Cattle slurry treatment by screw press separation and chemically enhanced settling: effect on greenhouse gas emissions after land spreading and grass yield

Five cattle slurry fractions with distinct characteristics were obtained using a combined separation process (screw press+chemically enhanced settling using polyacrylamide (PAM)). The purpose of the present study was to assess the effect of each fraction relatively to the untreated slurry (US) on the emissions of greenhouse gases (CH4, N2O) after grassland application and on the grass yield. Methane emissions occurred mainly in the first two days after application and were observed only in treatments with the US and liquid fractions. Significant N2O emissions were observed only in the US and liquid fractions treatments. A significant increase of the grass yield relatively to the US was observed in plots amended with the composted solid fraction and with the PAM-sup fraction resulting from the PAM sediment settling of the liquid fraction previously obtained by screw press separation, whereas in all other treatments, no significant differences were observed. Considering the overall separation process, the proposed scheme did not lead to an increase, relative to the US, of gas emissions after soil application of the fractions obtained except in the case of CH4 where a small increase was observed.