Mechanism of nanofiltration fouling in high hardness and dissolved organic carbon surface water based on chemical characterization of foulant

Nanofiltration (NF) membranes purifying high hardness and dissolved organic carbon (DOC) surface water are prone to much more severe fouling than those supplied by waters of better quality. We conducted analyses on an ultrafiltration/nanofiltration pilot plant supplied by high hardness and DOC water to investigate the NF foulant chemical composition and fouling mechanism. NF foulant was primarily organic (97%). Analyses of the physically (PRF) and chemically (CRF) removable foulant components unveiled their different compositions. CRF, i.e., the fouling substances directly adsorbed on the nanofilter surface and pores, mainly comprised hydrophobic and low molecular weight & building blocks of humic substances. PRF, i.e., the gel-layer formed by the substances continuous deposition, contained more compounds with hydrophilic character and a wide range of molecular weights, from low molecular weight & building blocks (<1 kDa) to humic substances (1 – 20 kDa) and biopolymers (>20 kDa), showing a more uniform distribution of those features. Calcium and magnesium may have promoted the bridging of the organics with the NF membrane, and between the organics, strengthening the foulant structure. The presence of a humic- and EPS-conditioning film may have interfered with or contributed to bacterial adhesion, respectively, forming biofilm patches. • High DOC and hardness water produces a NF gel-like foulant with some bacteria • Hydrophobic low molecular weight humic substances adsorb first on the NF membrane • Gel-layer forms next from varied molecular weight hydrophilic & hydrophobic compounds • Biopolymers favor bacterial attachment while humic substances interfere with it • Bacteria may form biofilm patches directly attached to the NF membrane surface