A Treatment Reliability-Based Method for Supporting Infrastructure Asset Management of Wastewater Treatment Plants

A simple and consolidated reliability-based method widely used to unveil the real reliability and stability of wastewater treatment plants (WWTPs) is herein proposed to trigger decision making on operational improvements and asset management for maintaining or improving treatment effectiveness, reliability, and efficiency. Five-year data (2015-2019) from 16 Portuguese activated sludge WWTPs were used. For the 73% of the yearly data which fitted a lognormal distribution, Niku's coefficient was computed to assess the plant annual reliability for biological oxygen demand (BOD5), chemical oxygen demand (COD), and total suspended solids (TSS). The standard deviation of the annual concentrations was used to characterize the plant stability, and the maximum standard deviations allowed to comply with the European discharge requirements for urban WWTPs were derived. The results demonstrate extended aeration WWTPs were more reliable and stable than conventional aeration WWTPs (0.98 reliability vs. 0.82 for BOD5, 0.97 vs. 0.91 for COD, and 0.94 vs. 0.89 for TSS). Furthermore, the lower reliabilities and stabilities were found for the smaller WWTPs. These results are important for strategic asset management for designing and rehabilitation of the wastewater treatment system. At tactical and operational levels, for resources' allocation and operating conditions set up, the computed WWTP's coefficient of variation allows establishing the mean effluent concentrations required for compliance with a given reliability for different scenarios of discharge requirements.