Certain strains of microalgae have high lipid yields and appear to be good choices for the production of biodiesel, however, one of the major difficulties is the lack of a large-scale cost effective harvesting technique. Microbial flocculation has the potential to provide a solution without the use of contaminating metallic ions such as Al3+. However, such flocculation would require the mixing of mega- or even giga-litres of microalgal culture on a daily basis and therefore it is important to estimate the energy required. By incorporating a baffled hydraulic flocculator into a proposed large scale system that was designed on the basis of laboratory data, the mixing energy required for the flocculation is estimated to be equivalent to 0.893 kWh per 10(3) kg of dry mass flocculated, the overall cost of the process is A$ 0.13 m(-3) of the culture medium and the net footprint area of the flocculating system is 0.7% of the proposed 1 km(2) high rate algal pond. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.