Ordinary sewage may soon be used to produce biocrude oil in wastewater treatment plans as researchers at the US Department of Energy’s (DOE) Pacific Northwest National Laboratory (PNNL) study a new approach that simplifies and accelerates the process.
The technology, hydrothermal liquefaction (HTL), uses high temperature and pressure to mimic the geological conditions the Earth uses to create crude oil, achieving in minutes something that takes Mother Nature millions of years.
The resulting material is similar to petroleum pumped out of the ground, with a small amount of water and oxygen mixed in. Conventional refining operations can then be used to refine the resulting biocrude.
Wastewater treatment plants across the US treat approximately 34 billion gallons of sewage every day, which could produce the equivalent of up to approximately 30 million barrels of oil per year. PNNL estimates that a single person could generate two to three gallons of biocrude per year.
Sewage, or more specifically sewage sludge, has long been viewed as a poor ingredient for producing biofuel because it’s too wet.
However, PNNL’s technology eliminates the need for drying required in a majority of current thermal technologies which historically has made wastewater to fuel conversion too energy intensive and expensive. HTL may also be used to make fuel from other types of wet organic feedstock, such as agricultural waste.
Using hydrothermal liquefaction, organic matter such as human waste can be broken down to simpler chemical compounds. The material is pressurized to 3,000 pounds per square inch (psi)—nearly one hundred times that of a car tire. Pressurized sludge then goes into a reactor system operating at about 660 degrees Fahrenheit. The heat and pressure cause the cells of the waste material to break down into different fractions—biocrude and an aqueous liquid phase.
Corinne Drennan, who is responsible for bioenergy technologies research at PNNL, said that there is a lot of a carbon in municipal waste water sludge and there are also plenty of fats. “The fats or lipids appear to facilitate the conversion of other materials in the wastewater such as toilet paper, keep the sludge moving through the reactor, and produce a very high quality biocrude that, when refined, yields fuels such as gasoline, diesel and jet fuels,”Drennan added.
In addition to producing useful fuel, HTL could give local governments significant cost savings by virtually eliminating the need for sewage residuals processing, transport and disposal.
Drennan said the process is simple, describing the reactor as “literally a hot, pressurized tube”.
“We’ve really accelerated hydrothermal conversion technology over the last six years to create a continuous and scalable process which allows the use of wet wastes like sewage sludge,” she said.
An independent assessment for the Water Environment & Reuse Foundation (WE&RF) calls HTL a highly disruptive technology that has potential for treating wastewater solids. WE&RF investigators noted the process has high carbon conversion efficiency with nearly 60% of available carbon in primary sludge becoming biocrude.
The HTL technology has been licensed by PNNL to Utah-based Genifuel Corporation, which is now working with Metro Vancouver, a partnership of 23 local authorities in British Columbia, Canada, to build a demonstration plant.
According to Darrell Mussatto, chair of Metro Vancouver’s Utilities Committee, Metro Vancouver wants to be the first wastewater treatment utility in North America to host the HTL technology in one of its treatment plants.
Mussatto also added that Metro Vancouver will be providing nearly one-half of the pilot project cost, which is estimated to be between C$8 million to C$9 million. The remaining balance will be subject to external funding.
Once funding is in place, Metro Vancouver plans to move to the design phase in 2017, followed by equipment fabrication, with start-up occurring in 2018.
In addition to the biocrude, the liquid phase can be treated with a catalyst to create other fuels and chemical products. A small amount of solid material is also generated, which contains important nutrients. For example, the ability to recover phosphorus, which can replace phosphorus ore used in fertilizer production, has been demonstrated in early efforts.