# Economies of Scale and Efficiency in the Wastewater Treatment Sector: A Decision Tree Approach
## 5 Conclusions  
The existence of economies of scale in the wastewater treatment sector is closely related to efficiency assessments. This study assessed 362 WWTPs in the Valencian Community and analyzed their efficiency according to the plant size, measured in terms of P.E. In order to evaluate the plants' efficiency, the efficiency analysis trees (EAT) model was used, which overcomes some limitations of previously used models. The main results obtained confirm that larger WWTPs tend to be more efficient than smaller ones, which is in line with previous studies; the results also indicated a strong correlation between efficiency and WWTP size. This study also included other variables that affect the economies of scale, such as the economic costs and the most common pollutants removed, as a proxy for the environmental benefits associated with avoiding the discharge of nitrogen and phosphorus into receiving water bodies. Including these variables in a scale economy assessments suggests that installing a WWTP for each municipality may not be the most efficient solution, due to the high operating costs of smaller WWTPs. Furthermore, including the importance of pollutants into the assessment revealed the high influence that economies of scale have on the three groups of WWTPs analyzed. According to these results, the minimum optimal size for WWTPs is the capacity to treat 10,000 P.E., achieving high levels of pollutant removal and lower operational costs. The detailed disaggregation of smaller WWTPs confirmed that there is no significant economic benefit when smaller facilities are installed. These results represent a valuable contribution to the literature, as they provide a more accurate assessment of economies of scale in the wastewater treatment sector. A direct consequence of these results is the modification of existing legislation. Specifically, there should be changes in the requirements for small municipalities, as defined in Directive 91/271/EEC. According to the results obtained for smaller communities, we \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**WWTP LD.**} & \multirow{2}{*}{**P.E.**} & \multicolumn{2}{c}{**Current**} & \multicolumn{2}{c}{**Potential**} & \multirow{2}{*}{**Potential**} \\  & & **Flow Treated** & & & **Operational** & **Total Costs** & **Operational** \\  & & **(m\({}^{3}\)/Annual)** & & **Costs** & **(EUR/Year)** & **Costs** & **Costs** \\  & & & **(EUR/m\({}^{3}\))** & & **(EUR/Year)** & **(EUR/Year)** \\ \hline
68 & 12,115 & 732,958 & 0.30 & 219,887 & & 202,296 \\
69 & 13,436 & 1,573,368 & 0.31 & 487,774 & & 434,250 \\
70 & 14,319 & 2,449,930 & 0.41 & 1,004,471 & & 676,181 \\
71 & 11,584 & 1,874,452 & 0.37 & 693,547 & & 517,349 \\ \hline  & & & & 2,405,650 & & 1,830,075 \\ \hline \end{tabular}
\end{table} 
Table 8: Potential cost saving for Group 2.  
recommend designing WWTPs according to the minimum size proposed here, thus ensuring their technical and economic efficiency. In the case of disaggregated WWTPs, we advise using joint facilities to ensure the optimal performance of the wastewater treatment process. In this regard, this study quantifies the benefits of implementing larger facilities to take advantage of lower operational costs. Thus, based on a sample of WWTPs with specific characteristics, decision makers can calculate the benefits of centralizing services in terms of the increased investment in pipelines to bring wastewater to a specific point for treatment and subsequent discharge. While it may be difficult to modify the existing small WWTPs, sustainable management strategies related to the use of soft treatment can be applied to help increase the efficiency of discharges while ensuring their efficacy. On the other hand, the possible self-generation of energy can help to reduce the economic costs of smaller facilities, ensuring their economic, environmental, and social sustainability. V.H.-C.: Writing--original draft, resources, methodology, investigation, formal analysis, and conceptualization. A.B.-D.: Writing, review and editing, methodology.