Study On New Strategy For Extraneous Water Assessment And Engineering Application In Sewer System

The urban drainage system is one of the most critical infrastructures,which is of great significance for maintaining the ecological environment and public health.The number of sewage facilities is increasing year by year in China,but there are typical problems such as high wastewater level and low concentration of water quality.These problems are related to the intrusion of "extraneous water" like groundwater and rainwater.Reducing the extraneous water volume to realize the “sewage quality and efficiency improvement” of the urban sewer system is an urgent problem in China.This study combines the extraneous water assessment and engineering project of sewage quality and efficiency improvement.The main work includes the following aspects:(1)The methods for extraneous water evaluation and detection were summarized,the principles,characteristics as well as limitations of each method were expounded.Moreover,extraneous water evaluation and detection were classified according to quantitative or qualitative principles,principles of detection methods,and spatial scale.(2)Four methods were selected to evaluate the extraneous water from three sewer systems in W city.The proportions of extraneous water in the three areas were15.8%～62.7%,4.6%～13.1%,and 43.8%～100%,respectively.There were specific differences in the evaluation results of the different methods.The estimation results based on the dry weather flow method and the triangle method were similar.The results based on the minimum night flow method were quite different from other methods.The sensitivity analysis of different methods was carried out,and the influence of the input parameters on the assessment results was discussed.(3)A positioning and quantitative study of extraneous water of the GA sewer system in W city was carried out.Based on the node water quality monitoring and field tests,it was determined that the primary source of extraneous water in the GA sewer system was the river water intrusion.Furthermore,the intrusion volume was measured,accounting for more than 40% of the total wastewater flow.When river water intrusion serves as the primary source of extraneous water,it brings new challenges to existing extraneous water assessment methods.And based on this discovery,a new strategy for extraneous water assessment was proposed.(4)Combined the estimation of extraneous water with the pre-diagnosis of the sewage pipe network,the pre-diagnosis technology based on the assessment of the extraneous water was proposed.And it was used in the sewage quality and efficiency improvement project of a southern city in China.According to the estimation results of the extraneous water volume of the sewer system,the sub-catchments withs serious problems were located,which is meaningful to guide the inspection and renovation of the pipe network.