The effectiveness of ultrasonic irradiation on coagulation and inactivity of algal cells was evaluated in the laboratory. Experiments were carried out under different conditions with or without ultrasound using polyaluminium hydroxychloride (PACl) as coagulant and Microcystis as test species. The results suggested that ultrasonic irradiation increased cells’ inactivity and obviously improved the coagulation process of algal cells. An increased rate of coagulation and reduced dosage of PACl required to cause efficient flocculation for algae removal were observed in coagulation experiments with ultrasound irradiation. At the same time, compared with samples without irradiation, the characteristics of flocs were improved and the settling rate was increased greatly. A very short period of ultrasonic pretreatment with 40 W input power was enough to achieve substantial removal efficiency under the condition that there was no breakage in algal cells. The changes of growth curves for algal samples during the seven days’ culturing suggested that ultrasound was effective for the inactivation of the test algae. The possible mechanisms for the enhanced effects on the coagulation and inactivity of Microcystis were that ultrasonic irradiation changed internal and external characteristics of algal cells, which co-promoted their immediate and accelerated coagulation and settlement.

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