# Economies of Scale and Efficiency in the Wastewater Treatment Sector: A Decision Tree Approach
## 4 Results
### Individual Efficiency Analysis  
Based on the results obtained in the EAT analysis (Table 4), the sample was subdivided into different smaller groups to determine whether the presence of economies of scale follows a pattern (Figure 1). \begin{table}
\begin{tabular}{l c c c c c} \hline
**EAT Input** & **Energy** & **Staff** & **Maintenance** & **Residues** & **Others** \\ \hline Global results & \(-0.452\)** & \(-0.634\)** & \(-0.680\)** & \(-0.252\)** & \(-0.947\)** \\ \hline Group 1 & \(-0.421\)** & \(-0.573\)** & \(-0.636\)** & \(-0.241\)** & \(-0.948\)** \\ Group 2 & \(-0.359\)** & \(-0.584\)** & \(-0.593\)** & \(-0.446\)** & \(-0.939\)** \\ Group 3 & \(-0.610\)** & \(-0.524\)** & \(-0.657\)** & \(-0.505\)** & \(-0.981\)** \\ Group 4 & \(-0.414\)** & \(-0.533\)** & \(-0.429\)** & \(-0.103\) & \(-0.636\)** \\ \hline \end{tabular}  
* The correlation is significant at the 0.01 level (two sided).
* The correlation is significant at the 0.05 level (two sided).  
\end{table} 
Table 5: Coefficient correlation between variables and score (non-parametric). The mean values of the inputs of each subgroup are shown in Table 6. This table breaks down the data for the various subgroups by range, showing various indicators such as P.E., costs (energy, staff, maintenance, waste, and other), and pollutants removed (TSS, COD, BOD\({}_{5}\), TN, and TP). The objective is to analyze the behavior of smaller structures in order to corroborate the existence of economies of scale. The sample was divided into subgroups by specific volume ranges (Figure 1): 1.A (under 3000), 1.B (3000-6000), and 1.C (6000-10,000), with further breakdowns under 1.A into 1.A.1 (\(<\)1000), 1.A.2 (1000-2000), and 1.A.3 (2000-3000). Regarding P.E., Table 6 shows the P.E. treated by each group. Group 1.A treats a total of 630 P.E., group 1.B 4400 P.E., Group 1.C approximately 8300 P.E. Subgroups 1.A.1, 1.A.2, and 1.A.3 break down the first group into smaller subgroups, treating 335, 1400, and 2600 P.E., respectively. Table 6 shows the values per unit for each input (costs). For instance, energy costs per unit range between 0.116 in 1.A and 0.093 in Group 1.C. This decrease in costs implies that the facilities acquire larger dimensions according to the P.E. treatedand, consequently, achieve lower energy costs. Likewise, staff and maintenance costs follow the same trend; when the size of the WWTP is increased, a significant reduction is achieved in the different costs required by the wastewater treatment process. As for the volumes of pollutants removed, these generally increase with the size of the group. Focusing on nutrients (TN and TP), WWTPs can increase the quantities of these  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Group** & \begin{tabular}{c} **1.A** \\ **(\(<\)3000)** \\ \end{tabular}  & \begin{tabular}{c} **1.B** \\ **(\(3000\)–6000)** \\ \end{tabular}  & \begin{tabular}{c} **1.C** \\ **(\(6000\)–10,000)** \\ \end{tabular}  & \begin{tabular}{c} **1.A.1** \\ **(\(<\)1000)** \\ \end{tabular}  & \ \begin{tabular}{c} **1.A.2** \\ **(\(1000\)–2000)** \\ \end{tabular}  & \begin{tabular}{c} **1.A.3** \\ **(\(2000\)–3000)** \\ \end{tabular}  \\ \hline P.E. & 631 & 4413 & 8375 & 335 & 1442 & 2625 \\ \hline Energy & 0.116 & 0.082 & 0.093 & 0.141 & 0.083 & 0.069 \\ Staff & 0.737 & 0.214 & 0.208 & 1.016 & 0.323 & 0.228 \\ Maintenance & EUR/m\({}^{3}\  
pollutants removed. Regarding Group 1.A, it removes 0.045 and 0.004 kg/m\({}^{3}\) of nitrogen and phosphorus, respectively. These values increase in Group 1.C to 0.102 and 0.012 kg/m\({}^{3}\), which implies significant growth. Focusing on the group of smaller WWTPs, the nutrient removal quantities show a similar trend, where larger facilities not only reduce operating costs but also enhance the amount removed per cubic meter treated. In summary, larger groups (those with the largest volume) tend to have higher initial values, generally lower resource costs per unit, and higher production volumes.