# Assessment of the Water Quality of WWTPs' Effluents through the Use of Wastewater Quality Index
## 4 Discussion  
The intercept (b\({}_{0}\) = 1.8947) indicates that, on average, WWQI\({}_{\rm Exp}\) results are almost two units higher than WWQI\({}_{\rm PCA}\) ones (Figure 4). This outcome contradicts the observations in [55] that the PCA method gives different results than the expert judgement (sometimes referred to as the Delphi method) for weight assignment and is considered more accurate in the results' evaluation. It is noteworthy to highlight that the calculated WWQIs using expert judgement and equal weights follow the same trend and are in good agreement (Figure 5), which partially contradicts the findings in [32], where the use of unequal parameter weighting is suggested. The only exception is PER where WWQI\({}_{\rm N}\) is higher than WWQI\({}_{\rm Exp}\) by 11 units. This difference leads to the different status classifications of treated wastewater at WWTP located close to Pernik. This result is in line with [32] and confirms the robustness of WWQI model when unequal weights are used (i.e., expert judgement, PCA weighting) compared to the equal weights assignment. \begin{table}
\begin{tabular}{c c c c} \hline
**WWTP** & **WWQI\({}_{\rm Exp}\)** & **WWQI\({}_{\rm PCA}\)** & **WWQI\({}_{\rm N}\)** & **Exceedings** \\ \hline BLG & & & \\ DMG & & & & EC \\ DPN & & & & TP \\ GAB & & & TP, TN \\ KNL & & & TP, Pb \\ K2N & & & TP \\ MON & & & TP \\ MZD & & & TP \\ PAZ & & & EC \\ PBN & & & \\ PDV & & & TP \\ PER & & & TP \\ POP & & & & EC \\ PVN & & & EC \\ RDN & & & & EC \\ SEV & & & & \\ SMK & & & & \\ SOF & & & & \\ SZG & & & & \\ TRO & & & & \\ \hline \end{tabular} Note. – – very good; – good; – moderate. \end{table} 
Table 7: Water status classification of investigated WWTPs based on applied WWQIs and water quality parameters’ exceedings. To assess the effectiveness of the three approaches used to calculate WWQI, it would be beneficial to compare the water status of wastewater outlets and excursions from environmental water quality standards. Out of 21 WWTP effluents, 11 are classified as having very good status (TRO, BLG, KNL, DPN, PBN, KZN, SZG, SMK, SOF and SEV). This group includes WWTPs of varying sizes and treatment facilities. The excellent/very good status aligns with the absence of excursions from environmental water quality standards. The only exception is the effluent from TRO, where the pH of 8.51 slightly exceeds the very good status range of 6.5-8.5. The second group of WWTP effluents (seven out of 21) is classified as having good water status. Among these, the effluents of two WWTPs (LOV and MZD) show no deviations from environmental standards, while the other five are associated with excursions from these standards. The effluents from PVN, RDN and DMG exhibit EC values exceeding the environmental standard, while those from GAB and POP have TP values above the corre  
Figure 4: Comparison of WWQIs calculated using expert judgement and PCA-based approaches (n = 21). Figure 5: Comparison of WWQIs calculated using equal weights and expert judgement approaches (n = 21). sponding standard. It can be concluded that the WWQIs of WWTPs with excursions are predominantly influenced by water quality parameters that meet environmental standards rather than those that do not. The elevated TP levels exceeding the limit for GAB and POP suggest the need for implementing phosphorus treatment facilities at both WWTPs. Two WWTPs, PDV and PZD, have two water quality parameters that do not meet standards. Both effluents are classified as having a moderate water status, aligning with a previous study [42] that identified issues at these facilities. The only WWTP effluent classified into different classes by the three approaches used is PER. The expert judgment approach assigns a moderate status, while both the equal weighting and PCA-based methods underestimate the TP excursion, resulting in a classification of a good water status for the treated wastewater.