Framework for Assessing Flood Reliability and Resilience of Wastewater Treatment Plants

Water Infrastructures play an important role in serving societies, and ensuring the proper performance of these facilities has received great attention recently. Growing hydrometeorological hazards, including coastal flooding, have posed a challenge to the management of water infrastructures. Reliability and resilience are two concepts for analyzing systems performance and behavior. Although there are various and sometimes even contradictory definitions in literature, in general, reliability relates to the successful performance of infrastructure within a defined period, whereas resilience relates to the capability of a system to return to normal operation when it is subject to malfunction due to the impact of hazards. This paper presents a framework for assessing the resilience and reliability of a typical wastewater-treatment plant under coastal flooding. Two main causes of the plant malfunction include increasing flow rate of influent to the plant (operational failure) and inundation of unit operations (structural failure). The performance of the plant is analyzed during a storm, assuming the full operation of each unit operation and the effluent concentration is obtained and compared with the standards limit. Next, the impact of structural failure is considered by lowering the removal rate of those unit operations that become inundated. Finally, the effect of some interventions in wastewatertreatment plant control strategy is assessed by comparing the resilience and reliability after implementing these adaptations. For resilience assessment, the concept of performance curves is used and three indicators are computed; to estimate reliability, the load-resistance concept is used and compared with the classical expected compliance percentage method. The methodology is applied to a wastewater treatment plant in New York City. Results show high resilience and reliability of biological parameters, which means that they are not greatly affected by the storm compared with total suspended solids and the nitrogen parameter. Furthermore, although the duration of the structural failure is short, the consequence of flooding is severe, which is demonstrated by low resilience based on magnitude. The interventions have various effects on different effluent parameters, improving some and degrading others, so evaluating trade-offs and making informed decisions will be necessary.