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INTRODUCTION
More than 2% of the world's electrical energy is a rough estimation of the energy used for water supply and wastewater treatment worldwide (Olsson 2012; Plappally & Lienhard 2012). Municipal wastewater treatment plants (WWTPs), as a major sub-sector of the water utilities, accounts for a significant amount of the overall energy consumption in this field. A large amount of primary energy, mainly originated from fossil sources, is used in WWTPs to meet stringent targets on effluent water quality, but contributes to environmental problems such as global warming and climate change. In this context, measures simultaneously aimed at maintaining a good quality of effluents but improving energy efficiency in WWTPs are imperative.
The benchmark of energy consumption in WWTPs represents a powerful management tool which uses specific indicators to find the optimal performance or to evaluate the energy efficiency of a plant in comparison with other plants or a standard value (inter alia Lindtner et al. 2004; Krampe 2013; Torregrossa et al. 2016). The benchmarking analysis helps to identify potentials for energy savings and may help in prioritising optimisation efforts (Krampe 2013). Currently, a universal benchmarking of energy performance in WWTPs does not exist yet at an international level (Belloir et al. 2015; Longo et al. 2016), and the rare benchmarking studies appear very fragmented and piecemeal because they were carried out locally on the basis of national/regional surveys.
Up to now, energy benchmarks have been referred to in the literature only for Austria (Lindtner et al. 2008; Haslinger et al. 2016), Germany (Baumann & Roth 2008; Haberkern et al. 2008), Australia (Krampe 2013; de Haas et al. 2015), the USA (WEF 2009; WERF 2011), Japan (Mizuta & Shimada 2010), and some plants in North Europe such as in Sweden (Lingsten et al. 2011), Denmark, Norway and Finland (Gustavsson & Tumlin 2013).
Among them, the Austrian Benchmarking System was developed in 1999 and regards more than 130 municipal WWTPs with population equivalent (PE) from 2,000 to 1,000,000 PE, and includes various financial costs expressed in Euros PE−1 year−1 (Lindtner et al. 2008). More recently, the energy consumption of the 104 Austrian municipal WWTPs in the period 2003-2013 were detailed by Haslinger et al. (2016).
The energy benchmark of German plants is based on the ‘target...