# Assessment of the Water Quality of WWTPs' Effluents through the Use of Wastewater Quality Index
###### Abstract  
Evaluating the efficiency of wastewater treatment plants (WWTPs) and their impact on receiving surface water bodies is a complex and highly significant task due to its regulatory implications for both environmental and public health. The monitoring of many water quality parameters related to the compliance of treated wastewater with environmental standards has led to the development of a unitless metric, the Wastewater Quality Index (WWQI), which serves as a practical tool for regulatory authorities. The aim of this research is to propose an appropriate WWQI methodology, incorporating a set of water quality indicators and a weighting approach, to evaluate wastewater effluents under operational monitoring. In this study, WWQI was successfully applied to access the operation of 21 WWTPs' effluents within a single monitoring campaign, outside the mandatory monitoring schemes. The WWQI was computed for physical-chemical parameters including chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), total suspended solids (TSS), electrical conductivity (EC) and pH, priority substances (Cd, Ni and Pb) and a specific contaminant (Cr) using the weighted approach in the WWQI calculation, based on equal weighting, expert judgement and PCA weighing using factor loadings. The three approaches give similar results for the calculated WWQI. The expert judgment approach is more suitable for evaluating WWTP performance during a single monitoring campaign due to its simplicity compared to the PCA-based approach and its ability to prioritize specific water quality parameters over an equal weightage method. W 2024 16 August 2024 10 September 2024 13 September 2024 2024 2024 2024 2024 2024 2024 2024 202  
Legislation in the field of water quality is vast. Depending on the water's intended use or the geographical region, numerous legislative documents apply. In Bulgaria, next to the national legislation, the European Directives also apply. Directive 91/271/EEC deals with the requirements (concentrations or percentage of reduction) for discharges from urban WWTPs [8]. Directive 2013/39/EU for priority substances sets limits for Cd, Ni and Pb in surface water bodies (SWBs) [9]. Finally, the maximum allowable concentrations for the most common physical-chemical parameters in the surface waters are set by Ordinance N-4 [10]. The use of so many legislative documents is necessary to ensure that treated waters do not negatively affect the SWBs. To optimize the treatment processes, a unitless number [11]--the Wastewater Quality Index (WWQI), was introduced as an assessment tool of the overall wastewater quality. It takes into consideration all the monitored chemical, physical and microbiological quality parameters. Often, the interpretation of such huge quality sets is very difficult and requires the application of sophisticated statistical methods. In contrast, a single number (ranging from 1 to 100) is preferred in checking compliance with the established water quality standards [12]. The higher the WWQIs, the more efficient the treatment is, and the wastewater effluents are meeting the WWTPs' design objectives. In contrast, the influents generally have low WWQI values. This renders them harmful if released untreated directly into the water bodies. Therefore, the use of the WWQI benefits the decision makers to rapidly assess the wastewater quality and compare different treatment processes. Different sets of parameters are usually selected for devising the WWQI. Some authors select the minimum required four parameters [6; 13], other investigators prefer fewer than 10--i.e., eight [7; 11; 14; 15] or nine [16]--and few research groups use 10-20 (13 [17], 14 [18] or 15 [19]), while there are reports that use 21 [20; 21] and even 23 water quality parameters [22]. The most parameters required by a method is 26 [23].