Technological breakthroughs

Scientists are exploring ways to convert straw, manure, and other types of agricultural waste into useful fertilizers through catalytic technologies. This approach has the potential to reduce pollution and conserve resources, but it still faces numerous technical, economic, and regulatory barriers that must be overcome.

Current fertilizer production relies heavily on non-renewable resources: natural gas for nitrogen synthesis and phosphate rock for phosphorus fertilizers. The production of nitrogen fertilizers via the Haber–Bosch process—a foundational industrial technology of modern agriculture—consumes about 2% of global energy and accounts for nearly 1.8% of global CO₂ emissions. Meanwhile, global agriculture generates billions of tons of residues each year—from straw, bagasse, and rice husks to manure and sludge—most of which are burned or disposed of at significant cost.

A review article published in Applied Catalysis O: Open (July 2025) by a research group from Wrocław University of Science and Technology (Poland) examines catalytic technologies capable of converting agricultural waste into fertilizers. The authors consider this a promising pathway aligned with circular economy principles, while also highlighting the many challenges that must be addressed before these technologies can be widely applied in practice.

Agricultural waste: a promising but complex feedstock

Agricultural waste encompasses various categories: straw and cereal residues, fruit and vegetable peels, oilseed cakes and shells, livestock manure, rice husks, and sludge. A common feature is that all contain essential nutrients for plant growth—particularly nitrogen (N), phosphorus (P), and potassium (K)—but in forms not directly available to plants.

However, these materials are heterogeneous. Their chemical composition varies significantly depending on crop type, geography, and storage conditions. Some wastes also contain heavy metals, residual antibiotics, microplastics, or pathogens, requiring additional treatment before being used as fertilizers. Moreover, agricultural waste is often spatially dispersed with low density, making collection and transport to centralized processing facilities challenging. All these factors increase costs and complicate production.

The authors estimate that only about 20–30% of total crop residues can realistically be utilized for fertilizer production, as the remainder must be retained to maintain soil fertility or serve other purposes such as animal feed or fuel.

Key catalytic technologies under investigation

Catalytic pyrolysis

This process involves heating biomass (plant and animal-derived organic material) at high temperatures in the absence of oxygen. It produces three main products: syngas (used as an energy source), bio-oil (used as fuel or chemical feedstock), and biochar—a carbon-rich porous material used for soil improvement. Catalysts—typically zeolites or metal oxides—are added to enhance product quality and control nutrient distribution.

Biochar is the most widely studied product in the context of fertilizer production. It is rich in carbon, has a porous structure, and can retain water and nutrients in soil. When enriched with nitrogen, phosphorus, and potassium, biochar can function as a slow-release fertilizer, enabling more efficient nutrient uptake and reducing environmental losses.

Studies show that pyrolysis temperature strongly influences biochar properties. Lower temperatures (below 500°C) yield biochar richer in nitrogen with slower nutrient release, while higher temperatures drive more nutrients into the gas phase. However, optimizing these parameters for specific feedstocks remains an active area of research.

Hydrothermal carbonization

This method treats organic materials in hot, pressurized water to produce hydrochar. Compared to pyrolysis, hydrothermal carbonization is more suitable for wet feedstocks such as manure or sludge. It operates at lower temperatures and retains more phosphorus in the final product.

Real-world implementations in Denmark and Germany demonstrate that integrating this technology into biogas plants can convert digestate into nutrient-rich fertilizers while reducing N₂O emissions by about 30% compared to conventional treatment methods.

Electrochemical recovery of nitrogen and phosphorus

Rather than burning or thermally processing organic matter, electrochemical methods use electricity to convert nitrate in wastewater into ammonia—a form of nitrogen that can be directly used as fertilizer. This approach is considered more environmentally friendly, as it can be coupled with renewable energy sources and operates under much milder conditions than the Haber–Bosch process.

However, the authors note that electrochemical ammonia synthesis is still in its early stages, with low energy efficiency and production costs that remain higher than those of conventional methods.

