Effect of mechanical separation of digested slurry on greenhouse gas and ammonia emissions during storage

In our days biogas plants are more and more common at farm facilities and the management of the digestate raise some concern for the possible emissions to air. The aim of this study was to evaluate how the mechanical separation affects ammonia (NH3) and greenhouse gas (GHG) emissions from the storage of the different fractions obtained from separation of codigested slurry. Flux measurements were estimated with static and dynamic systems on untreated digestate (UDS) and the solid (SF) and liquid (LF) fractions after separation during 32 days of storage. In the static system the UDS emitted less than the treated digested slurry (TDS = SF + LF), evidencing an important role of the total solids content in the emission process. On the contrary in the dynamic system UDS emitted more than the others fraction; in this case it seems that the emission driving force is the nitrogen content. Results also demonstrated that, not always NH3 emissions can be effectively measured both with photoacoustic gas analyser or the widely diffused acid trap method.