# Challenges of Hydrodynamic Cavitation of Organic Wastes
## 1 Introduction  
Currently, the rapid population rise and related increased food demand are leading to the progressive development of the agricultural sector. It is recognised as a significant contributor of anthropogenic global warming, as well as a source of substantial amounts of organic wastes [1]. It should also be pointed out that agricultural production is responsible for 90% of global freshwater and 30% of global energy consumption [2]. Importantly, the majority of its energetic demand is met from non-renewable sources [3]. Therefore, to reduce its harmful impact on environment, sustainable solutions should be applied in this sector. In recent years, hydrodynamic cavitation (HC) has proven its effectiveness in many fields [4, 5, 6]. It has been widely applied in water and wastewater treatment to remove bio-refractory compounds, such as pharmaceuticals, dyes, pesticides, pharmaceuticals, organic sulfide, phenol, cyanide, bitumen, oil and grease, fuel and chlorocarbon [7]. It has been commonly used as a disinfection method [8] and in landfill leachate treatment [9]. Moreover, HC has been also employed in many industrial applications, e.g., biodiesel generation [10], active compound extraction [11] as well as food production [12]. It has been used as pre-treatment method with lignocellulosic biomass, waste activated sludge, chitosan and alga. However, compared with other emerging technologies (microwave, ultrasound, gamma ray, and pulsed electric field) there is a limited number of studies in this area. Despite the recent progress in this field, many mechanisms within HC are still not well understood [8]. HC involves the formation, sequential growth and collapse of microbubble cavities, occurring when the local liquid pressure drops below the saturated vapour pressure. Large magnitudes of energy are released over a very small location. The emerging high temperature of 5000-10,000 K and pressure of 1000-2000 atm initiate various physical processes and chemical reactions. Additionally, this phenomenon is also accompanied by the formation of free radicals that may accelerate the chemical reactions or/and influence their mechanisms [4; 6; 8]. Due to its complexity and multi-stage nature, it is difficult to estimate the effect of cavitation on the removal of a given pollutant [5]. Therefore, each case should be considered individually. Many factors may influence the results of HC, such as type of cavitating reactor, operating inlet pressures, cavitation number, temperature, pH as well as the initial concentration of the pollutant. Moreover, the physicochemical properties of the applied liquid, including its vapour pressure, viscosity and surface tension, play a key role [6; 7; 13; 14]. Additional factors should be considered when HC is applied as a pre-treatment method for lignocellulosic waste (LB). In this process there may be more difficulties and disturbances compared with HC of sole liquids. In this case, additional aspects should be considered, e.g., the characteristics of the treated biomass. Nonetheless, the crucial factor is the applied liquid (carrier) in which the solid waste is suspended [6]. With the aim of ensuring sustainable development, the utilisation of waste streams for this purpose is highly recommended. The application of an inappropriate carrier could possibly lead to secondary pollution caused by the toxic products generated within HC. This concern may limit the subsequent use of pre-treated lignocellulosic biomass [12; 15]. Another limiting factor is the dry matter content in the cavitated mixture (solid/liquid ratio, S/L ratio). Depending on the construction of cavitation reactor, the characteristics of the treated biomass and operational parameters, the typical dry matter content varies between 1 and 16% [16; 17]. Importantly, previous studies indicate that low-solid loading (<5% solids, _w/w_) allowed for higher efficiency in terms of destruction of refractory compounds. This effect is achieved mainly due to the high mass transfer rate and mixing effect [13].