Abstract: Ultrasonic was adopted for inactivating Giardia in water. The effect of organic substance, turbidity and inorganic ions on ultrasonic inactivation was investigated. The mechanism of ultrasonic inactivation of Giardia was studied by scanning electron microscopy. Results indicated that the inactivation rate decreased with the increase of humic acid concentration, when the concentration of humic acid was 50.0 mg/L, the inactivation rate reduced to 80.6%. In the range of 0-20 NTU turbidity, the inactivation rate first increased and then decreased, when the turbidity was 1.0 NTU, the inactivation rate reached maximum (98.6%). The effect of Ca²⁺, Zn²⁺ and SO^2-₄ content on the inactivation was little, and low concentration of Ca²⁺(1.0 mg/L) could increase the inactivation rate to 96.3%. The increase of HCO^-₃ concentration could reduce the inactivation rate, when the concentration was 100 mg/L the inactivation rate was 83.8%. It was obvious that ultrasonic damaged the cell structure of Giardia by scanning electron microscopy morphological observation, which suggested that the mechanical destruction due to the cavitation was the major factor of inactivating Giardia by ultrasonic.

Key words: ultrasonic inactivation, Giardia, inactivation rate, scanning electron microscopy