Hazardous cyanobacteria integrity response to velocity gradient and powdered activated carbon in water treatment plants

Highlights: • D. circinale total metabolite concentration should be used to estimate PAC dosage. • The reservoir's dominant species should guide the operations in WTP. • D. circinale and R. raciborskii were the most and least sensitive cells in the experiment, respectively. Although some studies have investigated the impact caused by chemicals used on water treatment (coagulants and oxidants) on cyanobacteria integrity, the isolated effect of shear stress during coagulation is still not fully understood. This study evaluated the impact of different velocity gradients, mixing times, and the addition of powdered activated carbon (PAC) on the integrity of Microcystis aeruginosa, Raphidiopsis raciborskii, and Dolichospermum circinale, known producers of toxin and taste and odor (T&O) compounds. No association was found between R. raciborskii cell lysis and velocity gradient, with or without PAC, demonstrating the high resilience of this taxon to shear stress. In contrast, an association was found for M. aeruginosa at the highest velocity gradient evaluated (1000 s⁻¹) and for D. circinale above the lowest velocity gradient studied (600 s⁻¹). After PAC addition, there was a reduction in the chances of finding M. aeruginosa intact cells above velocity gradient 800 s⁻¹ at 45 s, while D. circinale show cell lysis in all the scenarios expect at 600 s⁻¹ and 10 s of agitation. The additional impact of PAC on cell lysis may lead to more release of metabolites and shows the need to adjust the hydraulic conditions in the rapid mixing stage, especially when more "fragile" cyanobacteria are present. Neither cyanobacterial cell size nor morphology was shown to be relevant to shear stress sensitivity, indicating that cell wall composition might have been an important factor in controlling cell lysis.