The Multiple Change Mechanism Of Pollutants In Sewage Pipe Network Under Confluent Conditions

The urban sewage network is an important part of the urban sewage system,the inflow of branch sewage will cause flow turbulence,microbial metabolism change and biofilm fall off,resulting in the change of sewage quality in the pipe network,thus affecting the water quality of sewage treatment plants.To study the transformation laws and mechanism of pollutants under the condition of multi-point confluence of urban sewage pipe network,a set of sewage pipeline pilot system with multiple confluent points was established.The transformation characteristics of the three main pollutants of carbon,nitrogen and sulfur in the sewage transportation process were explored.The distribution characteristics of microbial community structure in the biological phase of the system,as well as the effects of branch confluence and microorganism on pollutant transformation respectively were analyzed.The results would provide theoretical basis for further probing the transformation characteristics of sewage water quality in the actual urban sewage pipe network and its impact on sewage treatment plants.Through the analysis of the change process of carbon,nitrogen and sulfur pollutants along the pipeline in the initial confluence pipe network system,the results showed that under the influence of microbial metabolism before the confluence point,the concentration of dissolved chemical oxygen demand（SCOD）and sulfate(SO₄^2-)decreases,while the concentration of ammonia nitrogen（NH₄⁺-N）increases.After passing the confluence point,microbial metabolism changes and biofilm fall off due to the inlet of branch sewage,leading to a significant increase in the concentration of three types of pollutants.In the later stage,the water quality reached stability.Under the condition that the increase of various pollutants was basically unchanged due to the inlet of branch sewage,the concentration of SCOD decreased from about 320 mg/L at the inlet to about 280 mg/L at the outlet.The total increase of NH₄⁺-N under the action of ammonification was about 15 mg/L,which was higher than the total increase of 12.5mg/L due to the confluences.The results showed that the consumption and metabolism of microorganisms in the confluent pipe network was the dominant factor for the change of carbon and nitrogen pollutants.The concentration of SO₄^2-at the inlet and outlet of the confluent pipe was about 20 mg/L,indicating that the confluent pipe and biochemical metabolism maintained the content of SO₄^2-in a state of dynamic equilibrium.Further on the biological characteristics of phase structure of the microbial community distribution network test system analysis showed that Proteobacteria,Bacteroidetes and Fimicutes formed the main microbial community structure in the confluence sewage pipe network.Ferment bacteria（FB）,hydrogen-acetic acid producing bacteria（HPA）,methanogen bacteria（MA）and sulfate reducing bacteria（SRB）played an important role in the change of sewage quality.In the mature biofilm in the middle and back section of the pipeline,due to the change of sewage quality and environmental factors,the diversity and relative abundance of microorganisms decreased gradually,but the decreasing rate before the confluence point was significantly higher than that after the confluence point,which indicated that the intake of branch sewage had a certain promotion effect on the growth of microorganisms.Based on the variation law of pollutants and the distribution characteristics of microbial community in the sewage network,it is revealed that under the action of the fluctuation of sewage quality caused by multi-point confluence,the multiplication and proliferation of microorganisms in the biological phase of the sewage network are promoted,and the dominant role of their metabolism in the transformation of pollutants in the sewage network is enhanced.