Occurrence of trace organic contaminants in wastewater sludge and their removals by anaerobic digestion

• 18 out of 36 monitored TrOCs were consistently detected in raw primary sludge.
• These TrOCs occurred predominantly in the solid phase.
• TrOC removal by anaerobic digestion was governed by their molecular structure.
• An increase in SRT led to an increase in biogas production and VS removal.
• SRT increase did not lead to any discernible increase in TrOC removal.

This study aims to evaluate the occurrence of trace organic contaminants (TrOCs) in wastewater sludge and their removal during anaerobic digestion. The significant occurrence of 18 TrOCs in primary sludge was observed. These TrOCs occurred predominantly in the solid phase. Some of these TrOCs (e.g. paracetamol, caffeine, ibuprofen and triclosan) were also found at high concentrations (>10,000 ng/L) in the aqueous phase. The overall removal of TrOCs (from both the aqueous and solid phase) by anaerobic digestion was governed by their molecular structure (e.g. the presence/absence of electron withdrawing/donating functional groups). While an increase in sludge retention time (SRT) of the digester resulted in a small but clearly discernible increase in basic biological performance (e.g. volatile solids removal and biogas production), the impact of SRT on TrOC removal was negligible. The lack of SRT influence on TrOC removal suggests that TrOCs were not the main substrate for anaerobic digestion.