Post-aerobic treatment to enhance the removal of conventional and emerging micropollutants in the digestion of waste sludge

Low content of micropollutants in sewage sludge, essential to allow its safe re-use in agriculture, requires effective removals during the digestion phase. To this purpose, we investigated the performance of the anaerobic-aerobic sequential digestion process applied to real waste sludge in the removal of several classes of standard pollutants, i.e. extractable organic halogens (EOXs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), di(2-ethylhexyl)phthalate and alkylphenolethoxylates. In addition, emerging pollutants were also investigated based on their widespread occurrence and on their physicochemical characteristics and eco-toxicological relevance: quaternary ammonium compounds, a number of pharmaceuticals and selected biocides. The anaerobic step was conducted at mesophilic conditions, while two operating temperatures were tested for the post-aerobic treatment, i.e. 20 and 37 °C, respectively. Results showed that the post-aerobic digestion step enhanced the removal of all investigated standard and emerging micropollutants, even in presence of high accumulation in the anaerobic digestate (occurred for some PAHs and PCB congeners). Increased removals (up to 30%) have been generally observed at 37 °C aerobic temperature in comparison with tests at T = 20 °C for all investigated organic micropollutants, with the only exception of halogenated compounds (i.e. EOXs and PCBs). Low biodegradability and high bioaccumulation of the investigated pollutants were successfully faced by the sequential process, which has been demonstrated as an effective alternative solution to produce digested sludge for safe agricultural re-use.