Optimization of substrate composition in anaerobic co-digestion of agricultural waste using central composite design

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Biochemical methane potential (BMP) tests and modelling of anaerobic co-digestion (AcoD) of ternary mixtures of agricultural waste, i.e., sugar beet root waste (BW), cow dung (CD), and poultry manure (PM), are presented. According to a two-factor central composite design (CCD), 12 BMP tests were conducted under mesophilic conditions for 30 days (d). CD mass fraction (0.146–0.854) in the animal manure mixture and C/N ratio (13.515–30.485) of ternary waste mixture were selected as process factors. Experimental values of ultimate cumulative methane yield as process response were in the range of 105.32–356.10 mL/g VS. A first-order rate model was applied to predict the process kinetics. Enhanced values of methane production rate, evaluated as rate constant (0.044–0.123 d−1), were obtained for lower levels of CD mass fraction and higher levels of C/N ratio. Response surface methodology (RSM) was applied to assess the effects of process factors on the response and to find the optimum factor levels leading to a maximum performance. A maximum value of response variable of 347.48 mL/g VS was predicted for a CD fraction of 0.323 and a C/N ratio of 26.24. Values of experimental ultimate methane yield of 358.45 ± 33.40 mL/g VS, obtained in the BMP tests at optimum levels of process factors, were close to that of the maximum response predicted by the statistical model, demonstrating the model validity. Moreover, a significant synergistic effect (an increase in methane yield by 41.2%) related to the weighed experimental BMP of substrate mixture was obtained under optimum conditions.