# A Preliminary Study on the Use of Xylit as Filter Material for Domestic Wastewater Treatment
## 3 Results and Discussion
### Efficiency of Wastewater Quality Indicators Removal  
#### 3.2.1 Chemical Oxygen Demand  
The average value of inflow COD at preliminary stage was 717.8 \(\pm\) 113.0 mg/L (\(n\) = 9). In experiment I: 284.1 \(\pm\) 33.4 mg/L (\(n\) = 7), 290.0 \(\pm\) 30.8 mg/L (\(n\) = 7), and 287.7 \(\pm\) 31.2 mg/L (\(n\) = 7) for AH1, AH2, and SCR, respectively. In experiment II, average inflow values for COD were as follows: 537.2 \(\pm\) 47.9 mg/L (\(n\) = 6) and 548.0 \(\pm\) 46.3 mg/L (\(n\) = 6) for AH1 and AH2, respectively. The inflow COD values are presented in Tables 1-3 and Figures 3-5.  
Figure 4: Inlet and outlet COD and removal efficiencies at stage I.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample No.**} & \multicolumn{3}{c}{**AH1**} & \multicolumn{3}{c}{**AH2**} \\ \cline{2-7}  & **Inlet** & **Outlet** & **Efficiency** & **Inlet** & **Outlet** & **Efficiency** \\ \cline{2-7}  & **mg/L** & **mg/L** & **\%** & **mg/L** & **mg/L** & **\%** \\ \hline
1 & 685 & 45 & 93.4 & 695 & 46 & 93.4 \\
2 & 685 & 54 & 92.1 & 690 & 62 & 91.0 \\
3 & 484 & 108 & 77.7 & 485 & 64 & 86.8 \\
4 & 498 & 106 & 78.7 & 483 & 70 & 85.5 \\
5 & 445 & 47 & 89.4 & 440 & 67 & 84.8 \\
6 & 426 & 57 & 86.6 & 495 & 83 & 83.2 \\ \hline Avg. & \(537.2\pm 47.9\) & \(69.5\pm 12.0\) & \(86.3\pm 2.7\) & \(548.0\pm 46.3\) & \(65.3\pm 4.9\) & \(87.5\pm 1.6\) \\ \hline \end{tabular}
\end{table}
Table 3: Inlet and outlet COD and removal efficiencies at stage II.  
Figure 3: Inlet and outlet COD and removal efficiencies at preliminary stage.  
The average efficiencies of COD removal at preliminary stage were as follows: \(71.5\pm 5.0\%\) (\(n=9\)) for angel hair (AH) and \(70.8\pm 4.7\%\) (\(n=9\)) for crushed material (SCR). The details related to COD removal efficiencies at preliminary stage are presented in Table 1 and Figure 3.  
In experiment I, the average efficiencies of COD removal were as follows: \(85.0\pm 2.5\%\) (\(n=6\)), \(82.5\pm 2.1\%\) (\(n=7\)), and \(83.6\pm 1.0\%\) (\(n=7\)), for AH1, AH2, and SCR, respectively. COD removal efficiencies at stage I are presented in Table 2 and Figure 4.  
The average efficiencies of COD removal at stage II were as follows: \(86.3\pm 2.7\%\) (\(n=6\)) for AH1 and \(87.5\pm 1.6\%\) (\(n=6\)) for AH2. The details related to COD removal efficiencies at stage II are presented in Table 3 and Figure 5.  
The removal efficiencies of organic compounds expressed as COD were relatively high during all stages of the study (above \(70\%\) at the preliminary stage and above \(80\%\) at stages I and II). Similar results (especially compared to stages I and II), although for a different but indirectly related indicator--total organic carbon (TOC), obtained by [PERSON] et al. [7]--was about \(89\%\). In contrast, [PERSON] et al. [11] obtained for this material much lower TOC removal efficiency, only about \(70\%\).  
Higher efficiencies at stages I and II (comparing to the preliminary stage) resulted from a lower hydraulic load and a more even distribution of wastewater on the filter surface (periodic cleaning of sprinklers).