PCDD/F determination in sewage sludge composting. Influence of aeration and the presence of PCP

Abstract

Composting of sewage sludge is a common practice for sludge disposal. Some previous studies found high levels of polychorodibenzo-p-dioxins and polychorodibenzofurans (PCDD/Fs) after composting, especially octachlorodibenzo-p-dioxin (OCDD) but also 1234678-heptachlorodibenzo-p-dioxin (1234678-HpCDD) to a lesser extent. In this work, the concentrations of OCDD, 1234678-HpCDD and the rest of the 17 toxic congeners of PCDD/Fs were determined in compost obtained under different conditions. Although the toxicity of the two compounds mentioned above is small, their generation may reach undesirable levels. The PCDD/F content was analyzed in a composting plant and in a laboratory test. In both cases, the composted material was a mixture of sewage sludge, straw and sawdust. The composting plant was a tunnel with air turbine aeration and with a turner to homogenize and move the mixture upwards. The laboratory tests were carried out with Dewar vessels (with air dispersion at the bottom and controlled temperature) and with small vessels inside a controlled oven with non-forced aeration. The laboratory runs were also carried out with the addition of pentachlorophenol in some runs, as a dioxin precursor. The highest OCDD levels were found in three samples of the composting plant (30000–90000 pg/g dry matter or dm), with toxicity values surpassing the limit level for soil amendment (17 pg I-TEQ/g dm). Their formation was analyzed considering their concentration vs. that of octachorodibenzofuran (OCDF), which is not formed during composting. In the laboratory, in experiments carried out in a vessel with non-forced aeration conditions and with the addition of pentachlorophenol, the formation of OCDD was significant (e.g. from 80 to 1500 pg/g dm). That means that these two factors, non-forced aeration and the presence of pentachlorophenol, can cause the OCDD formation.