# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Biomass Pretreatment under Hydrodynamic Cavitation
The authors also performed a subsequent sonication of HC-pretreated WS with US (25 and 80 kHz) but no improvements were observed. UAT alone were also performed on the biomass (25-35 \({}^{\circ}\)C, 30-60 min, 25-80 kHz, and a combination of them) to compare their efficiencies in cellulose isolation. Overall, the US delignification percentage was higher (30-45%) than the HC alone (25%) but alterations of the lignin aromatic structural units' ratio were observed. According to [PERSON] et al., this feature could be a negative factor for further cellulose saccharification, making HC a more suitable process for this purpose. To compare the efficiency of HC and US in the pretreatment of agri-food residues, [PERSON] et al. (2019) [119] exploited these two technologies to improve the biogas production in an  
Figure 14: Klason lignin and py-lignin quantification approaches for raw WS and post HCAT WS. agricultural biogas plant (ABP) using a pretreated WS and cattle manure as fermentation substrate. Before UAT and HCAT, the two biomasses were mixed and introduced in a substrate preparation tank (SPT) as follows: 0.2/0.05/0.035 Mg fresh/TS (total solids)/VS (volatile solids) was daily introduced into the SPT, while the cattle slurry was pumped by 10 min with 0.20 m\({}^{3}\)/d of rate (0.2/0.01/0.008 Mg fresh/TS/VS per day). The UAT was carried out into 5 linked square hollow tubular sections (total volume of 40 L) and located in an US system composed by 60 transducers (24 kHz and 10 kW for each transducer) as shown in Figure 15. The hydrosonic pump used for HCAT was constituted by a cylindrical rotor fixedly connected to the shaft and placed in a tank of 25 L (Patent No PL 214,335 B1). The rotor was powered by a 4 kW electric motor at 2800 rpm (Figure 15). Different experiments have been performed:  
* ABP-90 days of operations (without feedstock pretreatment)
* ABP-UAT-90 days of operations
* ABP-HCAT-90 days of operations  
To evaluate the efficiency of pretreatment processes, biogas production rate (BPR) and cumulative biogas yield (CBY) have been monitored. In the ABP-90 days the BPR reached 15.9 m\({}^{3}\)/d, which was lower to the BPR reached with the exploiting of the two pretreated feedstocks. In fact, during ABP-UAT-90 days the BPR maximum value was 19.8 m\({}^{3}\)/d, while the ABP-HCAT-90 days led to a BPR value of 18.5 m\({}^{3}\)/d. Collective biogas yield (CBY) during ABP-90 days operation was of 370 L/kg VS, 460 L/kg vs. in the ABP-UAT-90 days and 430 L/kg vs. in the ABP-HCAT-90 days. The authors demonstrated that both UP and HCP improved biogas production by 25% and 17%, correspondingly. Moreover, [PERSON] et al. evaluated the energy consumption (to produce 1 m\({}^{3}\) of biogas) of the different set-up: comparing the two methods, lower energy demands for HC pump  
Figure 15: ABP scheme: 1-SPT, 2-cavitation-based pretreatment unit, 2A-US apparatus, 2B-HCP device, 3-MFT, 4-PFT, 5-digestate, 6-shredder, 7-pump, 8-digestate recirculation, 9-biogas outlet. Reprinted from Energy for Sustainable Development, 49, 21–26. Copyright (2019), with permission from Elsevier. operation resulted in lower energy requirement per unit of biogas produced in ABP-HCAT (1.65 kWh/m\({}^{3}\) vs. 2.26 1.65 kWh/m\({}^{3}\)). Energy consumption was the lowest in ABP-90 days (1.25 kWh/m\({}^{3}\) of produced biogas). Overall, experiments performed into the small-scale ABP proved that HC offers an energy-efficient method for the treatment of agri-food residues, leading to high biogas productivity. [PERSON] et al. (2018) [120] also claim that HCAT is an advantageous technology in terms of energy consumption if compared with other pretreatment technologies. In their study, they evaluated the efficiency of combined HC and enzymatic lignin oxidation (HCE) in the reduction of lignin from corncob. In fact, lacase enzyme exploited in this work can oxidize a variety of phenolic subunits of lignin and other aromatic compounds via radical-catalysed mechanism.