Customized different Ultrasonic algae removal transducer and generator for ultrasonic green algae control

There is an alternating cycle of positive and negative pressures in the propagation of ultrasonic waves in water. At the positive pressure phase, the water molecules are squeezed to increase the density of water, and the negative pressure phase causes the water molecules to be sparsely dispersed and the water density to decrease. When the amplitude is large enough, the average distance between water molecules in the negative pressure phase will exceed the critical molecular distance of water, causing the continuity of the water body to be destroyed and forming a cavitation nucleus. The cavitation nucleus rapidly expands to form cavitation bubbles, which are rapidly compressed or even collapsed in the subsequent positive pressure phase, which is the ultrasonic cavitation effect.

The pseudo-cavitations inside the algae cells cavitation under the action of ultrasonic waves, forming cavitation bubbles, and then the cavitation bubbles collapse. The shock wave and high temperature and high pressure caused by the collapse can destroy the active substances inside the algae cells, and also destroy the structure inside the cells and interfere with the synthesis of chlorophyll. At the same time, the free radicals generated can also oxidize the algal toxins released by cell rupture.

2.2 Principle of mechanical action

At the appropriate frequency, the shock wave, jet and radiation pressure caused by ultrasonic waves can make the algae cells elastically vibrate. When a certain sound intensity is reached, the mechanical vibration of the algae cells exceeds its elastic limit, causing the cell wall and pseudo-cavitation to rupture, making the algae cells Loss of partial activity and viability. This is the effect of the ultrasonic mechanical effect.

3 Factors affecting the effect of ultrasonic algae removal

In the ultrasonic algae removal process, the frequency, sound intensity and radiation time (frequency) of the ultrasonic wave will directly affect the algae removal effect.

3.1 Ultrasonic frequency is a key factor in ultrasonic algae removal technology

When the ultrasonic frequency is low, the cavitation bubble has sufficient time to generate the maximum negative pressure without breaking, so that the cavitation bubble will generate a stronger shock wave when it collapses. When the frequency of the ultrasonic wave is high, the pseudo-cavitation does not have enough time for cavitation, and the impact force generated during the collapse is relatively weak. When the frequency of external action is close to the frequency of the algae itself, it will cause resonance and cause cracking of the cavitation bubble.

According to the Rayleigh-Plessent formula:

R is the bubble radius; η is the viscosity coefficient; σ is the surface tension; ρ is the medium density; and P0 is the ambient pressure. It can be estimated that the optimal inhibition condition of microcystis and anabaena common in water is between 20-80 kHz.

3.2 Ultrasonic sound intensity (power) is also an important factor in ultrasonic algae removal

Sound intensity is generally expressed in W/cm?. At 15 kHz, the required sound intensity for cavitation bubbles is 0.16-2.6 W/cm?, and at 500 kHz, the required sound intensity for cavitation bubbles is 100-400 W/cm?. In theory, the increase in sound power at the same frequency can make the cavitation more intense and the algae removal effect is better. However, in practical applications, algae cells have a certain saturation effect on sound waves. At the same time, excessively high sound intensity will not only affect the aquatic organisms, but also generate a large number of cavitation bubbles, which will hinder the spread of sound waves in the water body and affect the range of algae removal. Moreover, too strong cavitation will The algae cells are directly broken, releasing the algal toxins in the cells into the water, which in turn affects the water quality.

Studies have shown that low-intensity ultrasonic radiation does not affect aquatic plants and animals, and it is also an ideal combination of algae removal and algae removal. Therefore, in practical applications, low-intensity ultrasonic radiation is selected.

3.3 Ultrasonic radiation time (frequency) will also affect the performance of ultrasonic algae removal

With the increase of ultrasonic irradiation time, the effect of ultrasonic algae removal will also increase. After reaching a certain dose, the removal rate of algae will not change much. This is because the cavitation intensity generated by the fixed ultrasonic device under certain power and frequency radiation is constant, and the extended ultrasonic treatment time cannot change the ultrasonic cavitation intensity, so it should be reasonably controlled in terms of ultrasonic treatment time.

In addition, different treatment modes have different inhibitory effects on algae. Multi-frequency ultrasonic irradiation of algae can enhance the inhibition of algae. The higher the frequency of treatment, the better the immediate removal of algae, and the multiple removal effect can be increased by 59.09% compared with the single action. Therefore, in the actual treatment, multi-frequency intermittent ultrasonic treatment will be an effective method for controlling algae and inhibiting algae growth.

4 Multi-frequency low frequency low intensity ultrasound actual treatment effect and conclusion

In cooperation with the Institute of Hydrobiology of Jinan University and the Guangdong Reservoir Blue Algae Control Center, a small eutrophic algae cultivation enclosure was established in the Liuxihu Reservoir in Conghua, Guangzhou, and an ultrasonic algae inhibition experiment was carried out.

4.1 Processing effect of different frequency segments

Three wide-band ultrasonic transducers with a frequency bandwidth of 20-50 kHz, 60-100 kHz, and 100-150 kHz are used with an intelligent ultrasonic power source to treat the water body rich in anabaena, 30 minutes a day for 6 days.