Insights into coagulation, softening and ozonation pre-treatments for reverse osmosis membrane fouling control in reclamation of textile secondary effluent

Abstract

Pre-treatment was an effective strategy to mitigate membrane fouling, while controversial fouling control performance and mechanism still restricted its application. Herein, coagulation, softening and ozonation pre-treatments were applied to mitigate reverse osmosis (RO) fouling in reclamation of textile secondary effluent. The results showed coagulation pre-treatment negligibly alleviated membrane fouling by dosing FeCl3 coagulant due to the low coagulation efficiency and dissolved organic carbon (DOC) removal (only 11 %) toward effluent organic matter (EfOM) with low molecular weight (2300–6145 Da). Unexpectedly, pre-ozonation caused more severe RO fouling with the increasing ozone doses owing to the enhanced bridging action of divalent cations between membrane and ozonated EfOM; however, the opposite phenomenon was observed when divalent cations were removed by softening due to the preferential removal of biopolymers, the decrease in molecular weight and hydrophobicity of EfOM. Especially, at ozone dose of 1.0 mg O3/mg DOC, flux loss and irreversible fouling resistance enhanced by 8.60 % and 25.4 %, but markedly reduced by 24.0 % and 41.2 % in the absence of divalent cations, respectively. Extended DLVO (XDLVO) theory further unveiled the different roles of pre-treatments in RO fouling control was mainly due to the variation of adhesion interfaces between RO membrane and foulants, especially for the Lewis acid-base (AB) interaction energy. We proposed that softening + ozonation pre-treatment was effective to control RO fouling. More importantly, the reclaimed water satisfy the national standard of China (GB/T 19923–2005) for industrial water reuse after softening + ozonation + RO membrane treatment, which is promising to reclaim textile wastewater.