Energy efficiency in wastewater treatment plants: A framework for benchmarking method selection and application

Utilities produce and store vast amount of data related to urban wastewater management. Not yet fully exploited, proper data analysis would provide relevant process information and represents a great opportunity to improve the process performance. In the last years, several statistical tools and benchmarking methods that can extract useful information from data have been described to analyse wastewater treatment plant (WWTP) energy efficiency. Improving energy efficiency at WWTPs is however a complex task which involves several actors (both internal and external to the water utility), requires an exchange of different types of information which can be analysed by a broad selection of methods. Benchmarking method therefore must not only be selected based on whether they provide a clear identification of inefficient processes; it must also match the available data and the skills of those performing the assessment and objectives of stakeholders interpreting the results. Here, we have identified the requirements of the most common benchmarking methods in terms of data, resources, complexity of use, and information provided. To do that, inefficiency is decomposed so that the analyst, considering the objective of the study and the available data, can link each element to the appropriate method for quantification and benchmarking, and relate inefficiency components with root-causes in wastewater treatment. Finally, a framework for selecting the most suitable benchmarking method to improve energy efficiency in WWTPs is proposed to assist water sector stakeholders. By offering guidelines on how integrates and links data, methods and actors in the water sector, the outcomes of this article are expected to move WWTPs towards increasing energy efficiencyThis work was financially supported by ENERWATER Coordination Support Action, as part of European Union's Horizon 2020research and innovation programme under grant agreement No 649819. A. Hospido and M. Mauricio-Iglesias belong to a Galician Competitive Research Group (GRC) under reference ED431C-2021/37, a programme co-funded by the FEDER (EU)S

Designing a robust index for WWTP energy efficiency: the ENERWATER Water Treatment Energy Index

The development and use of composite indexes has exploded in the last 15 years as a tool to summarise the large amount of information available nowadays. To ensure that composite indicators reflect faithfully the purpose of evaluation and are widely accepted and used, the users must understand the relationship between individual sub-indicators and the result (transparency) and the evaluation should not depend on weights, lack of information, etc. (robustness). It is proposed here for the first time a composite index to measure energy efficiency in a wastewater treatment plant, from the definition of the individual sub-indicators to the assignation of an energy label, easy to communicate to a broad public. Using as a demonstration real data from 44 wastewater treatment plants, the index robustness is tested and improved by uncertainty and sensitivity analysis results, finally achieving a robust algorithm which can be used by the large majority of wastewater treatment plantsThis work has been financially supported by ‘ENERWATER’ Coordination Support Action (www.enerwater.eu), as part of European Union's Horizon 2020 research and innovation programme under grant agreement No 649819. Authors belong to the Galician Competitive Research Group ED431C2017/029 and to the CRETUS Strategic Partnership (ED431E 2018/01), both programmes co-funded by FEDER (EU)S

Benchmarking energy use in wastewater treatment plants

IWA Pi Conference, May 15th 2017, ViennaAcademic research on benchmarking energy efficiency predominately focuses on two main streams of alternatives techniques: parametric and nonparametric. Since there is no agreement on the consistency of their estimates, the purpose of this paper is to investigate whether parametric ordinary least squares (OLS) and nonparametric data envelopment analysis (DEA) generate consistent wastewater treatment plant (WWTP) efficiency estimates. Our findings confirm that energy efficiency is function of several variables, including operational and exogenous factors. For it, a two-stage DEA process was followed in order to observe the overall effect produced by operation size and environmental conditions on efficiency index estimated by DEA. Based on the results of our analysis, the simply DEA in the variable return to scale assumption may not reflect the real efficiency of a WWTP. On the contrary a good consistency between OLS and 2-stage DEA was found. Conclusion of this study is that WWTP efficiency estimates may be sensitive to the method employed, and the use of multiple approaches for robustness checking is recommende

