# Challenges of Hydrodynamic Cavitation of Organic Wastes
## 3 Results and Discussion  
### Characteristic of Mixtures Applied in HC  
In the present study, four mixtures with differing properties were used in HC. The results are presented in Tables 1 and 3, as well as in Figure 3. MW and MLL were used as sole waste streams. The MW sample presented a typical composition for municipal wastewater after mechanical treatment. Compared with the other mixtures in the study, it was characterised by a high BI index as well as low contents of COD, SCOD, TS, VS and VFA. In turn, MLL had a composition as expected for a mature leachate characterised by high alkalinity and a low BI (<0.1) [25]. Moreover, this side stream showed high VFA, COD and N-NH4\({}^{+}\) contents, typically observed for earlier stage of decomposition of landfill [26]. The high concentrations of VFA and N-NH4\({}^{+}\) might be especially problematic for AD of this substrate [27; 28]. An additional factor that might limit the further use of this substrate in biological processes is a significant content of recalcitrant organic matter, which is also confirmed by the low BI (Table 1).  
Figure 3: Mixture composition before and after cavitation: (**a**) COD; (**b**) SCOD; (**c**) TS; (**d**) VS; (**e**) BOD5; and (**f**) BI (average values and standard deviations are given).  
As shown in Figure 3, the addition of LB to MW and MLL led to increases in COD, SCOD, TS, and VS in the raw mixtures. This fact is related to the LB characteristic of being a substrate rich in organic compounds (Table 1). A different trend occurred only in the case of LB diluted in MLL with regard to BOD\({}_{5}\). In this case, a decrease of 12% was observed compared with sole MLL. Importantly, the implementation of LB to both carriers, MW and MLL, led to the deterioration of BI. The significant declines of 81 and 60% were observed for the mixtures of LB and MW as well as LB and MLL, respectively. This was related to the application of the substrate characterised by a high content of refractory compounds (Figure 1). This fact is also highly unfavourable from the point of view their subsequent application in biological processes.  
It should be noted that especially anaerobic processes are highly sensitive to various environmental factors, including the pH values, ALK, VFA as well as N-NH\({}_{4}\)\({}^{+}\) contents, which might inhibit or accelerate the activity of methanogens [29,30]. Therefore, these parameters were also analysed in the present study (Table 3).  
Importantly, compared with sole MW and MLL, an enhanced VFA content was observed in the presence of LB. Its concentration increased by over 325% and 54% in LB + MW and LM + MLL, respectively. Previous studies indicate that VFA inhibition is the most frequent factor contributing to process instability and even leading to AD failure [31,32]. Its accumulation in the digester resulted in a decrease in the pH value and the subsequent reduction of methane production. On the other hand, regarding the pH values as well as the N-NH\({}_{4}\)\({}^{+}\) content, there were no noticeable effects of LB application on MW and MLL compositions. Apart from the previously mentioned parameters, ALK is considered as a crucial monitoring parameter within AD [29]. It should be noted that the optimum pH range for methanogens is relatively narrow and should be in the range 6.8 to 7.8. Therefore, the application of substrates with high ALK might ensure the adequate buffering capacity within the digester, which is particularly recommended. Moreover, previous studied indicate that high ALK allows for better process performance, increased methane yields, and reduced retention time [33]. Additionally, this fact might play a key role in conducting the multicomponent AD of various agro-industrial wastes, as well as organic fraction of municipal waste [34]. It should be noted that in the present study, the application of LB led to minor decreases in this parameter, compared with sole waste streams.