The operation and management of Wastewater Treatment Plants (WWTPs) is of paramount importance to guarantee the sustainability of water resources. Just like any other production process, wastewater treatment process involves different costs, such as: energy, personnel, maintenance and reagents. Regarding maintenance costs, they have become an issue of great concern to many operators in the last years. Maintaining the facilities in good condition ensures the proper performance of these infrastructures and reduces the risk of failures. It should be known that equipment breakdowns imply an increase in operating costs, with a high risk of generating an environmental damage due to the malfunction or the stoppage of the process. To address this situation, preventive maintenance polices are portrayed as the best strategies to reduce equipment breakdowns and repairs. Although maintenance costs are a relevant issue for wastewater treatment plant operators, there are not many studies that assess the influence of maintenance on the efficiency of the facilities. In order to fill this gap an empirical study is proposed for a sample of wastewater treatment plants facilities. (C) 2020 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.