Impacts Of Discharging Wastewater Effluent On Dissolved Organic Matter Composition In Receiving Water Body And Its Mechanisms

With the improvement of the pollution reduction scheme,our national sewage treatment capacity has been rapidly improved,and it reached as high as 1.88×10~9 m~3/d.It is a common way to discharge effluent from sewage treatment plants into nature water bodies including rivers and lakes,and which is also an important way for effluent reuse.However,we would not ignore the potential ecological risks of discharging effluent on the receiving water bodies,although the stricter discharge standards have been implemented.In the present study,we aimed to characterize variations of dissolved organic matter(DOM)from sewage effluent and its receiving water body through the field investigation and combined laboratory simulation experiment.We investigated the photobleaching and biodegradation processes for the discharged effluent organic matter(EfOM),and also probed the impacts of discharging EfOM on reshaping DOM composition in the receiving water bodies.A series of important results were obtained:Along urban river(Nanfei River),DOC content and chromogenic DOM(CDOM)content stayed at the same level,but its fluorescence intensities of fluorescent DOM(FDOM)and its fractions exhibited obvious variations along Nanfei River.Which suggested that excitation-emission-matrixs fluorescence spectroscopy combined with fluorescence regional integration(EEMs-FRI)technology could trace DOM variations sensitively on watershed scale.Based on the hydrophilic or hydrophobic functional groups,the hydrophobic DOM(HPO-DOM)was the dominating fraction,accounting for 52.33%±5.38% for DOM from Nanfei River,which was significantly higher than the hydrophilic DOM(HPI-DOM)and transphilic DOM(TPI-DOM)fractions.In addition,the fraction of<30 kDa was the dominating fraction based on the molecular weight,which accounted for about 73%of the total DOM from Nanfei River.Moreover,we found that there was chemical fractionation phenomenon for different DOM fractions along urban river.After sewage EfOM discharged into the receiving water body,both ultraviolet light(UVA and UVB)photobleaching and degradation by indigenous microorganisms could reduce EfOM content,and the degradation dynamics followed the first-order degradation kinetics.In addition,we found that ultraviolet photobleaching could promote the bioavailability of EfOM for indigenous microorganisms,and even increased microbial-derived DOM in receiving waters.It has been calculated that ultraviolet photobleaching(4 d)and degradation(30 d)by indigenous microorganisms could reduce about 67%of discharged EfOM into the river,but there was about 33%of discharged EfOM existed in the receiving water stably which was refractory fraction.In addition,photobleaching and indigenous microorganisms could alter the DOM composition in receiving water body.Due to photobleaching or degradation by indigenous microorganisms,discharged sewage EfOM would played significantly affects on DOM composition in the receiving water.We found that the biodegradable substances(such as soluble microbial by-product-like materials and tyrosin-like substances)were significantly decreased(P<0.05 or P<0.01),but the fulvic acid-like substances were significantly increased(P<0.05 or P<0.001),while the refractory substances(tryptophan-like and humic acid-like substances)stayed at the same level in receiving water body.Simultaneously,inconsistent environmental behaviors were observed for different DOM fractions(including HPI-DOM,HPO-DOM and TPI-DOM)at watershed scale.In conclusion,discharging effluent from sewage treatment plants would reshape DOM composition in receiving water body,and chemical fractionation phenomenon was observed along water course for different DOM fractions.These results could improve understanding potential ecological risks of discharging effluent on the receiving water bodies,and also provide novel ideas for watershed environmental management.