Biorefineries: Toward sustainable and efficient livestock manure management

Economic, environmental, and social sustainability is vital for any production industry. In this sense, the swine industry generates a large volume of manure, and its management is crucial to ensure such sustainability.

A large part of this livestock manure is applied directly to the fields as fertilizer, following agronomy criteria. However, the concentration of livestock farms in certain areas can generate an imbalance between the manure production and the available agricultural surface area. Without proper management, this excess can have environmental and social consequences, such as nitrate contamination of groundwater, which poses a risk to human health, inter alia.

In these areas where there is an imbalance between livestock manure and the availability of agricultural land, it is necessary to implement management and treatment strategies that allow the nutrients present in manure to be exported to areas with a demand for it. To do this, centralized treatment plants can be a good alternative, as they facilitate more efficient management and have reduced costs compared to on-site treatment on farms or industries.

The biogas sector has recently gained increased interest due to factors such as the ecological transition, decarbonization, geopolitical conflicts, and fluctuating fossil fuel prices. Energy companies see organic waste as an opportunity to produce biogas, which can be purified into biomethane, a renewable alternative to natural gas. This interest in biomethane could boost waste treatment and valorization projects that, until now, have lacked the necessary economic viability.

Although the production of biogas and biomethane offers an opportunity to improve the management of waste and organic by-products, the anaerobic digestion process does not affect the concentration of nutrients, it only modifies their oxidation state. Therefore, it is necessary to combine this technology with others that process the digestate (the material resulting from anaerobic digesters) to obtain value-added products that can be marketed and exported outside the territory, transitioning from the concept of biogas plants to biorefineries or eco-plants.

Biogas, and especially biomethane, already have a more or less consolidated market; however, there is still a long way to go in terms of other bioproducts that can be obtained in biorefineries. In this sense, it is essential to promote value chains ranging from biogenic carbon dioxide, recovered in the biogas purification process to produce biomethane, to fertilizer products, biostimulants, organic amendments, and reclaimed water, obtained from the treatment of digestate.

Regarding biogenic carbon dioxide, it is necessary to invest in research and innovation to develop processes to capture and valorize it. Some possible applications of this gas include the production of biofuels or its use in different industrial sectors, such as in the agri-food sector, for example in the stunning of animals by anoxia in slaughterhouses or its inclusion in gas mixtures for modified atmospheres in packaged fresh produce.

Regarding biofertilizers, biostimulants, and organic amendments, it is essential, first of all, to determine their environmental footprint and use the results, which are expected to be environmentally favorable, to promote their use instead of mineral and non-circular products. In addition to reducing the carbon footprint due to their production, their circular origin and potential contribution to carbon sequestration when applied to soil should also be considered. In this regard, it would be necessary to promote a political-legal framework to encourage substituting less sustainable and non-renewable products with these bioproducts.

A prime example of these products is biochar, a carbon-rich solid material obtained by pyrolyzing biomass, i.e. heating organic matter in the absence of oxygen. It is mainly used as a soil amendment to improve soil fertility, sequester carbon, and contribute to climate change mitigation. Increased soil organic matter also enhances biodiversity, improves water retention, and reduces erosion, among other benefits. These ecosystem services must be valued to increase the competitiveness of these products in the market.

Ammonium sulfate is another very interesting biofertilizer that can be obtained from the treatment of digestate, specifically through a stripping and absorption process with sulfuric acid. This fertilizer has purity levels comparable to those of its mineral analog; however, from a regulatory standpoint, it is currently considered to be of organic origin, limiting its application. This classification limits its competitiveness in the market compared to conventional mineral fertilizers.

The RENURE (Recovered Nitrogen from Manure) initiative seeks to reclassify certain recovered nitrogen fertilizers as mineral equivalents, which would allow their application without the restrictions imposed on organic fertilizers in nitrate-vulnerable areas. This regulatory change could increase the economic value of ammonium sulfate and other products derived from digestate, promoting their valorization within a circular economy model.

It is also essential to invest in research and innovation to develop organic products that offer the same qualitative advantages as inorganic products, such as the formulation based on N, P, K, and other micronutrients, or the presentation of the final products, for example, in the form of pellets or granules.

Finally, the main challenge regarding reclaimed water, obtained from tertiary treatment of the liquid fraction of digestate, is to develop a clear legal framework to regulate its possible uses. It is essential to establish control measures to guarantee its quality and to determine whether its final destination can be the cleaning of facilities, use in cooling towers, irrigation, or fertigation, as well as its discharge into collectors or even into watercourses. Given the evident current and future scenario of water scarcity, it would also be appropriate to consider more ambitious uses, such as its use for supplying water to animals or in food industry processes.

In summary, the challenges for making biorefineries a reality are not only technological, but also political, legal, and research-related, and involve coordination between the different agents in the value chains of the bioproducts obtained.