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

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.

Effect of free nitrous acid on extracellular polymeric substances production and membrane fouling in a nitritation membrane bioreactor

The membrane bioreactor (MBR) with nitritation based nitrogen removal processes has attracted growing interest in recent years, although membrane fouling in the nitritation MBR is a challenging issue. In this study, the inhibitory effect of free nitrous acid (FNA) on microbial extracellular polymeric substances (EPS) production and membrane fouling in a nitritation MBR was investigated. Results showed that EPS played a critical role in the biofouling process, and EPS production was affected by FNA concentration. As FNA concentration increased from 5.10 × 10-3 mg N/L to 1.34 × 10-2 mg N/L, protein (PN) and polysaccharide (PS) contents increased from 8.20 to 60.28 mg/g VSS and 4.74-30.46 mg/g VSS, respectively. However, when FNA concentration was 1.48 × 10-2 mg N/L, PN and PS reduced by 20.0% and 10.9%, respectively, indicating that the higher FNA concentration could reduce EPS production. The EPS reduction could be attributed to reduction in the loosely bound (LB) and tightly bound (TB) EPS but not the soluble microbial products (SMP). It was further revealed that higher FNA concentrations up to 1.48 × 10-2 mg N/L consequently mitigate trans-membrane pressure (TMP) rate in terms of dTMP/dt by 25.5% in the nitritation MBR. High throughput sequencing analysis revealed that the increase in FNA led to enrichment of Nitrosomonas but reduction in heterotrophic bacteria. This study showed that the appropriate FNA concentration affected EPS production and hence membrane fouling, leading to the possibility of membrane fouling mitigation by in-situ generated FNA in the nitritation MBR.