## 3 Results
### Analysis of Variance (ANOVA) and Response Surface Methodology (RSM)
#### 3.4.1 ANOVA and RSM for Percentage of Diesel Oil Removal  
The results of the two-way ANOVA done on percentage of diesel oil removal in 35 days are shown in Table 9.  
Both the u% and C/N variables were statistically different in the studied confidence level. Considering the F and \(p\) parameters, the C/N effect on the objective variable (pollutant removal) was greater than that of water content.  
The third-degree polynomial regression was:  
\[\begin{array}{c}\%\mbox{ removed diesel oil}=\\ 50.28-21.36\mbox{x}_{1}+1.011\mbox{x}_{2}+1.029\mbox{x}_{1}^{2}+0.1244\mbox{x} _{1}\mbox{x}_{2}-0.01006\mbox{x}_{2}^{2}+\\ -0.007591\mbox{x}_{1}^{2}\mbox{x}_{2}+0.0001458\mbox{x}_{1}\mbox{x}_{2}^{2}+0. 00001472\mbox{\cdot}\mbox{x}_{2}^{3}\end{array} \tag{11}\]  
where x\({}_{1}\) is the water content (u%) and x\({}_{2}\) is the carbon to nitrogen ratio (C/N).  
Considering the linear terms, the water content negatively influenced the diesel degradation (\(\beta_{1}=-21.36\)), while the nutrient concentration had a positive effect (\(\beta_{2}=1.011\)). The second- and third-order terms were very small and could be considered negligible.  
The combined effect of the two variables was positive (\(\beta_{5}=0.1244\)), but the value was small; therefore, this term can be considered negligible, too.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **u\% (By Weight)** & \multicolumn{2}{c}{
\begin{tabular}{c} **Second-Order Model** \\ **k (kg of soil-g\({}^{-1}\)-d\({}^{-1}\))** \\ \end{tabular} } & **R\({}^{2}\)** & **t\({}_{1/2}\) (d)** \\ \hline \multirow{3}{*}{Theoretical C\({}_{0}\)} & 8 & 0.0002 & 0.90 & 71 \\  & 12 & 0.0002 & 0.81 & 71 \\  & 15 & 0.0002 & 0.90 & 71 \\ \hline \multirow{3}{*}{Experimental C\({}_{0}\)} & 8 & 0.0002 & 0.90 & 76 \\  & 12 & 0.0002 & 0.85 & 81 \\  & 15 & 0.0002 & 0.90 & 83 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Second-order reaction rate constant and half-life time for microcosms with C/N = 180.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **SS** & **Df** & **MS** & **F** & \(p\)** \\ \hline u\% & 0.005 & 2 & 0.004 & 0.42 & 0.67 \\ \hline C/N & 0.039 & 3 & 0.013 & 1.27 & 0.37 \\ \hline Error & 0.06 & 6 & 0.01 & & \\ \hline Total & 0.108 & 11 & & & \\ \hline \end{tabular} SS = sequential sum of squares; Df = degrees of freedom; MS = mean squares; F = Fischer’s variance ratio; \(p\) = probability value.  
\end{table}
Table 9: Analysis of variance (ANOVA) for percentage of diesel oil removal.  
The same results were obtained by [PERSON] et al. [17]. They did an experimental design to evaluate the effects of diesel concentration, moisture content, and biomass dose on amount of removed diesel oil and CO2 produced during the process. The study was carried out with 25 g of soil polluted with diesel oil concentration in the range of 5000 to 15,000 mg\(\cdot\)kg\({}^{-1}\) of soil.  
The response surface is shown in Figure 12.  
It is evident that a maximum occurred in the area C/N = 100-180 and u% = 8%-10% b.w. By calculation, the theoretical optimum was identified: after 35 days, the maximum percentage of removed diesel oil was 24.9%, with u% = 8% b.w. and C/N ratio = 123.  
Deepening the analysis only for microcosms with C/N = 120 and C/N = 180, it was possible to monitor the yellow area's variation throughout the test (Figures 13-16). It was noted that the greatest percentage of diesel was removed in the space C/N = 120-130 and u% = 8%-10% b.w. After 138 days, the amount of pollutant was reduced by 70% in all microcosms with C/N = 120, while in the microcosms with C/N = 180, the diesel oil removal was still heavily influenced by the water content.  
Figure 14: Response surface for diesel oil removal by microcosms with C/N = 120 and C/N = 180 at t = 71 days.  
Figure 12: Response surface for diesel oil removal efficiency at t = 35 days.  
Figure 13: Response surface for diesel oil removal by microcosms with C/N = 120 and C/N = 180 at t = 35 days.