Giving sewage sludge a second life

A team of scientists from the University of Córdoba (UCO) has given sewage sludge a second life by turning it into activated carbon.

Sewage sludge is the solid waste that results from wastewater treatment. According to data from Spain’s Ministry for the Ecological Transition and the Demographic Challenge, 1.2 million tons of this waste were produced in Spain in 2021 alone, and its management is a growing problem. While some of the waste may have agricultural applications, such as being used as fertiliser after composting, its high concentration of metals limits its use, generating environmental problems.

By transforming the sludge into activated carbon, the researchers, from UCO’s Inorganic Chemistry and Waste Bioengineering and Green Engineering groups, produced a material with useful industrial applications. The high porosity of activated carbon means that it can adsorb molecules on its surface, making it especially suited to decontamination processes, such as water purification and gas treatment.

Transforming sewage sludge into activated carbon is not a new process, but the team’s innovation lies in producing the carbon “under more favourable and sustainable conditions, and obtaining a high-quality product”, according to María Carmen Gutiérrez, one of the authors of the study.

Compared to similar research carried out previously, this study managed to decrease the temperature required to carry out the process, leading to lower energy costs for the waste recovery procedure. The team also managed to reduce the amount of the activating agent — the substance that activates or accelerates the thermochemical reaction through which the waste becomes a useful product for society.

During the process, the sludge is initially dried before being mixed with the activating agent. The compound then undergoes pyrolysis (heating to high temperatures in the absence of oxygen, which carbonises the residue) and a treatment that purifies and removes certain minerals.

“From a practical point of view, it is important to propose solutions that can then be carried out on an industrial scale,” said María Ángeles Martín, a professor of Chemical Engineering at the University of Cordoba. In addition to using fewer resources, “it is one of the simplest procedures in the literature, and uses technologies that already exist on the market on an industrial scale”, she added.

At this stage, the researchers’ work, which originated from a doctoral thesis by researcher Hansi Martínez, has focused on verifying the quality of the activated carbon that can be obtained from sewage sludge. The next step, the research team explained, is for the group to develop applications appropriate for this material.

Image caption: The two UCO research groups that participated in the study. Image credit: University of Córdoba.