## 3 Results and Discussion
To confirm this evidence, analogous tests without the addition of the inoculum were performed with a foam dosage of 0.20%. In such condition, a TOC decrease was observed, as a consequence of the stripping of volatile substances from the mixture. In particular, after only 1 h of aeration, a TOC removal of 10% was observed for the C mixture, while a negligible TOC removal (~2%) was measured for the other mixtures, suggesting the limited concentration of volatile compounds on both formulations. After 1 h, a stabilization of the foam was observed and no variation in TOC trends was measured. For this reason, a different trend in TOC removal was observed (Figure 3c) before the first 20 h treatment and, after this time, a plateau was reached for all the concentrations investigated. The presence of volatile compounds justifies the addition in C formulation of stabilizers with high viscosity and low volatility (hexylene glycol) necessary to maintain such volatile compounds in the liquid phase. Due to the presence of these compounds, biological treatment is less effective for C, especially when the solution is more concentrated (0.20%) [36]. When mixture B was used at the lower dosages (0.10 and 0.13%) and for all the solutions prepared with mixture C, a TOC removal of 50% was not achieved during the 50 h tests, as a consequence of the limitations previously discussed about the presence of the additives (Figure 3). To evaluate any possible interference of the biodegradation process due to the presence of additives, further investigations, similar to those previously discussed, were performed on solutions with different concentrations of the mixtures A and B at the same initial TOC value, corresponding to the TOC measured for mixture A at 0.20% dosage (Figure 4). From Figure 4, it is possible to observe the effect of the additives present in B, as reported in Table 1. Mixture A showed a greater predisposition to biodegradation compared to B: in particular, after 72 h of treatment, the TOC removal efficiency values were 80 and 64% for A and B, respectively (first cycle). No further removal over time was observed after that point. To verify if this arrest on biodegradation could be correlated to the low availability of an active inoculum, after 72 h of treatment, 2.0 mL of fresh _Bacillus Clausii_ were added to both solutions and the TOC was monitored. In this second cycle, an additional TOC removal was observed for mixture A (up to 90%), while the persistence  
Figure 4: TOC removal % during accelerated biodegradation tests of conditioning mixture A (circle) and B (triangle) with 2.0 mL of _Bacillus Clausii_. First cycle of treatment (full symbols) and second cycle (empty symbols). According to the results displayed in Figure 5 and the values of the kinetic parameters reported in Table 4, it is possible to observe that, in the investigated range, temperature did not significantly influence the biodegradation process of mixtures A and B, whilst a remarkable influence was observed for mixture C.  
The temperature was not effective for the biodegradation of mixture A, and the TOC removal of 50% was reached in similar times. When mixture C was tested, a remarkable difference in the estimated time required for a 20% removal of TOC was observed, and, to achieve a removal of the 50%, a substantial difference in the estimated time values was determined for both the conditioning mixtures B and C. This result suggests that the high temperature (40 \({}^{\circ}\)C) combined with the high additives dosages (Table 1) could cause a loss of microorganism viability [34]. Similar considerations were also conducted by evaluating the effect of pH. Variations of this parameter were more significant than those observed for temperature. By the analysis of the TOC removal trends reported in Figure 6, a reduction in biodegradation rate under acidic pH condition (pH = 4) was observed in the treatment of mixture A (Figure 6a), while a slight significative effect was detected in the treatment of mixtures B and C (Figure 6b,c).