Combining anaerobic digestion and ozonation to remove PAH from urban sludge

Combination of anaerobic digestion and ozonation was investigated in order to reduce sewage sludge production. No previous data concerning the impact of such combined process on sludge quality are available. Moreover, control of sludge quality, particularly its concentration in metals or trace organic pollutants is fundamental to limit potential risks owing to sludge land application. Amongst these pollutants, polycyclic aromatic hydrocarbons (PAH) are particularly monitored because of their toxic and carcinogenic properties. Previous results have shown the partial removal of PAH (50%) during anaerobic sludge stabilisation. The aim of this study was to optimise PAH removal by combining anaerobic digestion with ozonation. Ozonation of anaerobically digested sludge improved the PAH removal rate (61%). An additional enhancement (up to 81%) of the PAH removal rate was obtained by the addition of hydrogen peroxide during ozonation. Similar performances were achieved by the use of surfactants, which enhanced PAH bioavailability. Finally, in order to evaluate the feasibility of the combination of the anaerobic digestion with ozonation, the biodegradability of the digested sludge ozonized in presence of surfactants and H2O2 was measured.