# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Biomass Pretreatment under Hydrodynamic Cavitation  
Under alkaline conditions, phenolic hydroxyl groups in lignin dissociate, forming a phenoxy radical stabilised by resonance, which is the initiator of depolymerization reactions. HC in alkaline conditions improves the formation of hydroperoxyl and hydroxyl radicals required for delignification purposes, thus intensifying the overall reaction rates. In addition, shock waves produced during HC are responsible for the breaking of several chemical bonds among lignin and (hemi-)cellulose in the lignocellulosic biomass. Likewise, to US, the key advantages of HC are the local and transient generation of high temperature and pressure (due to the collapse of shock waves) that are necessary for biomass deconstruction and delignification purposes, while the overall batch conditions still remain unaltered, thus preventing the needs of heating (or cooling) devices. These characteristics make HC a valuable tool for agri-food waste treatment [108], the key step being to recover cellulosic fraction, getting rid of the recalcitrant matrix structure, by means of delignification and detekturization. Polysaccharides resulting from HC-assisted treatment (HCAT), could be furtherly exploited in the lignocellulosic pretreatment, the key step in the biochemical processes to biofuels, for instance, an alkaline HCAT (3.0% NaOH) of reed, employed as starting raw material for the bioethanol production [109]. A cylindrical cavitation reactor connected to a centrifugal pump was used, with a radial form of orifice plate, leading to a maximum Glu yield of 326.5 g/kg biomass in 40 min. US treatment was conducted for the sake of comparison in the same conditions at 300 W power and 20 kHz dissolved less lignin (24.5%) than HC (42.3%). The HC highest delignification led to higher digestibility yield of pretreated samples (85.0%) compared to untreated (16%) and US treated ones (73.3%). Furthermore, the HC pretreated samples were used as substrate for batch saccharification and fermentation (SSF) leading to ethanol yield of 90%. An alkaline HCAT exploiting Venturi effect was also employed to enhance the efficiency of alkaline pretreatment of sugarcane bagasse (SCB) for EtOH production (Figure 13) [110]. A cylindrical cavitation reactor with a radial orifice plate with 27 holes of 1 mm diameter was used. Biomass was kept in a cylindrical wire cloth placed within the cavitation zone, in order to aid each particle in fully experiencing the cavitational effect. Under continuous feeding of 0.48 M NaOH, 60% of lignin removal was reached after 45 min. In addition, 52% of glucan content and 97.2% of enzymatic digestibility were achieved. Furthermore, EH afforded a yield of 82% and 30% higher than the untreated and alkaline-treated controls, respectively. More recently, the same authors reported the alkaline pretreatment of SCB recirculating through an orifice plate having 16 holes at a fluid velocity of 54.49 m/s. With only 30 min of pretreatment, 93% and 95% of cellulose and hemicellulose hydrolysis were achieved at 70 \({}^{\circ}\)C using less NaOH (0.3 M) [111]. This result was supplemented by the drastic change of biomass structure created by the dramatically disruptive cavitation effect. An alkaline-HCAT of SBC for bioethanol production was also investigated by [PERSON] et al. (2018) [112], which exploited a 3 L HC system composed by a centrifugal pump (1.5 CV), a cavitation zone and a recirculation tank. The system flowrate was adjusted to 5 m\({}^{3}\)/h with a resulting cavitation number of 0.017. The process fluid temperature was set to 60 \({}^{\circ}\)C thanks to water-jackets while the upstream pressure for cavities generation was set to 3 bar. A Box-Benhken design was utilized to assess the influence of different variables: NaOH concentration (0.1-0.3 M), H\({}_{2}\)O\({}_{2}\) concentration (0.2-1.0% \(v/v\)) and HC treatment time (2-10 min). For each experiment, 20 g of dry SBC was used.