Evolution of humic acid molecular weight as an index of compost stability

This paper proposes an index for the evaluation of compost maturity based o n the evolution of molecular weights of humic acids (HA) during composting. The evolution of HA molecular weight was followed during the composting of both olive mill wastewater (OMW) and olive mill pomace (OMP). The wastes w ere composted in forced aeration static piles. Samples of the compost were collected at different times during composting. The elution profiles of HA obtained by gel filtration (Sephadex G-150) showed the disappearance of fra ctions- with molecular weights less than or equal to 50 KDa and the contemp orary increase of fraction with molecular weights greater than or equal to 10(2) KDa. In this range, two fractions can be separated: the first one (Al ) with molecular weights greater than 10(2) KDa and below 2 10(2) KDa, and the second one (AZ) with molecular weight greater than 2 102 KDa. During co mposting, the ratio A2/A1 tends to reach a constant value which indicates t he evolution toward the polymerization of HA. The ratio A2/A1 was named HAE I (Humic Acid Evolution Index). It varies with the material composted and t he composting process and represents the maximum possible degree of HA poly merization. A comparison between HAEI and the usually used maturity indexes is also presented.