# Optimized Analytical-Numerical Procedure for Ultrasonic Sludge Treatment for Agricultural Use
###### Abstract
However, the high-energy requirements and the need for sophisticated instrumentation and calibration make this method not always advantageous. In addition, the effectiveness of sonication can be limited by specific characteristics of the sludge, such as the presence of substances resistant to cavitation (the collapse of tiny bubbles that transmit significant mechanical forces to the solid material in suspension), making it less effective in some contexts [22; 23]. In addition to the above procedure, a less costly method, the hydrodynamic cavitation, can be used for the same purposes. This method takes advantage of the formation and subsequent collapse of microbubbles of steam in a fluid, creating intense forces that break up the particles in the sludge. Specifically, when liquid passes through a constriction or valve at high velocity, it makes a low-pressure zone that induces bubble formation. The collapse of the bubbles releases enough energy to break down cellular structures and facilitate the solubilization of organic materials. While this technique reduces particle size with low energy consumption and less expensive equipment, it is poorly applicable to high-density slurries and is subject to wear of parts exposed to cavitation [24; 25]. Also, intensive heat treatment, which produces sludge dewatering by destroying pathogens, is possible; however, the high energy consumption and possible loss of essential elements limit its use [26; 27]. Another technique well established in the literature for dewatering and breaking down sludge structures is microwaves, which provide selective, rapid, and uniform heating while reducing treatment time but require expensive equipment without preventing or limiting the formation of unpleasant odors or toxic compounds [28; 29]. As far as contaminant removal is concerned, the application of a suitable electric current to the (very dense and highly conductive) sludge, which, by weakening the binding forces between the particles, helps to separate the solid particles from the aqueous phase, thus reducing the use of any chemical agents [30; 31]. Suppose the objective is the degradation of organic matter in low-density sludge. In that case, we are helped by photoanalytical treatment, in which the sludge is exposed to a suitable source of light (constant and prolonged), aided using specific catalysts (usually titanium dioxide) capable of reducing harmful organic compounds and pathogens. However, the continuous exposure to light and the use of expensive catalysts limit its applicability [32; 33]. In order to degradate the organic matter in low-density sludge, the photoanalytical treatment can be used. In this case, the sludge is exposed to a suitable source of light (constant and prolonged), aided using specific catalysts (usually titanium dioxide) capable of reducing harmful organic compounds and pathogens [32; 33]. Widespread is the mixing of sludge with flocculants/coagulants, which aggregate the particles to form heavy agglomerates that precipitate to the bottom of the tank and rapidly promote dewatering. However, such chemicals are often expensive and produce residues that require further treatment [33; 34; 35]. Studies have investigated the combined use of US and ozone to improve sludge conditioning and increase dewatering. Ozone, which is a powerful oxidant, facilitates the rupture of cell membranes and the solubilization of organic matter, therefore enhancing the effect of ultrasound. This combination reduces the sludge's viscosity and its filtration resistance [36]. Other studies have focused on the use of cationic polymers that, when combined with sonication, reduce the concentration of heavy metals and limit the time for sludge conditioning by making them stable during dewatering [15]. Again, the combination of thermo-alkaline conditioning with sonication assists in the degradation of complex organic compounds (due to high temperatures and high pH), facilitating the dewatering of the sludge.