Studying the Effects of Two Various Methods of Composting on the Degradation Levels of Polycyclic Aromatic Hydrocarbons (PAHs) in Sewage Sludge

The research comprised of studying the effect composting sewage sludge with sawdust and vermicomposting with earthworm Eisenia fetida has on the degradation of 16 polycyclic aromatic hydrocarbons (PAHs). Raw rural sewage sludge prior composting was more contaminated with PAHs than urban sewage sludge, in both cases exceeding EU cutoff limits of 6 mg/kg established for land application. Dibenzo[a,h]anthracene (DBahAnt), acenaphtylene (Acy) and indeno[1,2,3-c,d]pyrene (IPyr) were predominant in rural sewage sludge, whilst the urban sewage sludge contained the highest concentrations of benzo[b]fluoranthene (BbFl), benzo[k]fluoranthene (BkFl) and indeno[1,2,3-c,d]pyrene (IPyr). Thirty days of composting with sawdust has caused a significant reduction of 16 PAHs on average from 26.07 to 4.01 mg/kg (84.6%). During vermicomposting, total PAH concentration decreased on average from 15.5 to 2.37 mg/kg (84.7%). Vermicomposting caused full degradation of hydrocarbons containing 2 and 6 rings and significant reduction of PAHs with 3 aromatic rings (94.4%) as well as with 5 aromatic rings (83.2%). The lowest rate of degradation (64.4%) was observed for hydrocarbons with 4 aromatic rings such as fluoranthene, benzo(a)anthracene, chrysene and pyrene. On the other hand, the highest level of degradation was determined for PAHs with 2 rings (100%), 3 rings (88%) and 6 aromatic rings in the molecule (86.9%) after composting with sawdust. Acenaphthene and pyrene were found to be the most resistant to biodegradation during both composting methods.