Cyanobacteria harmful algal blooms (cyanoHABs) are of critical concern to drinking water treatment. Cyanobacteria, Microcystis sp. and Anabaena sp., are photosynthetic prokaryotic organisms that are dominant species in cyanoHABs and freshwater sources, including drinking water treatment reservoirs in Ohio. Due to increasing temperatures, nutrient-loading, and carbon dioxide emissions, the frequency and occurrence of cyanoHABs are more prevalent in water sources worldwide. CyanoHABs are nuisance organisms as the toxins produced negatively impact human health and recreational and economic resources. These toxins are also challenging to degrade in typical drinking water treatment. Commonly used algaecides such as copper sulfate can cause the release of intracellular toxins from cyanoHABs. In addition to the release of toxins, the chemicals can cause harm to non-target organisms that are beneficial to a healthy water ecosystem.
Hydrogen peroxide (H2O2) is proposed as an environmentally friendly algaecide because of its rapid decomposition into water and oxygen. In addition, hydrogen peroxide selectively suppresses cyanobacteria. Non-target algal species, such as green algae, have antioxidant capacities that increase their defense against hydrogen peroxide. Ultrasound has also been proposed as an alternative to chemical algaecides for cyanoHAB reductions. Manufacturers of these ultrasound units promote their capability to reduce cyanoHABs by diminishing their buoyancy in the water column and limiting their ability to participate in photosynthesis. Varying sound pressures from ultrasound units have been reported in lab and field applications. We propose that applying hydrogen peroxide during ultrasound treatment will reduce the amount of chemical algaecide needed for the initial interaction for treatment or provide a synergistic effect on ultrasound treatment.
Commercially available ultrasound units were purchased for this project. An anechoic chamber was built using soundproof tiles to absorb sound waves, mimicking drinking water reservoir conditions in the field. The ultrasound unit deployed in the tank cycles through frequency bandwidths of 24 kHz - 55 kHz, 100 kHz – 127 kHz, 145 kHz – 176 kHz, and 195 kHz – 205 kHz. The sound pressures were measured using a hydrophone at sample exposure points in the anechoic tank.
This study explored the instantaneous effects of low concentrations of hydrogen peroxide and ultrasound on the reduction of cyanobacteria over 48 hours. Initial concentrations of 106 cells/mL of cyanobacteria, Microcystis sp. and Anabaena sp., were treated with 2 and 4 mg/L H2O2 concentrations with and without exposure to ultrasound. Cell concentrations were diluted individually in bags and the bags were deployed in an anechoic tank in the laboratory. The experiment was evaluated at 0, 2, 4, 8, 24, and 48 hours. The algal reduction will be measured through cell enumerations and in vitro chlorophyll-a and pheophytin-a analysis by fluorescence. Cell enumerations will evaluate changes in particle concentration, chlorophyll-a will evaluate changes in algal biomass, and pheophytin-a will evaluate changes in chlorophyll-a degradation.
The results of our research indicate that ultrasound combined with the use of hydrogen peroxide does not provide an enhanced effect for cyanoHAB suppression. There were no significant changes in chlorophyll-a, pheophytin-a, or cell concentrations from hydrogen peroxide or ultrasound treatments. The peak sound pressure observed was 4.27 kPa around the sample holder, below the threshold for the physical effects of ultrasound to occur. In addition, experiments were conducted using high algal densities during the exponential growth phase, potentially causing excessive chlorophyll-a growth during treatments. Lastly, experiments were conducted using non-toxic cyanobacteria strains which degrade hydrogen peroxide more rapidly than toxin-producing cyanobacteria, making them less sensitive to oxidative stresses. The research will support water treatment utilities in an effort to provide the best management practices for cyanoHAB suppression.