Effectiveness of single-stage and sequential sludge digestion on removal of recalcitrant pharmaceuticals and conventional pollutants

Abstract The fate of pharmaceuticals, azithromycin (AZI), carbamazepine (CBZ), diclofenac (DCF), mefenamic acid (MFA), ibuprofen (IBP) and one ibuprofen metabolite (1-hydroxy ibuprofen (1-OH IBP)), were investigated in single-stage anaerobic (AN) and sequential anaerobic/aerobic/anoxic (AN/AERO/ANOX) sludge digestion under thermophilic conditions (55 °C). Intermittent aeration was used for the sequential AERO/ANOX vessel. Digesters showed an increase in CBZ, MFA and DCF at a hydraulic retention time (HRT) of 18 days as organic matter is degraded. However, the rate of increase for MFA was 10 times lower in sequential digestion compared to single-stage AN digestion (control). Sequential AN/AERO/ANOX digestion showed a 50% decrease in IBP levels while increasing IBPs aerobic metabolite 1-OH IBP by 12%. The control AN digester showed a 62% increase in IBP and a 34% decrease in 1-OH IBP. Sequential digestion provided other benefits, including reduced ammonia generation (44.5%), improved solids removal (4.1%) and enhanced digestate dewaterability (20%) compared to the control.