# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Acoustic Cavitation (AC) as Effective Biomass Pretreatment  
Although a variety of new technologies have been developed at the lab and industrial scale for biomass valorisation, there is still a long way to go to efficiently transform biowaste into biochemicals and bioenergy [76]. In particular, the development and application of even innovative, energy effective, scalable end inexpensive technologies could make the difference. In particular, there is a crucial need to design and develop sustainable cellulose recovery processes. These criteria are largely addressed by sonication, being a physical treatment that can increase size of pores and accessible surface area and decrease cellulose crystallinity and its degree of polymerization, enhancing the biodegradability or the enzymatic hydrolysis of agri-food residues. Furthermore, the significant decrease in process time and temperature are the most attractive characteristics of US-assisted treatments (UAT). In fact, US could induce fast structure damages due to several mechanisms: local shear stress, erosion, fragmentation, sonoporation and sonocapillary effect. Recently, many researchers have exploited US to improve biomass delignification, in particular under alkaline conditions. Despite these attempts, the exact physical mechanism of the US-assisted delignification is not yet recognized. Additionally, it can be stated that to date, a single and exhaustive biomass delignification process has not been established to carry out a complete cellulose recovery from agri-food waste in an economic and environment friendly manner. A comprehensive review of the recent literature on the application of cavitation for delignification has been given in the work of [PERSON] et al. (2012) [77] reporting the different investigated systems, highlighting reactors features and the important results achieved. Therefore, extensive research has been carried out for developing various US and HC delignification treatments of biomass. However, after a critical analysis it is possible to claim that pretreatment methods are \"tailor-made\" processes for every individual matrix which should be carefully chosen and proposed based on the characteristic properties of considered starting material. In this context, [PERSON] et al. (2014) [78], attempt to give mechanistic insight into UAT delignification of _Parthenium hysterophorus_, commonly known as carrot grass. The approach of their study was to couple simulations of cavitation bubble dynamics to the delignification experiments of this agri-food residue. The best result has been described under US irradiation for 15 min (20 kHz, 500 W) at 30 \({}^{\circ}\)C in presence of NaOH (1.5% \(w/v\)), starting from a biomass consistency of 2% \(w/v\). They noticed that both physical and chemical effects of transient cavitation contribute to final delignification (90%), that was 2-fold higher compared to mechanical treatment. Furthermore, the lignin depolymerization takes place through homolytic cleavage of phenyl ether \(\alpha-\)O\(-\)4 and \(\beta-\)O\(-\)4 bonds induced by US lignin. The chemical effect of radical generation induces oxidation/hydroxylation of the main lignin aromatic moieties and in addition, side chain elimination phenomena. Due to these exclusive mechanisms, UAT gives efficient delignification at room temperature and with minor requirement of delignifying agents. They finally observed that cavitation also causes recrystallization of cellulose due to its partial depolymerization. [PERSON] et al. (2017) [79] also evaluated the UAT effects on biomass structural alterations. Particularly, Eucalyptus wood with a particle size of 0.5 mm was submitted to AC (0.5-1.5 h) with an ultrasonic bath working at 28 kHz and 300 W. The protocol was tested with distilled water, aqueous NaOH solutions or acetic acid.