Author(s)

J. Havukainen, V. Uusitalo, K. Koistinen, M. Liikanen & M. Horttanainen

Abstract

The prevailing large-scale use of chemical fertilizers has been affecting environmental degradation. a broken nutrient cycle has caused problems worldwide, which are related to the question of how to feed 9 billion people by 2050 while limiting human operations within the planetary boundaries. Indispensable nutrients, phosphorus (P) and nitrogen (N), often leak because of human activities, such as food production. efficient  nutrient recycling can alleviate the problem. This study focuses on biofertilizers as a solution for the problem of a broken nutrient cycle. The study quantified the environmental benefits of using biofertilizers by calculating the carbon footprints of P and N in organic fertilizers by using the life cycle assessment (lCa) method on an existing biogas plant. The emissions from common production processes are allocated between products and co-products. however, whether a side flow is regarded as a co-product or waste is sometimes unclear. according to ISO 14040 and the greenhouse gas (GhG) protocol, if a substance does not have a value or the holder intends to dispose it, it can be regarded as waste. allocation of emission can be done according to parameters such as energy content, mass, or monetary value. The composted digestate was considered valuable; the allocation between biogas and nutrients was conducted according to the value of biogas and recycled fertilizers.

The calculated carbon footprints were 0.8 kg_CO2,eq./kg for N and 1.8 kg_CO2,eq./kg for P, whereas the carbon footprints for mineral fertilizers were 1.9–7.8 kg_CO2,eq./kg for N and 2.3–4.5 kg_CO2,eq./kg for P. The reduction of GhG emission in organic fertilizer production in comparison with the emission in mineral fertilizer production was on average 78% for N and 41% for P. On the other hand, inclusion of N₂O and Ch₄  emissions from composting increases the carbon footprints of nitrogen and phosphorus but there is high uncertainty included with these emissions. The value of nutrients in the biofertilizers is also uncertain but the interest towards using of them is increasing in Finland.

Keywords

anaerobic digestion, biofertilizer, biogas, carbon footprint, compost, digestate.