# Photo-Transformation of Effluent Organic Matter by ZnO-Based Sunlight Irradiation
## 4 Conclusions  
This study investigated the impact of EOM on receiving waters in terms of the properties of the organic matter and its removal efficiency by sunlight-driven photocatalysis. The experiments were conducted with water samples that were collected upstream and downstream of a WWTP and from wastewater effluent, while mixtures of upstream and wastewater effluent samples were mixed at different ratios in the lab. EEM-PARAFAC analysis decomposed the fluorescent organic matter into three components: a humic-like component (C1), a tyrosine-like component (C2), and a terrestrial-like humic component (C3). The DOC, UV\({}_{254}\), and F\({}_{\text{max}}\) of the PARAFAC components were used to calculate the removal efficiency via photocatalysis. The results showed that the EOM had a higher SUVA\({}_{254}\), a higher FI, and more abundant humic-like components than did the DOM. The analysis of the relative distribution of the PARAFAC components revealed an increase in both humic-like components and a simultaneous decrease in the protein-like component as the contribution of EOM increased. The photocatalytic removal efficiency differed between the DOM, EOM, and EOM-impacted samples due to the change in the properties of the organic matter in the source waters. The photocatalytic degradation of organic matter followed pseudo-first-order kinetics and the order of C3 \(>\) C1 \(>\) C2, which indicated that the humic-like components were preferentially removed as compared to the protein-like component under sunlight irradiation. The DOC and UV\({}_{254}\) removal efficiency decreased as the contribution of EOM increased, suggesting that EOM has potentially negative impacts on the efficiency of drinking water treatment. Additionally, a method based on the PARAFAC components incorporated with the flow data was established to compute the contribution of wastewater effluent in the receiving water. Applying this method at the point where we collected the downstream samples, the wastewater effluent contribution was determined to be \(3.8\pm 1.7\%\) and \(3.7\pm 1.6\%\), respectively, in terms of %EfOM and %WW\({}_{\text{flow}}\). This suggested that combining the PARAFAC components with the flow data can be successfully used to assess the impact of wastewater on receiving waters. The following are available online at [[http://www.mdpi.com/2076-3417/10/24/9002/s1](http://www.mdpi.com/2076-3417/10/24/9002/s1)]([http://www.mdpi.com/2076-3417/10/24/9002/s1](http://www.mdpi.com/2076-3417/10/24/9002/s1)), Figure S1: Experimental setup for the sunlight-induced photocatalytic degradation of organic matter present in receiving waters using ZnO as a photocatalyst, Figure S2: The UV/Vis absorption and EEM fluorescence spectrum for upstream, wastewater effluent, and downstream samples: (a) Fluorescence EEM spectra and (b) Changes in UV/Vis absorption spectra, DOC, UV\({}_{254}\), SUVA\({}_{254}\), FI, HIX, and BIX during the mixing between upstream and wastewater effluent (WW effluent), Figure S3: Changes of UV/Vis spectra during the photocatalytic degradation of organic matter with 0.2 g/L of ZnO dosage at pH 7: (a) DOM (upstream sample), and (b) EfOM (wastewater effluent), Figure S4: Changes of the optical parameters for upstream, wastewater effluent, and downstream samples by photocatalysis with 0.2 g/L of ZnO at pH 7: (a) SUVA\({}_{254}\), (b) \(\epsilon_{280}\), (c) E2/E3, (d) FI, (e) HIX, and (f) BIX, Figure S5: Changes in the relative distribution of PARAFAC components during the mixing between upstream and wastewater effluent: (a) percentage contribution (%) and (b) PARAFAC component ratios, Table S1: Basic water properties of upstream, wastewater effluent, and downstream samples, Table S2: Spectral characteristics of the three PARAFAC components identified by EEM-PARAFAC in this study and the comparison with those previously identified.