Occurrence of estrogens in sewage sludge and their fate during plant-scale anaerobic digestion

Estrogens, which contribute greatly to the endocrine-disrupting activity in sewage, are partially sorbed onto particulate matter during sewage treatment. We thus investigated the occurrence of estrogens in different kinds of sludge and throughout a plant-scale anaerobic digestion process. The analytical method was first validated when sorption interaction between spiked estrogens and sludge could occur. Hence, the recovery ratio of estrone (E1), 17β-estradiol (E2), estriol (E3) and 17α-ethinylestradiol (EE2) were determined when added to liquid sludge and mixed under various conditions. We show that minor non-extractable residues were formed (5–10%), suggesting that the sorption interaction established with sludge did not limit estrogen extraction. Estrogen concentrations measured in collected samples varied with sludge type. Secondary sludge showed higher E1 contents than primary sludge: respectively, 43 and 8 ng g⁻¹ dry weight (dw). Two pathways of E1 production during secondary treatment are proposed to explain such a result. Higher estrogen concentrations were found in secondary sludge from a conventional plant (55 ng g⁻¹ dw) compared to those from an advanced plant (13 ng g⁻¹ dw). Based on estimated estrogen concentrations in sewage, we conclude that operating parameters play a role in the sorption of estrogens during secondary treatment. Also, the hydrophobic properties of the estrogens influenced the individual adsorption of each molecule. Thus, E3 showed the highest estimated concentrations in sewage but very low concentrations in sludge. Finally, plant-scale anaerobic digestion showed low efficiency (<40%) for removing estrogens and, regarding the final dewatering process, concentrations increased for E2 and EE2.

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