# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Biomass Pretreatment under Hydrodynamic Cavitation
The yield of biogas was increased from 31.8 mL (untreated biomass) to 77.9 mL under HC working at 2500 rpm. Furthermore, the combined pretreatment using KOH and HC gave maximum yield of biogas (172.3 mL). [PERSON] et al. (2018) [118] exploited a rotor stator HC reactor to enhance biomethane production from agriculture biomass in a full-scale agricultural biogas plant (ABP) composed by three stirred anaerobic digesters (AD1, AD2, AD3) in series with a total volume of 4200 m\({}^{3}\) and an overall hydraulic retention time of 30-35 days to produce electrical energy and heat. The ABP feedstock consisted in agriculture byproducts (beet molasses and corn meal), energy crops (maize silage and tritticale silage) and pig slurry. The solid biomass  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Agri-Food Residue** & \ \begin{tabular}{c} **Pretreatment** \\ **Process** \\ \end{tabular}  & **Conditions** & \begin{tabular}{c} **Results** \\ **(g of Glu/J)** \\ \end{tabular}  \\ \hline \multirow{2}{*}{Sugarcane bagasse} & NaOH-HCAT & NaOH (1.9\(\%\)\(w\)/\(v\)) & \(6.43\times 10^{-5}\) \\  & NaOH-UAT & 45 min & \(2.61\times 10^{-5}\) \\ \hline \multirow{2}{*}{Reed} & NaOH-HCAT & NaOH (3.0\(\%\)\(w\)/\(v\)) & \(6.51\times 10^{-5}\) \\  & NaOH-UAT & 40 min & \(2.58\times 10^{-5}\) \\ \hline \multirow{2}{*}{Corn stover} & SP-HCAT & NaCO\({}_{3}\) (0.4 mol/L) & \(2.24\times 10^{-5}\) \\  & SP-UAT & \(\begin{array}{c}\text{H}_{2}\text{O}_{2}\text{O}_{2}\text{O}_{2}\text{O}_{feedback was introduced into AD1 on regular time during the day while both AD1 and AD2 were fed with pig slurry. Almost 96% of AD1 effluent was treated only once with the rotor stator type HC system during its transfer to AD2 with an inlet pressure of 2 bar and equal inlet and outlet flow rates as there was no accumulation in the reactor. Three batch experiments with an increasing HC system electrical power (470, 740 and 954 kJ/kg of total solid) have been performed to investigate the effect of the HC rotor speed on the anaerobic biodegradability. All the HCAT set-up enhanced the biomethane yield and degree of degradation (Volatile Solids removed, %) of the digestate from AD1 compared to the same untreated digestate. The highest increase of biomethane yield (+14%) has been obtained with an HC system energy input of 740 kJ/kg while the 470 and 954 kJ/kg experiments allowed for the increase of fermentation yield by 3.5 and 2.9%, respectively. According to [PERSON] et al. the enhancement of anaerobic digestion with the increasing of HC system energy input can be attributed to an increase of cavitation intensity with consequent increase of biomass surface area. Moreover, the electrical energy used for the ABP operation increases about 100 kWhel/day with the HC reactor implementation. As the total energy production was nearly constant through the whole experimental time, the lower energy consumption has been associated to energy saving in biogas plant (17%). A further delignification process performed with a rotor-stator HC system was also evaluated by [PERSON] et al. (2021) [47]. For this purpose, wheat straw (WS) was pretreated for 30 min at 50 \({}^{\circ}\)C in a NaOH solution (10% mass, based on dried biomass) with a 1:50 S/L ratio. The solid fraction was recovered and freeze-dried to evaluate its residual lignin content by wet-chemistry Klason procedure. The HC allowed a delignification removal of 25%, if compared to the untreated WS, and an increase of hexose carbohydrates by up to 16%. However, the Klason procedure could overestimate the residual lignin content due to the formation of pseudo-lignins, the condensation/coupling reaction product between lignin and other degraded biomass polymer compounds during the analytical procedure. Therefore, the residual lignin of raw WS and pretreated WS was also determined with the Py-GC/MS/FID method. A comparison of the two analytical methods' results confirmed the pseudo-lignin formation during the cellulose isolation process (Figure 14).