Cyanobacteria Cell Response to Combined Preoxidation and Mechanical Shear in Drinking Water Treatment

As source waters for many drinking water treatment plants are increasingly subject to intermittent blooms of cyanobacteria, utilities must adapt treatment methods to remove associated cyanotoxins. There is concern that treatment methods applied to remove dissolved extracellular toxins may damage whole cells, leading to the release of intracellular toxins and other secondary metabolites. The objective of this study was to determine whether sodium hypochlorite and potassium permanganate preoxidation treatments cause increased susceptibility of cyanobacterial cells to mechanical shear using a method that can be easily adapted by utilities. Bench-scale simulations examined Microcystis sp., Planktothrix sp., and Dolichospermum sp., which are three types of microcystin-producing cyanobacteria that are prominent in many bloom-impacted waters. We found that chlorine led to increased susceptibility to shear in Dolichospermum and permanganate led to increased susceptibility to shear in Planktothrix, each for shear rates of 750 s(-1) (a typical rate for in-line mixers used in drinking water treatment). We found that shear does not cause immediate critical damage to cells, regardless of preoxidation conditions. However, shear appears to cause a stress response in cyanobacteria, indicating the possibility of delayed toxin release. The results of this study will help water treatment utilities to manage stressors within the treatment process that could lead to release of cellular compounds during cyanobacterial blooms. Additionally, utilities can adapt the applied method of intracellular phycocyanin analysis to independently evaluate real-time cell response to their treatment processes.