Fractionation of compost-derived dissolved organic matter by flocculation process

A new approach for a simple and rapid fractionation of compost-derived dissolved organic matter (DOM) has been tested. Aqueous extracts of municipal solid waste composts were fractionated by flocculation with alum (Al2(SO4)3•18H2O), similar to the flocculation process used in water treatment. Flocculation curves of diluted compost extracts (100 mg/l DOM-carbon) were characterized by an initial rapid decrease in DOM-carbon content and UV absorbance at a relatively low alum dosage, then by a slow gradual decrease at increasing alum dosages. Between 10 and 50% of the DOM-carbon, and 35 and 65% of the UV absorbance were removed from the extracts by flocculation. Higher removal was obtained with the more mature compost extract, yielding good correlations between compost maturity and the percentage of DOM-carbon removal and UV absorbance reduction. The total amount of flocculated DOM (on a dry matter basis), was similar in all compost samples (about 500 mg DOM-carbon/100 g dry compost). On the other hand, the total amount of non-flocculated DOM decreased dramatically with compost maturity, from 4500 to 500 mg DOM-carbon/100 g dry compost. It is believed that the relatively hydrophobic high MW fraction (humic-type material) is precipitated by the flocculation process, whereas the relatively hydrophilic low MW fraction tends to remain in solution. Flocculated DOM may represent the more resistant, less biodegradable fraction, and the more reactive toward hydrophobic organic contaminants.