Impact of humic substances on the bioavailability of copper and zinc during composting of sewage sludge in reduced structural material content conditions

In this paper the results of research work on the transformation of Cu and Zn chemical forms bound with humic acids that occur during sewage sludge composting process are presented. A factor influencing the direction of the observed transformations was the volume of added structural material (including its elimination) to the mix with sewage sludge during the compost batch preparation stage. Composting was carried out in industrial conditions, in periodically turned over windrows differing in the proportion of dehydrated sludge, straw and wood chips 4:1:(0.5 + 0.5) – P1, 8:1:(1 + 1) – P2, respectively and in a windrow without straw added (P3). Test results confirmed impact of the organic matter humification process on the reduction of Cu bioavailability in compost. Cu percentage in humic acids (HA) fraction was increasing during the composting process and varied from 21% to 28% whereas the organic matter humification process did not play a fundamental role in Zn immobilization. The percentage of Zn bound with humic acids (HA), in the final day of composting in particular windrows, was 9.43% – P1, 1.85% – P2 and 3.40% – P3, respectively. Cluster analysis has demonstrated that samples (P3) of the composted sewage sludge without straw added differed fundamentally from the samples taken from P1 and P2 windrows. The samples taken from P3 windrows that presented the lowest contents total of Cu and Zn, carbon contents in humus substances and humic acids. Affiliation of the tested samples to given group (clusters) was influenced, first and foremost, by total Zn and Cu content as well as the content of carbon bound with humus substance and carbon bound with humic acids.