The high fertilizer demand for biodiesel production from microalgae is a significant challenge facing the commercialization of this promising technology. We investigated a processing strategy called hydrothermal carbonization (HTC) to convert wet algal biomass into a lipid-rich hydrochar and aqueous phase (AP) co-product. By reacting biomass at 200°C for 15 min, about 50% of the algae biomass became a solid hydrochar and roughly 40–70% of the C, N, and P in the reactant material dissolved into the AP. For the first time, an AP co-product of this nature was analyzed by HPLC, GC-MS and FT-ICR-MS to identify and characterize the dissolved organic matter. Using a unique marine bi-culture suspected to contain a green algae (Nannochloris) and a cyanobacteria (Synechocystis), we demonstrated that this AP co-product can support biomass growth better than a medium containing only inorganic nutrients. To manage unwanted contamination and optimize AP utilization, we employed a two-stage growth process and fed-batch additions of the AP co-product. The effect of media recycling and nutrient supplementation, as well as a production model for a large-scale facility, are discussed. Our work suggests that HTC can play a critical role in making algal biorefineries more sustainable by obviating biomass drying for fuel processing and recycling nutrients. © 2013 American Institute of Chemical Engineers Environ Prog, 32: 962–975, 2013
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