(424a) Commercial Pilot for Closed-Loop Wastewater Treatment and Biofuel Production Using Microalgae

In traditional wastewater treatment, significant energy is used to dilute, discharge, or landfill valuable resources including carbon, nitrogen, phosphorus, and freshwater. This situation is untenable due to energy costs and global scarcity of clean water. Development of new technologies requires a systems approach, in which the output from one process becomes the input for the next.

Algae Systems has developed and patented a novel system for treatment of municipal wastewater with algae grown in low-cost floating photobioreactors. In partnership with Daphne Utilities in Daphne, AL, Algae Systems has built and operated a pilot plant in which wastewater and concentrated CO₂ are converted to clean water and liquid fuel. Up to 40,000 gallons of municipal wastewater is initially subjected to solids removal and disinfection. The resulting primary effluent is deployed to photobioreactors floating on Mobile Bay. Use of floating photobioreactors eliminates competition for arable land and allows collocation with existing wastewater sources. Mixing is provided by wave energy, further reducing energy demands. The treated effluent has no detectable nitrogen and phosphorus, >60% reduction in chemical oxygen demand, and no measurable fecal coliform, making the process competitive with traditional secondary and tertiary treatment alternatives. CO₂ is harvested from the air, using low-cost technologies, further enhancing the sustainability of our process.  

Harvested algae slurry is processed via hydrothermal liquefaction (HTL) to produce fuel. Conversion of wet biomass to liquid fuel at high temperature and high pressure via HTL circumvents the need for dried biomass. The resulting fuel, termed biocrude, is useful as a near-direct replacement for crude oil. One of the challenges in using this biocrude is its high concentration of nitrogen and oxygen when compared to petroleum crude. Algae Systems has developed and patented a novel method for reducing nitrogen content of the biocrude while simultaneously boosting carbon content. During the modified HTL, more of the nitrogen is distributed to the aqueous phase so it can be further processed and recovered as valuable products for nutrient recycle.

We will present results from our operations in Daphne, where we have successfully demonstrated the conversion of algae to biocrude via HTL at the pilot scale. We will present data from our pilot facility highlighting the operational parameters that have the most effect on hydrothermal processing of the algae. Additionally, we will show that the Algae Systems process can reduce the nitrogen content of the biocrude to <3 wt %, a >50% reduction in nitrogen compared to previously reported values [1]. Lower nitrogen content in the biocrude reduces the need for downstream processing and is therefore a critical improvement for the feasibility of commercial-scale HTL processes. We will also discuss some of the critical challenges for the scale-up of HTL processing of algae and show the overall balances of carbon and nitrogen in our pilot plant.

(1) PJ Valdez, MC Nelson, HY Wang, XN Lin, PE Savage. Biomass Bioenergy 46 (2012) 317-331.