A framework for assessing flood reliability and resilience of waste water treatment plants

Abstract

Water Infrastructures play an important role in serving the societies and ensuring the proper performance of these facilities has received great attention recently. The growing hydro-meteorological hazards, including coastal flooding, have posed a challenge to the management of water infrastructures. Reliability and resilience are two concepts for analysing systems performance and behavior. While there are various and sometimes even contradictory definitions in literature, in general, reliability relates to the successful performance of infrastructure within the defined time period whereas resilience relates to the capability of a system for returning 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 waste water 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). First the performance of the plant is analyzed during a storm, assuming the full operation of each unit operations 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 wastewater treatment plants control strategy is assessed by comparing the resilience and reliability after implementing these adaptations. For resilience assessment, the concept of performance curve is used and 3 indicators were computed while for estimating the reliability the load-resistance concept is utilized and compared with classical expected compliance percentage method. The methodology has been applied to a wastewater treatment plant in New York City. Results show high resilience and reliability of biological parameters which means they are not greatly affected by the storm in comparison with total suspended solids and nitrogen parameter. Furthermore, although duration of the structural failure is short but the consequence of flooding is severe which is demonstrated in 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 decision will be necessary.