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Every year, millions of tonnes of agricultural and food waste are generated globally. Crop residues such as stalks and husks as well as food waste and other organic byproducts are often burned, dumped in landfills, or just left to rot, releasing copious amounts of greenhouse gasses into the atmosphere. However, these materials are rich in stored energy that can be converted into a valuable renewable energy source, biogas.
Biogas is a green fuel with several applications such as cooking, heating, and even electricity generation. Biogas is generated during the process of anaerobic digestion, where micro-organisms break down organic waste such as food and agricultural waste in the absence of oxygen. A more water-efficient version of conventional anaerobic digestion is dry anaerobic digestion (DAD), which relies onhigh-solid waste containing more than 20% total solids and limited water, making it ideal for water-scarce regions.
Problems associated with dry anaerobic digestion
While DAD is waterwise, it is more challenging than conventional anaerobic digestion. High-solid systems have the tendency to become chemically and biologically unstable. Limited water results in the slower breakdown of organic material. In addition, volatile fatty acids (VFAs), which are produced during the DAD process, tend to accumulate. As these acids build up, they lower the system’s pH, creating conditions that are harmful to the methane-producing microbes. If these microbes become stressed or die, methane production drops, and the digester can even fail altogether. This instability is one of the main reasons why DAD systems are not yet widely adopted, despite its many advantages.
A small material with massive potential
To address these challenges and make DAD more feasible, scientists are turning to an unusual helper: micro- and nano- sized biochar. Biochar is a carbon-rich material produced by heating organic biomass in an oxygen-free environment. When it is engineered to a much smaller size, it develops a highly porous structure with an enormous surface area.
This tiny material can serve as a microbial support system or microbial carrier. Its numerous pores provide safe spaces where microbes can attach and grow, forming stable communities. Small biochar can also adsorb toxic compounds that would otherwise inhibit microbial activity. Additionally, it creates micro-environments that buffer sudden changes in acidity, helping to keep conditions inside the digester stable. Perhaps most importantly, small biochar may enhance interactions between microbes, which boosts methane production.
The W3M-Dry AD project
The potential of small biochar on DAD is the basis of the initiation of the W3M-Dry AD project: Water wise waste management: Two ends of the size scale, macro and nano augmentation for dry anaerobic digestion optimisation. One of the key focus areas of the project is to explore whether small biochar can act as a microbial modulator – shaping and stabilising the microbial community inside dry anaerobic digesters and improving the overall process and biogas yield.
The team hypothesises that adding small biochar will not only increase methane production but also strengthen the system’s resilience to stress and prevent process failure. To test this, controlled laboratory experiments are being conducted using high-solid agricultural residues as substrates for DAD in the presence and absence of different small biochar types. By measuring biogas production, acid levels, and pH, and by sequencing microbial DNA, the researchers will track how biochar influences system performance, microbiome dynamics and overall biogas productivity.
Figure 1: Overview of W3M-Dry AD project
Why does this matter for the real world?
More stable and efficient DAD systems could be transformative, especially in regions experiencing water scarcity and limited energy access. Farmers and communities could turn organic waste such as crop residues into fuel, reduce waste disposal costs, and lower greenhouse gas emissions, all at the same time.
Beyond energy, this research also supports the idea of a circular bioeconomy, where waste is no longer discarded but re-used as a valuable resource. By understanding how biochar interacts with microbial communities, scientists can design smarter, more resilient technologies that work with nature rather than against it. By combining materials, science, microbiology, and renewable energy research, this work shows that even the smallest materials can have a massive impact.
This research forms part of the W3M-Dry AD project, a collaboration between South Africa, Mozambique, and Japan. It reflects a growing international push toward sustainable, low-water, low-carbon technologies that can meet the needs of both people and the planet. The SA project team is funded by the National Research Foundation (NRF) with team members affiliated to the Agricultural Research Council (ARC), University of South Africa (UNISA) and University of Venda (UNIVEN). – Dr Ashira Roopnarain and Dr Haripriya Rama, ARC-Natural Resources and Engineering
For inquiries and comments kindly contact Dr Ashira Roopnarain (email: RoopnarainA@arc.agric.za) or Dr Haripriya Rama (email: RamaH@arc.agric.za) at the ARC-Natural Resources and Engineering.
