城市污水集中收集率是城市污水系统运行效率的关键指标，城市污水污染物排放量是计算城市污水集中收集率的必需参数，但城市污水系统较高溢流量和缺乏溢流监测带来计算城市污水排污量的困扰。为此，应用城市污水系统模型和用水、污水处理、常住人口数等常规市政数据，首次提出了根据污水处理厂进水流量和负荷计算城市污水污染物排放量和污水集中收集率的2种方法。以面积约110 km2、常住居民近100×104 人口的城区污水系统为例，展示了应用2种方法的所需数据、具体步骤和计算结果，分析了2种方法各自的适用性。对流动人口、工商业活动和人日排污量等因素对城市污水污染物排污量的影响和估算进行了讨论。2种方法对当下国内和国外一些国家的城市污水排污量、污水集中收集率的计算和城市污水系统运行效率的评估，具有现实意义。

The centralized collection rate of urban sewage is a key indicator of the operational efficiency of urban sewage systems, urban sewage pollutant loading is an essential parameter for calculating the sewage collection ratio. The high overflow of urban sewage systems and lack of monitoring data pose difficulties in calculating sewage collection ratio. By using the urban sewage system model and regular domestic water consumption and sewage treatment information, this study first time proposed two methods, which were based on the influent flow and pollutant loadings of the sewage treatment plant to calculate the urban sewage pollutant loadings and sewage collection ratio. The applicability of two methods was demonstrated including the required data, specific steps, and calculation results by a case of an urban area with almost 110 km2 square meters and nearly 100×104 inhabitants in a city in the Yangtze Delta area. The applicability of each method was discussed. Furthermore, the effects of the commuters, industry and commercial activities, personal discharge load on the estimation of sewage collection ratio in the cities at different levels of economic development are discussed and analyzed. The methods introduced in this study had practical significance for calculating the sewage collection ratio and evaluating the operation efficiency of urban sewage systems in China and part of the other countries.

Combining the practices and experiences learned in the recent years in China and the relevant international practices this paper raises and discusses the four topics, which are related to improve the efficiencies of urban sewage systems up-grading and operation in China, including: extraneous water intrusion and overflow of separate sewer system, sewage concentration, the necessity to expand sewage treatment capacity, self-clean flow velocity in sewers. A summary and some suggestions are presented at the end of the paper.

China has achieved significant progress on wastewater treatment and aquatic environmental protection. However, leakage (in- and exfiltration) of sewer systems is still an issue. By using the statistical data of water and wastewater in 2016 in China, and the person loads (PLs) of water and wastewater in Singapore, the leakage fractions of hydraulic flow, organic carbon (COD), nitrogen (N) and phosphorus (P) mass loading, and in-sewer COD biological removal in the sewer systems of China (except Hong Kong, Macau and Taiwan), Shanghai, Guangzhou and Beijing were reported for the first time. The fractions of hydraulic flow infiltration (13%, Shanghai and Guangzhou) and exfiltration (39%, China) were calculated. Except Beijing, whose sewer networks are under appropriate management with small leakage fractions, the exfiltration fractions of COD (including in-sewer biological COD removal) ranged from 41% (Shanghai) to 66% (China) and averaged 55%; N ranged from 18% (Shanghai) to 48% (China) and averaged 33%; and P ranged from 23% (Shanghai and Guangzhou) to 44% (China) and averaged 30%. The exfiltrated sewage, COD, N and P not only wastes resources, but also contaminates the aquatic environment (especially groundwater) and contributes to 'black and odorous water bodies'. In- and exfiltration in the sewer network leads to low influent COD concentration, C/N ratio and high inorganic solids and inert particulate COD concentrations of many municipal wastewater treatment plants (WWTPs) causing high cost for nutrient removal, poor resource recovery, additional reactor/settler volume requirement and other operational problems. Therefore, tackling sewer leakage is of primary importance to today's environment in China. Recommendations for the inspection of sewer systems and the rehabilitation of damaged sewers as well as the development of design and operation guidelines of municipal WWTPs tailored to the specific local sewage characteristics and other conditions are proposed.