Life cycle assessment of pig slurry treatment technologies for nutrient redistribution in Denmark

Animal slurry management is associated with a range of impacts on fossil resource use and the environment. The impacts are greatest when large amounts of nutrient-rich slurry from livestock production cannot be adequately utilised on adjacent land. To facilitate nutrient redistribution, a range of different technologies are available. This study comprised a life cycle assessment of the environmental impacts from handling 1000 kg of pig slurry ex-animal. Application of untreated pig slurry onto adjacent land was compared with using four different treatment technologies to enable nutrient redistribution before land application: (a) separation by mechanical screw press, (b) screw press separation with composting of the solid fraction, (c) separation by decanter centrifuge, and (d) decanter centrifuge separation with ammonia stripping of the liquid fraction. Emissions were determined based on a combination of values derived from the literature and simulations with the Farm-N model for Danish agricultural and climatic conditions. The environmental impact categories assessed were climate change, freshwater eutrophication, marine eutrophication, terrestrial acidification, natural resource use, and soil carbon, nitrogen and phosphorus storage. In all separation scenarios, the liquid fraction was applied to land on the pig-producing (donor) farm and the solid fraction transported to a recipient farm and utilised for crop production. Separation, especially by centrifuge, was found to result in a lower environmental impact potential than application of untreated slurry to adjacent land. Composting and ammonia stripping either slightly increased or slightly decreased the environmental impact potential, depending on the impact category considered. The relative ranking of scenarios did not change after a sensitivity analysis in which coefficients for field emissions of nitrous oxide, ammonia and phosphorus were varied within the range cited in the literature. Therefore, the best technology to implement in a given situation depends on the environmental problem in question, local policy, cost and practicality.