Effect of chlortetracycline concentration on mesophilic anaerobic digestion characteristics and antibiotic degradation of chicken manure Qiang

Anaerobic digestion is a promising robust technology, which can not only reduce pollution of chicken manure but also produce energy, especially mesophilic anaerobic digestion of chicken manure has been widely used due to energy saving and long-term stability. In recent years, excess amount of antibiotics has been used in raising broilers to promote fast growth and prevent from diseases. The antibiotics residues problem in China rising day by day as compared to other countries, of which tetracycline antibiotic residues has been considered dangerous for living organisms. In China, it is reported that chlortetracycline residues especially in chicken manure was 563.8 mg/kg. Although, the toxicity threshold of antibiotics in soil regulated by Veterinary International Conference on Harmonization is 100 μg/kg, it has been found that, the chlortetracycline residues more than 50% of the soil sample in China has been found higher than the threshold. Anaerobic digestion technology plays an important role in degrading antibiotics resides, this process not only wasaffected by the antibiotics concentration and category, but also more importantly by substrate category. Chicken manure has greatly potential to produce higher biogas production per organic matter than swine and cattle manure, but the effect of chlortetracycline contamination on biogas production of mesophilic anaerobic digestion of that is still unclear, removal of chlortetracycline during anaerobic digestion of chicken manure also unknown. The effect of chlortetracycline on hydrolysis, acidogenesis, acetogenesis and methanogenesis during anaerobic digestion of chicken manure has not been deeply studied yet. Therefore, this study aimed to explore the effects of antibiotics on mesophilic anaerobic digestion of chicken manure. Influence of a wide range of chlortetracycline concentrations (4-200 mg/L CTC) on anaerobic digestion of chicken manure were investigated by batch experiments. The obtained results indicated that mesophilic methane production was increased when CTC concentration was less than 20 mg/L. More specifically, the ultimate increase rates of hydrolysis, acidogenesis, acetogenesis and methanogenesis for methane production were 12.69%, 11.55%, 11.31% and 9.82%, respectively. Indeed, the removal efficiency of CTC was increased from 59.87% to 71.95%. Further analysis showed that methane production was increased as a result of the transformation of extracellular polymer substances (EPS) from bound EPS (LB-EPS and TB-EPS) to slime EPS (S-EPS), which promoted the hydrolysis, In addition, the degradation of CTC produced the carbon sources for microorganisms. However, mesophilic methane production was inhibited when CTC concentration was higher than 60 mg/L. Moreover, the inhibition rate was increased with raising CTC concentration, among them the inhibitory effect of 200 mg/L CTC concentration was the strongest than others, the largest inhibition rate of 200 mg/L CTC concentration for hydrolysis, acidogenesis, acetogenesis and methanogenesis was 16.48%, 18.54%, 18.96% and 19.94%, respectively, and the removal efficiency of CTC reduced up to 43.4%-51.44%. EPS increased by 13.81%-39.23% compared with control, of which the concentration of EPS protein increased from 943.01 mg/L to 1 083.69-1 338.20 mg/L, it is assumed that bacterial resistance for CTC was increased by stimulating EPS secretion. Analysis of the distribution of residual CTC within the digester indicted that 0.46%-3.13% of CTC was existed in liquid digester, while 96.87%-99.54% of CTC was found in solid digester, which posed a significant threat to the environment. Further safe treatment was needed before solid digester used as organic fertilizer into field. © 2019, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.