Recycling of separated pig manure: characterization of maturity and chemical fractionation of elements during composting

Composting of separated pig manure (SPM) was studied to evaluate criteria indicating compost maturity and to determine the effect of composting on the fractionation of trace elements in SPM compost. Composting was performed in turn piles and the following parameters were measured in 10 samples during 122 days of composting: temperature, C/N ratio, ash content, metal contents, humic substance contents, and fractions (humic acid, fulvic acid, and nonhumic fractions - HA, FA, and NHF, respectively). A sequential extraction scheme was used to partition Cu, Mn, and Zn in SPM compost. The C/N ratio and ash content exhibited a typically high rate of change during the first 33 days and levelled off thereafter. The fresh SPM was enriched with Cu, Mn, and Zn due to feed additives. All metal concentrations increased approximately 2.6-fold in the final compost due to decomposition of organic matter. The HA content increased to a maximum at 80 days, representing the degree of humification and maturity of the compost. During the composting process, the major portions of Cu, Mn, and Zn were found in the organic, oxide, and carbonate fractions, respectively. Metal distributions in different chemical fractions were generally independent of composting age and, thus, respective total metal concentrations in the composts.