CHANGES OF CADMIUM, NICKEL AND LEAD CHEMICAL BONDS IN A SEWAGE SLUDGE-BASED COMPOST

The aim of the performed investigations was to determine changes which take place in chemical bonds of cadmium, nickel and lead fractions during sludge composting with the addition of sawdust and straw under conditions of an outdoor composting plant. For this purpose, a tractor compost-heat aerator was used which mixed, chopped and aerated the compost. Heap composting lasted 110 days and then the experimental composts were subjected to maturation (250 days). Samples for analyses were collected on day: 1, 54, 110 and 360 of the experiment using the BCR method (recommended by the Commission of the European Bureau Community of Reference) for metal sequential analysis. It was found that quantitative changes as well as the proportion of individual metal fractions concerned, primarily, chemical bonds of soluble fractions, while metal bonds with silicate minerals (residual) were affected only slightly. In the discussed composting conditions, it was observed the highest increase in the proportion of the exchangeable and easily-soluble fractions of cadmium in the mature compost which should be considered as an unfavorable process. On the other hand, cadmium and lead decreased proportions of their chemical bonds with organic matter, while nickel increased them. The residual fraction in the mature compost constituted 38.2 % (cadmium), 63.1 % (nickel) and 84.9 % (lead) of their total contents.