Microalgal harvesting for biofuels – Options and associated operational costs

Abstract

Microalgae are a source of a range of natural products, including biofuels, but financial and technical limitations can make large-scale production challenging to commercialize. A significant limitation is the practical and economical bulk harvesting of microalgae from the cultivation medium. Based on chemical and energy demands, common harvesting processes can have high costs due to microalgae having a similar specific gravity as their medium, negative surface charges and significant variability between strains.Determining the cost-effectiveness of harvesting methods is complicated as many publications commonly quote harvesting as costing 20–30 % of the operational cost, regardless of the method of biomass production (e.g., open ponds, bioreactors, etc.) or the harvesting method selected. This non-specific statement does not reflect that specific harvesting methods will have different operational costs. This paper aims to provide a more accurate, method-specific cost analysis of the harvesting stage operating costs to help in the economic analysis of microalgae production systems.The four primary harvesting mechanisms currently used, centrifugation, flocculation, flotation, and filtration, are assessed for their energy and chemical consumption. Centrifugation, flocculation, flotation, and filtration had an average biomass recovery of 92 %, 95 %, 92 % and 91 % and an energy requirement range of 1–8 kWh m−3, 0.1–6.7 kWh m−3, 0.015–7.6 kWh m−3 and 0.1–5.9 kWh m−3, respectively. Furthermore, the operational costs, based on energy requirements and the use of flocculants, were 0.448–1.086 USD m−3, 0.014–16.65 USD m−3, 0.08–31.3 USD m−3 and 0.013–0.791 USD m−3, respectively. Finally, the biomass production costs were calculated as ranging between 0.02–0.177 USD kg−1, 0.006–35.05 USD kg−1, 0.169–26.95 USD kg−1 and 0.02–3.00 USD kg−1 for centrifugation, flocculation, flotation, and filtration microalgae harvesting technologies, respectively.