Modeling Cumulative Methane Volume Produced in Anaerobic Co-Digestion of Poultry Litter and Wheat Straw using Developed Novel General Gompertz Models

Abstract. Modified Gompertz model has been widely used to simulate the kinetics of microbial growth and bio-products. However, the intrinsic shortcoming of this model rests with its inability of meeting the initial condition of bio-products, i.e., the bio-product mass is zero at the beginning. A general Gompertz model for bacterial growth was developed in this study based on a defined specific bacterial growth rate, which can relate to the original Gompertz model. The developed model for bacterial growth was further extended to account for bio-products and substrate consumption by means of the corresponding yield coefficients. All the models developed herein could meet the initial conditions of bacterial growth, bio-products, and substrate consumption. Batch experiments of anaerobic co-digestions of poultry litter and wheat straw at 2% total solids level consisting of 100, 75, and 50 VS% of poultry litter were conducted to verify the model. The maximum methane volumes and specific methane yields were obtained (1699 mL and 207 mL (g initial VS)^‑1 for 50% co-digestion, 1016 mL and 134 mL (g initial VS)^-1) for 75%, and 35.8 mL and 5.18 mL (g initial VS)^‑1) for 100%). The lowest methane volume of 100 VS% of poultry litter could be attributed to its lowest C:N ratio (9.63) and highest pH value (7.71). Volatile solid removals by 46.2, 54.4, and 53.3% were achieved by 100, 75, and 50 VS% of poultry litter co-digestions, respectively. The developed models for bio-products were successfully applied to cumulative methane volumes produced from anaerobic co-digestion.