Application of microwaves in the treatment of polymeric-containing wastewater

Ultrasonic water treatment technology and processes: In the 1990s, Mason conducted research on the use of ultrasonic cavitation to degrade harmful organic substances in water. The studies showed that ultrasonic waves are effective in breaking down organic compounds in water, which attracted the interest of many scholars. University laboratories and research institutes in countries such as the United States, Japan, Canada, Germany, and France are actively engaged in research on the ultrasonic degradation of organic compounds. In the late 1990s in China, research on the ultrasonic degradation of organic compounds achieved excellent results at Tongji University. To date, ultrasonic degradation has been applied to fatty hydrocarbons, aromatic hydrocarbons, phenols, esters, alcohols, ketones, amines, acids, and natural organic compounds, achieving excellent results. The emergence of ultrasonochemistry as an interdisciplinary science is a development that has taken place over the past decade or so. The effect of ultrasound on chemical reactions stems primarily from the phenomenon of ultrasonic cavitation; the collapse of these cavitation bubbles generates localized high temperatures, high pressures, as well as intense shock waves and jets, thereby providing a very special physicochemical environment that makes chemical reactions possible under conditions that would otherwise be difficult or impossible to achieve. The use of ultrasound in water treatment represents a new development in the field of sonochemistry in recent years. I. Mechanism of power ultrasound: When ultrasonic waves of a certain intensity propagate through a medium, a series of effects such as mechanical, thermal, optical, electrical, and chemical effects are generated. These effects can be summarized into the following three basic mechanisms: 1. Mechanical action. Ultrasonic waves are a form of propagation of mechanical energy; they are related to wave phenomena and generate vibrations that cause linear deformation. When ultrasound propagates in a liquid, although the displacement amplitude of the same particle is very small, the acceleration of these particles caused by ultrasound is extremely large. When ultrasonic waves with a frequency of 20 KHz and a power density of 1 W per square centimeter propagate in water, the amplitude of the sound pressure generated is 173 KPa. This means that the sound pressure amplitude changes 20,000 times per second between +173 KPa and -173 KPa, and the maximum acceleration of the particles involved is 1.44 million meters per second squared, which is approximately 1500 times the acceleration due to gravity. Such intense and rapid mechanical movements constitute the mechanical vibration effect of power ultrasound. 2. Cavitation. When ultrasound propagates through a liquid medium, when the sound intensity reaches a certain level, a local temporary negative pressure is generated in the area affected by the sound field in the liquid. This causes the microbubbles present in the liquid to grow and expand until they suddenly burst, resulting in strong shock waves in the liquid surrounding those bubbles. This leads to high temperatures and pressures in that localized area. When the cavitation bubbles collapse, transient temperatures of 5000 K and pressures of around 50 MPa are generated in the very small space surrounding them. The ultrasonic cavitation effect refers to the generation of a transient high temperature of 5000 K and a high pressure of about 50 MPa in a very small space surrounding the cavitation bubble; the temperature can drop at a rate of 10 to the power of 9 K/s, accompanied by intense shock waves and jets moving at speeds of up to 400 km/h. 3. Thermal effect. As ultrasound propagates through a medium, its vibrational energy is continuously absorbed by the medium and converted into thermal energy, raising the temperature of the medium. The continuous absorption of acoustic energy can cause overall heating in the medium, as well as localized heating outside the boundaries. When cavitation creates shock waves, there is also localized heating at the wave front and elsewhere outside those boundaries; this is the thermal effect of power ultrasound. II. Ultrasonic chemical mechanisms 1. Ultrasonic catalysis. Ultrasonic catalytic reactions are an emerging field of research. At present, research on reaction models and mechanisms remains somewhat unclear, but numerous scientific findings have confirmed the significant effects of catalytic reactions. Its main functions are as follows: first, high temperature and pressure conditions facilitate the breakdown of reactants into free radicals and secondary carbons, thereby forming more reactive species. Secondly, shock waves and microjets have a desorbing and cleaning effect on solid surfaces. Third, shock waves can destroy the structure of the reactants and disperse the reaction mixture. Fourth, ultrasonic cavitation causes deformation of the metal lattice and the formation of internal strain regions, thereby increasing the chemical reactivity of the metal. The reaction rate under ultrasonic conditions was 100,000 times higher than without ultrasound, and the reaction time was longer yet also **shortened**. 2. Ultrasonic degradation. Ultrasonic treatment can degrade macromolecules, with a more significant degradation effect on high-molecular-weight polymers. Ultrasonic degradation stems from the mechanical, cavitation, and thermal effects of ultrasound. III. Applications of power ultrasound in water treatment: The cavitation effect of power ultrasound creates a unique physicochemical environment for the degradation of harmful organic substances in water, thereby enabling the achievement of the goal of treating water using ultrasound. The harmful organic substances in ultrasonic cavitation bubbles create a unique physicochemical environment, thereby enabling the achievement of the goal of treating wastewater using ultrasound. The high energy generated by the collapse of ultrasonic cavitation bubbles is sufficient to break chemical bonds. In aqueous solution, the collapse of cavitation bubbles generates hydroxyl groups (OH?) ) and hydrogen group (H? ), it undergoes an oxidation reaction with organic substances. The unique physicochemical environment created by cavitation opens up new pathways for chemical reactions, significantly increasing the rate of these