Researchers remove heavy metals from biosolids

Engineers in Melbourne have developed a method to remove heavy metals, including copper and zinc, from biosolids.

The environmentally friendly method was led by RMIT University in collaboration with South East Water and Manipal University in India. It recycles the acidic liquid waste that is produced during the recovery phase instead of throwing it away.

Kalpit Shah, lead senior researcher from RMIT, said the metals found in biosolids — treated sewage sludge — can be valuable and the team’s approach can recover metals such as copper and zinc.

According to Shah, the innovation helps ensure that biosolids do not leach heavy metals into the environment, retaining the nutrients that can be used for land applications.

“With further processing, the biosolids can be turned into high-grade biochar, which is a renewable energy resource and has a range of applications including as a fertiliser,” Shah said.

The research is published in the international journal Hydrometallurgy.

The overall metal-removal process occurs over three stages: extraction, purification and recovery. Prior to the team’s work, this had not been fully explored among researchers beyond the first stage.

Ibrahim Hakeem, the first author of the study, said biosolids can contain several metals locked within organic matter, making purification and metal recovery challenging. The new approach can recover metals one by one and is a closed-loop solution with minimal environmental harm.

Abhishek Sharma, co-author from Manipal University, said the work was beneficial for improving the efficiency of the conversion of biosolids to biochar via pyrolysis.

According to Shah, the work complements the team’s biochar-producing technology, which South East Water, Intelligent Water Networks (IWN) and Greater Western Water are trialling at the Melton Recycled Water Plant.

“Pyrolysis is a process that uses heat to break down organic materials into valuable products and occurs without oxygen to prevent the materials from burning. The team uses this process to remove pathogens and contaminants of emerging concern from biosolids,” Shah said.

Aravind Surapaneni, co-author and Deputy Director (Industry) of the ARC-funded Training Centre for the Transformation of Australia’s Biosolids Resources at RMIT, said water industries are considering technologies such as pyrolysis to address concerns over contaminants.

The challenge is that reducing organic matter through pyrolysis results in a higher concentration of heavy metals in the biochar, which the team’s technique helps resolve.

In Australia, guidelines and regulations specify limits for heavy-metal concentrations, ensuring that biosolids can be used as fertiliser. The team aims to work with water authorities to use its heavy-metal removal technique prior to pyrolysis.

“We have previously seen our sludge as waste, but now through research like this we are able to see that it’s possible to clean it up and convert it into potential materials with value and further applications,” said David Bergmann, South East Water R&D Manager.

Shah said the team’s innovation could also be used for other waste streams such as stormwater lagoon sludge and mine tailings.

“We are planning to work with South East Water to do a techno-economic analysis which will hopefully lead to pilot-trialling,” Shah said.

“We’re also keen to work with companies who manage stormwater lagoons as well as mine tailings. The next step of engagement with them could be testing their samples in our lab, followed by the pilot-trialling.”

Image credit: RMIT University.