Translating removal efficiencies into operational performance indices of wastewater treatment plants Catarina Silva a , Sı´lvia Quadros b , Pedro Ramalho a , Helena Alegre a , Maria Joa ˜o Rosa a, * a Urban Water Division, Hydraulics and Environment Department, LNEC e National Civil Engineering Laboratory, Av. Brasil 101, 1700-066 Lisboa, Portugal b Climate, Meteorology and Global Change Group (C_CMMG, CITA-A), Agricultural Sciences Department, Azores University, Rua Capita ˜o Joa ˜o D’ Avila, 9700-042 Angra do Heroı´smo, Portugal article info Article history: Received 15 October 2013 Received in revised form 12 March 2014 Accepted 13 March 2014 Available online 25 March 2014 Keywords: Performance indicators Performance indices Removal efficiencies Wastewater treatment plants Modeling Model curves abstract Removal efficiencies are often used to assess the performance of a single or a group of unit operations/processes (UOPs) of a wastewater treatment plant (WWTP). However, depend- ing on the influent concentration (C in ), the same efficiency of removal (Er) may be insuf- ficient or excessive to achieve the UOP or WWTP effluent quality requirements, expressed by concentration limit values (LVs). This paper proposes performance indices (PXs), Er- based, as new metrics for benchmarking, i.e. for assessing and improving the perfor- mance of each UOP or treatment step and ultimately of the WWTP as a multi-barrier system, and comprehensively describes the stepwise method of translating Ers into PXs. PXs are dimensionless and vary between 0 and 300 to define three performance levels: unsatisfactory (0e100), acceptable (100e200) and good (200e300) performance. The method developed takes into consideration C in and LV, and the reference values for judging the performance are given from Er-C in typical ranges and Er vs. C in model curves, LV based and field data based. The general equations of the Er model curves are derived. A set of six curves is calibrated for TSS (Total Suspended Solids) and COD (Chemical Oxygen Demand) removal by primary sedimentation and activated sludge systems (carbon or combined carbon and nutrients removal), using 5-year (2006e2010) field data from five Portuguese WWTPs. A statistical analysis of the PX results is additionally proposed to assess treatment reliability. The new method is applied in two WWTPs and the PX results are compared with those of conventional measures e Er and performance indicators (PIs). The results demonstrate that, whereas a simplistic Er-driven or PI-driven management of the WWTPs shows limitations, the developed PXs are adequate measures for benchmarking removal efficiencies towards WWTP reliability and sustainability. ª 2014 Elsevier Ltd. All rights reserved. * Corresponding author. E-mail address: mjrosa@lnec.pt (M.J. Rosa). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres water research 57 (2014) 202 e214 http://dx.doi.org/10.1016/j.watres.2014.03.025 0043-1354/ª 2014 Elsevier Ltd. All rights reserved.