## 3 Results and Discussion
Comparing the results obtained with the two inocula, a strong difference in terms of biodegradation index between soil humus and _Bacillus Clausii_ was observed. In the tests performed with _Bacillus Clausii_, lower biodegradation indexes were calculated with respect to those of soil humus. In the latter case, after only five days of test, BOD\({}_{5}\) exceeded the correspondent initial COD\({}_{0}\) value, thus suggesting the presence of other species not chemically oxidable but biodegradable, such as some Nitrogen Organic Compounds (NOCs). This effect could be explained by the possible development of the nitrification process during the BOD closed bottle tests, due to the possible presence of nitrifying microorganism in a mixed consortium such as soil humus [32]. This last consideration supports the conclusion that using a single species inoculum helps ensure reproducible data. Meanwhile, as showed in Figure 2c, no substantial differences in TOC removals were calculated and this highlights that the adopted bacterial strain inoculated in the BOD test had the same ability to remove the organic compounds of foaming agents. The simplicity of the study and the possibility of obtaining results in a short time make this approach applicable during excavation activities, as a support to the evaluation of biodegradation processes of conditioned soils. Until now, the use of a specific inoculum in closed bottle tests at five days did not allow obtaining exhaustive information about the biodegradation of the products or to make comparisons with other products. Clearly, according to OECD guidelines and regulations, the use of a specific inoculum such as _Bacillus Clausii_ is not required. However, from a scientific and practical point of view, the development of a rapid and easy test that yields reproducible and reliable results might allow selection of the most  
Figure 2: 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 0.10% as concentration of the conditioning mixtures A (black), B (gray) and C (white) in demineralized water solutions. proper (most biodegradable) foaming agent, once the geotechnical requirements have been established, without the necessity of performing BOD\({}_{28}\) tests. For this reason, biodegradation tests were developed maintaining the system under a continuous air supply and in the presence of _Bacillus Clausii_ as a bacterial inoculum. These tests were termed accelerated because they required less time to develop than the 5, 10 and 28 days required by the closed bottle technique. The ABT tests were performed by considering an additional dosage (0.13%) helpful for the kinetic study. In Figure 3, the results of ABT of the three foaming agents (A, B and C) at three dosages (0.20, 0.13 and 0.10%) at 30 \({}^{\circ}\)C and neutral pH condition (pH = 6) are reported. In Figure 3, it is possible to observe that the concentration had a significant effect in the first hours for mixture B (Figure 3b). In fact, the removal rate increased with increasing organic carbon content, with a removal of about 60% after 47 h of treatment when B was used at the higher dosage (0.20%). With the decrease in the initial TOC (0.13 and 0.10%), the process showed slowing down as a consequence of the low availability of organic compounds to be consumed during the biological process. A similar trend was observed when mixture A (Figure 3a) was used at different concentrations, confirming that in the adopted range, the use of such mixtures did not compromise the development of a bioremediation process and did not affect the removal. Tests conducted on mixtures A and B have not indicated an inhibition for a high substrate concentration; conversely, such interference was clearly evident in the case of the third foaming agent (C), where the concentration of the additives was decidedly higher than A and B (Table 1).