# Challenges of Hydrodynamic Cavitation of Organic Wastes
## 3 Results and Discussion
### Influence of HC on the Properties of the Applied Mixtures
Regarding the LB pre-treatment, a single concentric hole seems to be a more advantageous design. Such a design promotes the oxidation of volatile hydrophobic substances in the gas phase, but limits the diffusion of hydroxyl radicals outside of the bubbles [44]. A different trend was observed with regard to SCOD; in this case, increases in all cavitated mixtures were achieved (Figure 3b). The greatest enhancements in this concentration were recorded for both mixtures with MW. For sole MW, it was 11%, while for the mixture of LB and MW, it reached over 20%. This was related to the solubilisation effect, a release of organic substances into the liquid phase within HC [8]. HC might also contribute to changes in organic compound structures and conversion of recalcitrant compounds in pre-treated materials to more biodegradable forms, which was confirmed by previous studies [35; 37]. It should be noted that in almost all cases, an increase in BOD\({}_{5}\) concentrations was found after HC. The exception was the series with MW; in this case, the parameter was reduced by approximately 40%. Importantly, for sole MLL and for a mixture of LB and MLL, enhancements of 23% and 24% were observed, respectively. The results obtained for BOD\({}_{5}\) corresponded with the BI. As previously, in almost all experimental series, an improvement of the BI in cavitated mixtures occurred. Only for MW was the BI deterioration observed. In turn, for the cavitated mixture of LB and MW, BI improved by 24%. It should be noted that the HC of LB suspended in MLL resulted in an improvement by 56% in the BI, compared with the mixture before HC. For sole MLL, a minor enhancement of 36% was found. The observed improvements in BI were related to high temperature and pressure, and to a lesser extent, with the activity of highly oxidizing radicals generated within HC [45]. It should be noted that increased temperature is especially recommended for LB. The studies conducted by [PERSON] and [PERSON] [46] and [PERSON] et al. [47] indicate enhanced removal rates of hemicellulose and lignin. However, the optimal temperature largely depends on the boiling point as well as vapor pressure of the applied medium [48]. Importantly, the results obtained for BI are consistent with previous studies. The improvement in BI within HC of various types of side streams, such as distillery products [49], pigments [50] and cork [51] as well as municipal wastewaters [7; 52] has been reported. Moreover, such beneficial outcomes have been also found within HC of landfill leachate [9]. The achieved results for BI are particularly important because the effective treatment of the analysed substrates still constitutes a serious technological and environmental issue. The management of lignocellulosic biomass is an especially urgent topic [18]. The results obtained also confirm that HC could be applied as an effective pre-treatment of these substrates. Moreover, these results indicate the importance of optimisation studies in HC to determine the most suitable inlet pressure and cavitation time [14]. Especially the first parameter is considered as a crucial factor in the cavitation intensity. For orifice plates, it should be in the range from 2 to 8 bar; however, the optimal value depends on shape, size and number of holes [39]. Generally, with the enhancement of this parameter, the amount of liquid passing in a given treatment time increases, which extends the residence time of the pollutants in the cavitation zone and results in an improved degradation rate of specific pollutants. However, exceeding the pressure to a certain level may lead to supercavitation, which can yield poor HC results. Moreover, it should be noted that high pressure is not beneficial for every pollutant [35,37]. This was confirmed in the present study. As previously mentioned, a decreasing trend in terms of BI was obtained only for MW.