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Microalgae are considered to be a promising alternative feedstock for next generation biofuels because of their rapid photosynthetic growth rates and less impact on land-use for food production compared with grain and other lignocellulosic biomass. In this study, a fast-growing, low-lipid, high-protein microalga species, Chlorella pyrenoidosa, was converted via hydrothermal liquefaction (HTL) into four products: bio-crude oil, aqueous product, gaseous product, and solid residue. The effects of operating conditions (reaction temperature and retention time) on the distributions of carbon and nitrogen in HTL products were quantified. Carbon recovery (CR), nitrogen recovery (NR) and energy recovery in the bio-crude oil fraction generally increased with the increase of reaction temperature as well as the retention time. The highest energy recovery of bio-crude oil was 65.4%, obtained at 280 C with 120 min retention time. Both carbon and nitrogen tended to preferentially accumulate in the HTL bio-crude oil products as temperature and retention time increased, but the opposite was true for the solid residual product. The NR values of HTL aqueous product also increased with reaction temperature and retention time. 65-70% of nitrogen and 35-40% of carbon in the original material were converted into water soluble compounds when reaction temperature was higher than 220 C and retention time was longer than 10 min. The CR of gas was less than 10% and is primarily present in the form of carbon dioxide. This study also introduces a novel treatment process (Environment-Enhancing Energy) that integrates algal growth for wastewater treatment with HTL of algal biomass, which provides synergistic recycling of carbon dioxide from the HTL gaseous product and the nutrients from HTL aqueous product to support multiple stages of algae production.

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This study introduces and analyzes a novel system for algal biofuel production that synergistically integrates algal wastewater treatment with hydrothermal liquefaction (HTL) of wastewater biosolids and algae into bio-crude oil. This system maximizes the biofuel potential of wastewater inputs by internally capturing and recycling carbon and nutrientsa powerful concept referred to as multi-cycle nutrient reuse, which amplifies waste nutrients into multiple cycles of algal biomass and bioenergy production. We call this system "Environment-Enhancing Energy" (E 2 -Energy) because it can simultaneously improve conventional wastewater treatment by nutrient removal and production of a large amount of biofuel co-products.Moreover, E 2 -Energy resolves several key bottlenecks commonly associated with large-scale algal biofuel production including: contamination of target high-oil algal cultures, high nutrient costs/usage, unsustainable fresh water usage, and large energy inputs for dewatering/extraction. A series of algal cultivation and HTL experiments were conducted to confirm the primary steps and performance characteristics of the E 2 -Energy system. These experiments showed: (1) low-oil, mixed algal-bacterial biomass can be successfully cultured with the recycled HTL aqueous product; (2) both organics and nutrients are removed from wastewater during algal-bacterial biomass production (63-95% reduction);(3) this low-oil, algal-bacterial biomass can be converted into bio-crude oil via HTL with a high yield ($50%) and a net positive energy balance; and (4) the HTL step re-releases nutrients to an aqueous phase product that can be recycled back to step (1). This repeating loop of steps 1-4 facilitates multi-cycle reuse of nutrients and thus provides biomass amplification. ‡ A mathematical model was also developed using STELLAâ to simulate mass balances for long-term E 2 -Energy operations with internal recycling of nutrients and carbon. The model results showed that E 2 -Energy can amplify the biomass and biofuel production from wastewater by up to 10 times, which gives it the potential to replace all US petroleum imports using only current wastewater feedstocks and carbon dioxide from the atmosphere or point sources. Thus, E 2 -Energy represents a major paradigm shift-where wastewater treatment systems become optimized biofuel producers with enhanced effluent quality, which provides a viable and advantageous pathway to sustainable, carbon-neutral energy independence.