## 4 Discussion  
In two-stage backwashing, a proportional increase in backwashing efficiency to the increment concentration of NaOCl was not observed, which is due to the complex interaction between factors in the experimental conditions. External stress for the microorganism can interfere with the induction of the dispersion of CDA. [PERSON] (2021) showed biofouling inhibition by adding 300 nM CDA and 125 mg/L vanillin as a quorum sensing inhibitor to the influent of MBR. However, adding 2% NaCl to the influent caused salinity stress to the microorganisms, reducing the biofouling inhibition effect from 11.8% to 9% [61]. The increment in the NaOCl dosage was expected to enhance the oxidizing capacity and increase oxidative stress on the microorganisms, which could lead to cell lysis at higher concentrations. Additionally, an increase in pH, resulting from a higher NaOCl dosage, could lead to a greater proportion of hypochlorous ions, affecting the behavior of chlorine. While it can be expected that oxidizing capacity, oxidative stress, and cell lysis can antagonize each other and cause backwash efficiency to fluctuate, the effect of pH is not as simple. Under the condition with 100 mg/L NaOCl, where the lowest pH was expected, the longest extension of operating time was observed. The biofilm, already dispersed by CDA, reduces EPS, making microorganisms planktonic and susceptible to HOCl. The synergetic effect of CDA and antibiotics indicates that germicidal action alone is sufficient to remove biofilm [26]. Compared to HOCI, OCl\({}^{-}\), which cannot penetrate cell membranes, may have given planktonic microorganisms more time to respond against oxidizing stress. As the concentration of NaOCl increased, the pH also rose, potentially causing fluctuations in the effectiveness of biofouling reduction. Therefore, when introducing two-stage backwashing for biofouling control, it is expected to be more efficient to perform it under weak acid conditions. The pH variation could be critical for the efficiency of NaOCl backwashing; however, its impact on biofilm dispersion by CDA is likely minimal when the pH level does not interfere with the biological treatment process. This is attributed to the relatively low external stress caused by pH change, which allows CDA to disperse biofilm effectively. It is notable that the DOC reduction at 300 mg/L is high, which may be attributed to the longer run time during the fifth cycle at 300 mg/L, as shown in Figure 1. This result suggests that organic fouling plays a significant role in membrane fouling under long-term operation, highlighting the importance of organic matter removal over extended periods. Therefore, it is likely that the optimal NaOCl concentration with CDA may vary throughout the operating period. This study aimed to introduce and evaluate MBR two-stage backwashing using CDA. The cleaning process utilizing CDA is effective in reducing NaOCl consumption, thereby mitigating the risk of membrane damage and minimizing adverse effects on microorganisms. This approach is expected to extend membrane lifespan and ensure stable water quality. Although the current application of CDA is more costly than the NaOCl-based method, the cost is likely to decrease in the future as commercial products are developed for industrial use. Conceptualization, S.P. and W.S. ; methodology, S.P. and W.S. ; investigation, S.P., C.K., Z.J. and J.H. ; data curation, S.P. ; writing--original draft preparation, S.P. ; writing--review and editing, S.P. and J.K.; visualization, S.P. ; supervision, J.K. All authors have read and agreed to the published version of the manuscript. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2021R1A2C2014255). This research was supported by the Carbon Neutrality Specialized Graduate Program through the Korea Environmental Industry & Technology Institute (KEITI) funded by the Ministry of Environment (MOE) (No. 202401260001). Not applicable.