# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Acoustic Cavitation (AC) as Effective Biomass Pretreatment
The results indicated higher EtOH production (820 mg/L) when cavitation was combined with acid medium. In addition, 911 mg/L of EtOH was reached when all the pretreatments were utilized, and SCB was treated with US (50 or 109 W) and acid (3%) and then with alkaline (23%) and US (50 or 109 W). Cellulosic fraction recovered from several biomasses can also be exploited as a fermentation substrate for hydrogen production. The biological dark fermentation represents a more sustainable and less expensive process than lignocellulosic matrix pyrolysis or gasification. [PERSON] et al. (2019) [87] combined US with dilute acid as a pretreatment process to increase fermentative hydrogen production from grass. To evaluate the efficiency of the synergistic process, the two protocols were tested separately as well. In particular, 2 g of grass suspended in 40 mL of distilled water were sonicated with an immersion horn for 30 min with an output power of 260 W, without any addition of chemicals. For the conventional acidic treatment, the same quantity of biomass was added of 40 mL of HCl solution (1% \(w/w\)) and then refluxed for 30 min. The same biomass to HCl solution (1% \(w/w\)) ratio, sonication time and US output power were exploited to investigate the efficiency of US + dilute acid pretreatment towards grass carbohydrate and soluble chemical oxygen demand (SCOD) increase. The combined protocol allowed the increase of SCOD and carbohydrate content, respectively, by almost 99% and 237% if compared to the raw matrix; the SCOD and carbohydrates recovery was 88.5% and 231.5% higher compared to UAT alone, while it was around 17% and 36% higher related to the dilute acid alone. [PERSON] et al. also performed different fermentation batch exploiting raw and pretreated grass as unique carbon source. The highest hydrogen quantity (84.4 mL) was documented for the US + dilute acid pretreatment, which was 190.0% and 35.0% higher compared to US and diluted acid alone pretreatments, respectively (Figure 7). According to the author, this result indicates that combined pretreatment intensified the hydrogen production via fermentation because of the higher carbohydrates availability and the disruption of recalcitrant structure of the lignocellulosic biomass. To improve enzymatic hydrolysis of rice straw (RS), [PERSON] et al. (2017) [88] have lately exploited the US-assisted cellulose recovery process combined with [PERSON]'s reagent (U/F-RS) by means of a horn type sonotrode, working at 22 kHz in power range of 200-600 W.  
Figure 7: Cumulative hydrogen production for control and pretreated groups. The authors confirmed that combination of acoustic cavitation together with the oxidizing agent enabled a reliable and effective pretreatment for RS, suitable for subsequent EH step (Figure 8). Firstly, U/F-RS led to the sample's lowest degree of polymerization and largest specific surface area. Furthermore, this sample had the highest enzymatic activity after 48 h of saccharification, with 4- and 1.5-times higher sugars yield with respect to raw RS and [PERSON]'s reagent alone (F-RS), respectively. Post-treated analysis revealed that the application of US did not change the average content of hemicellulose and lignin in comparison with F-RS, hence authors concluded that the rice straw higher specific surface area and lower DP obtained after combined pretreatment led to the higher enzymatic activity observed on U/F-RS. Compared with the conventional [PERSON]'s reagent process which generally takes dozens of hours, the U/F-RS treatment only takes 3 h to obtain similar EH yield. Since it is well known that cellulose recovery from lignocellulosic matrixes can be improved by means of delignification under alkaline condition at high temperature, the heating generated by US irradiation could be useful to save time and energy. In this context, [PERSON] et al. (2017) [89] showed that the energy dissipated into heat from a horn-type US device working at 300 W and 22 kHz for 70 min, was an effective and reliable method for RS alkali-cellulose recovery.