Acidification of raw and co-digested pig slurries with alum before mechanical separation reduces gaseous emission during storage of solid and liquid fractions

•Acidification of slurry with alum reduces NH3 and GHG emissions during storage.•The use of alum reduces NH3 emissions during mechanical separation.•The emissions from both acidified fractions is lower than from acidified slurry.•More nitrogen and organic matter is kept in acidified slurries after storage.•The use of alum improves the welfare of farmers and animals. Acidification of livestock slurries is used to reduce ammonia and methane emissions, and mechanical separation is applied to concentrate organic matter and nutrients in the resulting solid fraction (SF). The fractions obtained after separation are normally stored on farms during long periods before they can be applied to soil or transported to fields far away. During this storage period and/or transport, the emissions of ammonia and greenhouse gases (GHG) can be high, causing environmental problems. The aim of this study was to assess the effects of acidification before the separation of raw and co-digested pig slurries on the gaseous emissions during the storage of the resulting liquid fraction (LF) and SF. The emission rates of NH3, N2O, CO2, and CH4 were followed during 70days of storage. Acidification applied before separation significantly reduced the emissions of NH3 and GHG during the storage of the fractions from raw and co-digested slurries. The main reductions were observed in the acidified fractions, relative to the non-acidified fractions (that led to the highest gaseous emissions), in both slurries. Thus, NH3 and CH4 emissions were significantly reduced in acidified LFs and N2O emissions were significantly reduced in acidified SFs. The CO2 emissions were significantly reduced in both acidified fractions from both slurries, since most of the inorganic C was released during the acidification process. It can be concluded that acidification before separation is a good abatement practice to minimize ammonia emissions during separation and to reduce gaseous emissions during the storage of slurry fractions.