Comparative Evaluation of Treatment Efficiency, DOM Transformation, and Microbial Profile in Two Typical Pretreatment Systems of a Full-Scale Nanofiltration

The pretreatment process is critical to the efficiency of nanofiltration (NF) water plants; however, comprehensive evaluations of pretreatment options have been notably lacking. This study investigated two typical pretreatment systems, ultrafiltration (PSUF) and sand filtration + microfiltration (PSSF), in a full-scale NF plant by comparing treatment performance, organics transformation, and microbial profiles. PSUF demonstrated superior performance in removing turbidity, the silt density index, dissolved organic matter (DOM), and biomass. PSSF caused greater pressure losses and higher operational costs due to an elevated fouling potential. DOM analysis via Fourier transform ion cyclotron resonance mass spectrometry revealed that PSUF contained more refractory polyaromatic hydrocarbons and polyphenols with higher aromaticity, whereas PSSF exhibited enhanced biodegradability characterized by more building blocks and low molecular weight compounds (19%) in its permeate. PSUF generated more organic molecular fragments, resulting in a greater number of new organic products with dominant reactions, including oxygen addition, dealkylation, and decarboxylation. The higher proportion of nitrogen- and sulfur-containing components in PSSF (76.2%) compared to PSUF (66.1%) suggests an increased level of generation of bio-DOM, supported by the more diverse microbial communities and greater abundance of biofilm-forming microbes (Gaiellales, Sphingomonadaceae, and Saccharimonadales) in PSSF. Protective microfiltration acted as a potential fouling source due to the release of accumulated DOM. This study provides an in-depth evaluation of two common pretreatments and offers practical insights to support the application of NF.