Using chitosan-modified clays to control black-bloom-induced black suspended matter in Taihu Lake: Deposition and resuspension of black matter/clay flocs

•We analyzed the difference of the black matter/clay flocs formation between jar-test and flow speed simulated experiments.•We confirmed the relationship between shear stress in Taihu Lake and the resuspension of the black matter/clay flocs.•We demonstrate that spray quartz sand can inhibit the resuspension of the black matter/clay flocs in a certain extent.

A cyanobacteria-induced black bloom in Taihu Lake, 2007, subjected nearly one million Wuxi City residents to a drinking water crisis. This black bloom attracted wide attention in China and the rest of the world. However, black bloom is a highly weather-dependent event, and its rapid movement in lakes makes it difficult to predict where it will occur. Therefore, jar-tests and simulated flow experiments were performed to investigate the flocculation, deposition, and resuspension of black-bloom-induced black matter. At a dosage of 0.2 g L−1 chitosan + 1 g L−1 diatomite, 90% of the turbidity was removed within 1 h in jar-tests and in low flow speed simulated experiments. However, the black matter/clay flocs did not fully sink, and stratification of turbidity apparently occurred in the lower part of the tank during the simulated flow experiments. The resuspension experiments under simulated flow speeds showed that at a wind speed of 1–6 m s−1 in Taihu Lake produced currents that did not greatly affect floc resuspension, but a wind speed of 3–4 m s−1 produced waves that could induce floc resuspension. Quartz sand was sprayed over the flocs to verify its effect on inhibiting resuspension. A shear stress of 0.37 N m−2 did not induce floc resuspension, which indicated that a wind speed of 6 m s−1 would not induce floc resuspension in Taihu Lake.