Insights into in-situ free nitrous acid induced extracellular polymeric substances changes and membrane fouling mitigation in a nitritation membrane bioreactor

Abstract

This study investigated the effectiveness of free nitrous acid (FNA) on mitigating membrane fouling, with the associated mechanisms, in two nitritation membrane bioreactors (MBRs) operated with Nitrosomonas-enriched culture. Results showed that FNA stress, regulated by pH and nitrite concentration, maintained a low-level fouling as opposed to the control MBR where trans-membrane pressure (TMP) exceeded 30 kPa. Compared to the control MBR, production of biofilm in the FNA stressed MBR was reduced by 68.1% in terms of mass and 78.2% in terms of thickness. Suspended biomass and biofilm extracellular polymeric substances (EPS) characterized by liquid chromatography (LC-OCD-OND) indicated FNA stress reduced the amount of low molecular weight neutrals and hydrophobic dissolved organic carbon. These components would have had high fouling potential. Excitation emission matrix (EEM) fluorescence contours indicated that exposure to FNA stimulated the production of tyrosine-like proteins but reduced those of SMP like and humic acid-like substances. This could have affected the adhesion between bacteria and membrane and so contributed to the reduced biofilm and fouling. X-ray photoelectron spectroscopy (XPS) analysis revealed marked differences in intensities of the main functionalities in the EPS for both sludge and biofilm, due to the oxidative effect of FNA, e.g. FNA stress resulted in more aliphatic C-OH, amines and amides while the control had more C=O, amino acids and amino sugars. This study showed that in-situ generated FNA could be employed to mitigate membrane fouling effectively via its biocidal and oxidative effect.