Effect of the pig slurry separation techniques on the characteristics and potential availability of N to plants in the resulting liquid and solid fractions

Solid–liquid separation of slurry is used to obtain a solid (SF) and liquid fraction (LF), both used as organic fertilisers. Previous works showed that the separation technique used influences the composition of the resulting SF but no data are available relative to the LF. Four commonly used separation processes were considered – centrifugation, sieving, enhanced settling by the addition of cationic polyacrylamide (PAM-), and sediment settling as well as a recent sequential process – sieving followed by PAM addition to the resulting LF. The resulting LFs and SFs were comprehensively characterised and the amount of N potentially available for plants after incorporation to soil was also evaluated. Except for slurry that was only sieved which was very similar in composition to whole slurry (WS), the SFs and LFs had, respectively, higher and lower, concentrations of nutrients and organic matter (OM) than the WS. The ratio of ammoniacal to total N in the LFs varied from 0.33 to 0.92 whereas the values in the SFs varied between 0.16 and 0.31. The N:P:K ratio was also significantly influenced by the separation technique. Less than 10% of the total N applied in the WS, SFs or LFs was found to be potentially available for plants and, in some LFs, N immobilisation occurred after soil application. Since separation technique strongly influences nutrient distribution between LF and SF, as well as N speciation, the choice of the technique has to consider the final use of the resulting fractions as well as acquisition and operating costs.