Removal of Anabaena by ultrasonic pretreatment enhancing-coagulation and water treatment processes

Abstract

Cyanobacteria bloom brings great challenges to the safety of the drinking water process. Ultrasonic pretreatment can effectively enhance coagulation to remove algae cells from algae-laden water. However, due to the special chain-like structure of Anabaena, the removal characteristics of Anabaena in water treatment process are quite different from those of unicellular cyanobacteria. No study has yet systematic studied on the enhancement effect of ultrasonic enhanced water treatment process on Anabaena. Therefore, this paper studied the characteristics and mechanism of ultrasonic pretreatment to enhance the removal of Anabaena, and carried out pilot experiments. The results showed that ultrasound irradiation at 740 kHz and 1.12 MHz had a great effect on the destruction of the chain-like structure and cell structure of Anabaena. After 15 min, the removal rate of Anabaena cells reached 16.0% and 30.4%, respectively, and the release of intracellular organic matter (IOM) reached 4.3 and 6.0 mg/L, in which phycocyanin released more. The damage of dynamic ultrasound irradiation on algae cells is weak, which can avoid the influence of uneven ultrasound irradiation on algae cells. The enhanced coagulation effect was better after 740 kHz ultrasonic pretreatment for 1 min, and the removal rates of algal cells, turbidity and dissolved organic carbon (DOC) increased by 7%, 9.4% and 12.6%, respectively. Pilot scale experiments demonstrate that ultrasonic pretreatment leads to a notable enhancement in the removal efficiency during the coagulation and sedimentation stages. After 4 h of operation, the turbidity and algal cell removal in the sedimentation stage increased by 6.1% and 14.0%, respectively. In addition, the removal of DOC in the final treated water increased by 6.7%. Furthermore, the disinfection by-products (DBPs) concentration experiences a reduction exceeding 200 μg per liter. Together, these results show a substantial improvement in filtration efficiency and effluent quality. This study provides guidance for the ongoing study of ultrasonic pretreatment to remove chain-like cyanobacteria.