Combined sewer overflow (CSO) pollution poses a serious threat to the urban water environment and is more severe in old urban areas. This research uses the old urban area in the sponge city pilot area in Tongzhou District, Beijing, as the study area. The United States Environmental Protection Agency (USEPA) storm water management model (SWMM) was used to establish the hydrologic and hydraulic model of this area. The model parameters were calibrated and validated based on the measured rainfall and runoff data. The results show that the Nash-Sutcliffe efficiency coefficient for calibration and validation is more than 0.74. Thirty-two sets of systematic CSO control schemes are formulated, which include the "gray (includes the pipes, pumps, ditches, and detention ponds engineered by people to manage stormwater) strategy" and "gray-green strategies", and the regularity of CSO control for "low impact development (LID) facilities at the source", "intercepting sewer pipes at the midway", and "storage tank at the end", are quantitatively analyzed. The results show that the LID facility has an average annual reduction rate of 22% for the CSO frequency and 35% to 49% for the CSO volume. The retrofitting of intercepting sewer pipes has an average annual reduction rate of 11% for the CSO frequency and 4% to 15% for the CSO volume, and the storage tank has an average annual reduction rate from 3% to 36% for the CSO volume; furthermore, the reduction rate decreases with the increase in the CSO volume reduction rate by LID facilities. When the CSO control target is stricter, the control effect of the "end" segment is more obvious, but the control efficiency is lower. By studying the variability of the storage tank volume under different control targets, it can be concluded that it is reasonable to set the CSO control target because the number of overflow events does not exceed four times per year for the study area.

Combined sewer overflow pollution has gradually become the limiting factor for the further improvement of river water quality during rain events. Setting up a comprehensive regulation method based on synchronous monitoring is essential for combined sewer overflow management. However, current studies mainly focus on single monitoring and lack a correlation between control objectives and control effects. This study establishes a new aspect of a comprehensive regulation and control method based on overflow characteristic analysis, a calculation model, and control target determination. Through synchronous monitoring of the pipe network, the sewage treatment plant, and the river course in the Liangshui River basin of China, rainfall thresholds of outlets in a combined pipe network, pre-treatment overflow, and simple-treatment overflow were 14, 9, and 16 mm, respectively, and the overflow volume was positively correlated with the rainfall. The COD (chemical oxygen demand) concentration from the pre-treatment overflow was much higher than that from the combined pipe network, and the EMC (event mean concentration) in heavy rain was higher than in rainstorms. The shortest time exceeding the water quality by overflow pollution was 1 h, and the longest time was more than 7 days. Overflow load proportions of the three links were 43.4%, 32.8%, and 23.8%, accounting for 66.3% of the total pollutant load of the river, and the best scheme of input–output ratio was to regulate the first three outlets of overflow load. Our results provide comprehensive guidance and a systematic approach for the monitoring and control of combined sewer overflow.

Antecedent soil moisture determines how much storage remains in bioretention media before rainfall, but its relationship with runoff control is poorly quantified. Three wet seasons of monitoring were analyzed for two field-scale bioretention systems with contrasting media depths, runoff loading, and drainage boundaries. For each event, antecedent available storage was calculated from the initial soil moisture profile relative to field capacity and, separately, to the 95th percentile of peak water content at each depth. Event-activated storage was defined as the greatest synchronous increase in water stored across the monitored profile following rainfall onset. Both storage metrics were expressed as equivalent rainfall depths over the contributing roof area. Runoff volume reduction ranged from 62.3% to 100% in BR1 and from 43.3% to 100% in BR2. Median event-activated storage over the media area was 4.9 mm in BR1 and 19.1 mm in BR2. However, after conversion to the contributing roof area, median antecedent storage was larger in BR1 than in BR2 under both the field capacity reference (3.8 versus 0.6 mm) and the high water reference (11.0 versus 5.0 mm). Greater antecedent storage was associated with a lower probability of drainage activation in both systems and greater runoff volume reduction in BR1. In BR2, greater event-activated storage was associated with lower runoff volume reduction, indicating that stronger within-event media wetting did not necessarily correspond to greater facility-scale runoff retention. Dynamic storage should therefore be interpreted in relation to hydraulic loading, media configuration, and drainage pathways.