Study On The Evaluation Of Self-purification Capacity And Its Influencing Factors Of Urban Sewage Plant Tail Water Supply River

In recent years,with the continuous decrease of rainfall and the increasing demand for drinking water,the natural recharge of urban rivers is becoming less and less.Most small and medium-sized rivers have gradually evolved into urban sewage treatment plant tail water recharge rivers.Different from the general natural rivers and lakes,the tail water supply rivers are mostly shallow water bodies,which are greatly disturbed by various external factors.Therefore,the self-purification ability of general rivers and lakes cannot be applied.The self-purification ability and influencing factors of tail water supply rivers in urban should be studied.In this paper,Shuangji River of Xinmi that was typical urban sewage tail water supply river was taken as the research object.First of all,according to the evaluation of water quality status,the main pollution factors of the river were identified.Then the degradation ability of main pollutants in different water periods and different section were calculated through the field measurement experiment.And then the influence of initial concentration of pollutants,temperature,dissolved oxygen and flow rate on degradation ability was explored.Finally,16S r RNA gene sequencing technology was used to explore the correlation between microbial community and river self-purification ability.The main conclusions are as follows:（1）The pollution situation of each section in Xinmi section of Shuangji River was basically expressed as follows:dry season>wet season>normal season.The main pollution factors were COD,ammonia nitrogen and total phosphorus.The comprehensive degradation coefficient of COD is between 0.228-0.277 d^-1,the comprehensive degradation coefficient of ammonia nitrogen is between 0.289-0.323d-1,and the comprehensive degradation coefficient of total phosphorus is between0.306-0.320 d^-1.Totally the comprehensive degradation coefficient of ammonia nitrogen and total phosphorus was high,the purification ability was good,and the purification ability of COD was weak.The comprehensive degradation coefficients of each section in different water periods were different,COD:normal season>wet season>dry season,and ammonia nitrogen:wet season>normal season>dry season,and the comprehensive degradation coefficients of COD and ammonia nitrogen were close in wet season and normal season,which were significantly greater than those in dry season.The comprehensive degradation coefficient of total phosphorus was not obvious in each water period,but slightly higher in wet and normal season.According to the river section,the degradation ability of COD and ammonia nitrogen was expressed as follows:Suzhai section-Manshuiqiao section（section 2）>Suwanqiao section-Maandong section（section 3）>Wanzi River estuary-Dongdianqiao section（section 1）.The total phosphorus was the highest in section 2,and the difference between other two sections was small.（2）The degradation ability of COD and ammonia nitrogen in Xinmi section of Shuangji River was mainly affected by temperature,initial concentration and flow rate.Within the range of various influencing factors of the river,the degradation ability of COD and ammonia nitrogen was improved with the increase of temperature,initial concentration and flow rate.The degradation ability of total phosphorus was mainly affected by temperature and initial concentration.The degradation ability decreases with the increase of temperature,and the degradation ability decreases with the increase of initial concentration.Dissolved oxygen has little effect on the degradation of main pollutants.（3）The abundance and diversity of bacteria in Xinmi section of Shuangji River varied seasonally.The abundance was highest in summer,followed by autumn and lowest in winter.Diversity was highest in autumn,followed by summer and lowest in winter.According to the river section,microbial diversity was as follows:section 2>section 3>section 1.The discharge of tail water from urban sewage treatment plant and papermaking wastewater treatment plant reduces the microbial diversity.The degradation ability of COD and ammonia nitrogen in Xinmi section of Shuangji River was positively correlated with the abundance and diversity of river microorganisms（p<0.05）,and the correlation between the degradation ability of total phosphorus andαdiversity index was weak.Increasing the abundance and diversity of microorganisms can help improve the self-purification ability of the river.（4）At the phylum level,Proteobacteria was the first dominant phyla,followed by Bacteroidetes and Actinobacteria.The composition of bacterial communities in rivers was different in different seasons,and the relative abundance of dominant bacteria was affected by the seasonal variation.The average abundance of Proteobacteria in summer was as high as 46.98%,1.73 times that in autumn and 1.98times that in winter.The average abundance of actinomycetes in autumn was 3.06times higher than that in winter.The abundance of bacteroidetes varied little in different seasons.At genus level,Flavobacterium was the first dominant genus,followed by Rhodobacter and Dechloromonas.In different seasons,the composition of bacteria was similar and the abundances were significantly different.The abundances of nitrogen and phosphorus removal bacteria such as Flavobacterium,Rhodobacterium and Dechloromonas were significantly higher in summer and autumn,while the relative abundances of dominant nitrogen and phosphorus removal bacteria were significantly lower in winter.The abundance of dominant nitrogen and phosphorus removal bacteria were reduced because of discharge of tail water from domestic and industrial sewage plants.The abundance of denitrification and phosphorus removal bacteria such as Flavobacterium,Rhodobacter and Dechlormonas in section 2 and 3 were significantly higher than those in section 1.It’s helpful to improve the degradation ability of COD,ammonia nitrogen and total phosphorus in the Xinmi section of Shuangji River by increasing the abundance of Proteobacteria,Bacteroidetes and Actinobacteria at the genus level.