INFLUENCE OF COMPOSTING SEWAGE SLUDGE ON CHANGE IN LEAD CONTENT IN SEQUENTIALLY SEPARATED FRACTIONS

This article discusses lead quantitative changes occurring in sequentially separated fractions of sewage sludge composted together with straw and sawdust. The experiment was carried out in controlled conditions in four bioreactors. Experimental composts were prepared using sewage sludge obtained from a biological-chemical sewage treatment plant (composts 1 and 2) and sewage sludge derived from a mechanical-chemical sewage treatment plant (composts 3). Each compost mixture was characterized by the same proportion of sewage sludge (65 %), sawdust (30 %) and straw (5 %). Lead sequential extraction in experimental composts was performed using a modified BCR method. The effect of composting of sludge derived from biological-chemical types of sewage treatment plants was 2.5-3.0 times lower lead content in water-and acid-soluble bonds (Fr. I). On the other hand, in the case of sludge obtained from the mechanical-chemical sewage treatment plant, the amount of lead in fraction I increased by 27 % in comparison with the content of this metal in samples collected at the initiation of the experiment. The composting process of the examined sewage sludge together with straw and sawdust exerted a similar impact on changes in the metal content in organic bonds (Fr. III) and residual fraction (Fr. IV). The quantities of lead found in fraction III of mature composts 1, 2 and 3 were, respectively, by 2.0, 1.5 and 4.0 times higher in comparison with the amounts of this element in experimental mixtures determined on the day of trial establishment. A reverse direction of quantitative changes was observed in the case of Pb determined in fraction IV of the experimental composts. Irrespective of the applied sewage sludge, content of the metal continued to increase up to the phase of cooling down and then, they declined during the phase of compost maturation. The above-described process was the strongest in conditions of compost 3. Share of lead in fraction I to IV reflected its quantitative changes expressed in absolute values. The experiments confirmed preferential development by Pb of poorly-and non-soluble bonds with the solid phase of composts.