PNNL turns sewage into biocrude

RESEARCHERS at Pacific Northwest National Laboratory (PNNL) in the US have developed a process to turn human sewage sludge into biocrude that can be refined like conventional petroleum.

In the US alone, sewage treatment works process around 155bn l/y of sewage, and the PNNL team, led by Corinne Drennan, believes that this could be used to produce the equivalent of 30m bbl of oil. Sewage sludge is too wet to be used in conventional biofuel production and must generally first be dried out, which is costly. PNNL’s new hydrothermal liquefaction technology eliminates the need for a drying step.

In hydrothermal liquefaction, the sewage sludge is first pressurised to 3,000 psi (20,684 kPa) before being passed into a reactor at around 350?C. The heat and pressure breaks down the cells and other material in the waste, which separates into an aqueous liquid phase and the biocrude.

Some solid waste is generated by the process, but only a small quantity, and the team says that this waste is very high in phosphorus, an important plant nutrient, and could replace phosphorus ore in fertiliser production. The process could save regional governments money, as it virtually eliminates the need for sewage residuals processing, transport and disposal. The researchers add that the technology could be used for any kind of wet organic waste, such as agricultural waste.

Drennan says that the real beauty of the process is its simplicity.

'There is plenty of carbon in municipal waste water sludge and interestingly, there are also 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,' she added.

PNNL has licensed the technology to renewable energy firm Genifuel. Genifuel and Metro Vancouver, a network of 23 local authorities in British Columbia, Canada, will work together to build a demonstration plant. The project is expected to cost C$8–9m (US$6–7m) and will be funded by Metro Vancouver and external sources. It is hoped that the design phase will begin in 2017, with startup in 2018.