Influence of in-situ NaClO backwashing on the biological performance and membrane fouling behavior in a granule-based SNAD-MBR process

A granule-based simultaneous nitrification, anammox, and denitrification process in a membrane bioreactor (SNAD-MBR) was performed to investigate the influence of in-situ NaClO backwashing on biological performance and membrane fouling. First, the limit of the NaClO concentration for the SNAD granules was determined (20 mg/g center dot SS) in a short-term batch test. Then, three in-situ NaClO backwashing strategies (NaClO concentrations of 5-15 mg/g center dot SS with backwashing frequencies of 1/10-1/30 min) were implemented during long-term SNAD-MBR process. The results showed that the removal efficiencies of total nitrogen (TN) and chemical oxygen demand decreased from 92.69% and 94.19% to 81.82% and 85.39%, respectively, with an increase in the NaClO concentration in spite of a reduction in backwashing frequency. The deterioration of the biological performance could be explained by the inhibitory activities of the functional bacteria after long-term exposure to NaClO. Nevertheless, the stable contributions of autotrophic and heterotrophic processes to TN removal (Eanammox of 91.23%, Edenitrification of 8.77%) indicated a negligible impact of NaClO back washing on the balance of the nitrogen removal pathways. Furthermore, membrane fouling was significantly suppressed when in-situ NaClO backwashing was performed with a relatively high frequency (1/1-10/20min) and a low concentration (5-10 mg/g center dot SS). However, a higher NaClO concentration (15 mg/g center dot SS) exacerbated membrane fouling, although the backwashing frequency was low. Meanwhile, the high fouling potential of the SNAD granules resulted from the high production of soluble microbial products (especially tryptophan protein-like substances) under NaClO stress. The possible underlying mechanism was the trade-off between the membrane scrubbing effect by NaClO and the variable physico-chemical properties of the SNAD granules. The optimal in-situ NaClO backwashing strategy that considered the process performance, membrane fouling, and permeate production was a NaClO concentration of 15 mg/g center dot SS and a backwashing frequency of 1/20 min. The results provide guidance for in-situ NaClO backwashing of the SNAD-MBR process.