Nutrient recycling in large-scale microalgal production: Mass and energy analysis of two recovery strategies by process simulation

Nutrient recycling is of paramount importance for microalgal cultivation aimed at biofuels production. Though different techniques have been assessed at lab-scale level, less information is available on the actual nutrient recovery yields and on the energy balance of a large-scale closed-loop process. In this work two technologies, namely anaerobic digestion (AD) and flash hydrolysis (FH), were investigated by process simulation analysis and compared in terms of both nutrients recovery and energy profitability (EROEI index). FH performs better in terms of nutrients recycling, with up to 70% and 60% of phosphorus and nitrogen recovered in bio-available forms, while incomplete biodegradability limits the nutrients recovery extent in AD. On the other hand, AD requires much lower energy inputs, achieving EROEI values always favorable (2.3-5), while the high thermal requirements of FH, even with heat integration, highlight the need of improving the operating conditions to reduce the energetic burden of this technology (EROEI = 0.8-2.4).