Fossilfri gödselproduktion – biobaserade gödningsmedel: effektiv användning av näringsrika biprodukter för att ersätta mineralgödsel inom jordbruket

Biofertilizers is generated as a co-product to biogas when digesting organic material in anaerobically in a biogas plant. In this study, the climate impact of biofertilizers and biogas systems were studied using life cycle assessment methodology. The study was loosely based on data from two real facilities co-digesting organic waste materials. The effects on soil carbon stocks were also assessed, when applying biofertilisers continuously on agricultural soils. This effect was based on results from respirometer trials performed in a laboratory setting, using digestates from the two biogas plants that were modelled in the life cycle assessment. In this report, a background to the biogas production and use of biofertiliser in Sweden today is given, as well as a detailed description of the life cycle assessment, including the soil experiments and calculations that formed the basis for the soil carbon modelling. The results indicate that biofertilizers and biogas systems contribute to reducing greenhouse gas emissions from the Swedish food production system already today. The contribution to soil organic carbon stocks most likely makes well designed and controlled biogas systems close to carbon neutral. Systems with very low resource use and emission levels might even achieve a global cooling effect due to the potential soil carbon stock changes. The exact climate impact from a biogas plant is however highly variable, and depends to a high degree on its ability to avoid leakage of methane, which is a highly potent greenhouse gas. Even small leakages can increase the impact from the system significantly. It is therefore important to be aware of the risk of leakage, both from the pant and the storage of the digestates. The later can cause very high emissions if the material leaving the biogas process has not been well digested. The potential of biofertilisers and biogas systems co-digesting municipal waste contributing further to achieving a climate neutral food production system in Sweden by 2045 is however limited. Generated waste should definitely be processed to produce biogas and biofertilizers, which is being done to a large extent already today. Reducing the amount of waste should however be an overarching goal for society, since that increases resource use efficiency and minimizes environmental impacts, including climate impact. This inevitably reduces the amount of available feedstock for municipal waste biogas plants.