## 3 Results
### EEM Peak Composition by Backwashing  
EEM analysis can characterize organic matter in EPS samples and indicate microbial behavior during the backwashing process. Approximate wavelengths of specific peak areas of organic matter are as follows: tyrosine-like substances [T1 peak; Ex/Em = 220-240/330-360 nm], fulvic acid-like substances [A peak; Ex/Em = 230-260/400-450 nm], tryptophan-like substances [T2 peak; Ex/Em = 270-280/330-360], and humic acid-like molecules [C peak; Ex/Em = 300-340/400-450 nm]. The intensity changes in the T1 and T2 peaks infer a microbial activity change in the biofilm. The C peak indicates ready-made EPS, which are composed of higher-molecular-weight substances, while the A peak indicates relatively lower-molecular-weight substances [48; 49; 50]. The contour plots of EEM are presented in Figure 2. As shown in Figure 2, the backwashing process significantly flattened each peak in the presence of NaOCl, despite normalization by DOC [51; 52]. This suggests that NaOCl can damage functional groups of organic matter and reduce fluorescence. The fluorescence regional integration (FRI) analysis was applied to perform a quantitative comparison of EEM data, analyzing the microbial reaction and behavior of the backwashing process [48]. The average emission intensity of each area is defined as the peak intensity, and the sum of the peak intensity of SEPS and BEPS is analyzed. The reduction in peak intensity is compared in Figure 3, and the fraction change tendency is compared in Figure 4.  
Figure 1: Time TMP graph for each two-stage backwashing condition. Figure 2: Contour image of F-EEM analysis of bound EPS. The EEM peaks are identified as T1 (tyrosine-like substances), T2 (tryptophan-like substances), A (fulvic acid-like substances), and C (humic acid-like substances). (**a**) before washing with CDA and DW, (**b**) after washing with CDA and DW, (**c**) before washing with CDA and 100 mg/L NaOCl, (**d**) after washing with CDA and 100 mg/L NaOCl, (**e**) before washing with CDA and 200 mg/L NaOCl, (**f**) after washing with CDA and 200 mg/L NaOCl, (**g**) before washing with CDA and 300 mg/L NaOCl, (**h**) after washing with CDA and 300 mg/L NaOCl, (**i**) before washing with CDA and 400 mg/L NaOCl, (**j**) after washing with CDA and 400 mg/L NaOCl, (**k**) before washing with CDA and 500 mg/L NaOCl, and (**l**) after washing with CDA and 500 mg/L NaOCl. In the CDA with the DW case of Figure 3, the T1 and T2 peaks increase, and the A and C peaks decrease. This indicates that biofilm dispersion occurred as a result of gene expression triggered by the application of CDA. As the microbials inside the biofilm changed their lifestyle from sessile to planktonic, microbial activity increased and fulvic acid and humic acid, which are components of the biofilm, degenerated. On the other hand, a significant decrease in all peaks can be observed with the application of NaOCl. It is also interesting that peak reduction decreases with increments of the NaOCl concentration. Despite the increase in the oxidizing capacity with the increment in NaOCl, the decrease in peak reduction indicates the unwanted effect with the NaOCl application. The most significant peak reduction at 100 mg/L suggests that it most effectively weakened the biofilm matrix. The normalized percent fluorescence response (P\({}_{\text{i,n}}\)) distribution is shown in Figure 4a, and the variation in each distribution is organized in Figure 4b. In Figure 4b, the sum of each variation is always zero. The variation in the distribution can define the characteristics of the EPS that changed from before to after the backwashing, and this reveals the backwashing behavior. For the C peak, the distribution decreases in the presence of NaOCl, which means that larger molecules are primarily decomposed by the sacrifice reaction [36; 54; 53].