New hydrolysis products of oxytetracycline and their contribution to hard COD in biological effluents of antibiotic production wastewater

With the increasing demand for stringent discharge regulations, the efficient removal of non-biodegradable organic (hard COD) from the biological effluent of pharmaceutical wastewater is becoming increasingly important. Enhanced hydrolysis pretreatment has been used to remove antibiotic potency in antibiotic production wastewater before biological treatment, while the resulting transformation products (TPs) and their contribution to hard COD are largely unknown. Herein, we combine target and non-target analysis aiming to find the major oxytetracycline (OTC) TPs after enhanced hydrolysis pretreatment and to quantify their contribution to the hard COD in biological effluents. Three literature-known and 15 yet unknown TPs, most of which could not be biodegraded, were identified during lab-scale hydrolysis of OTC under neutral conditions. Nuclear magnetic resonance and combined density functional theory were used to determine and infer the molecular structures and possible hydrolysis pathways of them. Furthermore, two stable and dominant TPs, M390s (M390-1 and M390-2), were separated, purified, and structurally identified, which could account for 21.0% and 14.9% -20.4% (molar ratio) of the initial OTC in the lab-scale and field studies, respectively. More importantly, M390s account for 4.5---7.8% of the hard COD in biological effluents. The results indicate that the contribution of antibiotic TPs to hard COD in wastewater could not be ignored, and provide more knowledge regarding the efficient treatment of antibiotic production wastewater.