As part of a three-year project led by the US Army Corps of Engineers Research and Development Center (ERDC) to mitigate growth of toxic algae blooms, Pacific Northwest National Laboratory (PNNL) is investigating these blooms as a viable source of biocrude fuel and fertilizer.

One year into the ERDC project, the team demonstrated a three-step approach that “cleans” the water, removes the algae and entrained nutrients such as nitrogen and phosphorus from the water, and transforms the algae into a potential energy source.

The project is called Harmful Algal Bloom Interception, Treatment and Transformation System (HABITATS). The goal is to remove blue-green algae from large bodies of water while simultaneously recovering energy and nutrients such as nitrogen and phosphorus—an approach that mitigates near-term impacts from harmful algal blooms and helps reduce future blooms over time.

To achieve this goal, the HABITATS team has deployed a three-stage process: interception, treatment, and transformation.

• The first stage—interception—involves collecting algae from the surface of a natural body of water using a skimming device and boom that guide the algae to a collection point.

• In the treatment stage, the algae are separated from the water using a technology called dissolved air floatation (DAF), which was developed by project partner AECOM. The newly cleaned water is then returned to the environment, while the algae are further concentrated to maximize the potential for energy and nutrient recovery.

• The final stage involves transforming the concentrated algae biomass into biocrude using a PNNL-developed process called hydrothermal liquefaction, or HTL. HTL uses heat and pressure—250–380 °C and 5–28 MPa—to convert a variety of feedstocks to biocrude.

Algae biorefinery concept through HTL. Figure from Tian et al. (2017) in the book Algal Biofuels.

In 2019, the ERDC conducted a baseline pilot study of the three stages of HABITATS. The location—Lake Okeechobee, Florida—is a shallow lake covering roughly 740 square miles where massive blooms form annually and have increased in severity during the past decade.

In the pilot, the research team discovered that the DAF consistently removed around 95 percent of the algae from the water and greatly concentrated the algae, reducing phosphorus, nitrogen, and organic carbon concentrations by greater than 95, 65, and 50 percent, respectively. In other words, with removal of the algae and associated nutrients such as phosphorus, the water was much cleaner when returned to the lake.

The resulting algae paste was then transported to PNNL for processing using HTL to determine if biocrude could be produced. HTL mimics the geological conditions the Earth uses to create crude oil, using high pressure and temperature to achieve in minutes what has typically taken millions of years.

There were some less-than-favorable properties with the algae concentrated from Lake Okeechobee, like higher-than-normal viscosity and high ash content resulting from the flocculants used in the DAF process. But despite those challenges, we were able to generate HTL biocrude from nuisance algae harvested under realistic field conditions. It was a great proof of concept.

—chemical engineer Justin Billing, who led the PNNL portion of the study

In the second year of HABITATS, the team made adjustments to the algae concentration process as identified during the pilot and will perform further testing to refine the process. PNNL will apply its expertise in catalytic upgrading to make diesel-range renewable fuel blendstocks from the HTL biocrude.

This research was conducted for the Aquatic Nuisance Species Research Program, sponsored by the US Army Corps of Engineers.