From bulk sewage to biocrude oil

Wastewater treatment plants across the United States may one day be turning ordinary sewage into biocrude oil, thanks to research at the Department of Energy’s Pacific Northwest National Laboratory (PNNL).

Sewage sludge has long been viewed as a poor ingredient for producing biofuel because it is too wet. PNNL’s approach 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.

The technology — hydrothermal liquefaction (HTL) — mimics the geological conditions the Earth uses to create crude oil, enabling organic matter such as human waste to be broken down to simpler chemical compounds. The material is pressurised to 3000 psi — nearly 100 times that of a car tyre — before the pressurised sludge then goes into a reactor system operating at about 315°C.

The heat and pressure cause the cells of the waste material to break down into biocrude and an aqueous liquid phase. This biocrude can then be refined using conventional petroleum-refining operations.

“There is plenty of carbon in municipal waste water sludge and, interestingly, there are also fats,” said Corinne Drennan from PNNL. “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.”

Biocrude oil, produced from wastewater treatment plant sludge, looks and performs virtually like fossil petroleum. Image courtesy of WE&RF.

Water Environment & Reuse Foundation (WE&RF) investigators said the process has high carbon conversion efficiency, with nearly 60% of available carbon in primary sludge becoming biocrude. Furthermore, 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, early efforts have demonstrated the ability to recover phosphorus, which can replace phosphorus ore used in fertiliser production. HTL may also be used to make fuel from other types of wet organic feedstock, such as agricultural waste.

“The best thing about this process is how simple it is,” said Drennan. “The reactor is literally a hot, pressurised 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.”

PNNL has licensed its HTL technology to Utah-based Genifuel Corporation, which is now working with Metro Vancouver to build a demonstration plant at one of the latter’s wastewater treatment plants. Once funding for the plant is in place, Metro Vancouver will move to the design phase in 2017, followed by equipment fabrication, with start-up occurring in 2018.

Top image caption: Sludge from Metro Vancouver’s wastewater treatment plant has been dewatered prior to conversion to biocrude oil at PNNL. Image courtesy of WE&RF.