The effects of chlortetracycline on anaerobic digestion of chicken manure and the role of extracellular polymeric substances

Abstract

In this study, the process stability and the protective role of extracellular polymeric substances (EPS) were evaluated during the anaerobic digestion (AD) of chicken manure (CM) suffering chlortetracycline (CTC) concentrations from 0 mg/L to 100 mg/L in a continuous stirred tank reactor (CSTR) operated for 270 days. Results showed that during the phases with low levels of CTC (0, 1.25, and 2.5 mg/L), the volume biogas production rate increased from 0.77 ± 0.01 L/(L·d) to 1.15 ± 0.02 L/(L·d) with an obvious decrease in the concentration of VFAs from 2.57 ± 0.07 g/L to 1.17 ± 0.08 g/L. The reactor remained stable with continued operation as the CTC in the CM spiked to 5 mg/L and 10 mg/L, respectively. During the phases with high levels of CTC (25, 50, and 100 mg/L), despite a reduction in biogas production of 18%, 43% and 56%, respectively, compared to the steady stage, the reactor finally could maintain the inhibition-steady stage. This stage was characterized by steady biogas production along with accumulating VFAs and ammonia nitrogen, which increased the inhibition thresholds of CTC on the AD of CM. An appreciable increase in EPS was identified. Spectral analysis results showed the fluorescence of protein-like in EPS was quenched with increasing dosages of CTC. The quenching constant (Kq) induced by CTC for tyrosine was found to be 3.969 × 1012 (R2 = 0.942), implying that the increased CTC prompt anaerobic microbes excrete more EPS to protect themselves from the CTC toxins by the formation of the complex tyrosine-CTC. This protection led to a decrease in the biodegraded CTC from 61.6 to 40.9%, and an increase in the adsorbed CTC on EPS in sludge from 35 to 56.1%. The results indicated that the absorbed CTC on EPS may pose a risk to the soil ecosystem due to the agriculture utilization of the biogas residue.