From volume to pollution: a review of real-time control potential for combined sewer systems

ABSTRACT The evolution of RTC from uncontrolled over VBRTC to PBRTC is illustrated alongside the relevant potentials and key findings and knowledge gaps identified in the literature review. Real-time control (RTC) is widely adopted in combined sewer systems to mitigate combined sewer overflows (CSOs), with volume-based RTC (VBRTC) being the predominant approach in both research and practice. While in research, pollution-based RTC (PBRTC) has been shown to achieve greater pollutant load reductions into receiving waters than VBRTC, the adoption of PBRTC to full-scale systems remains limited due to challenges such as the lack of a systematic methodology to evaluate its potential benefits. This study addresses this gap by conducting a literature review of PBRTC case studies. A standardised assessment framework is established by defining the load reduction achieved by PBRTC compared to the VBRTC baseline as the PBRTC–VBRTC relative potential (PVP). Based on the limited number of available studies, preliminary hypotheses are formulated regarding the factors influencing the PVP. According to the existing body of research, the main influences are the inclusion of pollution information in the VBRTC reference, inter-event pollutant load variability during CSO events, spatial pollutant concentration variability, catchment size, and the specific network configuration. The literature synthesis reveals multiple knowledge gaps, regarding the quantification of the influence of the listed parameters to identify key use cases in which PBRTC may outperform VBRTC.