Identification and fate of antibiotic residue degradation during composting and vermicomposting of chicken manure

Oxytetracycline (OCT) is a broad-spectrum antibiotic commonly used for broiler production to enhance growth and feeding efficacy. Concern has been expressed that the parent compounds of antibiotics such as OCT or their bioactive metabolites may be excreted through animal feces or urine with negative consequences for the environment when applied to agricultural lands. An integrated system involving a combination of thermophilic composting and vermicomposting using Eisenia fetida was investigated as a strategy for reducing the concentrations of oxytetracycline and its metabolites (4-epi-oxytetracycline EOTC], α-apo-oxytetracycline α-Apo-OTC] and β-apo-oxytetracycline β-Apo-OTC]) in chicken manure. Treatments consisted of combinations of chicken manure and waste paper to produce waste mixtures with C: N ratios of 20, 30, 40, 50, 60 and 70 (T1, T2, T3, T4, T5 and T6, respectively). These were subjected to thermophilic composting for 20 days followed by vermicomposting with E. fetida for 7 weeks. The oxytetracycline concentration at the start of the experiment was in the range of 123.3–35.2 mg/kg, which reduced to 44–25.3 mg/kg after the thermophilic composting, and further reduced to 35.4–20.7 mg/kg at the end of the vermicomposting stage. The maximum overall percentage reduction of residual oxytetracycline in the final product was recorded in T3 (C:N ratio 40). The maximum percentage reduction of 84.3% was recorded in C: N ratio of 40 (T3) in which the metabolite degradation products were reduced to 3.1 mg/kg of 4-epi-oxytetracycline (EOTC), 3.9 mg/kg of α-apo-oxytetracycline (α-Apo-OTC) and 4 mg/kg of β-apo-oxytetracycline (β-Apo-OTC).