Effects of Lighter Dose of Oxytetracycline on the Accumulation and Degradation of Volatile Fatty Acids in the Process of Thermophilic Anaerobic Digestion of Swine Manure

Oxytetracycline (OTC) is a commonly used antibiotic in livestock farming for controlling intestinal and respiratory infections in farm animals. However, the absorption of antibiotics by animals is limited, and most antibiotics are excreted in the original form with manure, which will have an impact on the environment. The removal of antibiotics from swine manure could generally be performed via anaerobic digestion (AD). In this study, the effect of oxytetracycline (OTC) at doses of 0.1, 0.5, and 1.0 mg/L on the thermophilic anaerobic digestion of swine manure (55 °C) in batch digesters was studied. The methane production, volatile fatty acid (VFA) levels, and dissolved organic matter (DOM) were determined and compared with the control (0 mg/L of OTC). The results indicate that (1) OTC at 0.1 mg/L had no inhibitory effect on methane production or on the accumulation of VFAs, while 0.5 mg/L and 1.0 mg/L inhibited methane production, with inhibition rates of 4.03% and 14.12% ( p < 0.05), respectively; (2) the VFAs of each reactor peaked on the first day of the reaction, and as the OTC dose increased from 0 to 1.0 mg/L, the maximum VFA accumulation increased from 1346.94 mg/g to 2370 mg/g of volatile solids (VS); and (3) oxytetracycline (0.5 and 1.0 mg/L) could promote the temporary accumulation of propionic acid, which did, however, not result in significant VFA accumulation. Further, OTC at 1.0 mg/L can promote DOM production, and therefore, VFA accumulation.