# Assessment of the Water Quality of WWTPs' Effluents through the Use of Wastewater Quality Index
## 2 Materials and Methods  
### Sampling and Sample Preparation  
Samples of 21 treated wastewater effluents at the outlets of the studied WWTPs were collected in August 2018 in typical hot weather conditions (>30 \({}^{\circ}\)C) without stormy events (Table 1). The plants were selected based on their wide range of p.e. and the rivers they discharge in (Figure 1). Figure 1: Sampling map. Based on the sensitivity of the receiving SWBs, maximum allowable concentrations (MACs) for certain parameters are devised in the respective legislation (Table 2). Directive 91/271/EEC establishes the maximum allowable concentrations for chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP) and total suspended solids (TSS) for the discharges. Directive 2013/39/EU sets the limits for the priority pollutants Cd, Ni and Pb. The national legislative document (Ordinance N-4) determines the maximum allowable concentrations for achieving at least a moderate status of the SWBs (lakes and rivers) for pH, electrical conductivity (EC) and Cr. According to the same document, the MAC for Cr is set at 32 \(\mu\)g/L for both the oxidation states--Cr (III) and Cr (VI). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Acronym**} & \multicolumn{4}{c}{**WWTP**} \\ \cline{2-7}  & **Sampling** & **Population** & **Q** & **Sampling Date** & **Receiving SWB** \\  & **Location** & **Equivalent** & **m\({}^{3}\)/2018** & & & \\ \hline BLG & Blagoevgrad & 87,520 & 3,468,098 & 13 August 2018 & **Struna River** \\ DMG & Dimitrovgrad & 70,350 & 2,233,651 & 17 August 2018 & **Maritsa River** \\ DPN & Dupnitsa & 55,240 & 5,915,566 & 13 August 2018 & **Struna River** \\ GAB & Gabrovo & 99,780 & 8,458,611 & 29 August 2018 & **Tantra River** \\ KNL & Kyustendil & 70,000 & 1,587,885 & 13 August 2018 & **Struna River** \\ K2N & Kazanlak & 80,000 & 9,988,652 & 16 August 2018 & **Tundhaz River** \\ LOV & Lovech & 85,600 & 5,945,412 & 10 August 2018 & **Osam River** \\ MON & Montana & 98,617 & 8,468,560 & 17 August 2018 & **Ogosta River** \\ MZD & Mezdra & 15,984 & 381,028 & 17 August 2018 & **Iskar River** \\ PAZ & Pazardzhik & 156,000 & 31,17,426 & 27 August 2018 & **Maritsa River** \\ PBN & Pavel Banya & 3000 & 597,656 & 16 August 2018 & **Tundhaz River** \\ PDV & Plovdiv & 596,000 & 17,101,385 & 17 August 2018 & **Maritsa River** \\ PER & Pernik & 82,000 & 3,277,632 & 14 August 2018 & **Struna River** \\ POP & Popovo & 37,000 & 1,583,874 & 30 August 2018 & **Cherni Lom River** \\ PVN & Pleven & 188,000 & 22,058,863 & 10 August 2018 & **Vit River** \\ RDN & Radnevo & 18,346 & 1,814,123 & 16 August 2018 & **Maritsa River** \\  & & & & & & (through Sazilyka River) \\ SEV & Sevlievo & 54,000 & 3,269,429 & 29 August 2018 & **Tantra River** \\  & & & & & (through Rositsa River) \\ SMK & Samokov & 125,000 & 9,863,285 & 23 August 2018 & **Iskar River** \\ SOF & Sofia & 2,037,000 & 133,505,643 & 23 August 2018 & **Iskar River** \\ SZG & Stara Zagora & 256,300 & 7,671,645 & 16 August 2018 & **Maritsa River** \\  & & & & (through Bedechka River) \\ TRO & Troyan & 80,000 & 2,158,541 & 10 August 2018 & **Osam River** \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Acronyms, locations, population equivalent (p.e. ), discharged volumes (Q), sampling dates and the receiving surface water bodies the WWPTs discharge in.