Pitfalls in international benchmarking of energy intensity across wastewater treatment utilities

Nathan L Walker,A Prysor Williams,David Styles

doi:10.1016/j.jenvman.2021.113613

Nathan L Walker, A Prysor Williams + Show 1 more

Open Access

https://doi.org/10.1016/j.jenvman.2021.113613

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Abstract

The collection, treatment and disposal of wastewater is estimated to consume more than 2% of the world's electrical energy, whilst some wastewater treatment plants (WWTPs) can account for over 20% of electrical consumption within municipalities. To investigate areas to improve wastewater treatment, international benchmarking on energy (electrical) intensity was conducted with the indicator kWh/m3 and a quality control of secondary treatment or better for ≥95% of treated volume. The core sample included 321 companies from 31 countries, however, to analyse regional differences, 11 countries from an external sample made up of various studies of WWTPs was also used in places. The sample displayed a weak-negative size effect with energy intensity, although Kruskal-Wallace analyses showed there was a significant difference between the size of groups (p-value of 0.015), suggesting that as companies get larger; they consume less electricity per cubic metre of wastewater treated. This relationship was not completely linear, as mid to large companies (10,001–100,000 customers) had the largest average consumption of 0.99 kWh/m3. In the regional analysis, EU states had the largest average kWh/m3 with 1.18, which appeared a result of the higher wastewater effluent standards of the region. This was supported by Denmark being the second largest average consuming country (1.35 kWh/m3), since it has some of strictest effluent standards in the world. Along with energy intensity, the associated greenhouse gas (GHG) emissions were calculated enabling the targeting of regions for improvement in response to climate change. Poland had the highest carbon footprint (0.91 kgCO2e/m3) arising from an energy intensity of 0.89 kWh/m3; conversely, a clean electricity grid can affectively mitigate wastewater treatment inefficiencies, exemplified by Norway who emit just 0.013 kgCO2e per cubic meter treated, despite consuming 0.60 kWh/m3. Finally, limitations to available data and the analysis were highlighted from which, it is advised that influent vs. effluent and net energy, as opposed to gross, data be used in future analyses. The large international sample size, energy data with a quality control, GHG analysis, and specific benchmarking recommendations give this study a novelty which could be of use to water industry operators, benchmarking organisations, and regulators.

Highlights

The collection, treatment and disposal of wastewater is a significant consumer of energy, with estimates suggesting that more than 2% of the world’s electrical energy is used for water supply and wastewater treatment (Plappally and Lienhard, 2012; Olsson, 2015)
The expectation is that larger wastewater treatment plants (WWTPs) and companies are more efficient due to economies of scale (Molinos-Senante et al, 2018)
That as regions with lower effluent standards improve to similar levels of advanced economies, their energy consumption will increase too

Summary

Introduction

The collection, treatment and disposal of wastewater is a significant consumer of energy, with estimates suggesting that more than 2% of the world’s electrical energy is used for water supply and wastewater treatment (Plappally and Lienhard, 2012; Olsson, 2015). Reducing the en ergy consumption of wastewater management is integral to efficient resource use within a circular economy and to reduce greenhouse gas (GHG) emissions. This task is more difficult considering WWTP elec tricity demand within developed countries is expected to increase by over 20% in the 15 years as controls on wastewater become more stringent (Wang et al, 2012; Hao et al, 2015); with the same trend expected in developing countries as wastewater quality becomes a greater priority (Lopes et al, 2020). The importance of improving the sustainability of wastewater treatment is highlighted by its inclusion in the United Nations Sustainability Development Goal 6 (2021a) that seeks to secure safe drinking water and sanitation, focussing on the sustainable management of wastewater, water resources and ecosystems.

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