## 3 Discussion
### The Fate of Chlorophyll-a, MCs and T&O Compounds in the Associated DWTP
Hence, it is necessary to increase backwash time and frequency to improve the filtration efficiency in the removal of cyanobacteria and the associated metabolites. The chlorination process is generally regarded as a step to reduce organic matter, which can breed harmful bacteria once released into the water distribution network. Incomplete removal of cyanobacterial metabolites by the preceding treatment processes requires more chlorine to guarantee enough residual chlorine in the water distribution network. Serious safety problems could occur if disinfection byproducts (DBPs) are formed in drinking water. Although the chlorination step was effective to remove three MCs congeners, the concentrations of T&O compounds increased after chlorination, especially for _d_-CYC and _d_-ION. This phenomenon contradicts the results found by [PERSON] et al. [24] that CYC and ION could be removed by chlorination if the process followed a pseudo first-order and a pseudo second-order kinetics mechanism, respectively. However, their previous kinetics experiments using pure odorant chemicals diluted in ultrapure water were not disrupted by other competitive organic compounds such as that found in natural (raw) water. [PERSON] and [PERSON] [25] found that CYC forms by an oxidative cleavage reaction of \(\beta\)-carotene and is catalyzed by \(\beta\)-carotene oxygenase bound on _cyanobacterial_ cells under aerobic conditions; thus, the results in this study might be because chlorination induced the production of CYC and ION via conversion reactions of carotenoids, which coincided with the increased CYC observed in the oxidation of \(\beta\)-carotene with permanganate oxidation [26]. Further investigation is needed to demonstrate the reaction mechanism to reveal the difference between the laboratory scale chlorination experiments and the operational conditions of raw water suffering from off-flavor problem. The Establishment of Chlorophyll-a (Chl-a) Limits at Intake (CLIs) for DWTP and Reservoir Management  
The conventional DWTP was effective for removing cyanobacterial cells but not for the dissolved cyanobacterial metabolites. Together with the release of metabolites by cyanobacterial cells in the C/F process and accumulated in sludge and filtering material, the dissolved organic matter could not be removed effectively before chlorine dosing disinfection generated DBPs [27]. Therefore, controlling cyanobacteria in a water source or before entering the DWTP is important to guarantee the safety of drinking water. In this study, the concentrations of metabolites including three MC categories (MC-LR, MC-RR and MC-YR) and three T&O compounds (MIB, ION and CYC) significantly correlated with chlorophyll-_a_ (Figure 1) and were used to calculate the CLIs for the DWTP associated with the EDWS. Correspondingly, the GEO was not chosen in the CLI system since no positive correlation was detected between GEO and algae. According to our previous paper, the guideline (\(G_{met}\)) of the MC-LR, MC-RR and MC-YR was 1.0 \(\upmu\)g/L, 1.0 \(\upmu\)g/L and 10.0 \(\upmu\)g/L, respectively [19]. The OTCs of MIB, ION and CYC were used as their guideline (\(G_{met}\)), respectively. The MT values for the six chosen metabolites in each season were calculated according to their removal rates in the studies as noted by Equations (2)-(5). The CLI for each season was the minimum value of calculated MTs. Finally, the CLIs were 4.0 \(\pm\) 0.9, 9.8 \(\pm\) 1.4, 26.9 \(\pm\) 3.9 and 9.1 \(\pm\) 1.7 \(\upmu\)g/L according to the MT value of CYC, MIB, MC-YR and CYC in the spring, summer, autumn and winter, respectively (Figure 5). These values were between the threshold of 1 and 50 \(\upmu\)g/L chlorophyll-_a_ for the Alert Level 1 and 2 [2]. The relatively low CLIs obtained in the present study were not only determined according to MC-YR in autumn but also according to T&O compounds in other seasons. It is worth noting that the CLIs might be unusually low for the DWTP which has been in operation for more than 30 years.