Combined Sewer Overflows: impacts, mitigation solutions and European perspective

Emanuele Quaranta, European Commission Joint Research Centre, Italy

Alberto Pistocchi, European Commission Joint Research Centre, Italy

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What are Combined Sewer Overflows (CSOs)?

Combined sewers (CS) collect the wastewater from buildings (e.g., sanitary flow), industrial wastewater and stormwater runoff, and convey the mixed wastewater to a Wastewater Treatment Plant (WWTP). However, when the flow exceeds the maximum conveyance capacity of the network and/or the capacity of the WWTP, typically during intense storm events, the wastewater surplus is discharged through Combined Sewer Overflows (CSOs) into the environment (Figure 1). The alternative is separate sewer systems, which collect and transport wastewater and stormwater separately. Separate sewers are more expensive, and Combined Sewers are therefore common.

Figure 1. Combined Sewer Overflow. c. Christopher Zurcher

What are the main CSO impacts?

CSOs are a widespread reality in Europe and elsewhere (in the European Union –EU27- roughly half of the sewer system is combined). The adverse impact exerted by CSOs on the receiving water bodies can be traced back to wastewater pollution load, substances transported by surface runoff (for example oil, car tyre particles, dog faeces)  and remobilization of in-sewer sediments and sewer biofilm. These substances worsen water quality, causing impacts on ecosystems. Impacts are also perceived on the economy and on tourism, as public authorities could impose a bathing prohibition in certain periods.

How can CSO impacts be reduced?

CSO impacts can be reduced by preventing the combined sewer arrangement, or by minimizing the associated impacts by treating the CSO volume. Prevention strategies can be classified into green strategies and grey strategies. 

Green strategies aim at limiting the runoff that reaches the combined sewers, through nature based solutions (NBS), in particular urban greening (e.g., green roofs, Figure 2). Urban greening stores stormwater and releases it gradually. Instead, grey strategies generally act by mitigating high flows within the sewer infrastructure (e.g. through storage capacity of buffer tanks installed within the CS network, that store and release wastewater). Real time control is also an emerging grey strategy aiming at preventing CSOs. CSO treatment strategies aim at processing the CSO before it is discharged into the receiving water bodies, e.g. by treating it in constructed wetlands*, which are classified green strategies too. 

*Constructed wetland: engineering systems designed and constructed to utilize natural processes (wetland vegetation, soil, and microbes) to treat wastewater. They have the aspect of ponds/natural storage tanks.

Green strategies generate multipurpose benefits, e.g. wastewater and urban runoff reduction, urban heat island mitigation, carbon reduction and biodiversity improvement.

Figure 2. Green roofs on private buildings (photos courtesy of Gayle Lynn Falkenthal of Facon Valley Group)

How much can CSOs be reduced in the European Union?

Considering a ‘green’ prevention strategy, it has been estimated that CSO volumes could be reduced by 20% – if 17% of the EU urban impervious surface served by a CS were covered with urban greening. If a ‘grey’ prevention strategy were implemented, providing the same storage capacity as the abovementioned green strategy, the CSO volumes would be reduced only by 7%. The treatment in constructed wetlands to achieve the same benefits of the abovementioned green strategy would cost 20 times less, but with a slightly lower ratio of benefits to costs. Real time control was estimated to reduce CSO by 21%, on average, when implemented at the large scale throughout the European CS system, and at a significantly reduced implementation cost. However, its benefits to cost ratio would be more than ten times lower than that of a green strategy.


Green solutions generally appear to cost a lot, as they need to be implemented on large areas. Nevertheless, for the European context, the ratio of benefits to costs of green solutions (i.e., urban greening) has been found to be more than ten times greater than that of grey solutions, due to the additional benefits discussed above. Furthermore, citizens are often willing to pay for benefits entailed by green solutions. However, some limitations also exist, for example the limited space availability for implementing urban greening. Treatment strategies, i.e. constructed wetlands, show to be cheaper and more cost-effective in terms of CSO reductions. While constructed wetlands may be still regarded mainly as investments by water managers, they require relatively large spaces compared to grey solutions (but less space, for the same water quantity, and similar ecological benefit, as a preventive green strategy) and have more apparent implications on the organization of the urban environment. They may bring benefits comparable to extensive greening in some respects, but they are typically less usable by citizens, hence they may not be attractive as investments outside routine water management. 

Cost-effective management of CSO requires solutions tailored to the specific conditions in each urban area, considering additional benefits, but also limitations and costs. CSO management strategies may be better accommodated in appropriately designed urban water management plans, taking into consideration the need to improve the ecological status of the receiving water bodies, together with multiple objectives including urban development, climate change adaptation, biodiversity support and pollution control, which are additional benefits of urban greening, that can potentially mobilise investments from a variety of actors.

Further readings

Quaranta, E., Fuchs, S., Liefting, H. J., Schellart, A., & Pistocchi, A. (2022). Costs and benefits of combined sewer overflow management strategies at the European scale. Journal of Environmental Management, 318, 115629.

Quaranta, E., Dorati, C., & Pistocchi, A. (2021). Water, energy and climate benefits of urban greening throughout Europe under different climatic scenarios. Scientific reports, 11(1), 1-10.