Potassium recovery from ash

Potassium is the third essential nutrient for plant growth. Ash from the combustion of plant residues—especially rice straw—contains relatively high potassium content. Studies indicate that the simplest and most cost-effective method is dissolving ash in water at room temperature and recovering potassium from the solution. However, this area has received less research attention compared to nitrogen and phosphorus recovery, and large-scale industrial applications remain limited.

Environmental and economic benefits: promising but requiring careful evaluation

The authors summarize several potential benefits of producing fertilizers from waste through catalytic technologies:

From an environmental perspective, these technologies could reduce CO₂ emissions from fertilizer production by up to 30%, decrease N₂O emissions—a greenhouse gas nearly 298 times more potent than CO₂—from fertilizer application, and mitigate water pollution caused by nutrient runoff. Biochar-based fertilizers can also sequester carbon in soils over long periods.

From an economic perspective, using waste as feedstock reduces dependence on volatile fossil-based resources. Slow-release fertilizers may allow farmers to apply fewer inputs more efficiently, lowering labor and input costs.

However, the authors emphasize that many of these benefits are based on controlled experimental conditions and may not fully reflect large-scale industrial realities. Initial investment costs for catalytic systems remain high, and few technologies are currently cost-competitive with conventional fertilizers.

Unresolved challenges

The authors categorize the barriers into four main groups:

Technical challenges: Catalysts can lose activity over time due to carbon deposition, poisoning by impurities, or structural degradation at high temperatures. Catalyst regeneration is costly and may disrupt production. In addition, limited data on heat and mass transfer at large scales complicates scale-up from laboratory to industrial systems.

Feedstock challenges: Variability in waste composition across seasons, regions, and storage conditions directly affects catalytic efficiency and product quality. The presence of contaminants such as heavy metals, antibiotics, and microplastics requires additional costly purification steps.

Economic and logistical challenges: Conventional fertilizers benefit from economies of scale and well-established supply chains. Small-scale waste-based fertilizer systems struggle to compete on cost with large industrial producers. The dispersed nature of agricultural waste further increases collection and transportation costs.

Regulatory and social challenges: In many regions, particularly in the European Union, fertilizers derived from waste must undergo strict testing for heavy metals, pathogens, and other contaminants before market approval. Regulatory inconsistencies across countries hinder large-scale commercialization. Moreover, farmers may be reluctant to adopt waste-derived fertilizers due to concerns about quality and safety.

Future research directions

The authors suggest several priority areas for future research: developing more durable and cost-effective catalysts, potentially using biochar or natural aluminosilicates instead of expensive synthetic materials; applying artificial intelligence to optimize catalytic processes and predict product properties; combining chemical catalysis with microbial processes to improve nutrient recovery; and conducting more pilot-scale experiments before large-scale deployment.

They also emphasize the critical role of policy: subsidies, tax incentives, and clear regulatory frameworks are essential to encourage investment in this high-risk field.

Implications for Vietnam

Vietnam is one of the countries in Southeast Asia with a large volume of agricultural waste, particularly rice straw from paddy cultivation, bagasse from sugar production, and manure from livestock farming. Open burning of rice straw after harvest remains common in many regions, causing air pollution and wasting valuable resources.

Biochar production technologies from rice straw and agricultural residues have been piloted in Vietnam, but have not yet been widely scaled up due to high initial investment costs and the lack of appropriate supply chains. Lessons from integrated systems in Denmark and Germany—combining biogas plants with hydrothermal carbonization—may offer useful references for large-scale livestock operations or rice-producing regions in the Mekong Delta.

However, it should be noted that most studies reviewed in the article were conducted under conditions different from those in Vietnam. Local adaptation experiments—considering feedstock types, soil characteristics, and tropical climate conditions—are necessary before widespread implementation.
----
Source: Skrzypczak, D., Pstrowska, K., Niciejewska, A., Mazur-Nowacka, A., Wilk, Ł., & Chojnacka, K. (2025). Catalytic innovations in fertilizer production from agricultural waste: Enhancing soil health and sustainability. Applied Catalysis O: Open, 206, 207064. https://doi.org/10.1016/j.apcato.2025.207064

https://doi.org/10.1016/j.apcato.2025.207064