Past studies have projected that global mean sea levels could be up to between 0.98 and 2.92 m higher by the year 2100 than pre-industrial levels, which could seriously affect wastewater treatment plants (WWTPs). However, there is currently a lack of guidelines regarding how these types of installations can adapt to sea level rise (SLR). The present research analyzes how SLR might affect WWTPs that are situated near the coastline and how they can adapt by using experiences of land subsidence as a proxy. The Tohoku region in northern Japan experienced severe land subsidence (up to − 1.14 m in Ishinomaki city) after the 2011 Tohoku earthquake. The authors conducted in-depth interviews with staff from three significant WWTPs in the area to elucidate the effects that land subsidence had on their operations and how they could adapt to an increase in land subsidence or SLR. The results suggest that for land subsidence of − 0.53 m (equivalent to a SLR of + 0.53 m), the surveyed WWTPs were considered to be able to operate normally, without undertaking any major adaptation actions. Critical levels that influence the vulnerability and adaptation strategies of WWTPs to SLR were identified. These critical levels can help differentiate between the three types of SLR-induced flooding that can affect the plants, namely coastal flooding, discharge flooding and groundwater inundation. WWTPs utilizing combined sewage systems may face more difficulties when adapting to SLR. Finally, the authors proposed limit-state adaptation pathways for WWTPs situated in low-lying coastal areas, including a sequence of possible countermeasures and a timeline for specific actions to take place.
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