Evaluation of low-frequency ultrasound as a safe and sustainable method for the removal and inactivation of cyanobacteria in water

Cyanobacteria are prokaryotic and photosynthetic organisms contained in various aquatic ecosystems. Multiple studies confirm the risks caused by their presence in aquatic bodies due to their ability to generate cyanotoxins. Similarly, it has been shown that these bloom-forming species can regulate their flotation through gas vacuoles, which represents a competitive advantage over other phytoplankton species, favoring their proliferation. The application of ultrasound (US) can break these gas vacuoles and facilitates the sedimentation of cyanobacterial cells, interrupting their vital functions, such as photosynthesis and nutrient absorption; however, it can also release cyanotoxins contained within cyanobacteria. In this work, low-frequency US was used to assess its potential in inactivating and removing cyanobacteria and cyanotoxins from a Colombian reservoir. The efficiency of the sonication process, using different exposure powers (10, 30, and 50 W) and times (5, 10, 20, and 30 min) at a frequency of 40 kHz was evaluated through microscopy, cytometry, and sedimentation tests, as well as by the analysis of cyanotoxins in water. The sonication process was optimized by using the response surface methodology through a full-factorial design of experiments. It was found that power and exposure time had a significant and directly proportional effect on the sedimentation of cyanobacterial cells after the sonication process (p-values equal to 0.0019 and 0.0050, respectively). A removal efficiency percentage of up to 74.9% was reached, while cell sedimentation was achieved in 5 min and its effect was significantly sustained for the following 72 h under optimal operating conditions (50 W of power and a contact time of 30 min). Nevertheless, under these conditions, cyanotoxins were not efficiently removed, since no significant degradation rates (< 10%) were observed for microcystins (MC) in the surface water matrix. In this regard, for scaling up purposes, even though US has been reported to be effective for cyanobacterial and cyanotoxin control, further and careful research must be conducted to avoid reaching erroneous conclusions that would limit the actual application of the US process in natural water matrices.