Constructing Loose Nanofiltration-Featured Process for Micropolluted Water Treatment: Influencing Effect from the Residual Natural Organic Matter

In this study, we propose a combined process consisting of loose nanofiltration (LNF), mainly functioning to remove natural organic matter (NOM) while largely preserving mineral salts, and a subsequent ozonation or activated carbon adsorption, applied mainly to remove organic micropollutants (OMPs), under expectedly greatly reduced competing effects from the residual NOM of lowered concentration. Six LNF and one conventional NF membrane were adopted in an attempt to more comprehensively investigate the influence of residual NOM. All NF membranes had high rejections (∼90%) of humic- and fulvic-like substances, while those with a larger molecular weight cutoff (MWCO) had lower rejections of soluble microbial products and protein-like substances, to below 50%. This characteristic benefits the subsequent ozonation treatment, with a greatly reduced ozone decay rate (by >80%) and a lower total organic carbon-specific decay rate than that in feedwater. The preferred removal of humic- and fulvic-like substances by LNF membranes also benefits the removal of OMPs, especially those of high hydrophobicity, by granular activated carbon (GAC) adsorption. The residual NOM could hardly be adsorbed by GAC, with a greatly reduced adsorption amount onto the adsorbents (by >90%) compared to that in feedwater, thus leaving sufficient adsorption sites for OMP removal. A combination process based on loose nanofiltration was used to maximize the benefits of each unit, resulting in the simultaneous high removal of NOM and OMPs.