Biogas is indeed circular

The debate about biogas has reached Catalonia loaded with mistrust, interests, and simplifications. It is normal for it to generate debate: we are talking about organic waste, livestock manure, industrial plants, transport, energy, and territory. But precisely because the topic is complex, it is advisable to avoid diagnoses that mix everything up and end up turning a waste treatment technology into the scapegoat for all the problems of the livestock model.Biogas is an organic waste treatment technology that allows a part of biogenic carbon (of biological origin) to be transformed into methane, a greenhouse gas much more potent than CO₂ in the short term, to be used energetically or molecularly, and to obtain a digested residue that can have fertilizing use if managed correctly. This process is not only for slurry: there is also municipal organic fraction, pruning and gardening waste, sewage sludge, agri-food waste, agricultural residues, food industry waste, and other organic flows that, in the context of climate emergency, must be treated anaerobically (in the absence of oxygen).This is the first misunderstanding. Presenting biogas plants solely as “energy factories” distorts the debate. Their primary function is environmental: to reduce methane emissions, a greenhouse gas dozens of times more potent than CO₂. Once digested, the waste does not release as much methane, it is a chemically more stable fertilizer, with less risk of pathogens and fewer volatile compounds that cause bad odors. The recovery of gas in the form of energy or molecularly is relevant, but it is a consequence of the valorization of a waste that needs to be treated, just like with wastewater treatment plants or municipal organic waste.Catalonia has a real problem of livestock concentration in certain areas, external dependence on feed, and pressure on soils and aquifers. Not all the impacts of the food and livestock model can be attributed to biogas: imported soy, cereals, mineral fertilizers, maritime transport, or agricultural machinery. These impacts exist and must be reduced. But they are not caused by the biogas plant. They are part of the livestock and food system. This problem must be addressed with livestock planning, territorial limits, herd control, strict compliance with nitrate regulations, and better connection between agriculture and livestock. But this debate is not resolved by rejecting anaerobic digestion outright. It is one thing to discuss what livestock model the territory can support, and another to decide how to best treat the manure and organic waste that are already generated.

Anaerobic digestion does not eliminate nitrogen or phosphorus from slurries. This point is extremely important. After the process, a digestate remains that still contains nutrients and is, therefore, still subject to the same agronomic limits: available land, crop absorption capacity, vulnerable areas, fertilization plans, and application controls. In other words, biogas is not a technological route that makes waste disappear and allows for indefinite growth of the pig herd. It does not create new land, it does not make nitrogen disappear, and it does not nullify the territorial balance required by current regulations. If there is not enough agricultural base to grow today, a biogas plant does not change this limit.There is also a strategic dimension that often remains outside the debate: the value of biogenic carbon. In a decarbonized economy, it will not be enough to replace fossil fuels with renewable electricity. There will be industrial uses that will continue to require carbon molecules: plastics chemistry, polymers, solvents, methanol, certain synthetic fuels, or other basic chemical products. Where will this carbon come from? If we do not recover it from organic waste or captured biogenic CO₂, we will continue to depend on oil and gas as raw materials. Therefore, anaerobic digestion should not be seen only as a way to produce a gas that is burned, but also as a link in a future renewable carbon economy: capturing methane can generate biomethane, but it can also open the door to recovering biogenic CO₂ and other useful streams for the chemical industry. In Catalonia, where the Tarragona chemical cluster has an enormous industrial weight, this issue is not insignificant: maintaining chemical activity in a climate neutrality scenario will largely depend on the ability to replace fossil carbon with renewable, recycled, or captured carbon. Understanding this function is key to valuing waste treatment plants with a broader perspective than the simple combustion of biogas.Therefore, stating that biogas cannot be circular because, like any industrial activity, it uses energy, materials, or auxiliary products is an inconsistent statement. By this criterion, practically nothing would be circular: not recycling, not composting, nor water treatment. Circularity is not the absolute purity of a process without any externalities, but rather the ability to recover material and energy flows that would otherwise be lost or cause impacts. In the case of biogas, all products, both from the gas phase and the liquid-solid phase, allow for complete circularity of carbons as green carbons, undeniably closing a full circle. A circular economy cannot afford to waste essential renewable materials. The idea that slurry and manure are "not waste" because they have historically been fertilizers does not fit either the necessary solution to environmental problems or the necessary utilization of renewable resources to replace fossil fuels and confront the climate emergency.