reactions. It possesses a strong ability to degrade organic substances; through continuous ultrasonic treatment, harmful organic compounds can be broken down into non-toxic or low-toxic substances such as inorganic ions, water, carbon dioxide, or organic acids. The ultrasound degradation technology for organic pollutants in water can be used alone, or the ultrasound cavitation effect can be utilized to combine this technology with other treatment methods in order to degrade and remove organic pollutants. The combined technologies include the following types: the combination of ultrasound and ozone, which uses ultrasound for degradation and sterilization along with ozone for disinfection to treat polluted water. The combination of ultrasound and magnetization treatment techniques enables magnetization to achieve solid-liquid separation in polluted water, degrade organic substances such as COD and BOD, and also remove color from dyed water. Ultrasound can also be used directly as an auxiliary technique to conventional chemical disinfection processes. When large-scale water treatment is carried out using traditional chemical methods, the addition of ultrasonic radiation can **reduce the amount of chemical agents required. IV. Related processing technologies used in conjunction with ultrasound IV. Introduction to related processing technologies used with ultrasound 1. The solid-liquid separation of wastewater is a prerequisite for ultrasound treatment. Wastewater usually contains suspended solids or impurities; therefore, a collection system is necessary. This can be in the form of a wastewater tank or basin, where larger particles and solids should be separated from the wastewater. For smaller suspended particles, polyacrylamide coagulants or inorganic coagulants can be used. Anionic and cationic non-ionic polyacrylamide flocculants are inorganic flocculants. It is a water-soluble polymer or electrolyte. Due to the presence of a certain number of polar groups in its molecular chain, it can adsorb the suspended solid particles in wastewater, bridge these particles together, or cause them to aggregate into larger flocs through charge neutralization. This accelerates the sedimentation of particles in the suspension, resulting in a significant acceleration of solution clarification and improved filtration efficiency. If inorganic flocculants (polyferric sulfate concentrate, polyaluminum chloride, iron salts, etc.) are used simultaneously, a greater effect can be achieved. The amount of flocculant added is generally 0.0110 g per cubic meter, and it dissolves completely in cold water as well. Its main functions are clarification and purification, sedimentation promotion, filtration enhancement, and thickening; it is a commonly used substance in the treatment of wastewater and liquid waste. 2. Filtration is a necessary condition in the treatment of sewage and wastewater. Its purpose is to filter out suspended particulate matter and colloidal particles with a concentration of 20 mg/L or less in the wastewater. No expensive and sophisticated activated carbon devices are required for filtration here; ordinary mechanical filters are more than sufficient to meet the requirements of the subsequent processing steps. 3. The role of water treatment agents in ultrasonic sewage treatment. The aforementioned ultrasonic chemistry mechanism and the role of ultrasound in water treatment are even more effective when water treatment agents are added together with ultrasound. The mechanism of solid chlorine dioxide water treatment agents in water treatment consists of two parts: hypochlorous acid oxidation and reactive oxygen generation. The oxidizing action of hypochlorous acid. This is the main mechanism of sterilization. When the solid chlorine dioxide water treatment agent is dissolved in water, it undergoes hydrolysis to produce hypochlorous acid, which acts on the proteins in bacterial cells. Hypochlorous acid can not only act on cell walls, but also, due to its small size and lack of charge, it can easily penetrate cells to exert its effect or destroy their phosphodehydrogenase, thereby disrupting sugar metabolism and leading to the death of bacteria. A new dehydrogenase that disrupts sugar metabolism, leading to bacterial death. Reactive oxygen species effect. Since hypochlorous acid decomposes to form reactive oxygen species, which oxidize the proteins in bacterial cells, these reactive oxygen species combine with the cellular components, thereby killing microorganisms. 4. Combination of ultraviolet light and ultrasound. UVC sterilization technology. Ultraviolet light is a type of light wave that is invisible to the naked eye; it is divided into three bands, A, B, and C, based on different wavelength ranges. 253.7 nm represents the peak wavelength within the UVC band. Over long distances, it is divided into A; ultraviolet light primarily affects the nucleic acids of microorganisms, such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), causing them to be damaged. UV-C water disinfection technology utilizes the disinfecting principles of ultraviolet light to physically destroy the DNA of viruses, bacteria, and other pathogenic agents in water, preventing them from multiplying and reproducing. UV-C can completely replace ozone for disinfecting large volumes of water, and filtration can also replace ozone, as well as traditional methods such as filter membranes and oxidants; it is safe and reliable, with no risk of secondary pollution. In summary, the wastewater treatment technologies and equipment used in combination with ultrasound include: (1) Equipment that combines solid-liquid separation filters with ultrasound degradation ； (2) Equipment combining solid-liquid separation, ultrasonic degradation, and ozone disinfection ； (3) Equipment combining solid-liquid separation, ultrasonic degradation, and UV disinfection ；

Reports on the use of microwaves in wastewater treatment appeared a few years ago; however, according to technical articles, they are mainly used in small-scale water treatment facilities. They require high energy consumption and result in high treatment costs. Yet they are suitable for certain types of wastewater, such as those with poor biodegradability like pesticide wastewater – microwave treatment can improve their biodegradability, facilitating the subsequent biological treatment processes. Microwave treatment of wastewater is not suitable in all cases; selection is necessary.