Microalgae have been considered as a promising feedstock for biofuels and greenhouse gas reduction. A low-cost harvesting technology without secondary contamination for down-stream extraction is a key requirement to make algal biofuel commercially viable. A novel harvesting method using ammonia as a flocculant to make the algal biomass settable was devised and studied. Another major advantage of this approach is that the ammonia added will be reused as fertilizer in the subsequent cultures. The results indicated that ammonia-induced flocculation led to more than 99% removal of algae at 12 h. The OD600 of algae growing in the ammonia-enriched flocculation medium treated with heating and CO2 was 2 times than that of initial after 6 days. These results suggested that this flocculation method was efficient, convenient and allowed the reuse of the flocculated medium, therefore providing an option for economic harvesting and cultivation of microalgae. (C) 2012 Elsevier Ltd. All rights reserved.