Continuously operated process performance of anaerobic digestion of chicken manure with feeding high solid concentration

Abstract: Chicken manure contains a lot of easily degradable organic matter suitable for anaerobic digestion (AD). But, a high concentration of ammonia nitrogen has posed a great challenge to the AD process. Diluting with water can be used to increase the influent volume. Correspondingly, the subsequent digestate yield can be reduced with the decrease of the nitrogen content, as well as the nutritional value of organic fertilizer. It is very necessary to balance between the dilution water and the solid concentration of feedstock for a higher solid AD process. However, only a few studies are reported on the high solid chicken manure AD at present. It is still lacking on the performance of AD under the feedstock with solid concentration, organic load, ammonia nitrogen concentration, and their synergistic effects. In this study, a long-term experiment was carried out to establish the stable and operational high solid AD process of chicken manure. The chicken manure with a high total solid (TS 15%) was also disposed of by the AD process under the hydraulic retention time (HRT) of 60 d through 155 days. The experimental results showed that the methane yield was 326 mL/g (Volatile Solids, VS) under the organic load rate (OLR) of 1.5 g/(L·d) and the total ammonia nitrogen (TAN) concentration of 7.5 g/L. The concentration of total volatile fatty acids (TVFA) was about 470 mg/L, and the pH was above 8.3, indicating the stable operation with low TVFA residues. The hydrolysis, acidogenesis, and methanogenesis efficiencies of chicken manure were 61%, 47%, and 47%, respectively. Among them, the same acidogenesis and methanogenesis efficiencies indicated that the TVFA maintained a low concentration because the TVFA production through the acidogenic phase was been mostly converted into methane. The relationship was also determined between the concentration of TS and TAN chicken manure with the conversion efficiencies of the hydrolysis, acidogenesis, and methanogenesis. The AD performance was also evaluated under different chemical oxygen demand (COD) concentrations of the influent and effluent. The total COD (TCOD) concentration of the feedstock was 171 g/L. The proportions of soluble COD (SCOD) and particulate COD (PCOD) were 26.5% and 73.5%, respectively. In the long-term AD process, the PCOD, VFA-COD, and CH4-COD accounted for 38.9%, 0.3%, and 47.4%, respectively. The biogas production was 2.5 L/d, including 76.3% CH4 and 23.7% CO2. The specific methanogenic activity (SMA) of the anaerobic sludge was 0.042 g/(g·d), indicating superior metabolic activity. The accumulation biogas yield and accumulation methane yield were 412 mL/g and 286 mL/g, respectively. The model fitting coefficient R2 was 0.998 by the Gompertz model. The kinetic constant was 0.202 d-1 and the correlation coefficient was 0.982 by the first-order model. Consequently, the degradation process of acetate was in line with the first-order kinetic characteristics, where the methane production rate was positively correlated with the concentration of acetate. The high solid AD of chicken manure can be widely expected to tolerate the extremely high concentration of ammonia nitrogen. Therefore, the improved AD can be used to greatly reduce the dilution water of feedstock and the digestate yield, compared with the conventional. This finding can also provide a promising potential for the engineering application of AD chicken manure in biogas plants.