The growth promotion effect of veterinary antibiotics on livestock enabled a factory farming system. It brought on the overuse of veterinary antibiotics in the livestock industry. The consumption of antibiotics for livestock increased from only hundreds of tons in 1950 to 63,200 tons in 2010. Since veterinary antibiotics are excreted with the livestock manure after the administration, the overuse of veterinary antibiotics has led to continuous contamination of the aquatic and terrestrial environment via the livestock manure. In the environment, veterinary antibiotics caused many problems such as microbial toxicity, the prevalence of antibiotic resistant genes and bacteria, the emergence of super-bacteria, and exposure of veterinary antibiotics to humans.

Anaerobic digestion has been getting more attention as a sustainable treatment method to treat organic wastes such as livestock manure and food waste in recent decades. At the same time, there has been a growing concern that anaerobic digestion systems treating livestock manure will be the reservoirs supplying veterinary antibiotics to the environment. The fate of veterinary antibiotics in AD will change and affect the environmental concentration and hazardous in the environmental metrics. In addition, even after excretion with livestock manure, veterinary antibiotics maintain their antimicrobial activity. Since the AD process is a series of microbial metabolizations, the antimicrobial effect can undermine the performance of theAD system. Understanding veterinary antibiotics’ fate and effects on anaerobic digestion are necessary to prevent the prevalence of veterinary antibiotics in the environment and mitigate the anthropogenic environmental problems.

In this regard, this study aimed to evaluate the fate and long-term effects of chlortetracycline on the AD of swine manure (SM). Chlortetracycline (CTC) was selected as representative veterinary antibiotics due to its highest usage, excretion rate, and microbial toxicity. Specific objectives were to predict environmental concentrations of chlortetracycline in swine manure and soil for evaluation of the ecological risk of the chlortetracycline, to evaluate the transformation of chlortetracycline in anaerobic digestion treating swine manure by developing a mathematical model, and to elucidate long-term effects of chlortetracycline on the performance and microbial community of the anaerobic digestion of swine manure.

In order to predict concentrations of CTC generated from slurry pit and evaluate its ecological risk in soil, slurry pit farm practices and their uncertainties were considered based on the European Medicine Agency guidelines. Sensitivity analysis was conducted on the exposure estimation of CTC in soil employing the Monte Carlo simulation. Transformation of CTC in swine manure and anaerobic digestate was investigated in batch test. Mass balance-based ordinary differential equations were established by assuming their reversible and irreversible 1 st order kinetics reactions. Simulation was conducted to predict CTC and transformation products in CSTR treating swine manure by using estimated kinetic constants and the estimated concentrations of CTC and transformation in swine manure from slurry pit. Two continuous-stirred tank reactors treating SM w/ and w/o CTC spiking (3 mg/L) were operated for 900 days to assess the long-term effects of CTC on the anaerobic digestion. The test concentration was determined based on the predicted environmental concentration of CTC in swine manure from slurry pit. Performance amd stability parameters such as methane generation, organic removal, volatile fatty acid concentrations, ammonia concentration, alkalinity, etc. were monitored. Microbial diversity and community were evaluated to elucidate the long-term inhibition.

The predicted environmental concentrations of CTC in the slurry pit and soil were in a range of 0.54 - 5.64 mg/kgmanure and 3.42 - 67.59 μg/kgsoil, respectively, for a 90% confidence level. The predicted ranges included the measured values reported in previous studies. The probability of risk quotient was estimated at 9.3% based on the Monte Carlo simulation. If anaerobic digestion is applied before fertilization/composing, the probability of ecological risk is significantly reduced to 0.6 %. The four most influential factors on the exposure to CTC in soil were identified as nitrogen in fertilizer/compost, daily cleaning water usage, the ratio of sick pigs requiring antibiotics, and pit emptying cycles.

In the swine manure and its anaerobic digestate, epimer, isomer, and epiisomer of CTC were transformed from the CTC. They accounted for 60 - 93 % (w/w) of the residual total CTCs. The CTC is expected to be transported by solid-phase of anaerobic digestate (> 70 % by wt.) while most of ECTC, EICTC, and ICTC remained in the liquid phase (> 60 % by wt.). A series of ordinary differential equations based on the 1 st order kinetics demonstrated the kinetics of degradation and transformation in the swine manure and its anaerobic digestate. The simulation included observed concentration of CTC from full-scale biogas plants treating swine manure.

At around day 300 after theCSTR operation, total chemical oxygen demand reduction and methane generation decreased in the test reactor due to the reduced methane generation rate and mineralization ratio of the SM. The methane generation was not recovered during 300 days even after the CTC exposure was stopped. The inhibition is caused by a reduction of microbial diversity and change of the microbial community to an inefficient state in terms of the AD performance.

The concentration of CTC in slurry pit and manure compost-applied soil can be efficiently estimated by considering uncertainties of field conditions. The ecological risk of CTC in the soil is probable and affected by slurry pit farm practices. It is difficult to reduce the risk under 5% by controlling the farm practices. AD of swine manure is recommended not only for organic waste treatment but also for the ecological risk reduction under 0.5%. Transformation of CTC during the AD implies that the environmental risk based on the concentration of CTC can be underestimated due to unnoticed reversible transformation from ECTC to CTC and the expression of ARGs by ICTC. The suggested model can be utilized for the prediction of the concentrations of CTC and the transformation products in a CSTR treating swine manure. Batch test for estimating transformation kinetic constants is recommended to be conducted without degradation until the equilibrium achieved. It is hard to predict the inhibition by using physicochemical indicators of the AD system and recover from the inhibition within 300 days. Continuous exposure to CTC for at least 100 days needs to be avoided for the sustainable management of AD plants treating SM.