## 3 Results and Discussion  
### The Efficiency of Pollutant Removal  
The chosen statistical values of pollutants in wastewater from the studied household WWTPs are presented in Tables 2 and 3. The concentrations of pollutants in wastewater inflowing to the chambers with activated sludge (after mechanical treatment) were relevantly lower in comparison to raw wastewater flowing into the preliminary settling tanks (the first element of the system), as described in an earlier paper [9]. The values of pollutant concentrations in wastewater inflowing to the activated sludge chambers in the studied WWTPs were close to those described in the literature for wastewater treated mechanically in the preliminary settling tanks [34; 35; 46; 48; 49; 50]. **TSS** is a measure of the floating solid content in wastewater, which indicates its clarity [51; 52]. At the researched facilities no. 1 and 2, the removal efficiency of TSS was low and amounted to 17% and 48%, respectively (Figure 2). The low efficiency of TSS removal in the analyzed systems is due to the fact that a significant proportion (>60%) is removed in the preliminary settling tanks, which constitutes the first elements of the studied WWTPs [9]. Better results for TSS removal (58-94%) were found by [PERSON] and [PERSON] [31] in three facilities with activated sludge but without preliminary settling tanks. Quite high TSS removal effects (90-96%) were also obtained by [PERSON] and [PERSON] [53] in household WWTPs operating with low activated sludge technology in a sequential batch reactor (SBR) system. A study conducted on two household hybrid CW WWTPs of VF/HF type (with vertical and horizontal flow) with common read and willow, operating in the RNP, showed that they also provide quite high efficiency (80-87%) of TSS \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multicolumn{8}{c}{**Statistical Indicators**} \\ \cline{2-10}  & \multicolumn{2}{c}{**Mean**} & \multicolumn{2}{c}{**Min**} & \multicolumn{2}{c}{**Max**} & \multicolumn{2}{c}{
\begin{tabular}{c} **Standard** \\ **Deviation** \\ \end{tabular} } \\ \cline{2-10}  & \multicolumn{2}{c}{**in**} & **out** & **in** & **out** & **in** & **out** & **in** & **out** \\ \hline pH & - & - & - & 7.90 & 6.29 & 8.22 & 7.72 & - & - \\ \hline \multirow{2}{*}{Dissolved oxygen} & mg & \multirow{2}{*}{1.62} & \multirow{2}{*}{8.02} & \multirow{2}{*}{0.23} & \multirow{2}{*}{3.15} & \multirow{2}{*}{7.02} & \multirow{2}{*}{10.56} & \multirow{2}{*}{1.63} & \multirow{2}{*}{1.97} \\  & O\({}_{2}\)/dm\({}^{3}\) & & & & & & & & \\ \hline TSS & mg/dm\({}^{3}\) & & 28.7 & 23.9 & 5.3 & 1.3 & 69.0 & 73.0 & 20.9 & 18.6 \\ \hline BODs & mgO\({}_{2}\)/dm\({}^{3}\) & & 53.0 & 9.0 & 12.3 & 1.0 & 80.0 & 53.0 & 22.1 & 12.9 \\ \hline COD & mgO\({}_{2}\)/dm\({}^{3}\) & & 180 & 63 & 111 & 20.7 & 236 & 130 & 34.3 & 27.43 \\ \hline Ammonium nitrogen & mg/dm\({}^{3}\) & & 136.0 & 39.9 & 111.0 & 15.0 & 172.0 & 80.5 & 17.2 & 18.1 \\ \hline Nitrate nitrogen & mg/dm\({}^{3}\) & & 1.97 & 61.17 & 0.09 & 25.5 & 5.70 & 87.7 & 1.89 & 17.88 \\ \hline Nitrite nitrogen & mg/dm\({}^{3}\) & & 0.31 & 1.36 & 0.03 & 0.59 & 1.17 & 3.97 & 0.43 & 0.81 \\ \hline TN & mg/dm\({}^{3}\) & & 160 & 127 & 121 & 94 & 207 & 159 & 24.4 & 15.3 \\ \hline TP & mg/dm\({}^{3}\) & & 12.0 & 11.5 & 8.2 & 9.5 & 17.2 & 19.0 & 3.0 & 2.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Pollutant concentrations in the inflow and outflow of facility no. 1. TSS, total suspended solids; BOD5, biochemical oxygen demand in 5 days; COD, chemical oxygen demand; TN, total nitrogen; TP, total phosphorus. removal [36]. Furthermore, [PERSON] et al. [49], in a CW with common reed, manna grass, and Virginia mallow, obtained an efficiency of TSS removal of more than 86%. The conducted study shows that the average concentrations of TSS in outflow from the analyzed WWTPs were 23.9 mg/dm\({}^{3}\) for facility no. 1 and 18.1 mg/dm\({}^{3}\) for facility no. 2 (Tables 2 and 3).