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WSU, PNNL researchers convert algal biofuel waste from hydrothermal liquefaction into commodity using anaerobes; sewage sludge next

Researchers at Washington State University and Pacific Northwest National Laboratory have devised a method of converting a waste product generated by the conversion of algae into bio-crude into a usable and valuable commodity. The results of the team’s research are published in the journal Bioresource Technology.

Converting algae to biofuels can utilize a two-step process. The first, developed by PNNL, applies high pressure and high temperature to algae to create bio-oil. The second converts that bio-oil into biofuel, which can replace gasoline, diesel and jet fuel. The first step—hydrothermal liquefaction (HTL)—produces waste; approximately 25 to 40 percent of carbon and 80 percent of nutrients from the algae are left behind in wastewater streams.

The wastewater is generally hard to process because it contains a variety of different chemicals in small concentrations, said Birgitte K. Ahring, professor at WSU Tri-Cities’ Bioproducts, Sciences and Engineering Laboratory. But Ahring and her team have found that adapting anaerobic microbes to break down the remaining residue is a viable option. Through this process, the material becomes degradable and gets transformed into a bionatural gas without the use of harsh chemicals. The solid material that remains can also be applied as a fertilizer or recycled back into the hydrothermal liquefaction process for further use.

In this study, semi-continuous anaerobic digestion is used to degrade the organic fraction of wastewater streams from HTL of the algae Tetraselmis (AgTet) and Chlorella (AgChlr). Results indicated high methane yields at 20–30% (v/v) HTL wastewater together with clarified manure, producing 327.2mL/gVSin (or volatile solids in feed) for AgTet and 263.4mL/gVSin for AgChlr. There was a significant reduction in methane production at concentrations higher than 40% (v/v) HTL wastewater in the feed, possibly due to the accumulation of chloride salts or inhibitory compounds such as pyridines, piperidines and pyrrolidines. This was further confirmed by comparing COD, salt and the ammonia concentrations of the effluents after anaerobic digestion at different concentrations of wastewater in manure.

—Fernandez et al.

The ability to convert a waste product into a usable commodity provides algal biorefineries with a solution to a large problem, Ahring said.

After removing the solids, about 10 percent of the output is bio oil, with the remaining 90 percent being a waste byproduct. The fact that we’ve developed an alternative method to recycle or treat the leftover material means it’s more economical to produce the bio oil, making the potential for commercial use of the process more likely.

—co-author Andrew Schmidt, of PNNL’s chemical and biological processes development group

Ahring said the team’s results were so promising that they are now partnering with PNNL on its conversion of sewage sludge to fuel using a similar strategy for the wastewater.


  • Sebastian Fernandez, Keerthi Srinivas, Andrew J. Schmidt, Marie S. Swita, Birgitte K. Ahring (2017) “Anaerobic digestion of organic fraction from hydrothermal liquefied algae wastewater byproduct,” Bioresource Technology, Volume 247, Pages 250-258 doi: 10.1016/j.biortech.2017.09.030



Digesters can make fuel, it is a matter of applying what works.

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