## 3 Results and Discussion  
In a previous investigation, BOD curves up to 28 days for the mixtures A, B and C (0.20%) were reported and a significative enhancement on biodegradation was observed by adopting soil bacterial inoculum [23]. The high activity of the microorganisms was highlighted already after 10 days of test, when, from the comparison of the TOC, COD and BOD data, a comparison between the different biodegradability of the foaming agents was possible to be made [23]. This type of investigation is required in the case of an a priori evaluation on the choice of the foaming agent to be used during the excavation activities' joint geotechnical assessments. In case of an on-line monitoring, a short methodology may be useful especially when, for example, adverse conditions on TBM operations, and a dosage change of foam is required. In this case, time is a crucial parameter for establishing the effects of a change in dosages. For this reason, five days of closed bottle tests as short tests were performed at different polymer concentration (0.20 and 0.10%) and two different inocula (_Bacillus Clausii_ and soil). In Figure 1, the results of the biodegradation tests of the mixtures A, B and C dosed at 0.20% are presented. The results obtained after five days showed that the presence of a mixed consortium (soil bacterial inoculum) resulted in an enhancement of substrate biodegradation compared to _Bacillus Clausii_ (Figure 1a), thus suggesting that the investigated conditioning mixtures exhibited a high response to biodegradation especially for B (BOD/COD = 0.92) and C (BOD/COD = 0.93). For sample A, the low value of BOD/COD index (Figure 1b) at the same dosage is due to the high TOC initial value of this foaming agent, compared to B and C. To support this observation, the results of organic mineralization in term of TOC are reported in Figure 1c. Only a slight TOC removal was observed both when soil humus supernatant and _Bacillus Clausii_ were used as inoculum in BOD tests. This result is not in accordance with the measured BOD: when _Bacillus Clausii_ was used, the reached BOD value after five days (15.9 mg O\({}_{2}\)/L) was lower than 86% compared to the BOD reached when the soil humus supernatant was employed. This result can be ascribed to the higher respiration activity of a mixed consortium such as soil humus supernatant with respect to a single bacterial strain [31]. This last aspect is extremely important for a reproducible and representative test of the process: the use of a specific inoculum guarantees the collection of more reliable and reproducible data. Figure 2 shows the results obtained at a dosage of 0.10%, where the inhibition due to the high concentration of conditioning agent did not occur. Figure 1: BOD (**a**), biodegradation index (**b**) and TOC removal (**c**) after five days in closed bottle tests. Conditions: 2.0 mL of _Bacillus Clausii_ or inoculum from soil humus (soil) and 0.20% as concentration of the conditioning mixtures A (black), B (gray) and C (white) in demineralized water solutions. Dilution of the conditioning mixtures resulted in an enhancement of biodegradation. As already observed, the BOD values (Figure 2a) strongly depended on the used inoculum. The BOD values measured after five days were 383.5, 210.1 and 211.3 mgO\({}_{2}\)/L, respectively, for the conditioning mixtures A, B and C, when soil humus was used as the inoculum, thus suggesting a higher biodegradation rate for the 0.10% mixtures with respect to the 0.20% mixtures. Conversely, when the _Bacillus Clausii_ was used as inoculum, a substantial increase in BOD values was not observed (23.54, 28.07 and 20.36 mgO\({}_{2}\)/L for A, B and C, respectively). Such remarkable difference in BOD values between the two used inocula was due to the different respiration activity, as previously discussed. Additionally, at 0.20% dosage, a notable enhancement of the biodegradation process was observed when mixed bacterial culture was used, as highlighted by the BOD/COD indexes reported in Figure 2b.