Characterization of the organic fraction of an uncomposted and composted sewage sludge by isoelectric focusing and gel filtration

Summary Isoelectric focusing was used to characterize the organic matter of composted and uncomposted sewage sludge. The technique was applied to organic matter extracts and to three fractions, obtained by ultrafiltration, with different molecular weights (<10³, 10³–10⁴, >10⁴). The elution curves of the extracts through Sephadex G-50 revealed a loss in the proportion of organic matter of low molecular weight as composting progressed, together with an enrichment of the high-molecular-weight proportion. Separation into fractions by controlled ultrafiltration proved to be valid, as deduced from the chromatograms obtained by Sephadex G-50 filtration. The extracts of uncomposted sludge showed a greater number of bands with a low isoelectric point than the composted extract, because there were more acidic molecules in the samples that had not undergone humification. The spectrum corresponding to the extract of the 210-day compost showed greater homogeneity with a lower number of bands. A great part of the organic matter extracted with 0.1 M Na₄P₂O₇ at pH 7.1 corresponded to an intermediate molecular weight. The ampholytes at pH 4–6 gave better resolution than those at pH 3.5–10, in the focusing of fractions with the lowest and the greatest molecular weight. A more homogeneous spectrum was observed for the high-molecular-weight fractions from extracts of the 210-day composted sample; in addition, the bands were displaced towards higher isoelectric points, which indicated that the molecules were more condensed, with a minor content of negatively charged groups and a spectrum similar to that of relative fractions of true humic acids.