## 4 Conclusions  
The biodegradation of conditioning mixtures injected into the soil during TBM-EPB tunneling was studied by performing laboratory tests on three conditioning mixtures containing SLES as surfactant agent (10-50%, 10-20% and 5-10% in mixtures A, B and C, respectively) and a limited dosage of additives. Results showed that the biodegradation of the selected mixtures strongly depended on the inoculum used as a bacterial source in BOD tests. According to the closed bottle tests (as laid down in the OECD guidelines), only in the presence of soil humus, it was possible to establish a comparison of the three formulations within five days.  
In order to propose a rapid and reproducible methodology for the comparison of biodegradation trends exhibited by different formulation of SLES and additives, a procedure involving ABT was proposed using the _Bacillus Clausii_ as the inoculum.  
The results obtained in the ABT tests were in line with those obtained using the standard procedure. In cases of mixtures rich in additives, an inhibition effect due to the high organic concentration was detected (mixture C), while in cases of high SLES concentration (mixtures A and B), this inhibition was limited because of easily biodegradable compounds. In such conditions, the estimated times required to achieve the TOC removal of about 50% were 19.25, 22.36 and 17.33 h for mixture A at dosages of  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \begin{tabular}{c} **Conditioning** \\ **Mixtures** \\ \end{tabular} & \begin{tabular}{c} **Temperature** \\ **[\({}^{\circ}\)C]** \\ \end{tabular} & \begin{tabular}{c} **K** \\ **[1/h]** \\ \end{tabular} & \begin{tabular}{c} **t\({}^{\circ}\)20\%** \\ **[h]** \\ \end{tabular} & \begin{tabular}{c} **t\({}^{\circ}\)50\%** \\ **[h]** \\ \end{tabular} & \begin{tabular}{c} **pH** \\ **[1/h]** \\ \end{tabular} & \begin{tabular}{c} **K** \\ **[1/h]** \\ \end{tabular} & \
\begin{tabular}{c} **t\({}^{\circ}\)20\%** \\ **[1/h]** \\ \end{tabular} &
\begin{tabular}{c} **t\({}^{\circ}\)50\%** \\ **[1/h]** \\ \end{tabular} \\ \hline \multirow{2}{*}{**A**} & 20 & 0.037 & 6.03 & 18.73 & 4 & 0.019 & 11.74 & 36.48 \\  & 40 & 0.035 & 6.37 & 19.80 & 8 & 0.037 & 6.03 & 18.73 \\ \hline \multirow{2}{*}{**B**} & 20 & 0.017 & 13.12 & 40.77 & 4 & 0.011 & 20.28 & 63.01 \\  & 40 & 0.015 & 14.87 & 46.21 & 8 & 0.013 & 17.16 & 53.32 \\ \hline \multirow{2}{*}{**C**} & 20 & 0.016 & 13.94 & 43.32 & 4 & 0.012 & 18.59 & 57.76 \\  & 40 & 0.009 & 24.79 & 77.01 & 8 & 0.015 & 14.87 & 46.21 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Blackman overall first order kinetic constant (K) and estimated time (t*) required to achieve the 20 and 50% of TOC\({}_{0}\) (t*\({}_{20\%}\) and t*\({}_{50\%}\)) at different temperature values (mixture concentration = 0.10% and pH = 6) and at different pH values (mixture concentration = 0.10% and T = 30 \({}^{\circ}\)C).  
0.10, 0.13 and 0.2%, respectively. For B, the estimated times required were 49.51, 31.50 and 21.00 h and for C were 43.32, 46.21 and 63.01 h at a concentration of 0.10, 0.13 and 0.20%, respectively.  
In all conditions adopted, the Blackman first order kinetic model well described the experimental results.  
Conceptualization, I.B. and G.V.; validation, G.V., D.S. and A.D.G.; investigation, M.D.F. and A.D.B.; data curation, I.B. and M.D.F.; writing--original draft preparation, I.B., G.V., D.S.; writing--review and editing, L.D.P.; supervision, L.D.P. and S.M. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.