Efficacies and mechanisms of different cleaning strategies for NF and RO membranes in a full-scale zero liquid discharge system

Abstract

Membrane fouling conditions can be distinct for multi-step nanofiltration (NF) and reverse osmosis (RO) membrane processes in zero liquid discharge (ZLD) system. Customizing cleaning procedures according to fouling characteristics is imperative to improve the overall cleaning efficiency of the system. Herein, we performed four different “alkaline + acid” cleaning procedures, including alkaline solution (NaOH), acid solution (citric acid), metal chelating agents (Na2EDTA), and surfactants (SDS), on five fouled NF and RO membranes from a full-scale ZLD system for cold-rolling wastewater brine treatment. The“alkaline + acid” cleaning procedures were effective for organic and inorganic combined fouling removal in two-stage NF process, but failed to remove SiAl complex scaling completely for the 2nd RO process. Due to the slight membrane fouling, both performance and surface properties of 1st and 3rd pass RO membranes did not change significantly after the cleaning. The high pH-induced solubilization were the major mechanism for the removal of the SiAl complex. Na2EDTA/SDS addition caused the decrease of foulant-foulant and foulant-membrane interactions. Customizing membrane cleaning procedures based on membrane fouling characteristics can facilitate the stable operation of hybrid membrane system for the practical industrial wastewater ZLD treatment.