# Economies of Scale and Efficiency in the Wastewater Treatment Sector: A Decision Tree Approach
###### Abstract  
In this study, we use the efficiency analysis trees (EAT) model to evaluate the efficiency of wastewater treatment plants (WWTPs), focusing on identifying the key variables that enhance their performance. While traditional methods consider factors such as the plant size, age, and technology, the EAT model improves the precision of and reduces errors in the efficiency estimation process. The results highlight the significance of facility size, particularly in areas with smaller populations, suggesting that economies of scale can play a crucial role in optimizing treatment processes. Centralizing the sector could lead to cost savings through ensuring better resource allocation and more effective management strategies. This study provides insights into how facility dimensions impact efficiency, aiding in strategic planning for wastewater treatment operations. The novelty of this study pertains to its implementation of the EAT model to assess efficiency from an economic point of view. Specifically, this makes it possible to identify which facility dimensions ensure better performance and, consequently, will help operators to establish criteria for intervention and geographical planning, both technically and economically. corporate sustainability; wastewater treatment plant; efficiency analysis trees; environmental economics Article  
Accademic Editors: [PERSON] and [PERSON] Received: 16 January 2005 12 March 2020 12 March 2020 19 March 2025 19 March 2025 19 March 2025 202  
characteristics of the wastewater and the quality requirements of the effluent. For example, the aeration process, which is necessary for the removal of pollutants in the water, can account for up to 70% of the total energy consumption [9]. Similarly, depending on the technology used, sludge treatment also generally has high energy demands. However, these facilities have the potential to contribute to the decarbonization of the energy system. This could be achieved through utilizing the thermal and chemical energy present in wastewater and sewage sludge. These processes, combined with other alternatives--such as the use of renewable resources and energy optimization of the facility itself--can significantly increase the balance of clean energy used, thus reducing the consumption of non-renewable energy sources [10]. Beyond self-consumption, it is also possible to integrate the energy production of these facilities as an agent of local energy supplies, thus reducing the consumption of energy from non-renewable sources, contributing to decarbonization and enhancing the circular economy. As WWTPs are an essential part of guaranteeing the circularity of water resources in urban water management, it is also necessary that wastewater treatment plants fulfill the principles of the circular economy; that is, improving the efficiency of the processes by reducing the amount of resources needed to perform optimally, keeping the products and materials in the loop as long as possible while reducing the amount of residue generated [11; 12]. In terms of WWTP processes, the treatment involves using a combination of physical, chemical, and biological technologies to reduce wastewater pollutants to meet the quality standards set by Directive 91/271/EEC. The performance of WWTPs is influenced by numerous factors such as the technology [13; 14], the size of the facilities [15; 16], their age [17; 18], and maintenance management [19]. However, the optimum size of wastewater treatment systems is an area that has received a great deal of attention in the literature; there is significant debate regarding the cost-efficiency of centralized and decentralized sewage solutions [20; 21; 22]. Economies of scale are well-known in urban water cycle management; numerous studies have shown the influence of the network (supply and/or sanitation) on the efficiency of the process [23; 24; 25; 26; 27]. This suggests that the most efficient way to achieve a reduction in wastewater treatment costs is to increase the size of treatment facilities.