Study On Sediment Nitrification And Denitrification Rates In Stream Reach Receiving Urban Sewage Treatment Plant Effluent

With the rapid increase in the number of wastewater treatment plants,the ecological environmental problems caused by the effluent of urban wastewater treatment plants are attracting more and more attention.To reveal the influence of tail water from sewage treatment plant on the nitrification and denitrification potential of receiving river sediments,four sampling points were set at the upper and lower reaches of the tail water discharge outfall of Caitianpu sewage treatment plant in the Banqiao River,Hefei City.The benthic sediment and its overlying water samples were collected every other month from October 2018 to August 2019.Subsequently,the sediment nitrification and denitrification rates and its spatiotemporal characteristics were analyzed,and then the major influencing factors were further identified.Moreover,the microbial carbon limitation was assessed according to the response of the sediment nitrification and denitrification rates to exogenous carbon?glucose?gradient.The results showed that:?1?There were high concentrations of nitrogen and phosphorus in the study river reaches.The concentrations of nitrogen and phosphorus pollutants in the lower reaches of the tail water outfall were higher than that in the upper reaches.In addition,the lower reaches were in an oxidation state,while the upper reaches were in a reduction state.The concentrations of NH₄⁺-N,TN,TP and SRP were higher in summer and lower in spring and autumn.Based on the single-factor water quality index,both TN and NH₄⁺-N exceeded the target of the water environment functional area,but according to the results of I_wq,the river reach was inferior to the level?.The ecological risk of ammonia nitrogen exposure was low.?2?The content range of TN and TP in the sediment was 1043.11?1850.5493mg·kg^-1 and 733.69?895.9393mg·kg^-1,respectively.The content of nitrogen and phosphorus in the sediment in the lower reaches was higher than that in the upper reaches.The range of sedimentary TP evaluation index was 1.22?1.49,suggested that the river was in a moderately polluted state.The sites S₀,S₂ and S₃ were basically in organic nitrogen pollution,and S₂and S₃ were in an organic pollution state.?3?The spatial variability of sediment ANR was S₁>S₂>S₃>S₀.The addition of exogenous carbon had an inhibitory effect on the surface nitrification process of river sediments in the spring and summer.The PNR in sediment was 0.045?0.691?g?g^-1?h^-1,and the seasonal variability was basically as follows:Summer was high,winter was low,spring and autumn were almost the same.The spatial variation was as follows:the PNR in the downstream sites were higher than that in the upstream sites in spring and summer,and there was no obviously spatial variability in other seasons.The nitrification rates were strongly correlated with sedimentary physicochemical indicators,while the correlation with water quality indicators were relatively weak There were significant or extremely significant differences in ANR between different sampling points,but almost no significant differences in PNR.?4?The rates of denitrification in the sediments decreased with the increasing of the culture time.The denitrification rates in the sediments in the downstream were rarely higher than that in the upstream.Among the overlying water indexes,the water temperature,TN and NO₃^--N concentrations had a greater effect on the sediments of denitrification rates.Furthermore,and the physical and chemical indexes of the sediment basically had a significant influenceon the denitrification rates.?5?The denitrification process was limited by carbon,and the limitation had significantly seasonal variations.The denitrification rates in the sediments had the most significant response to exogenous carbon in autumn,followed by summer and relatively weak in spring and winter by dry method.The wet method showed that the spring was most significant,followed by autumn,and winter and summer were weaker.