Wang Wenqi, Leitão João P, Wani Omar
Department of Hydrosciences, School of Earth Sciences and Engineering, Nanjing University, 210023, Nanjing, China; Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600, Dübendorf, Switzerland.
Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600, Dübendorf, Switzerland.
Environ Res. 2021 Nov;202:111688. doi: 10.1016/j.envres.2021.111688. Epub 2021 Jul 19.
Recurring combined sewer overflows (CSOs) can have a significant impact on the ecological condition of receiving water bodies. There are several structural measures, like adding retention basins and switching to low impact development solutions, that have been proposed to reduce the number of sewage overflows. Besides, several flow control strategies have been discussed in scientific literature that take advantage of the space within urban drainage networks, which is assumed to be adequate, for temporary storage. The adequacy of such storage space, however, is not a universally valid assumption as a large fraction of drainage networks frequently operate close to their design discharge. In this paper, we investigate the efficacy of flow control for a space-constrained drainage network. We employ a low-cost, heuristic real time control strategy with the use of flow control devices (FCDs) and available in-sewer space to reduce the magnitude of CSOs. We consider the performance of the proposed control strategy and discuss the effect of FCD location on CSO reduction. Our results, based on over 300 rainfall-event simulations, show that the flow control strategy using limited sewer capacity is more efficient during relatively small rainfall events, where the CSO is large enough to enable reduction using the chosen control rules. The CSO is reduced, to varying degrees, for around 80% of rainfall events with peak intensity between 10 and 20 mm h. For larger rainfall events, the flow control is more unstable in response to abrupt water release during control operation, which seems to be unavoidable because of the water accumulation effect and the transition to pressurized pipe flow in space-constrained networks. We also found that the flow control performance is highly sensitive to the FCD location - as it depends on the interplay of the peak rainfall intensity and the water level condition immediately upstream of the FCD. The efficacy of a location for flow control is determined by the unfilled capacity (i.e., effective in-sewer storage potential) in the pipe upstream of the FCD during the rainfall peak; furthermore, the location also has to be resistant to the water accumulation effect. Using our analysis, we substantiate two anticipated caveats to flow control strategies when the storage space is limited in a drainage network: diminished performance in CSO reduction and the appearance of additional control-related challenges, which are otherwise mitigated in more spacious networks.
合流制下水道溢流（CSO）的反复发生会对受纳水体的生态状况产生重大影响。人们已经提出了几种结构性措施，如增设蓄水池和转向低影响开发解决方案，以减少污水溢流的次数。此外，科学文献中还讨论了几种流量控制策略，这些策略利用城市排水管网内假定足够的空间进行临时存储。然而，这种存储空间的充足性并非普遍有效的假设，因为很大一部分排水管网经常在接近其设计流量的情况下运行。在本文中，我们研究了空间受限排水管网的流量控制效果。我们采用一种低成本的启发式实时控制策略，利用流量控制装置（FCD）和可用的下水道空间来降低CSO的规模。我们考虑了所提出控制策略的性能，并讨论了FCD位置对减少CSO的影响。我们基于300多次降雨事件模拟的结果表明，在相对较小的降雨事件中，使用有限下水道容量的流量控制策略效率更高，此时CSO足够大，能够使用选定的控制规则进行减少。对于峰值强度在10至20毫米/小时之间的降雨事件，约80%的事件中CSO得到了不同程度的减少。对于较大的降雨事件，在控制操作期间，由于积水效应和空间受限管网中向压力管流的转变，流量控制对突然放水的响应更加不稳定，这似乎是不可避免的。我们还发现，流量控制性能对FCD位置高度敏感——这取决于峰值降雨强度与FCD紧邻上游水位状况的相互作用。流量控制位置的有效性取决于降雨峰值期间FCD上游管道中的未填充容量（即有效的下水道存储潜力）；此外，该位置还必须能抵抗积水效应。通过我们的分析，我们证实了在排水管网存储空间有限时流量控制策略的两个预期警告：减少CSO的性能下降以及出现额外的与控制相关的挑战，而在更宽敞的管网中这些挑战会得到缓解。
