This study developed a magnetic nanoparticles (MNPs) harvesting and reactivation technique for rapid cyanobacteria Microcystis aeruginosa separation. The harvesting of raw MNPs achieved high efficiency of 99.6% with the MNPs dosage of 0.58 g MNPs/g dry-biomass, but gradually decreased to 59.1% when directly reused 5 times. With extra ultrasonic chloroform: methanol solvent treatment, the MNPs can be effectively reactivated for M. aeruginosa harvesting with 60% efficiency after 5 times reactivation and the separation efficiency kept above 93% with 0.20 g MNPs/g dry-biomass dosage. The cyanobacteria-MNPs complex can be effectively disrupted by ultrasonic chloroform: methanol solvent treatment and the zeta potential was recovered for MNPs electrostatic attraction. The MNPs adsorption followed the Langmuir isotherm, and the maximum adsorption capacity and Langmuir constant was 3.74 g dry-biomass/g and 311.64 L/g respectively. This MNPs reactivation technique can achieve low energy separation and reduce MNPs consumption by 67%, providing potential engineering implementation for cyanobacterial biomass harvesting. (C) 2015 Elsevier Ltd. All rights reserved.