Using agricultural waste to clean wastewater

In her doctoral research in Chemistry, Brigitte Mukarunyana demonstrates how biochars and hydrochars, carbon-based materials produced from biomass such as coffee husks, coffee pulp, wood, and sugarcane residues, can effectively remove harmful contaminants from wastewater. The research combines advanced laboratory experiments with field studies to address real-world water pollution challenges.

“Wastewater bears our mark; biomass restores,” says Brigitte Mukarunyana. “This research shows that locally available materials can become part of the solution to protect water resources and public health.”

Traps and adsorbs pollutants

The work focuses on water systems in Rwanda, where rapid urbanization, agriculture, and limited wastewater treatment contribute to widespread pollution. Field investigations revealed high levels of organic contaminants in both hospital wastewater and rivers. In hospital effluent, pharmaceutical concentrations reached 244,000 nanograms per litre, while river samples contained more than 50 different pharmaceutical compounds, alongside pesticides and plant-derived substances. These patterns showed that urban areas are mainly affected by pharmaceutical pollution, while rural and agricultural regions are more impacted by pesticides and natural organic compounds.

To address these challenges, the thesis investigates how agricultural waste can be transformed into water treatment materials through thermochemical processes such as pyrolysis and hydrothermal carbonization. The resulting biochars and hydrochars act as adsorbents, meaning they trap and retain contaminants from water. Experiments using real wastewater demonstrated that these materials can remove between 14 and 66 per cent of pharmaceuticals, approximately 75 per cent of pesticides, and, in some cases, achieve complete removal of specific compounds such as polyphenols. Under optimal conditions, some materials reached removal efficiencies above 90 per cent.

Reduces wastes and costs

A key contribution of the research is explaining why these materials are effective. Their performance depends on several factors, including the type of biomass used, the production process, and the resulting surface structure and chemistry. Rather than being driven by a single factor, pollutant removal is governed by a combination of mechanisms such as chemical interactions, hydrophobic attraction, and trapping within microscopic pores.

The study emphasizes the potential of transforming waste into a valuable resource, endorsing a circular approach where agricultural by-products are reused for environmental safety. This process reduces waste, lowers the cost of water treatment, and creates opportunities for local production of purification materials. It is particularly important for regions where centralized treatment systems are not feasible, as these materials can be produced locally and used in decentralized or small-scale applications.

The research, conducted at the Department of Chemistry at Umeå University in collaboration with international partners, provides a foundation for future development of sustainable water treatment technologies. Continued work will focus on scaling up production, testing the materials in real-life systems, and integrating them with existing treatment methods.

About the thesis defence

On June 12 Brigitte Mukarunyana defends her doctoral thesis titled Biochars and hydrochars for the adsorption of organic contaminants from wastewater. The location is Stora hörsalen, KBC Building, Umeå University. Faculty opponent is docent Sahar Dalahmeh, KTH Royal Institute of Technology.

Read the whole thesis