# Bioremediation of Hydrocarbon-Polluted Soil: Evaluation of Different Operative Parameters
## 3 Results  
### Statistical Analysis  
#### 3.1.1 Test M  
The decomposition of the variability of the TPH concentration into contributions due to Treatments, Time, and their interaction is presented in Table 2.  
Since the \(p\)-values are lower than 0.05 (statistical significance), the main effects have a statistically significant influence on TPH concentration with a 95.0% of confidence level. Furthermore, the interaction between Treatments and Time is statistically significant.  
Therefore, multiple comparisons were developed to determine which levels of each factor are significantly different from the others. Table 3 presents the result of the multiple ranks for the factor Treatments, where all TPH values of both replicas at all times of the test M were averaged.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Source** & **Sum of** **Squares** & **Df** & **Mean** **Square** & **F-Ratio** & \(p\)**-Value** \\ \hline
**Main Effects** & & & & & \\ Treatments & 537.43 & 5 & 107.47 & 72.21 & 0.000 \\ Time & 16,735.40 & 4 & 4183.84 & 2810.60 & 0.000 \\
**Interactions** & & & & & \\ Treatments-Time & 845.22 & 20 & 42.27 & 28.39 & 0.000 \\ Residual & 44.66 & 30 & 1.49 & & \\ Total (corrected) & 18162.70 & 59 & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: ANOVA for test M.  
The last column shows four homogenous groups, identified by columns of Xs. Within each column, the levels of the factor Treatment have no statistically significant differences. Looking at the data, results show that different combinations of water content and C/N ratio give a significant influence on diesel oil removal.  
Figure 3 presents a plot of TPH concentration by levels of the factor Time.  
Despite the levels of the factor Time present statistical differences regarding the means of TPH concentration, there is an evident change in the trend of the mean ranks after the third level of time (70 days). After this time, although the means of hydrocarbons concentration slightly continue to decrease, the values of TPH concentration have tiny changes compared with the previous level.  
Since different combinations of water content and carbon to nitrogen ratio influence the removal of the hydrocarbons, it is of interest to know which combination of treatment and time would ensure better results for any process period. Figure 4 shows the interactions of both factors.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Treatments**} & \multirow{2}{*}{**Mean (g\(\cdot\)kg\({}^{-1}\))**} & **Lower Limit** & **Upper Limit** & **Homogeneous** \\  & & **(g\(\cdot\)kg\({}^{-1}\))** & **(g\(\cdot\)kg\({}^{-1}\))** & **Groups** \\ \hline u8\%-C/N = 120 & 36.65 & 35.86 & 37.44 & X \\ u12\%-C/N = 180 & 37.25 & 36.46 & 38.04 & X X \\ u15\%-C/N = 180 & 38.15 & 37.36 & 38.94 & X X \\ u12\%-C/N = 120 & 38.48 & 37.69 & 39.27 & X \\ u8\%-C/N = 180 & 43.28 & 42.49 & 44.07 & X \\ u15\%-C/N = 120 & 44.36 & 43.57 & 45.15 & X \\ \hline \hline \end{tabular}
\end{table}
Table 3: Multiple rank tests for the levels of the factor Treatment in test M.  
Figure 4: Interaction plot for the factors in test M.  
Figure 3: Scatterplot of TPH concentration by levels of the factor Time.  
The lines which represent the levels of Treatments are not parallel, but they cut each other. This corroborates the interaction reported in the ANOVA table and indicates that during the process, some treatments alternated their order of degradation regarding the others. The model assumptions are verified in Appendix A.