Fouling of nanofiltration and reverse osmosis membranes during municipal wastewater reclamation: Membrane autopsy results from pilot-scale investigations

This study investigated membrane fouling during wastewater reclamation by collecting operational performance data and employing state-of-the-art techniques for characterization of membrane foulants. Utilizing a pilot-scale membrane unit, two nanofiltration membranes (NF-90 and NF-4040, Dow/Filmtec) and one low-pressure reverse osmosis membrane (TMG10, Toray America) were tested at two wastewater reclamation facilities treating microfiltered non-nitrified secondary effluent and nitrified/denitrified tertiary effluent. The membranes and foulants were characterized by environmental scanning electron microscopy coupled with energy dispersive spectroscopy, attenuated total reflection-Fourier transform infrared spectrometry, zeta-potential measurement, atomic force microscopy, phospholipids analysis, and contact angle measurement. During treatment of the nitrified/denitrified effluent, membrane fouling was dominated by biofouling in combination with organic fouling, colloidal fouling, and inorganic scaling. Sufficient chloramines residual was identified as being critical in controlling biofouling during wastewater desalination. During filtration of the non-nitrified effluent, fouling of the membrane lead-elements was mainly caused by adsorption and deposition of effluent organic matter while biofouling still observed in tail-elements despite the formation of chloramines. Rough and hydrophobic membrane with high permeability (e.g., NF-90) displayed more severe initial specific flux decline during filtration, while smooth and hydrophilic membrane (e.g., NF-4040) exhibited high and constant specific flux when biofouling was under control.