the NF membrane surface than CaCO3 at that NF permeate recovery. This scaling sequence in the loose NF module was different from that in traditional SWRO desalination processes.