Improving the performance of shipboard rotary drum filters in the removal of cyanobacterial blooms by cationic polyacrylamide flocculation

Abstract

Removing the extensively accumulated cyanobacteria from lake is a straightforward means to tackle cyanobacterial blooms. However, developing removal technology with both high removal efficiency and large treatment capacity remains a great challenge. We developed a shipboard rotary drum filter (RDF) with a 35-μm screen to remove cyanobacterial blooms at a flow rate of 900 m3/h in Lake Dianchi (Kunming, China). Nevertheless, the chlorophyll a (chl.a) concentration of the effluent was approximately 50 μg/L, which is higher than the control target of 20 μg/L. In this study, we aimed to improve the performance of RDF on cyanobacterial removal by the utilization of cationic polyacrylamide (CPAM). High molecular weight CPAM with a 30% cationic degree exhibited excellent flocculation effectiveness, with low flocculant doses, large sized flocs, short flocculation time, and high resistance to shear forces. Both the filtration rate and the cyanobacteria removal efficiency of the 35-μm screen were notably improved by 1 mg/L of CPAM. After being added before the water inlet pump and being agitated by the pump impellers, the CPAM could effectively flocculate cyanobacteria particles at a hydraulic retention time shorter than 20 s. Chl.a concentration and turbidity in the effluents of the shipboard RDF were reduced to lower than 20 μg/L and 12 NTU, respectively. Thus, a shipboard RDF coupled with CPAM flocculation is a promising approach to the removal of cyanobacterial blooms.