# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Biomass Pretreatment under Hydrodynamic Cavitation
The highest SCB cellulose increase (40 to 50%) was obtained with the recirculation of the 0.3 M of NaOH, 0.2% (\(v/v\)) of H\({}_{2}\)O\({}_{2}\) solution in 6 min of process time. On the other hand, the highest lignin removal (27.2 to 14.3%) was reached with a 0.3 M of NaOH, 0.6% (\(v/v\)) of H\({}_{2}\)O\({}_{2}\) solution and 10 min of time. The operating conditions have been further optimized to maximize the pretreated SBC hydrolysis yield: the maximum value (95.43% of hydrolysis in 24 h) has been obtained recirculating a 0.29 M of NaOH, 0.78% (\(v/v\)) of H\({}_{2}\)O\({}_{2}\) solution for 9.95 min and it was quite similar to the one predicted by the model (96.27 \(\pm\) 7.31%) but higher in comparison to the ones reached with the pretreated biomass with a 0.29 M of NaOH and 0.78% (\(v/v\)) of H\({}_{2}\)O\({}_{2}\) recirculating solutions, respectively. The SCB hydrolysate was evaluated in terms of ethanol production exploiting yeast species _S. stipitis_ NRRL-Y7124. The highest ethanol yield obtained after the fermentation process into a bubble column reactor was 0.49 g/g biomass (31.50 g/L and 0.68 g/Lh of volumetric productivity) after 46 h. At the same fermentation time Xyl has been completely consumed, contrary to Glu that was completely consumed after only 24 h. The efficiency of HC combined with sodium percarbonate was tested in the degradation of polysaccharides in corn stover, which was treated in a circular flow system, exploiting a Venturi tube (Figure 3) [113]. Figure 13: Schematic representation of HC system used for SCB pretreatment. The SP-UAT of corn stover led to lower digestibility compared to the HC-SP procedure, which was more effective in terms of both Glu and Xyl production. This discrepancy could be attributed to the 50% greater lignin removal during the HCAT. The mild protocol (30 \({}^{\circ}\)C) allowed for the avoidance of the generation of inhibitors, such as furfural. The comparison of energy efficiency during the HC- and US-assisted pretreatments of SCB, reed and corn stover is summarized in the Table 3.  
The efficiency of pretreatment is reported in terms of cavitation yield, calculated according to [PERSON] et al. (2016) in Equation (2) [114]. \[C_{Y}=\frac{G_{r}}{P_{m}\times t} \tag{2}\]  
where \(C_{Y}\) = cavitation yield (g of Glu/J), \(G_{r}\) = Glu released during EH (g), \(P_{m}\) = power of pump (W) and \(t\) = operation time (s). As displayed, HC systems are more energy efficient than UAT. HC resulted very useful also for other processes requiring a biomass pretreatment step. For example, the delignification of wheat straw in paper manufacturing was improved under HC alkaline conditions [115]. In this case, the hydrodynamic cavitating reactor used for the wheat straw delignification was a rotor-stator type device (Figure 3), with a solid cylinder bearing indentations on its surface [116]. Rotor can be rotated at different speeds of rotation. Due to high speed of the rotor, very high surface velocities are generated at the cylinder surface. Such high velocities (typically 18-20 m/s) are accountable for the formation of elevated turbulence, leading to the breakage/lysis of lignin, cellulose and hemicellulose matrix. After only 10-15 min of alkali pretreatment (0.3 M, KOH), an increasing of the tensile index (50-55%) of the synthesized paper sheets was achieved, suitable for paper board manufacture. HC delignification of WS employed for the manufacture of paper revealed some advantages, such as the reduction of treatment time, energy consumption (nearly 50%) and temperature (25 \({}^{\circ}\)C). In addition, the versatility of the process was improved, as different parameters (such as the speed rotation and straw consistency) could be adjusted according to the required properties of the target paper. A rotor stator type was also employed for the wheat straw pretreatment step in the production of biogas [117]. The design of experiments approach (DOE) helped the optimization of the process conditions.