Costs of slurry separation technologies and alternative use of the solid fraction for biogas production or burning – a Danish perspective

Abstract

Separation is an option when livestock are produced in livestock intensive areas producing a surplus of nutrients. Separation of the slurry into a liquid nitrogen rich fraction and a more solid phosphorus rich fraction, which is exported away from the farm, may alleviate this problem. Separation offers an alternative to exporting the slurry further away, renting more land or buying more land. Today the farmer can burn the solid fraction, use it in biogas plants or sell it to another farmer. At the same time the need for P-balance is stricter than before, but developments in feeding, regulation and the reduction of livestock numbers in Denmark have made separation less favourable. This article discusses the many options with focus on the dominant separation technologies in Denmark, such as decanter and flocculation, as well as source separation, in order to compare them with to traditional handling. Key parameters are livestock density, transport distance and cost of separation. The conclusion is that unless land prices or prices on slurry agreements are very high, traditional handling of animal manure has the lowest costs. Decanter separation can be the cheapest if area is limited and co-operation with neighbours is possible as large volumes reduce separation costs per tonne. Flocculation is the best if much P has to be stored in the solid fraction. Separation can be combined with biogas production and the solid fraction from flocculation seems to give the highest gas production per tonne.