A critical comparison of methods for benchmarking energy performance in WWTPs

The energy efficiency of wastewater treatment plants (WWTPs) is a subject of growing interests by water utility and WWTPs operators. As consequence, benchmarking in recent years has become an important tool to assess the performance of these facilities and help identify best practices. This study compares various benchmarking methodology employed for the assessment of the energy efficiency in WWTPs. The literature review revealed three main benchmarking approaches: normalization, statistical techniques and programming techniques. Advantages and disadvantages were identified for each one. It was found additionally that no single Key Performance Indicators (KPIs) used to characterize the energy performance could be used universally. In general, while these methods can be used for comparison, the diagnosis of the energy performance remains an unsolved issu

Preliminary optimization of the environmental performance of PHA downstream processing

As downstream processing has been identified as a bottleneck for the sustainable development of the polyhydroxyalkanoates (PHAs) value chains, this work aims to provide new insights for the optimization of its environmental performance. After identifying the most promising methods for PHA recovery and purification, four processes with two different system functions,high- and low-grade PHA, were defined and scaled up. The environmental performance of these processes was evaluated by life cycle assessment (LCA). Processes based on solvent extraction showed a worse environmental performance due to the high energy requirements of solvents recovery. Processes relying on chemical digestion showed a better environmental performance when combined with other technologies such as high pressure homogenization, which allows a reduction of the amount of chemicals employed. Therefore, two main improvement actions were proposed to overcome these hotspots and estimate the environmental burdens reduction: heat integration in larger facilities would reduce the heat duty, while introducing a chemical recovery unit or combining chemical digestion with other technologies would reduce those impacts related to chemicals consumption. Through this work, it is demonstrated that the environmental performance of PHA downstream processing can be improved if the process design include life cycle assessment from its conceptio

Comparative energy efficiency of Swiss wastewater treatment plants based on economic foundations

IAEE Conference, Groningen, June 10-13, 2018As the number of wastewater treatment plants (WWTPs) increases worldwide and the effluent quality requirements become more demanding, the issue of energy efficiency has been attracting increasing attention from an environmental and economic point of view. Earlier approaches to measuring WWTP energy efficiency such as Data Envelopment Analysis (DEA) have recently focused on controlling for exogenous variables ignoring the possible presence of omitted (not observed) variables. This omission can lead to biased efficiency index. Moreover, since the level of efficiency can be decomposed in two parts, one persistent and one transient, based on such approaches, water utilities may decide to invest in new machines and infrastructure, when instead the origins of inefficiency come from a non-optimal use of some machines or vice versa. The objective of this paper is to investigate how overall inefficiency of WWTPs is decomposed to persistent and transient inefficiency. This allows better evaluation of energy saving measures since both components convey different types of information. While persistent inefficiency reflects long-term structural problems due to, e.g., energy inefficient equipment used for wastewater treatment, transient inefficiency is associated with process operational practices or decisions that take place in the short term. Distinguish between persistent and transient inefficiency, while controlling for exogenous factors, is thus essential to deduce appropriate energy diagnosis and design useful energy efficiency strategies for WWTPs. This research applies a novel approach of Stochastic Frontier Analysis (SFA) for energy demand modelling to estimate the comparative energy efficiency of a comprehensive panel of WWTPs in Switzerland, as far as in known, for the first tim

Can nitrogen removal be a win-win situation?

10th World Congress of Chemical Engineering Barcelona, 3 October 201

An exploratory data analysis of energy consumption of WWTP. Influencing factors and possible methods for benchmarking

Wastewater treatment plants are relevant energy consumers. As they carry out complex operations submitted to large variability caused by, among others, the influent and loading variations, the assessment of the performance is challenging. In this study we perform a regression analysis over a sample of 185 WWTP data in order to establish a regression model that predicts the average energy consumption of the plant given plant design and operation predictors such as the average influent flowrate, the COD loading and the plant loading factor. The regression model is also used to ascertain whether the location of the WWTP in France, Spain and Germany has an impact over the energy consumptio

Measuring energy demand and efficiency at WWTPs: an econometric approach

Frontiers International Conference on Wastewater TreatMent. 21-24 may 2017. Palermo-Ital