Membrane autopsy as a sustainable management of fouling phenomena occurring in MF, UF and NF processes

Abstract

Our paper is focused on membrane autopsies conducted on hollow-fibre MF/UF membranes operated with raw surface water and with two selected and well-characterized hydrophobic (denoted HPOA) and hydrophilic (denoted TPIA) NOM fractions for a polyamide nanofiltration (NF) membrane, using a large panel of analytical tools including novel technologies such as electron spectroscopy for chemical analysis (ESCA) and pyrolysis gas chromatography/mass spectrometry in addition to field emission gun scanning electronic microscopy (FESEM), atomic force microscopy (AFM), contact angle and streaming potential measurements. Complementary information obtained from the analyses of the feed water, the permeate and the backwash water solution (DOC, UV254, high pressure SEC, total amino acids (AA) and amino sugar (AS), hardness, alkalinity) are also discussed. Our results showed more severe flux decline with MF membranes as compared to UF membranes. Also, MF PVDF which appeared to be more hydrophobic, more negatively charged with a higher roughness was more susceptible to fouling than MF PS. Identification of proteins and amino sugars in the organic foulants (i.e. bacterial cell wall residues) as shown by the pyrolysis GC/MS analysis permitted to suspect biofouling presence and FESEM and AFM images recorded from MF PVDF after backwash confirmed the presence of bacteria. The goal of our work was also to study the fouling properties of 2 humic fractions (i.e. hydrophobic, denoted HPOA) and non-humic (i.e. hydrophilic, denoted TPIA), NOM fractions isolated from the Blavet River (France) toward a commercial polyamide (PA) NF membrane (denoted NF-55). The comparison of the affinity of both selected NOM fractions with the PA NF membrane was conducted using contact angle, hydraulic permeability, streaming potential (SP) and NaCl aqueous solution permeation measurements. HPOA is more retained inside the pores as compared to the TPIA that was mainly sorbed on the membrane surface. Furthermore the membranes acidic–basic properties were amplified after foulants deposition in comparison to the cleaned membrane where a dominant specific sorption of monovalents and divalent ions occurred.