Long-term inhibition of chlortetracycline antibiotics on anaerobic digestion of swine manure

This study aimed to identify whether chronic effects are present in the anaerobic digestion (AD) of swine manure (SM) containing chlortetracycline (CTC), which is one of the major broad-spectrum veterinary antibiotics, and to elucidate the long-term inhibitory effects and recovery from the inhibition based on AD performance and mi-crobial community. Two continuous-stirred tank reactors treating SM with and without CTC spiking (3 mg/L) were operated for 900 days. Due to the degradation and transformation, the total concentration including CTC's epimer and isomer in the test reactor was 1.5 mg/L. The exposure level was determined according to probabi-listically estimated concentrations with uncertainties in field conditions. Until the cessation of CTC exposure on day 585, the methane generation of test reactor continuously decreased to 55 +/- 17 mL/g-VS/day, 53% that of control. The methane generation and organic removal were not recovered within 300 days after the CTC exposure was stopped. During the experiment, stability parameters such as pH, total ammonium nitrogen, the composition of methane and alkalinity were the same for both reactors. The concentration and composition of VFAs in the test reactor were different with those of control but not in inhibition level. Microbial profiles revealed that reduction in bacterial diversity and changed balance in microbial species resulted in the perfor-mance downgrade under the long-term antibiotic pressure. Since it is hard to recover from the inhibition and difficult to predict the inhibition using physicochemical indicators, continuous exposure to CTC needs to be avoided for the sustainable management of AD plants treating SM.

Automated summary

This study aimed to investigate the chronic effects and recovery from inhibition in the anaerobic digestion of swine manure containing chlortetracycline. Two reactors were operated for 900 days, with one reactor spiked with CTC. Methane generation and organic removal did not recover within 300 days after CTC exposure was stopped, due to reduced bacterial diversity and shift in microbial species under long-term antibiotic pressure. Continuous exposure to CTC should be avoided for sustainable management of AD plants treating swine manure.