Research explores phosphorus recovery from sludge-derived hydrochar

Using a combination of heat, water and phase separation, researchers are exploring a cost-effective method to concentrate phosphorus from wastewater sludge, according to a press release from the University of British Columbia’s Okanagan Campus (UBCO).

“Phosphorus is a non-renewable, but essential, element for life and has many industrial uses,” said Huan Liu, a doctoral student with UBCO’s School of Engineering and lead author of a new study investigating this method.

This researchers’ study looks closer at the processes behind hydrothermal liquefaction on concentrating phosphorus into an recoverable residue. It appears in the journal Water Research. Among its findings are the rates of metal release from hydrochar by nitric acid.

Phosphorus is a natural mineral crucial for human health and essential to food security as a commercial fertilizer; however, it's also listed as a critical raw material because many countries rely on imports.

“The uneven distribution of phosphate rock has created political and economic risks,” Liu said. “On the other hand, phosphorus discharge from waste sources, such as wastewater, is a major contributor to aquatic eutrophication, causing severe environmental challenges including algae blooms and dead zones in lakes.”

Liu and his supervisor, principal investigator Cigdem Eskicioglu, are investigating a process that integrates hydrothermal liquefaction.

The process converts organic components of municipal wastewater sludge into a petroleum-like bio-crude and concentrates the phosphorus into a solid residue called hydrochar. The hydrochar can have 100 times higher total phosphorus than raw sludge, making it comparable to the phosphate rock used in commercial fertilizers.

Liu describes the extraction process as mirroring what happens when you mix minerals and acids.

“We were able to identify, for the first time, the kinetic reactions of phosphorus leaching from hydrochar to optimize the recovery of useful materials, such as what is needed for fertilizer,” said Liu.

According to Eskicioglu, their latest findings are essential for wastewater utilities aiming to develop a process to recover usable nutrients from the system.

“At a time when we are seeking to be more sustainable and looking for alternative fuels, extruding useable materials from waste is essential,” Eskicioglu said. “Recovery and recycling is the solution that also provides the double benefit of providing a secondary source of phosphorus that can be globally distributed and also help with environmental conservation.”