Urban domestic sewage is one of the important nitrate (NO-3) sources for surface water; however, their NO-3 concentrations and nitrogen and oxygen isotope values (δ15N-NO-3 and δ18O-NO-3) remain unclear, and the factors affecting NO-3 concentrations and δ15N-NO-3 and δ18O-NO-3 values of effluents in the waste water treatment plant (WWTP) are still unknown. Water samples in the Jiaozuo WWTP were collected to illustrate this question. Influents, clarified water in the secondary sedimentation tank (SST), and effluents of the WWTP were sampled every 8 h. The ammonia (NH+4) concentrations, NO-3 concentrations, and δ15N-NO-3 and δ18O-NO-3 values were analyzed to elucidate the nitrogen transfers through different treatment sections and illustrate the factors affecting the effluent NO-3 concentrations and isotope ratios. The results indicated that ① the mean NH+4 concentration was (22.86±2.16) mg·L-1 in the influent and decreased to (3.78±1.98) mg·L-1 in the SST and continuously reduced to (2.70±1.98) mg·L-1 in the effluent of the WWTP. The median NO-3 concentration was 0.62 mg·L-1 in the influent, and the average NO-3 concentration increased to (33.48±3.10) mg·L-1 in the SST and gradually increased to (37.20±4.34) mg·L-1 in the effluent of the WWTP. ② The mean values of δ15N-NO-3 and δ18O-NO-3 were (17.1±10.7)‰ and (19.2±2.2)‰ in the influent of the WWTP, the median values of δ15N-NO-3 and δ18O-NO-3 were 11.9‰ and 6.4‰ in the SST, and the average values were (12.6±1.9)‰ and (5.7±0.8)‰ in the effluent of the WWTP. ③ The NH+4 concentrations of influent had significant differences compared to those in the SST and the effluent (P<0.05). The reduction of NH+4 concentrations in the SST was due to the above nitrification during the aerobic treatment process, which transferred NH+4 to NO-3. The NH+4 concentrations in the SST had no significant differences with that in the effluent of the WWTP (P>0.05). ④ The NO-3 concentrations in the influent had significant differences with those in the SST and the effluent (P<0.05), and minor NO-3 concentrations but relatively high δ15N-NO-3 and δ18O-NO-3 values in the influent were probably due to denitrification during the pipe sewage transportation. The obviously increased NO-3 concentrations (P<0.05) but decreased δ18O-NO-3 values (P<0.05) in the SST and the effluent resulted from water oxygen incorporation during the nitrification. The above results confirmed the impacts of aerobic and anaerobic treatment processes on NO-3 concentrations and isotope ratios of effluent from the WWTP and provided scientific basis for the identification of sewage contributions to surface water nitrate via average δ15N-NO-3 and δ18O-NO-3 values.
Keywords: ammonia; nitrate; nitrogen and oxygen isotopes; nitrogen transfer; waste water treatment plant.