Energy audits in wastewater treatment plants (WWTPs) reveal large differences in the energy consumption in the various stages, depending also on the indicators used in the audits. This work is aimed at formulating a suitable methodology to perform audits in WWTPs and identifying the most suitable key energy consumption indicators for comparison among different plants and benchmarking. Hydraulic-based stages, stages based on chemical oxygen demand, sludge-based stages and building stages were distinguished in WWTPs and analysed with different energy indicators. Detailed energy audits were carried out on five small WWTPs treating less than 10,000 population equivalent and using continuous data for 2 years. The plants have in common a low designed capacity utilization (52% on average) and equipment oversizing which leads to waste of energy in the absence of controls and inverters (a common situation in small plants). The study confirms that there are several opportunities for reducing energy consumption in small WWTPs: in addition to the pumping of influent wastewater and aeration, small plants demonstrate low energy efficiency in recirculation of settled sludge and in aerobic stabilization. Denitrification above 75% is ensured through intermittent aeration and without recirculation of mixed liquor. Automation in place of manual controls is mandatory in illumination and electrical heating.

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