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Management of Wastewater Treatment Plants (WWTPs) is a complex process, not only from a technical point of view but also from an economic point of view. There are several factors that influence the wastewater treatment process and can cause an increase in management costs. One of these factors is the treatment capacity, in particular the WWTPs’ oversizing problem. The difference between the design population equivalent (p.e.) and the actual p.e. treated generates imbalances in the wastewater treatment process costs. This paper analyses the influence that oversizing has on the maintenance costs for WWTPs in the Valencia region (Spain). Through an econometric approach, the behaviour of WWTPs including the oversizing variable has been modelled. Specifically, maintenance cost functions have been obtained to model the variability of maintenance costs according to oversizing influence. To date, this approach has not been considered in the published literature, reinforcing the novelty of this work. Results verify the significant influence of oversizing on maintenance costs in WWTPs. Modelling proposed in this work acts as an effective tool for WWTP managers to identify both the maintenance cost trend and how far the WWTP is from the design p.e. Likewise, results confirm the fact that WWTPs should work close to their design p.e. to improve the efficiency of the wastewater treatment process, reducing WWTPs’ costs.

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.