Organic substrates evaluation based on biochemical methane potential and degradation kinetic

Abstract: The purpose of this paper was to develop a standardized method for substrate evaluation process in biogas plant or relevant anaerobic digestion laboratory, which could increase the efficiency, strictness and scientificity of current substrate evaluation process when the substrate evaluation test standard was absent. The substrate assessment method was proposed by analyzing the data of accumulated gas production and the degradation dynamics of substrates with the help of mathematical model. The first-order model, modified first-order model and Gompertz model were used to fit the biochemical methane potential (BMP) curve of wheat straw, separated stem and leaf of maize, dry and fresh alfalfa, chicken manure, food waste, dead chick and adult chicken respectively, and in the meantime the kinetic constant and the estimated maximum BMP of these 9 substrates were estimated. The simulation performances of these 3 models were statistically analyzed and compared to find the suitable model for the substrate evaluation process. The modified first-order model had the highest simulation precision of BMP curve while the first-order model showed the lowest precision. The Gompertz model showed a moderate simulation precision of BMP curve, and the maximum BMP value simulated by the Gompertz model was more close to the experiment value among the 3 models. The BMP1% (accumulated biogas production when single-day biogas production accounted for 1% of BMP) was raised in German guideline VDI 4630, which was a standardized way to define BMP value and corresponding necessary experiment time, thus avoiding the subjective judgment during evaluation process. In the meantime, when adopting the BMP1% as judging index, the accumulated methane production accounted for more than 90% of the ultimate accumulated methane production, and the necessary experiment period was reduced by 27.94%-70.58% compared with the whole experiment period. The BMP values (based on BMP1%) of 9 evaluating substrates, wheat straw, separated stem and leaf of maize, dry and fresh alfalfa, chicken manure, food waste, dead chick and hen, were 220.49, 241.01, 262.10, 310.84, 305.80, 508.41, 520.90, 630.7 and 618.05 mL/g (volatile solid, VS) respectively. Fat-containing substrates, like dead chick and hen, possessed the highest BMP. The kinetic constant generated from the model simulation was objectively referred to the degradation speed of substrate. The analysis results showed that among 9 substrates, the dry and fresh alfalfa had the highest kinetic constant, which meant they were very easy to degrade. The kinetic constants of other cellulosic substrates (except maize stalk and wheat straw), chicken manure and food waste were just lower than alfalfa, and they were also easy to degrade in anaerobic digestion process, while the degradation of dead adult chicken was the slowest because of the presence of oil. By comparing the simulation performance of the 3 models, the Gompertz model was recommended for the maximum BMP estimation and the modified first-order model was more suitable for kinetic constant estimation. The introduction of BMP1% improved the efficiency of substrate evaluation for biogas plants. As the substrate degradation speed was also one of the important factors for substrate evaluation, the comparison of substrate's kinetic constants was necessary as well.