# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
Although this pretreatment is very effective, the high energy input required and the toxic by-products produced make this process less sustainable. At the same time, liquid hot water treatment process [65] is very close to steam explosion but uses water instead of steam to reach hemicellulose solubilization and lignin transformation. In general, liquid hot water pretreatments are attractive from a cost-savings point of view because no chemicals and corrosion-resistant materials are required for hydrolysis reactors. However, the high energy input required and the production of many toxic products make this eco-friendly method (as no corrosive material is used) less attractive. Furthermore, in the AFEX pretreatment [67], biomass is exposed to liquid NH\({}_{3}\) at room temperature and high pressure. AFEX does not require small particle size of biomass, enabling a good cellulose recrystallization and accessibility (by means of a good NH\({}_{3}\) induced swelling) to hydrolytic enzymes. AFEX increases the digestibility of lignocellulosic biomass by removing the least acetyl groups by deacetylation process. Drawbacks are the large amount of ammonia required and the ineffectiveness with biomass with high lignin content. The SC-CO\({}_{2}\) pretreatment [68] involves the use of supercritical CO\({}_{2}\) (1000-4000 psi) that could penetrate the biomass structure, dissolving available water and forming carbonate that could hydrolyse hemicellulose. No inhibitory compounds were produced during SC-CO\({}_{2}\) pretreatment, but the required high operating pressure (1000-4000 psi) and the high costs are a big drawback. Among chemical pretreatments, alkali delignification is more effective for low lignin content biomass, especially softwood [74]. Moreover, alkaline delignification is not as energy demanding as some of the other pretreatments being carried out at low pressures and temperatures [60]. Compared with acid pretreatment [61], alkaline delignification has less influence on sugar degradation, but the salts generated are often incorporated into the biomass during the process. Oxidizing agents, like hydrogen peroxide and ozone, have also been exploited for biomass pretreatment at ambient temperature. Although ozonolysis is one of the few methods capable of breaking down the aromatic structure of lignin without affecting cellulose, it requires a huge amount of ozone. On the other hand, the use of ionic liquid or the newly introduced (natural) deep eutectic solvent for the agri-food waste pretreatment is a growing practice. These sol \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Pretreatment for** & & & \\
**Cellulose** & **Processes** & **Possible Biomass Alterations** & **Remarks** \\
**Recovery** & & & \\ \hline Physicochemical and & & & High pressure requirements \\ Chemical & SC-CO\({}_{2}\) & Increases the accessible specific surface area & Easy CO\({}_{2}\) recovery after \\  & Organosolv & High delignification Separation and recovery & extraction \\  & process & of high-quality cellulose & High price of organic solvents \\  & Alkali & Decreases the cellulose crystallinity & High pollution and high chemical \\  & Acid & Decreases the polymerization degrees of & recovery cost \\  & & cellulose hydrolysis of hemicellulose & Rapid treatment rate \\  & Oxidative & High delignification Hydrolysis of & High cost of bleaching agent \\  & & hemicellulose & High delignification \\ Biological & Fungi & Degrades lignin and hemicelluloses & Low energy requirement \\  & & Decreases the polymerization degree & \\  & & of cellulose & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._vents are very effective in the disruption of the hydrogen bonds network among biomass components, facilitating the cellulase access to cellulose but they are very expensive [75]. Biotechnology can contribute to more efficient and greener deconstruction of plant cell wall by providing tailor-made biocatalysts based on the oxidative enzymes responsible for lignin attack in nature.