Dive Brief:

• Biosolids from wastewater should not undergo pyrolysis to create energy, because the process requires more energy than it produces, according to correspondence published in Resources, Conservation & Recycling by Andrew Neil Rollinson, of the University of Loughborough, and Jumoke Mojisola Oladejo, of the University of Nottingham in China.
• The publication is in reaction to a widely cited paper entitled "Pyrolysis of dried biosolids can be energy positive" from researchers at Marquette University that argued pyrolysis of biosolids can be energy positive.​ Rollinson told Waste Dive via email that the authors appear to have made "many errors" and should retract their work.
• According to Rollinson, pyrolysis of wastewater sludge solids is too energy intensive and concentrates toxins that are released into the atmosphere. Industry should work to create a more circular economy, "rather than sending more and more quantities of waste for disposal (including wastewater)," he wrote.

Dive Insight:

The gist of the new correspondence is that conclusions in the Marquette University paper were incorrect, and the heat and energy required to process wastewater biosolids is in fact higher than any energy output generated by drying and processing that material via pyrolysis – despite the paper claiming it can generate energy. The authors of the newly published correspondence detail more than a half-dozen items they list as "mistakes and limitations."

Rollinson said he takes particular issue with the paper because it "represents a false claim being permeated through the scientific community." Already, he wrote, the paper he's refuting has been cited nearly 20 times in the last two years. "When false claims are made, the whole repository of scientific knowledge is threatened."

While he thinks the paper should be amended or retracted, Rollinson was clear that he isn't trying to accuse the authors of fraud. The mistake, he said, "could be innocent, with the only impropriety being multiple mistakes, combined with zeal to present them in a positive light."

Handling wastewater is an essential service for any local government, and with beneficial options for biosolids sometimes challenging in certain markets the temptation to find sustainable uses is certainly understandable. However, as demonstrated by Rollinson and Oladejo, too much energy may be used to dry and prepare the biosolids for conversion, making pyrolysis much less sustainable than some have thought.

This does not mean, however, that pyrolysis as a technology for handling waste should be totally discounted in Rollinson's view. He said the process has a "niche, for it can produce useful chemicals from solid organic matter," but one should not forget that the process is very energy intensive.

The concept of pyrolysis or other chemical recycling technologies has begun to gain some traction for plastics in the U.S. recently. Yet, as described in recent research and a supplemental article, he's not convinced the technology makes sense in this application.

"Pyrolysis is definitely not the answer to plastic waste," Rollinson wrote.