# Economies of Scale and Efficiency in the Wastewater Treatment Sector: A Decision Tree Approach
## 3 Sample Description  
The study uses a sample consisting of 362 WWTPs located in the Valencia Region (Spanish Mediterranean coast), which treats an average annual volume of 301,516 m\({}^{3}\)/year. A basic requirement for the application of this methodology and the analysis of the economies of scale is that the influence of other factors should be isolated; for this reason, the sample should be as homogeneous as possible in terms of the technology of the wastewater treatment process. Accordingly, the WWTPs of the sample consist of a primary treatment followed by a secondary treatment based on extended aeration. None of the facilities are equipped with a tertiary treatment. The information to be used in the analysis, which describes the performance of these facilities, is shown in Table 1; it was provided by the Valencian Wastewater Treatment Agency (EPSAR). The parameters included in this study are different, such that we can differentiate between economic and technical variables. The first group includes variables related to the operation and maintenance costs of the WWTPs (EUR/year): energy (including the cost of the fixed part, power, and the variable part, energy consumed); staff (defined by salaries, fees, taxes, and social insurance); maintenance (the revision, repair, or replacement of the equipment and facility); residues (expenses associated with WWTP management); and others (reagents and consumables). The second group refers to the information related to the amount of pollutant material removed in the process (kg/year): total suspended solids (TSS), chemical oxygen demand (COD), biochemical oxygen demand (BOD\({}_{5}\)), total nitrogen (TN), and total phosphorous (TP). To analyze the effect of economies of scale, the sample is divided into four groups based on the P.E. treated by the facilities. It should be noted that the disaggregation of the sample into these four groups maintains the proportionality according to the characteristics of the territory. The design of a WWTP is influenced by different factors such as the climate, the receiving peak flows, the P.E., and pollutant loads [56]. Additionally, the geography of the region could become another influential factor, affecting the construction or the \begin{table}
\begin{tabular}{c c c} \hline \hline  & & **Average** \\ \hline P.E. & & 13,320 \\ Flow (m\({}^{3}\)/year) & & 301,516 \\ \hline \multirow{4}{*}{Inputs (EUR/year)} & Energy & 19,988 \\  & Staff & 58,107 \\ \cline{1-1}  & Maintenance & 13,154 \\ \cline{1-1}  & Residues & 6832 \\ \cline{1-1}  & Others & 13,431 \\ \hline \multirow{4}{*}{Outputs (kg/year)} & TSS & 267,815 \\ \cline{1-1}  & COD & 507,945 \\ \cline{1-1}  & BOD\({}_{5}\) & 283,287 \\ \cline{1-1}  & TN & 41,203 \\ \cline{1-1}  & TP & 5525 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sample description. operation of the process. As shown in Table 2, a large part of the Valencian territory is composed of small municipalities; in accordance with Directive 91/271/EC, which mandates that municipalities with more than 2000 P.E. must treat their wastewater through a biological wastewater treatment system, these municipalities have been equipped with WWTPs. Therefore, small WWTPs, represented by Group 1, which includes those facilities that treat less than 10,000 P.E., make up the majority of the sample, accounting for 81.4% of the WWTPs included in the study. Table 3 summarizes the economic and environmental variables (pollutants removed) for each of the groups. Considering the four groups into which the sample has been divided, it is evident that an increase in the P.E. leads to a reduction in process costs. Thus, from an economic point of view, the differences in costs (EUR/m\({}^{3}\)) in each group analyzed reveal a reduction in unit costs as the size of the facility increases. Similarly, the pollutants removed generally increase as the size of the facility increases. In the case of suspended solids, the increase from Group 1 to Group 3 offers an increase of approximately 30%.