Characteristics of hydrothermal treatment for the disintegration of oxytetracycline fermentation residue and inactivation of residual antibiotics

Oxytetracycline fermentation residue (OFR) can be managed by bioprocesses to recycle organic substances. However, the organic materials present are resistant to biodegradation due to protection by the OFR matrix structure, and the high levels of residual oxytetracycline have unavoidably negative impacts on microorganisms in bioprocesses and result in the poor biodegradability of OFR. Hydrothermal treatment (HT) is promising for processing OFR to improve its biodegradability, but there is still insufficient practical experience with HT of OFR. To explore the applicability of employing HT as a pretreatment to improve the biodegradability of OFR, HT processes at temperatures varying from 90 °C to 180 °C were performed on OFR to disintegrate its matrix and inactivate the high levels of residual oxytetracycline. The results demonstrated that performing HT clearly enhanced the production of hydrophilic matter including soluble chemical oxygen demand (SCOD), soluble carbohydrates and soluble proteins. Nevertheless, the drawbacks of HT for degrading soluble organics and mineralizing total organics were clearly observed at 180 °C. Efficient destruction of the OFR matrix structure was speculated based on the release of carbohydrates and proteins and was visually confirmed by photomicrographs and scanning electron microscopy graphs. The disappearance of oxytetracycline fit a pseudo first-order kinetics model and the toxic intermediates of oxytetracycline, including 4-epi-oxytetracycline, α-apo-oxytetracycline and β-apo-oxytetracycline, could not be persistent during HT. Antibacterial activity linked to oxytetracycline greatly decreased after HT. The conditions of 150 °C and 180 min resulted in relatively good results in SCOD release and oxytetracycline inactivation. All these suggest that HT effectively improves the biodegradability of OFR.