# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
###### Abstract
The conversion of agri-food lignocellulosic residues into biofuels or chemicals in integrated biorefineries includes three essential steps: (1) delignification (or pretreatment), (2) conversion of hemicellulose and into C5 and C6 monosaccharides and (3) their fermentation into raw biofuels. Sugars can also be used to produce platform chemicals such as acetone, butanol, ethanol, amino acid, itaconic acid and lactic acid by biological or chemical conversion processes [21]. The conversion of agricultural and forestry waste offers numerous technological challenges, notably in the pretreatment step [16; 22] and several processes have been developed for this purpose including mechanical, chemical and biological treatments [15]. During pretreatment, lignin is modified or partially separated and removed from cellulose and hemicelluloses, making them more accessible to hydrolytic enzymes. However, pretreatment process often involves the production of different byproducts such as aliphatic acids (formic and acetic acid), furan derivatives (furan aldehyde, 5-hydroxy methyl furfural \"HMF\") and several phenolic compounds which inhibit hydrolytic enzymes as well as fermentative microorganisms, thereby decreasing both the overall biomass conversion yields and fermentation capacity [23; 24]. For these reasons, pretreatment methods aiming to enhance the enzymatic hydrolysis (EH) of cellulose by reducing the collateral drawbacks should be developed [25; 26]. The key factors that should be optimised in the hydrolysis of cellulose are the degree of polymerization, the crystallinity, the surface area as well as the remaining lignin and moisture content. The main pretreatment options historically explored are steam explosion, ammonia fibre explosion, hydrothermal, wet oxidation, alkaline, acidic, organosolv, and, more recently, ionic liquid or deep eutectic solvent pretreatment [27]. However, these current methods are expensive and energy consuming and utilize chemicals which entail special disposal, handling or production procedures. Taking into account biofuel production, it is counterproductive to combine energy severe, wasteful processes. Furthermore, the process required to enhance cellulose accessibility and recovery will be strongly influenced by the nature of the native biomass [28]. Consequently, new technologies must focus on methods with low environmental and economic impacts and high efficiency [29]. In the last few years, the comprehensive utilization of lignocellulosic feedstocks has attracted considerable interest for the development of new sustainable processes established on nonconventional technologies. Although these tools have proven very efficient in biomass fractionation, depolymerization of (hemi)cellulose and lignin or sugar conversion into value-added chemicals, the energy consumption of these processes has to be assessed for a subsequent application on industrial scale [11]. In this context, the disclosure of even more effective delignification processes for the recovery of cellulose from agri-food wastes is desirable, and cavitational treatments could represent the breakthrough. Among them, acoustic (AC) and hydrodynamic cavitation (HC) could provide a harsh physicochemical environment that is hard to achieve with other engineering procedures [30; 31], especially in terms of energy efficiency, and could meet the challenges related to the processing of recalcitrant lignocellulosic biomass for cellulose recovery [3]. In this review, the recent advances in the application of acoustic and hydrodynamic cavitation on cellulose recovery from agri-food waste are explored, with a focus on the key parameters that could affect the sustainability of the overall processes.