Evaluation of Metals Content in Composted Sewage Sludge and Organic Food Waste

There are two main aspects for which bio-waste composting plays an important role in the transition to a bio-circular economy. The first one regards the use in agriculture and the food industry of the final product that could be obtained (compost), while the second one is linked to the reduction of the volume of waste that must be managed to decrease the green effect impact. The main objective of this study was to evaluate the compostability of wastewater treatment sludge (WTS) by using wood chips as a bulking agent (BA) and to determine heavy metals (HM) concentrations (Cd, total Cr, Cu, Hg, Ni, Pb, and Zn) in mature compost. The composting unit is a closed system, provided with a semipermeable membrane and positively forced aeration. Two composting experiments were set up to evaluate the effect of two different composting mixture recipes on compost quality. For the evaluation of heavy metals content in the obtained compost, the reference values according to the proposed technical norms of Romanian specific regulation were considered. In case of the first experiment, results showed that for most heavy metals, contamination of the analyzed compost exceeded the thresholds for the first class of quality but can be used on agricultural land. For the second experiment, results indicated a concentration of Ni of 43 mg/kgd.w.. Consequently, it falls into category III, a good quality compost, and will be able to be used as a soil improver for degraded soils.