Wastewater Treatment Plants (WWTPs) play a crucial role in maintaining ecological balance, a cornerstone of environmental health for thriving biodiversity and undisturbed natural processes. This balance is crucial for the sustainability of ecosystems, directly influencing human health, biodiversity, and the overall quality of our natural environment. WWTPs contribute to this equilibrium by efficiently removing pollutants and harmful substances from wastewater, thus averting the degradation of water bodies that are essential for numerous ecological processes. WWTPs encompass multiple stages of wastewater and sludge treatment and are significant energy consumers globally, especially in secondary treatment, particularly the activated sludge method which is the most common method. With an upcoming directive from the European Union aiming to reduce energy consumption in WWTPs, this paper focuses on a literature review examining global practices implemented across all stages of WWTP treatment processes. It summarizes the key points of each study, focusing primarily on the outcomes of each application. This document concludes with an in-depth review of each study and provides general conclusions for each group of studies. The objective is to identify methods that have effectively reduced energy consumption and enhanced the overall energy efficiency of WWTPs. The main conclusions indicate that the studies encompass a wide range of applications that achieve significant reductions in energy consumption. However, additional testing of these applications in more diverse operating environments through trials could further enhance their reliability and increase acceptance among WWTP operators.

Wastewater treatment plants (WWTPs) are crucial for environmental protection and public health; however, they are among the most energy-intensive facilities in the water sector. This study examines the correlation between energy consumption and treatment efficiency at the Metamorphosis WWTP (MWWTP) in Attica, Greece, during the years 2022 and 2023. By analyzing influent and effluent characteristics, energy consumption patterns, and the removal efficiencies of key pollutants—Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD₅), and Suspended Solids (SS)—this research provides valuable insights into optimizing wastewater treatment operations. The findings reveal that, despite seasonal variations and fluctuations in influent composition, the facility consistently achieved high pollutant removal rates while maintaining stable energy consumption. The influent BOD₅ increased from 992.8 mg L⁻¹ in 2022 to 1122.3 mg L⁻¹ in 2023. COD rose from 1925.4 mg L⁻¹ to 2594.4 mg L⁻¹, SS from 1280.8 mg L⁻¹ to 1421.2 mg L⁻¹, and total phosphorus from 14.2 mg L⁻¹ to 17.0 mg L⁻¹. Effluent concentrations remained consistently low, with BOD₅ at 6.1 mg L⁻¹ in 2022 and 4.7 mg L⁻¹ in 2023; COD at 23.8 mg L⁻¹ and 25.2 mg L⁻¹, respectively; total nitrogen at 20.2 mg L⁻¹ and 16.7 mg L⁻¹; total phosphorus at 2.4 mg L⁻¹ and 2.6 mg L⁻¹; and SS at 2.4 mg L⁻¹ and 3.5 mg L⁻¹. These results indicate removal efficiencies exceeding 90%. Energy consumption remained stable, recorded at 13,044.9 kWh (0.593 kWh m⁻³ influent) in 2022 and 13,126.1 kWh (0.598 kWh m⁻³ influent) in 2023. These results highlight the importance of integrating energy-efficient strategies and renewable energy solutions to enhance wastewater treatment plant (WWTP) sustainability. This study contributes to ongoing efforts to improve energy optimization in wastewater treatment, supporting global initiatives for carbon footprint reduction and advancing the principles of a circular economy.