# Economies of Scale and Efficiency in the Wastewater Treatment Sector: A Decision Tree Approach
## 4 Results
### Potential Operational Cost Savings in WWTP by Group  
We assessed the cost-saving potential of WWTPs, considering the impact that an increase in the size of the WWTP would have on the wastewater treatment process. In this sense, using Equation (5) and the current operating costs (EUR/m\({}^{3}\)) of each WWTP group, the savings that would be generated by belonging to the next group (next size) were assessed. This involves an assessment of the increasing influence of size, according to the P.E. treated. The results obtained are shown in Figure 3, which presents the costs generated in the wastewater treatment processes by each group. Considering the influence of scale economies, the centralization of services enables a reduction in operational costs, with potential savings estimated by applying the cost structure of larger facilities. As a result, the WWTPs in Group 1 would achieve a cost reduction similar to the WWTPs in Group 2, while Group 2 would achieve further reductions, similar to the performance of Group 3, and so forth. As shown in Figure 3, Group 1 (WWTPs treating less than 10,000 P.E.) incurs operating costs of around EUR 1.36/m\({}^{3}\), since this group of WWTPs treats the wastewater of small populations. Group 2 is made up of facilities that treat between 10,000 and 20,000 P.E. Their operating costs are EUR 0.32/m\({}^{3}\), on average. Group 3 includes those WWTPs that treat between 20,000 and 50,000 P.E., with lower than average operating costs, around 0.27 EUR/m\({}^{3}\). This trend continues to decline, reaching EUR 0.19/m\({}^{3}\) in Group 4 (>50,000 P.E.). This reduction in the operational costs, along with the increased amounts of pollutants removed (Kg/m\({}^{3}\)), allows for the quantification of the direct economic benefits of implementing economies of scale in the wastewater treatment sector. The reduction in operational costs would be possible if, during the design and investment stage, the sewage collection of multiple populations were centralized. In this regard, larger facilities would lower wastewater treatment costs, increasing plant efficiency through lower costs and higher pollutant removal rates. Based on the results obtained, centralizing services in smaller-scale facilities (Group 1) would achieve an average cost of EUR 0.32/m\({}^{3}\). Group 2 would achieve a reduction of  
Figure 3: Operational costs per group. approximately 14% and Group 3 of around 29%. These potential savings in operational costs must be quantified to assess the feasibility of centralizing the service. Some of the variables that ultimately justify sharing services depend on the distance between urban agglomerations and the topography; it is therefore necessary to analyze both the investment and the subsequent maintenance of the sewer networks that would enable their connection. The potential savings that could be obtained by using centralized sewage networks open up a new line of research. Specifically, research should consider the construction of new sewerage networks in areas where municipalities or urban agglomerations are close to each other. Table 7 shows an example of the potential savings that the facilities would obtain in the case of sharing services. The operational costs associated with each of the facilities fluctuate between EUR 0.63/m\({}^{3}\) and EUR 1.15/m\({}^{3}\). The heterogeneity of the costs may depend on the flow rates and organic loads, as well as variables related to the facilities themselves. However, the geographical proximity of these facilities would make it possible to share services and take advantage of the economies of scale generated by a larger facility. In the case analyzed, all of the WWTPs treat approximately 10,000 P.E., so they would reach the operational costs of Group 2, of EUR 0.320/m\({}^{3}\). More specifically, WWTP1 treats a total of 650 P.E. and has an annual flow of 57,786 m\({}^{3}\). The operational costs amount to EUR 46,807/year, which implies approximately EUR 0.81/m\({}^{3}\). WWTP1 treats a total of 652 P.E.