Controlling Cyanobacterial Blooms by Ultrasonic Irradiation – Lessons Learned from a Real-World Application in a Hypereutrophic Reservoir

Abstract

Excessive nutrient loading and climatic changes have increased the frequency and intensity of harmful algal blooms (HABs) in freshwater systems globally. These blooms are responsible for the impairment of many water bodies. The use of ultrasonic treatment has been proposed as a novel environmentally friendly technology to control HABs. While results from laboratory-based experiments have shown promise, there has been little work conducted towards assessing the efficacy of the technology at the field-scale. In this study, we quantify and assess the efficiency of one of the largest field-deployed systems of ultrasonic transmitters (10 ultrasound emitting floating buoys) deployed in a large (220 Million m3) hypereutrophic semi-arid reservoir. The collected ultrasonic recordings indicated that the transmitters were continuously emitting ultrasonic waves with dynamic frequencies, ranging between 27 and 47 kHz. An analysis of the propagation of waves in the water suggests that the ultrasonic influence of the system was largely confined to a radial distance of 30 m and to a depth of 5 m. Water quality data collected from the reservoir showed no statistically significant improvements in terms of chlorophyll-a, secchi disk depth, turbidity, and Microcystin-LR (MC-LR) levels in the treated areas as compared to a control site on the same lake. Moreover, water samples collected in close proximity to the ultrasonic buoys showed that algal concentrations tended to have a uniform distribution over the first 2 m below the water surface. Meanwhile, samples collected at the control site showed a typical drop in concentration with depth as a result of light limitation. Treated regions also tended to have a higher ratio of (MC-LR) toxin to chlorophyll-a in comparison to the control site. This suggests that the ultrasonic buoys impacted the buoyancy of the cyanobacteria and may have increased their environmental stress, leading to higher toxins levels. Our results provide a cautionary note to many water establishments that are currently planning to deploy commercially available ultrasonic buoys on their HAB-affected freshwater systems.