# Challenges of Hydrodynamic Cavitation of Organic Wastes
## 3 Results and Discussion
### Influence of HC on the Properties of the Applied Mixtures  
The results in terms of organic compounds are presented in Figure 3. Compared with the raw mixtures, HC resulted in minor declines in the COD content in all mixtures (Figure 3a). Major changes were observed for both mixtures with LB. Reductions of 15.7% and 11.8% were observed for LB suspended in MLL and MW, respectively. A similar observation was made for the TS content (Figure 3c). The exception was the series with MW; in this case, comparable results were achieved before and after HC. The observed changes resulted from the destruction of organic compounds that occurred within HC [9]. However, considering the VS content, there was no clear tendency in terms of the HC influence on the compositions of mixtures. In the case of MLL, as well as LB and MLL, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Unit** & & **MW** & **MLL** & **LB + MW** & **LB + MLL** \\ \hline ALK & mg/L & Before HC & 580 \(\pm\) 35 & 13,444 \(\pm\) 327 & 551 \(\pm\) 20 & 12,767 \(\pm\) 346 \\  & & After HC & 574 \(\pm\) 14 & 12,694 \(\pm\) 308 & 631 \(\pm\) 39 & 14,369 \(\pm\) 290 \\ pH & - & Before HC & 7.30 \(\pm\) 0.03 & 8.12 \(\pm\) 0.05 & 7.34 \(\pm\) 0.07 & 8.22 \(\pm\) 0.08 \\  & & After HC & 8.01 \(\pm\) 0.02 & 8.65 \(\pm\) 0.05 & 7.21 \(\pm\) 0.07 & 8.51 \(\pm\) 0.08 \\ VFA & mg/L & Before HC & 140 \(\pm\) 14 & 1213 \(\pm\) 124 & 595 \(\pm\) 38 & 1865 \(\pm\) 211 \\  & & After HC & 136 \(\pm\) 6.8 & 1198 \(\pm\) 132 & 740 \(\pm\) 45 & 1909 \(\pm\) 261 \\ N-NH\({}_{4}\)\({}^{+}\) & mg/L & Before HC & 68 \(\pm\) 0.5 & 1710 \(\pm\) 260 & 65 \(\pm\) 1.5 & 1729 \(\pm\) 285 \\  & & After HC & 63.5 \(\pm\) 0.5 & 1902 \(\pm\) 300 & 64.3 \(\pm\) 0.7 & 2810 \(\pm\) 310 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mixture composition in terms of AD application before and after cavitation (average values and standard deviations are given). comparable results were found before and after HC. The major change, as previously, was found for LB suspended in MW: a drop of 24% in the cavitated mixture was observed, indicating decomposition of LB. Previous studies indicate the great potential of this method for degradation of organic molecules [16; 35; 36]. It has been applied as a pre-treatment method for wheat straw, corncob, corn stover, grass silage, sugar cane bagasse, reed, and miscanthus grass [8; 35]. The changes within HC of various wastes are related to the physical and chemical effects that occur, as well as their combination. The physical effects include shockwaves, microcurrents, shear stress and increased temperature. These effects tend to break down the external structure, decrease the particle size, enhance the surface area, and finally lead to improvement in the accessibility and/or solubility of LB [8; 35; 36]. In turn, the chemical effects are related to the generation of hydroxyl radicals, which are highly reactive and are able to oxidise organic matter. In particular, the hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) produced from hydroxyl radicals has a proven effectiveness in reducing the organic and inorganic matter presented in substrates [9]. The crucial factors the process are the type of cavitation inductor, the characteristics of the treated material and the adopted operational parameters [35]. In the present study an orifice plate with one concentric hole was used as an HC inductor. It is known that orifice plate is one of the most effective HC devices allowing for the generation of the intense cavitation conditions, mainly due to the extended intensity of the formation of hydroxyl radicals [37]. This device is preferred in order to achieve intense chemical reactions [38]. Previous studies indicate that multiple-hole orifice plates result in enhanced cavitation effects compared with single-hole orifice plates [37; 39; 40; 41]. Generally, smaller size holes and a greater number of holes result in higher removal rate of various pollutants [42; 43]. However, such a HC design is not suitable for all applications.