Combining Ultrasonic with Other Water Treatment Technologies

Combining Ultrasonic with Other Water Treatment Technologies

1. Ultrasound – Traditional Water Treatment Technology

Ultrasound generates powerful shear forces and cavitation effects, effectively destroying pollutants in water, such as heavy metal ions, organic matter, and nutrients like nitrogen and phosphorus. Combining this with traditional water treatment methods, such as coagulation, sedimentation, and filtration, can further enhance water treatment efficiency. For example, petrochemical wastewater contains large amounts of organic matter and toxic substances, posing serious risks to the environment and human health. Ultrasound technology can effectively remove these organic and toxic substances from petrochemical wastewater through the synergistic effects of physicochemical and biological effects, achieving effective treatment. Dyeing wastewater contains large amounts of dyes and auxiliaries, making it difficult to treat. Traditional water treatment methods can only remove simple pollutants in the wastewater. Ultrasound technology can disrupt the chemical structure of dyes and auxiliaries, promoting their aggregation and precipitation. Ultrasound also activates dissolved oxygen in the water, generating strong oxidants such as hydroxyl radicals, which further degrade organic pollutants. Wu et al. treated radioactive uranium wastewater using an optimized ultrasound-flocculation-precipitation combined process. They found a significant synergistic effect between ultrasound and flocculant dosage, achieving a uranium ion removal rate of 95.4%.

2. Ultrasound-Membrane Technology

Membrane technology plays a vital role in drinking water treatment, but membrane fouling is a key issue facing membrane treatment. Research has shown that the mechanical vibrations, acoustic streaming, and acoustic cavitation generated by ultrasound not only enhance membrane separation capacity but also effectively clean the membrane surface, inhibiting concentration polarization and membrane fouling, thereby improving membrane flux to a certain extent. Furthermore, as a form of energy, ultrasound propagation in a solution can cause periodic compression and expansion of the solution, generating microvibrations in the water. While the amplitude is small, the acceleration is high, promoting the membrane separation process. Muthukumaran et al. believe that there are four mechanisms of enhancement in the ultrasound-enhanced membrane separation process: 1) Acoustic waves can agglomerate ultrafine particles, reducing membrane solute adsorption and pore clogging, thereby inhibiting membrane fouling; 2) Ultrasound can provide sufficient mechanical vibration energy to move some particles suspended in the solution away from the membrane surface, preventing particle deposition, effectively mitigating concentration polarization and the formation of a filter cake layer, and significantly reducing boundary layer resistance and filter cake resistance; 3) The microfluidics generated by ultrasound can break up the gel layer and filter cake layer formed on the membrane surface, dispersing them in the liquid; 4) Macroscopic turbulence caused by microjets, shock waves, and acoustic pulses can enhance diffusion within the main turbulent flow and also induce local turbulence in the boundary layer. This local turbulence transforms molecular diffusion in the boundary layer into eddy diffusion, ultimately enhancing convective mass transfer between the material and the interface.

3. Ultrasonic-Ozone Technology

Currently, there has been extensive research on ultrasonic-ozone technology. Ozone can generate chemically active oxygen free radicals under the action of ultrasound. These free radicals can combine with ozone to generate oxygen, or react with water to generate strong oxidizing species such as ·OH and ·H2O2 (Formulas (1) to (4)), thereby promoting ozone decomposition and improving reaction efficiency. Research by Helfred et al. [11] showed that ultrasound can crush ozone-containing bubbles into "microbubbles". The specific surface area of "microbubbles" is 101 to 104 times larger than that of ordinary bubbles, which increases the contact area between ozone and water and accelerates the dissolution rate of ozone in water. Ziylani-Yavas et al. [12] studied the ultrasonic-ozone method for treating paracetamol. The results showed that the combined technology enhanced the production of oxidizing species and improved the mineralization rate of pollutants.

4. Ultrasonic-photocatalytic technology

Photocatalytic technology refers to a technology that uses the redox ability of photocatalysts under light to purify pollutants and synthetic substances. Photocatalytic technology is highly popular because of its mild reaction conditions and wide application fields. The combination of ultrasound and photocatalytic technology can decompose hydrophobic substances and expand the transfer path of photogenerated electron holes. The research results of Hamdaoui et al. [13] showed that under the same conditions, the combination of ultrasound radiation and photochemical process led to an increase in the mineralization rate of chlorophenol compared with the use of separate treatment technologies. This means that there is a great synergistic effect between the three oxidation processes of direct photochemical action, high-frequency sonochemistry and ozone reaction generated by ultraviolet radiation air. The factors affecting ultrasonic treatment of water resources mainly include the use parameters of ultrasound, such as frequency, power and sound intensity, as well as the physical and chemical parameters of the wastewater to be treated, such as temperature, particles and pollutant properties. In addition, the ultrasonic treatment process is also affected by factors such as ultrasonic power intensity. During the degradation process, the reaction rate is not constant. Generally speaking, the greater the ultrasonic power intensity, the faster the reaction rate. As an environmentally friendly technology, ultrasound shows great application potential in the future water treatment industry.

 Although this technology has achieved certain research results, the problems of high energy consumption and low efficiency reduction when used alone need to be further solved. For example, how to optimize the structure and performance of ultrasonic equipment to improve its stability and efficiency, how to conduct in-depth research on the mechanism of ultrasound to achieve its efficient, safe, and environmentally friendly application, and how to develop new ultrasonic treatment processes to adapt to different types of sewage and water quality conditions and reduce ultrasonic energy consumption. Breaking through bottlenecks and overcoming barriers based on existing research will help us adapt to the ever-changing water quality issues.