Closing the loop on biosolids and algae

With rising costs and increased pressure on landfilling and land application of biosolids, many utilities are evaluating alternative processes for managing such residuals. Compounding these pressures are growing concerns about excess nutrients and emerging contaminants, specifically perfluorinated compounds, i.e., PFAS.

In this article, Dan Levy, Will Lovins and Terry Goss, detail an emerging and sustainable solution that offers exciting opportunities for beneficial transformation of wastewater biosolids and Harmful Algal Blooms (HABs) waste streams as a feedstock to produce renewable biofuel, with the added possibility for PFAS destruction.

To comprehend the solution, first we need to examine the component parts:

• Hydrothermal Liquefaction (HTL), which uses high-pressure (200 bar) and high-temperature (350°C) processes to produce a biocrude product from sludge, is a potentially game-changing technology. Added benefits are gained by further treating the residual liquid to generate renewable natural gas with minimal waste, nutrient capture, and both solids and chemical oxygen demand (COD) reduction that exceed levels achieved in conventional digestion processes. The HTL process is similar to the way fossil fuels are formed, but in minutes as opposed to millions of years and without adding new carbon dioxide to the atmosphere.
• Harmful algal blooms (HABs) are formed by an overabundance of nutrients in water and can produce toxins that threaten human health, aquatic ecosystems, and drinking water, making them one of the most scientifically complex and economically difficult environmental issues facing the world today. They cost an estimated $1 billion in economic damages and losses each year in the United States alone. With continued nutrient enrichment and the effects of climate change, HABs are now occurring more frequently, becoming more toxic and lasting longer.

Algae Harvesting Hydronucleation Flotation Technology (HFT)

At AECOM, we are advancing our patent pending HFT to address the growing HAB crisis. We developed the HFT to physically remove the overabundance of nutrients in our nation’s impacted waterways by harvesting algae. While extremely efficient at consuming nutrients in the water (specifically nutrient and phosphorus), algae is also extremely efficient in sequestrating CO₂ from the atmosphere. By physically removing the algae, the threat of HABs can be reduced and potentially eliminated, and the recovered algae biomass, which is rich in carbon, can be used to as a feedstock for the creation of biocrude via HTL.

The versatility and scalability of HFT for real world application has been proven effective in numerous demonstration tests conducted in Florida, Ohio and New York, including the award winning and federally funded USACE Engineer Research & Development Center (ERDC) Harmful Algal Bloom, Interception, Treatment and Transformation System (HABITATS) research program.

Our HFT provides a truly innovative and sustainable closed loop system that can restore our nation’s impacted waterways while providing a renewable green energy source for our future.

“Like biosolids, algae can be used as a feedstock in HTL for biocrude production.”

Two issues, one sustainable stone

Given that biosolids created during wastewater treatment and harvested algae both provide suitable feedstocks for HTL, locating these facilities at a wastewater treatment plant seems an obvious, economical and sustainable solution.

To put this to the test we set up a mobile pilot at the Altamonte Springs Water Reclamation Facility near Orlando, Florida. The pilot was supported by our strategic partner, Genifuel, using their state-of-the-art HTL technology.

Important findings from the HTL pilot

• The use of HTL technology to convert wastewater and wild HAB biosolids into biofuel is technically feasible.
• Wastewater treatment plants can enhance algae-to-biofuel return on investment with full-time plant operation and seasonal co-processing of harvested HABs.
• Literature shows promising use of HTL to reduce PFAS in contaminated biosolid waste streams. With wastewater treatment plants serving as a collection point for PFAS, and as governmental regulatory agencies start introducing rules on PFAS in biosolids, HTL may play an integral role in removing these persistent chemicals from biosolids.

Environmental and societal benefits of combining algae harvesting HFT with HTL

• The process mitigates the nutrient impacts to HAB-impaired water bodies.
• HAB harvesting reduces greenhouse gasses and improves oxygen levels in the water.
• Producing energy with existing waste streams creates a sustainable circular economy.

AECOM is committed to scaling the integration of these key technologies through its continued collaboration with research, industry, OEMs, and regulatory and community partners.