# Challenges of Hydrodynamic Cavitation of Organic Wastes
## 3 Results and Discussion
### Influence of HC on the Properties of the Applied Mixtures
Importantly, the prior research conducted by [PERSON] [52] indicates that the optimal effects for this medium were obtained with a lower inlet pressure of 5 bar using the same orifice plate as HC inductor as in the present study. Regarding the lignocellulosic biomass, for reed, the best results were found with an inlet pressure of 5 bar and 41.1 min [53]. In turn, for sugarcane bagasse, the HC was conducted under the following conditions: inlet pressure of 3 bar and total time of 10 min [54]. However, in both cases, the HC processes were combined with alkali pre-treatment. However, due to the complexity of HC, additional indicators should be considered before the implementation of this method on a technical scale. Often, a key concern in its successful application is the possible generation of toxic by-products leading to secondary pollution of the treated medium, thus excluding its subsequent application. This problem was previously reported in literature during the application of the lignocellulosic biomass using this technology [16,17,55,56]. Therefore, it is crucial to select an appropriate carrier for suspension of LB, especially when waste streams are used as carriers. Frequently, the HC of sole liquids and their mixtures with LB present divergent results, mainly due to additional interactions occurring during the process [45]. Therefore, more thorough evaluation of the composition of the treated material is required. Despite the beneficial effect observed within the HC of LB and MLL, previous research indicates that HC favours the formation of aromatic compounds that might inhibit the AD process, thus preventing MLL from being used as a carrier for lignocellulosic biomass [57]. On the other hand, the application of different operational parameters could lead to different results. Therefore, optimisation studies play a crucial role in successful HC; such evaluation should include temperature, pressure, duration as well as the type of the HC inductor. The pH value is a crucial factor within the HC process; it has a significant influence on the degradation of specific pollutants. The acidic pH values promote the formation of hydroxyl radicals, thus ensuring a higher oxidation potential and a lower rate of their recombination [37,58]. As shown in Table 3, a decrease in the pH value occurred only for the mixture of LB and MW, which could be related to a transformation of some organic compounds into low-molecular-weight acids [59]. In the remaining cases, HC led to an increase in the pH value in cavitated mixtures. The observed growth in the pH values within the HC of MLL and LB + MLL resulted from the decomposition of acid groups [45]. Compared with the raw mixtures, a growth in the VFA content by 24% was observed, indicating a deposition of lipids within HC was found only for the cavitated mixture of LB and MW. In the remaining cases, comparable values were observed before and after HC. Regarding the N-NH4\({}^{+}\) content resulting from HC, only for sole MLL and the mixture LB + MLL led to an increase in its concentration compared with the raw mixtures. This was related to the destruction of proteins. As was previously mentioned, the ammonia nitrogen is recognised as a major AD inhibitor that could lead to process failure [29]. Thisobservation also excluded the application of MLL as a carrier for LB. Considering the ALK within HC, sole MW and MLL resulted in minor reductions. In the series with LB, an enhancement in the ALK content occurred. Importantly, this beneficial effect was found for LB and MW, thus improving the properties of the mixture. The results obtained in terms of AD parameters are consistent with the previous studies, which indicated that HC favours a lower production of inhibitors [60]. Another crucial factor that should be considered before implementation of this technology on a technical scale are the operational costs related to energy consumption and reagent dosing. These costs are particularly important during the HC of lignocellulosic biomass.