The role shifting of organic, inorganic and biological foulants along different positions of a two-stage nanofiltration process

Abstract

Membrane fouling is an intricate problem that restrains the efficiency of nanofiltration (NF). In this study, we collected raw water from different elements of a local full-scale plant to investigate the fouling characteristics along a two-stage-module NF process. The fouled membranes at different flux decline ratios were autopsied according to scanning electron microscope (SEM), atomic force microscope (AFM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectrometer (XPS), adenosine triphosphate (ATP), contact angle and zeta potential measurements to track the dynamic evolution of organic, inorganic and biological fouling. Statistical approaches, including principal component analysis (PCA), redundancy analysis (RDA), and multiple linear regression coupled with variation partitioning analysis (VPA), were performed to delineate the spatial variation of fouling characteristics along different stages of the membrane modules. At the head of the 1st-stage module, biofouling and organic-biological binary interaction were the main causes of flux decline, contributing to 26.5% and 27.9% of the variance of flux, respectively. At the head of the 2nd-stage module, organic fouling became predominant (78.5%) including organic individual, organic-biological binary and organic-inorganic-biological ternary effects. At the tail of the 2nd-stage module, organic-inorganic interaction made the largest contribution (39.7%) due to metal-organic complexation. The dominant role of the foulants gradually shifted from biofouling at the head stage towards organic fouling and organic-inorganic binary fouling at the terminal stage due to the continuous change of water quality along the process. More targeted fouling control strategies (foulant-targeted or position-targeted) may be developed based on these spatially varied fouling features along the NF process.