Methane production potential and kinetic characteristics of the anaerobic digestion of erythromycin fermentation residues

Erythromycin fermentation residue (EFR) has been classified as a hazardous material, due to the high content of erythromycin and the spread of antibiotic-resistance genes. The removal of erythromycin from EFR can also be required for the hydrothermal pretreatment with the ultra-high temperature and high pressure for a long time (>160°C, >60 min), due to the stable 14-member-ring and the complex matric structure of mycelium. For instance, the spray drying flowered by incineration is expensive to afford, as well as organic wastes. Anaerobic digestion (AD) has been supposed as an alternative technology to reduce erythromycin with low energy consumption and bioenergy recovery. This study aims to investigate whether the EFR can be used as the substrate for AD without pretreatment. Methane potential experiments were then carried out via batch assays under different temperatures (20, 35, and 55℃). EFR with liquid phase (named LEFR) was chosen as the raw material. Modified Gompertz model and the first-order kinetic model were applied to verify the capacity and efficiency of EFR and LEFR for methane production. The content of nitrogen in EFR was 8.1% (using day matter), according to the elemental analyzer and Buswell equation. In order to avoid the influence of ammonia inhibition, 1 g of volatile solid (VS) of EFR or LEFR was added to each strum bottle. The highest concentration of ammonia nitrogen in theory was controlled within 1 250 mg/L. The methane potential experiment showed that the highest accumulated biogas production was 675.2 mL/g (methane of 460 mL/g) obtained from EFR under mesophilic conditions, which was 163% and 55.5% higher than that under psychrophilic and thermophilic conditions, respectively. While thermophilic conditions showed that higher maximal methane yield rates (R_max) were achieved in 32.30 and 11.70 mL/(g·d) for the EFR and LEFR than lower temperatures. In addition, the first-order kinetics of EFR presented outstandingly two-stage characteristics, compared with the LEFR. Among them, the first kinetics at 55℃ (k₁=0.142 2 d^-1 and k₂=0.091 0 d^-1) were significantly higher than that at 35℃ (k₁=0.036 3 d^-1 and k₂=0.031 3 d^-1) and 20℃ (k₁=0.042 9 d^-1 and k₂=0.0 295 d^-1). The lag period (λ) was ranked in the descending order of the thermophilic > psychrophilic > mesophilic conditions for the methane production from EFR. Even though λ under the thermophilic condition as long as 7.06 d, it was only 1.43 d at the mesophilic condition, which was common for the AD of high solid wastes. After mass balance calculation, the solid needed to be hydrolyzed, of which some was removed in LEFR together with soluble chemical oxygen demand (SCOD). The hydrolysis efficiency of EFR increased step by step with the increase of digestion temperature, with the highest hydrolysis efficiency of 87% at thermophilic conditions. The highest methane conversion efficiency of EFR was 72.2% under mesophilic conditions, rarely unconverted volatile fatty acids. In conclusion, the relatively high hydrolysis efficiency and methane yield indicated that the residual erythromycin had no inhibitory effect on the anaerobic bacteria and methanogens. The recommended hydraulic retention time (HRT) for AD of EFR under mesophilic conditions was 35 d, indicating the time reaching 80% of the maximum biogas production. Therefore, the EFR was suggested as a promising organic waste to be treated via AD technology directly without any pretreatment. The finding can also provide a new idea to reduce the resource treatment of antibiotic fermentation residue.