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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. [ABSTRACT FROM AUTHOR]

• 1,3,5-trimethoxybenzene can quench samples containing N-chlorinated MCLR product. • pH and temperature affect MCLR degradation efficiency during chlorination. • Free chlorine consumption correlated well with dissolved organic matter properties. • A model is built to predict MCLR decay in natural waters during chlorination. Microcystin-LR (MCLR) produced during certain cyanobacteria blooms can contaminate drinking water sources and pose a threat to public health. Previous studies of MCLR degradation by free chlorine may have artifacts from using strong reducing agents to quench chlorination reactions, and they also have not explored the influence of water quality characteristics such as pH, alkalinity, temperature and dissolved organic matter (DOM). Using a novel quencher, 1,3,5-trimethoxybenzene (TMB), the apparent MCLR degradation rate constants were found to be higher than those obtained with thiosulfate (S 2 O 3 2−), a traditionally used strong reducing quencher. Thiosulfate converted N-chlorinated MCLR degradation products back to the parent MCLR, thereby underestimating MCLR loss over time. The second-order rate constants for HOCl (k HOCl) and OCl− (k OCl-) during chlorination of MCLR were determined to be 72 ± 13 and 28 ± 1.8 M−1s−1, respectively, allowing for determination of the apparent MCLR rate constants (k app,MCLR) for any known pH condition. The MCLR reaction with free chlorine was strongly affected by temperature and the presence of DOM, while changes in ionic strength and alkalinity had little effect. Free chlorine in the presence of DOM, originating from both terrestrial and microbial sources, exhibited two-stage decay. The initial chlorine demand in the first 15 s of reaction can be determined by the dissolved organic carbon (DOC) concentration (initial chlorine demand = 1.8 × DOC), and the second-order rate constants for the later slower decay correlated well with SUVA 254 (k app,DOM = 0.73 × SUVA 254 - 0.41). The results yielded a practical model to predict the decay of MCLR during chlorination of waters with varied water quality characteristics. Image, graphical abstract [ABSTRACT FROM AUTHOR]