# Assessment of the Water Quality of WWTPs' Effluents through the Use of Wastewater Quality Index
###### Abstract
One of the pitfalls of the WWQI approach is that it establishes relationships between water quality indicators not allowing a straightforward interpretation of what causes the specific value of the WWQI, thus eclipsing or over-emphasizing a single bad parameter value. The selection of parameters to be used in devising the WQI models is often based on the data (un)availability [24] and obtainability, expert opinion and the environmental significance or application type (e.g., drinking water, surface water, underground water, wastewater, etc.) [25]. The Canadian Council of Ministers of the Environment (CCME) WQI is currently one of the most widely used methods to evaluate surface water for the protection of aquatic life in accordance with specific guidelines due to its simplicity, possibility to vary the study parameters at different locations and adaptability to different legal requirements and different water uses [26; 27; 28; 29]. The sampling protocol requires at least four parameters, sampled at least four times, which is suitable for water quality assessment of the mandatory sampling, but renders the method useless in random checks. Since many WQIs exist [30; 31], weighted [6; 13; 14; 15; 17; 21; 22] and unweighted [7; 11; 16; 18; 20] approaches are undertaken to assess the WWTPs' performance. The weights are assigned according to the parameters' relative importance for water quality. Usually, the highest weights are assigned to parameters that have major effects on water quality and are very important for water quality compliance. The minimum weight is usually assigned to parameters which are not considered harmful. A different approach utilizes the parameters with the lowest permissible limits as being the most harmful even on slight concentration fluctuations. Subsequently, the highest weights are assigned to low permissible limits' parameters, while the high permissible limits' parameters allow relatively fewer chances of pollution and, therefore, low weightings are assigned [32]. One approach is to assign equal weightage to all the studied parameters; another is based on an expert judgement [33]. Assigning equal weights to the selected parameters acknowledges their similar importance in the assessment and is close to the approach followed in the Water Framework Directive (WFD) [34]. Experts' opinions are used for the variables' choice and their weight assignments. In a third approach, the weights are defined as functions of the standards proposed in the water quality guidelines [35]. Regardless of the approach used,the weighting can deteriorate the final WWQI calculated value due to possible changes in the expert panel or when the guidelines are improved. Therefore, the weighting should be decided according to the use of water or to be determined locally. As different experts give sometimes different weights for the same parameters [21; 36], Principal Component Analysis (PCA) was used to assign the weight based on the estimated eigenvalues and component loading for each parameter using the recommended standards for the corresponding parameters [37]. The PC-weight assignment has been used to monitor temporal wastewater quality [38], overcoming the shortness of the CCME WQI approach. PCA-based WQI minimizes the subjectivity and uncertainty from overrating or underrating [39], despite its inconsistencies [40]. The present work aims to evaluate the water quality of WWTPs' effluents by combining physical-chemical parameters, specific contaminants, a priority substance and common wastewater quality indices, in a single monitoring campaign, outside the mandatory monitoring schemes. For this reason, a weighted approach incorporating the deviations from environmental legislation norms, based on (i) equal weightage, (ii) expert judgement and (iii) PCA estimated factor loadings was introduced to the calculation of WWQI.