Variation of efficiencies and limits of ultrasonication for practical algal bloom control in fields

Algal blooms are an increasing issue in managing water resources for drinking water production and recreational activities in many countries. Among various techniques, ultrasonication is known as a cost-effective method for control of harmful algal blooms (HABs) in relatively large area of water bodies. Most of engineering parameters for operating ultrasonication have been empirically determined based on laboratory scale tests, however, field or pilot tests in real environments are still rare. For field application, duration of ultrasonication is often on a monthly basis which is impractical for stream where there is flow and thus retention time is short. More realistic experimental approaches are required for practical applications of ultrasound. In this study, relatively low frequencies (36-175 kHz) of ultrasonication with low power intensity, less than 650 W, were tested for algal control in various pilot (100-750 L) and field (4 m³) tests in a short duration (<20 min). Generally, rapid decline of sound pressure (Pa) of ultrasonication was observed with distance (80% decrease even with 0.5 m difference). In a pilot test (100 L), the highest algae reduction was achieved at 36 kHz with 0.003 W mL^-1 of power density within 10 min duration, but there was a noticeable increase in microcystin due to damaged algal cells by the low frequency of ultrasound. In a short-term operation without flow, distance from the ultrasound system was an important parameter for effective algae reduction, while longer exposure time ensured sufficient algae reduction. In a circulation pond (4 m³) with flow, 108 kHz-450 W showed the greatest efficiency in algal control and approximately 50-90% algal cells reduction was observed at 36-175 kHz with less than 650 W power and 60 min duration.