A comparison of on-site nutrient and energy recycling technologies in algal oil production

•We compare AD and HTL integrated in an algae oil production system.•In a nutrient recycling comparison, AD recycles more N but less P than HTL.•AD provides slightly more useful energy, yielding lower algae oil energy intensity.•AD integration leads to slightly lower algae oil life cycle GHG emissions than HTL. Research on biofuel production pathways from algae continues because among other potential advantages they avoid key consequential effects of terrestrial oil crops, such as competition for cropland. However, the economics, energetic balance, and climate change emissions from algal biofuels pathways do not always show great potential, due in part to high fertilizer demand. Nutrient recycling from algal biomass residue is likely to be essential for reducing the environmental impacts and cost associated with algae-derived fuels. After a review of available technologies, anaerobic digestion (AD) and hydrothermal liquefaction (HTL) were selected and compared on their nutrient recycling and energy recovery potential for lipid-extracted algal biomass using the microalgae strain Scenedesmus dimorphus. For 1kg (dry weight) of algae cultivated in an open raceway pond, 40.7gN and 3.8gP can be recycled through AD, while 26.0gN and 6.8gP can be recycled through HTL. In terms of energy production, 2.49MJ heat and 2.61MJ electricity are generated from AD biogas combustion to meet production system demands, while 3.30MJ heat and 0.95MJ electricity from HTL products are generated and used within the production system. Assuming recycled nutrient products from AD or HTL technologies displace demand for synthetic fertilizers, and energy products displace natural gas and electricity, the life cycle greenhouse gas reduction achieved by adding AD to the simulated algal oil production system is between 622 and 808gcarbon dioxide equivalent (CO2e)/kg biomass depending on substitution assumptions, while the life cycle GHG reduction achieved by HTL is between 513 and 535gCO2e/kg biomass depending on substitution assumptions. Based on the effectiveness of nutrient recycling and energy recovery, as well as technology maturity, AD appears to perform better than HTL as a nutrient and energy recycling technology in algae oil production systems.