Nutrients recycling and energy evaluation in a closed microalgal biofuel production system

A closed process for microalgal biofuel production involving in lipid extraction, anaerobic digestion, and microalgal cultivation was proposed. Nutrients recycling and energy in the system were evaluated. During the anaerobic digestion process, 69.37% of nitrogen and 60.22% of phosphorus resided in the lipid-extracted microalgae (LEM) were released into the digested liquid (DL). Microalgae (Chlorella pyrenoidosa) could grow normally in mixed 10% DL and Selenite Enrichment (SE) medium (without nitrogen and phosphorus additives). The maximum biomass concentration was 1.25 g/L (dry biomass), which was higher than that obtained with the standard SE medium (1.18 g/L dry biomass). This indicates that the abundant nutrients in the DL can be recycled for more sustainable microalgae growth. Economic analysis evaluated that recycling of nutrients in the DL could potentially reduce nutrient cost by 41.77%. Additionally, 28.38% more energy was recovered as methane in LEM during the anaerobic digestion process. This would compensate for the cost of microalgae biofuel production.