## 3 Results and Discussion
### The Efficiency of Pollutant Removal
\begin{table}
\begin{tabular}{l l l l l l l l l l} \hline \hline \multirow{3}{*}{**Parameters**} & \multicolumn{6}{c}{**Statistical Indicators**} \\ \cline{3-10}  & \multicolumn{3}{c}{**Mean**} & \multicolumn{3}{c}{**Min**} & \multicolumn{3}{c}{**Max**} & \multicolumn{3}{c}{**Standard**} \\  & & \multicolumn{3}{c}{**in**} & \multicolumn{3}{c}{**out**} & \multicolumn{3}{c}{**in**} & \multicolumn{3}{c}{**out**} & \multicolumn{3}{c}{**in**} & \multicolumn{3}{c}{**out**} \\ \hline pH & - & - & - & 7.04 & 6.14 & 11.9 & 7.86 & - & - \\ \hline \multirow{2}{*}{Dissolved oxygen} & mg & \multirow{2}{*}{1.29} & \multirow{2}{*}{3.03} & \multirow{2}{*}{0.21} & \multirow{2}{*}{0.60} & \multirow{2}{*}{3.75} & \multirow{2}{*}{9.48} & \multirow{2}{*}{1.09} & \multirow{2}{*}{2.37} \\  & \(\text{O}_{2}/\text{dm}^{3}\) & & & & & & & & \\ \hline TSS & mg/dm\({}^{3}\) & 34.5 & 18.1 & 3.8 & 2.7 & 116 & 48.8 & 31.0 & 14.1 \\ \hline BOD\({}_{5}\) & mgO\({}_{2}/\text{dm}^{3}\) & 85.6 & 29.1 & 16 & 1.6 & 250 & 63.4 & 58.2 & 24 \\ \hline COD & mgO\({}_{2}/\text{dm}^{3}\) & 251 & 95 & 109 & 18 & 400 & 179 & 88 & 52.3 \\ \hline Ammonium nitrogen & mg/dm\({}^{3}\) & 97.6 & 53.4 & 58 & 1.1 & 134 & 102 & 19.7 & 33.0 \\ \hline Nitrate nitrogen & mg/dm\({}^{3}\) & 0.62 & 11.5 & 0.18 & 0.5 & 1.2 & 54.2 & 0.32 & 17.70 \\ \hline Nitrite nitrogen & mg/dm\({}^{3}\) & 0.14 & 0.31 & 0.05 & 0.02 & 0.76 & 1.68 & 0.20 & 0.49 \\ \hline TN & mg/dm\({}^{3}\) & 117 & 77.0 & 60.0 & 35.0 & 182 & 104 & 25.1 & 18.7 \\ \hline TP & mg/dm\({}^{3}\) & 17.3 & 12.1 & 11.4 & 9.6 & 30.9 & 15.8 & 5.7 & 2.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Pollutants concentrations in the inflow and outflow of the facility no. 2.  
facilities but lacking classical preliminary settling tanks, and they found higher COD values which amounted to 128-490 mg/dm\({}^{3}\). **Total Nitrogen. ** In the analyzed facilities no. 1 and 2, the efficiency of TN removal was low and amounted to 21% and 34%, respectively (Figure 2). Much lower effects of TN removal (\(<\)7%) were found by [PERSON] and [PERSON] [31] in three other facilities with activated sludge without preliminary settling tanks. Significantly higher effects of TN removal (51-83%) were obtained by [PERSON] and [PERSON] [53] in household WWTPs operating with SBR systems. High TN removal effects (73-86%) were also obtained by [PERSON] et al. [36] in hybrid household CW WWTPs of VF/HF type in the RNP. Furthermore, [PERSON] et al. [49], in a household hybrid CW, obtained more than 86% efficiency of TN removal. The average values of TN concentration in the outflow from the analyzed facilities no. 1 and 2 were high and amounted to 127 and 77 mg/dm\({}^{3}\), respectively (Tables 2 and 3). These values are several times higher than the permissible limit (30 mg/dm\({}^{3}\)) required in Poland for wastewater discharged into lakes and their tributaries [47]. Even higher concentrations of TN (124-320 mg/dm\({}^{3}\)) were previously found by [PERSON] and [PERSON] [31] in the outflow from three other facilities with activated sludge but without classical preliminary settling tanks. The efficiency of TN removal in activated sludge systems depends primarily on the course of processes such as nitrification or denitrification, among others [54; 55]. From the data presented in Tables 2 and 3, it can be seen that the nitrification process in the studied facilities proceeded properly, which is evidenced by a significant decrease in the concentration of ammonium nitrogen and an increase in the content of nitrate and nitrite nitrogen, as well as the concentration of oxygen in the treated wastewater. However, high concentrations of aerobic forms of nitrogen and TN in the outflow from the analyzed systems indicate that the household WWTPs with activated sludge are not able to create appropriate conditions for the denitrification process. **Total Phosphorus. ** In the studied facilities no. 1 and 2, the efficiency of TP removal was low and amounted to 4% and 30%, respectively (Figure 2).