## 3 Results
### Biofouling Reduction by CDA and NaOCI  
The total EPS extracted from the membrane modules before and after backwashing are compared in Figure 5. As shown in Figure 5a, CDA with DW reduced 41.7% of the polysaccharide, and the addition of NaOCI generally had a negative effect on the polysaccharide reduction. This result is hypothesized to be due to a conflict between the mechanism of CDA and the microbial response to NaOCI [31; 44; 45]. The highest polysaccharide reduction rate was 43.3% at 400 mg/L. However, for protein reduction, the addition of NaOCI significantly improved efficiency. The highest protein reduction rate was 76.4% of 400 mg/L (Figure 5b). This result can be attributed to the protein denaturation effect of NaOCI [56]. Interestingly, [PERSON] et al. (2022) reported that exposure to NaOCI increased the protein-to-TOC ratio in the EPS of NaOCI-resistant microbes and polysaccharide is preferentially degraded by NaOCI to protect microbes from reactive oxygen species (ROS). However, our results suggest that the addition of NaOCI, compared to CDA alone, significantly improved the removal of protein and generally had a negative impact on the removal of polysaccharides. Polysaccharide reduction by CDA is expected to counteract the preferential degradation of polysaccharides in the EPS by NaOCI. It is interpreted that the delayed response of microbes to oxidative stress caused an initial failure to prevent the denaturation of biofilm proteins [36]. As shown in Figure 5c, DOC reduction was comparatively lower than polysaccharide and protein reduction. For example, the reduction in DOC at 400 mg/L was 4.8%. This result indicates that backwashing with NaOCI at the experimental conditions degrades biomass into intermediate products but does not remove it from the membrane surface. On the other hand, the higher DOC reduction by CDA with DW than other samples suggests that the partial degradation of organic matter may have a negative impact on foulant removal from the membrane surface. The changes in the DNA content in EPS are shown in Figure 5d. In the sample, the EPS concentrations were compared before and after backwashing. The concentrations  
Figure 5: Comparison of EPS’ extraction from before and after backwashing membrane: (**a**) polysaccharide reduction rate; (**b**) protein reduction rate; (**c**) DOC reduction rate; (**d**) changes in DNA contents in EPS. of components such as protein and polysaccharide were reduced in samples after backwashing, indicating that the contents in the EPS were lower in the backwashed sample, which can be expected. In the case of DNA, however, an increase in the DNA content was observed in the sample treated with a NaOCI concentration of 200 mg/L, which is assumed to result from cell lysis caused by NaOCI during backwashing. The intracellular molecules released during cell lysis can serve as an additional foulant, and the released DNA can be utilized by the microbial community as extracellular DNA (eDNA). The eDNA can contribute to horizontal gene transfer in the biofilm community and subsequently contribute to chlorine resistance, including biofilm production [57; 58; 59]. In summary, two-stage backwashing with CDA and DW was effective in reducing polysaccharide and DOC, but not in reducing protein. This suggests that CDA is effective in weakening the slime structure of the biofilm [60]. However, in the case of two-stage backwashing with CDA and NaOCI, the reduction in polysaccharide and DOC was not as high as that of DW, but the reduction in protein was significantly improved. It is estimated that the reduction in protein impaired the integrity of the biofilm, which had a positive effect on fouling control. The increase in DNA in EPS with an increment in the NaOCI concentration indicates cell lysis, and this suggests that an increase in oxidation capacity and additional foulant may lead to conflict; thus, no improvement in operational efficiency was observed.