How Wastewater Treatment Plants Could Be Pumping Out Fuel Instead of Sludge
Research captured on cover of Water Environment Research
Next time you flush, consider this: Today's sewage is potentially tomorrow's fuel. Sludge from wastewater treatment plants could be transformed into a type of crude oil using a technique being developed at PNNL.
With the technique, called hydrothermal liquefaction, researchers can perform in about 30 minutes the same processes that naturally form fossil fuels over millions of years. The result is a biocrude that can be further refined into blendstock for conventional engine fuel. This untapped source of energy in the U.S. could produce the equivalent of 41 million barrels of oil a year—almost as much output as the entire state of Louisiana.
Sludge consists of the solids that get settled out of sewage—every year, the U.S. produces about 14 million tons of those solids on a dry basis. Wastewater treatment plants must dispose of the material somehow—typically by anaerobic digestion, incineration, sending it to landfills, or providing it to farms as fertilizer. All of these disposal methods pose significant costs and harbor potential ecological downsides.
Hydrothermal liquefaction, or HTL, converts the sludge to biocrude using heat and pressure—about 660 degrees Fahrenheit and 3,000 pounds per square inch. The process breaks down the waste, producing both a black, viscous biocrude and a watery liquid. The liquid can provide a second potential fuel stream: methane. That gas, along with some carbon dioxide, can be produced from the liquid using a process called catalytic hydrothermal gasification.
The puree-like consistency of the sludge makes it a good candidate for the HTL process, which can also be performed on algae, wood and other biomass.
"When the feedstock comes in wet, HTL has a huge advantage," says PNNL researcher Justin Billing. Other processes that might be used to treat solids, such as pyrolysis or gasification, require the material to be dried first, making those methods more energy intensive.
From the Lab to the Field
PNNL has been developing HTL for the past four decades, using various types of raw material. Yet even as the technology improved, finding a cost-effective, feasible feedstock remained challenging. By optimizing the process to work with sludge, PNNL is addressing that problem.
The economics bear out, Billing says, not only because sewage is plentiful and essentially free, but also because the process delivers a high energy return on investment. The biocrude, in other words, contains three or four times the energy required for conversion, lending it a value well higher the cost of heating the solids.
The biocrude that comes out of the HTL process looks a lot like heavy crude, but chemically, it isn't quite ready for refinery prime-time. Because of this, PNNL has also developed the upgrading step, called catalytic hydrotreatment, needed to make it a compatible biofuel blendstock.
"The biocrude itself comes out higher in nitrogen and oxygen than most refiners at this point are willing to consider as a feedstock," Billing says. "We have demonstrated that catalytic upgrading would then make it an acceptable, high-quality hydrocarbon stream." The upgrading and HTL are both described in a paper recently published in the journal Water Environment Research.
Using HTL on all of the sludge in the U.S. could save $2.2 billion in disposal costs. It would also cut emissions from conventional diesel fuel by at least half.
With a proof-of-concept accomplished, the next step is to test at a larger scale. The Utah-based company Genifuel has licensed PNNL's HTL technology and is now working with Metro Vancouver, a partnership of 23 local authorities in British Columbia, Canada, to build a demonstration plant. A second pilot project with the Central Contra Costa Sanitary District (CCCSD) in Martinez, California, funded in part by the Department of Energy, is in the design phase.