Characteristics Of Nutrient Retention And Ammonia Nitrogen Toxicity Risk Analysis Of Stream Predominated By Wastewater Treatment Plant Effluent

Wastewater treatment plant as a daily life and industrial wastewater treatment sites,The effluent water contains a large amount of nitrogen,phosphorus nutrients and other substances,and direct discharge will bring certain negative impacts to lakes and rivers.To investigate the retention of nitrogen and phosphorus nutrients after the effluent water enters the stream,eight field tracer experiments were carried out,in which a solution of conservative（NaBr）tracers and biologically active（NH₄Cl and KH₂PO₄）was added to the stream in short time,were conducted at creek,which is a first-order stream of Ershibu River in Nanfeihe River basin and predominated by wastewater treatment plant effluent.Based on the data sets of tracer experiments,the OTIS model was used to characterize the migration and transformation of water-borne solutes,to analyze the effect of mixed water on nutrient retention from transient storage.By calculating nutrient spiraling metrics to assess the capacity of transient storage zone in the syudy reach and multivariate statistical analysis of the parameters.Combined with the effluent and stream water quality measurements,to estimate nitrogen and phosphorus daily flux of streams and assess ammonia nitrogen toxicity risk,get the following results:（1）The average daily flux of nitrogen and phosphorus in the experimental reach were108.37 and 3.27kg,the mean values of retention rate were 3.43%and 4.09%,among them,NO₃-N accounts for about 70%of the total nitrogen,that mean most of the nitrogen nutrients come from the input of the wastewater treatment plant effluent and will eventually enter the Chaohu Lake with the water flow.After the effluent enters the streams,most of the ammonia-nitrogen reference values of the mixed water bodies are at a normal level,and the ammonia-nitrogen value is less risky.（2）For the hydrological parameters,the mean values of A,A_s,αand D in this study were 1.238m²,0.339m²,2.424×10^-3s^-1and 0.18m²·s^-1,and comparable to that of the other streams at home and abroad.For the transient storage metrics,the values of T_s,A_s/A,R_h andF_med²⁰⁰are all small,this means that the solute in this channel has a shorter residence time in the transient storage zone and the effect of transient storage zone is smaller than the mainstream zone.（3）The analysis of four different functional modules in OTIS model showed that,the solute concentration peak relative deviation of the scenario 2 as high as 13.53%to62.58%,which were much higher than the scenario 3,and that mean the transient storage has a greater impact on solute transport than the lateral inflow.Correlation analysis presented that there was a significant positive correlation between the RE₂ and the RE₄,and the flow rate（Q）was negatively correlated with the RE_4.（4）The analysis of nutrient retention showed that the average values of S_w-NH₄ and S_w-SRP were 12.38km and 13.61km,furthermore,the average values of S_w-NO₃ was7.84km,all of them were far higher than the total length of the source stream（4km）,indicating that the river no longer has the capacity to remove nitrogen and phosphorus nutrients.By comparing the existing research of the upstream,it has not been found that the wastewater treatment plant effluent brings obvious adverse effects on the retention of nutrient in the stream.（5）Estimates of mass loss indicated that the main channel accounted for 94.86%of the NH₄-N uptake,and 96.02%for SRP,indicating that the effect of the nutrient retention mainly from the main channel.Through regression analysis betweem the nutrient absorption coefficient and the parameter,it shows that most of the nutrient absorption coefficient have a significant correlation with Q and v,but less correlation with c andF_med²⁰⁰.