# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Biomass Pretreatment under Hydrodynamic Cavitation
To determine the optimal conditions for HCE pretreatment, response surface methodology (RSM) was utilized. Three independent variables were assumed to influence the process performed at pH 4.5 (acetate buffer): catalyst loading, biomass to liquid ratio and reaction time. Since the process design consisted in the supply of biomass slurry via holes in the orifice plate to cavitation zone, preliminary studies have been performed to evaluate the proper biomass loading rates to avoid orifice holes occlusion. The range of selected pretreatment variables was: biomass loading of 2.5-5.0% (\(w/v\)), enzyme loading of 3-10 U g\({}^{-1}\) (on dried biomass) and reaction time of 5-60 min. The HC system exploited for the experiments was composed by a 6 L circulation tank, an orifice plate, a centrifugal pump, an electrical motor, gate valves for priming and bypass and pipe accessories. Moreover, two orifice plate configurations have been tested: orifice plate 1 (OP1: 9 holes and 2 mm \(\O\)) and orifice plate 2 (OP2:4 holes and 3 mm \(\O\)). Regardless, the two plates did not influence the tests' outcomes, resulting in common optimized conditions for enzymatic HCAT: matrix loading at 5%, enzyme loading at 6.5 U/g of biomass and a process time of 60 min. However, the lignin reduction obtained with OP1 set-up (47.4%) was higher than the one obtained with OP2 (35.91%). [PERSON] et al. only reported an increase of cellulose content in treated corncob exploiting OP1, equal to 25.3%. Overall, HCE pretreatment represents a suitable and an \"energy saver\" technology for cellulose isolation from agri-food wastes but, according to the authors, 5% of biomass loading for scaling up seems to be low and may be increased to achieve an economic output. Taking into account all the potentiality associated with HC, the disclosure of semi-continuous and continuous HC-based technologies for agri-food residues pretreatment could be a viable alternative. For this purpose, [PERSON] et al. (2019) [121] used HC to perform an alkaline-assisted treatment over a residue of sugarcane bagasse (SCB) in sequential batches (SB-HC), semi-continuous (SC-HC) and continuous (C-HC) approach in presence of H\({}_{2}\)O\({}_{2}\). The efficiency of the different pretreatments have been evaluated as biomass cellulose increase and quantity of sugars released after pretreated biomass EH. For SCB pretreatment in sequential batches process, 25 g of dry SCB was maintained in a cylindrical wire cloth (40 mesh) and an alkaline H\({}_{2}\)O\({}_{2}\) solution (0.3 mol/L of NaOH, 0.8% \(v/v\) of H\({}_{2}\)O\({}_{2}\)) was recirculated (2 m\({}^{3}\)/h flow rate). After first pretreatment (fresh solution), the liquid fraction was collected and recycled in subsequent batches over fresh SCB (at list 14 cycles). The cellulose recovered from each batch climbed from 40% to 44% and no significant differences were observed by recycling the black liquor. The highest Glu and Xyl recovery after the EH of the fresh solution pretreated biomass are 42.33 g/100 g and 20 g/100 g, respectively, which are 3.5 times higher compared to Glu released in untreated biomass (9.3 g/100 g). The SC-HC process has been performed in the HC system shown in Figure 16. Three litres of the same solution used for batch process (0.3 mol/L of NaOH, 0.78% of H\({}_{2}\)O\({}_{2}\)) were placed in the cavitation reactor (N\({}^{\circ}\)2 in Figure 16) while 2 L of same solution containing SCB at 1% (S/L ratio) was constantly fed into the HC device (flow rate: 200 mL/min, residence time: 7.5 min). While the cellulose content, on average, after SC-HC process was slightly higher (48%) than the obtained in the SB-HC with fresh solution (44%), the recovered Glu amount was about 35 g/100 g, which is smaller than that recorded for fresh solution in batch process (42 g/100 g) but greater than that recorded for the untreated SCB (9 g/100 g).