Formation And Mitigation Of Low-Concentration Influent For Typical Urban Sewage Treatment Plant In Southern China

Urban sewage treatment plant influent concentration is low,not only affect the sewage treatment plant pollution treatment capacity to play（additional carbon source）,untreated water pollutants will also be disorganized into the surface water environment.The central environmental protection inspector clearly pointed out that"Nanchang City is lagging behind in the construction and transformation of domestic sewage pipeline network,the problem of direct sewage discharge is ineffective,the problem of mixed rain and sewage flow is prominent,a large amount of domestic sewage is not effectively collected and treated and discharged into the environment,and the inefficient operation of sewage treatment plants leads to a rebound trend of black smelly water bodies."The low concentration of influent water from the sewage treatment plants in southern towns may be caused by a variety of factors,resulting in an unclear bottom and unknown causes.For the phenomenon of low influent concentration of sewage treatment plants in southern cities with high annual rainfall,a sewage treatment plant in Nanchang,a typical southern city,as an example,combined with the analysis of the impact of rainfall characteristics on the influent concentration of sewage treatment plants,starting from the water quality and water quantity changes at key nodes along the source-pumping station-water plant,study the changes along the water quality（source-pumping station-water plant）,water quantity（pumping station-water plant）and historical evolution,using the water quantity The triangular balance method is used to estimate the groundwater,river water and rainwater mixed into the wastewater treatment plant,to investigate the causes of low influent concentration of wastewater treatment plant,and accordingly to propose countermeasures to solve the low influent concentration of urban wastewater treatment plant,in order to provide scientific and technological support to solve the low influent concentration of wastewater treatment plant in a typical southern city.The main conclusions are as follows:（1）Rainfall characteristics have an impact on low influent concentration of sewage treatment plant influent.Rainfall is intensive and large during the rainy season.The low concentration of pollutants in the influent water of wastewater treatment plants is mainly concentrated in March-October 5:00-10:00,followed by 18:00-24:00.In the rainy season with intensive rainfall from 15:00 to 24:00,the influent COD concentration is obviously low;in 5:00-10:00 the influent water concentration is low.The low concentration of inlet water is more obvious at 5:00-10:00,which may be related to the morning and evening peak characteristics of water use.（2）The main parameters of water quality along the source-pumping station-water plant COD_Cr,ammonia nitrogen,TP concentrations decayed significantly.The concentration of COD_Cr,ammonia nitrogen and TP along the flood rainy days residential community-pumping station decreased by 69.78%,75.21%and 86.23%respectively.The concentration of COD_Cr,ammonia nitrogen and TP along the journey from the community to the pumping station on sunny days during the flood season also changed significantly reduced by 66.78%,76.94%and 8.91%,respectively.（3）The variation of water volume from pumping station to water plant indicates infiltration of external water.The daily influent COD concentration of the wastewater plant was significantly negatively correlated with the daily flow rate of the pumping station（correlation coefficient of -0.428,p-value<0.01）.Pumping station A and pumping station D continued to climb in daily flow in 2020,and both monthly and monthly average flows in 2020 were highly significantly positively correlated（Pearson correlation coefficients of 0.914 and 0.969,respectively,both p-values<0.01）.（4）Revealed the causes of low influent concentration at a wastewater treatment plant in Nanchang City.The proportion of groundwater/river water and rainwater mixed into this wastewater treatment plant in 2018-2020 within the service area of this wastewater treatment plant is high.The water balance triangle method estimates that the external water accounts for 47%～57%,of which groundwater/river water accounts for about 29%～39%and rainwater accounts for about 11%～18%.Groundwater and river water infiltration to dilute wastewater is the main reason for low influent concentration of wastewater plants,and rainwater mixing is a secondary reason.（5）Countermeasures to solve the low influent concentration of sewage treatment plants are proposed,mainly including source modification,rainwater and sewage diversion,pumping station modification,and creating a smart drainage system.