Modification And Extension Of ADM1 For Model Investigation Of Methanogenesis In Anaerobic Digestion System

The anaerobic digestion model No.1（ADM1）is the most common model of anaerobic digestion（AD）,describing the major biochemical and physicochemical processes.With the development of AD process technology,the enrichment of application scenarios,and the progressive research on organic waste treatment and syngas biomethanation,the system boundary and framework structure of the initial ADM1 can no longer cover the existing AD systems.Due to many simplifications and assumptions,many experimental processes are also neglected in the initial ADM1.In addition,there are limited examples of practical applications of ADM1,and the potential of the model to simulate actual working conditions is not yet known.There are deficiencies and compromises in the biochemical structure,physicochemical structure,inhibitor term and practical application of ADM1,which need to be further modified and extended.Therefore,the main studies of this research are as follows:（1）The typical methanogenic pathways were extended and the ammonia inhibitor term was modified in ADM1.New established parameter estimation method provided a solution to the situation of complex models with no reference values for parameters or too many parameters needed to be estimated.The model was calibrated based on experiments with three typical methanogenic precursors（acetate,formate,and H₂/CO₂）,and the model was validated based on experiments with glucose methanogenesis（including all three typical methanogenic pathways mentioned above）.The model predicted good R²>0.91 for methane production and volatile fatty acids（VFA）.Further simulations based on modified ADM1 evaluated the carbon flux distribution of the glucose methanogenic system and found that ammonia inhibition led to a significant accumulation of propionate（21.31%of carbon accumulation）and that the formate methanogenesis was significant（28-34%of contribution to methane production）.The enrichment of Methanobacterium at high ammonia levels（41%relative abundance）indicated that formate-utilizing methanogenesis and hydrogenotrophic methanogenesis were more ammonia tolerant.The model simulation results and gene sequencing results confirm and support each other.（2）Biofilm kinetic equations were introduced into ADM1 to quantitatively assess the community function of biofilms on different exogenous media surfaces.Model calibration was based on the data from batch experiments and model validation was based on data from semi-continuous experiments,obtaining satisfied simulation results for methane production and VFA with R²>0.96.The maximum ammonia inhibition parameters(0.17–0.23kg COD·m^-3for K_INH4+)were obtained in the model describing the simultaneous addition of magnetite powder and foam filler,indicating the simultaneous addition of two exogenous media was the Pareto-optimal solution for increasing methane production and reducing propionate accumulation.Combining ADM1 simulations and gene sequencing results,it was found that biofilm methanogenesis was more kinetically advantageous than liquid-phase methanogenesis,with higher values of maximum specific uptake rate k_m(up to42 kg COD substrate·（kg COD biomass·d）^-1)and lower values of half-saturation rate K_s(down to 1.2×10^-5kg COD·m^-3),which was the inner kinetic mechanism for the effective mitigation of ammonia inhibition by magnetite powder and foam filler.（3）Lactate related process and metal ion kinetic equations were added to ADM1 to reveal the black box of methanogen’s response to metal ions from the kinetic perspective.Model calibration and validation were performed using data from the granular sludge experimental group and activated sludge experimental group,respectively,and good simulations of methane production and intermediate metabolites were obtained（R²>0.80）.The model describing the addition of Ca had the lowest value of 0.055 kmol·m^-3for the metal ion kinetic parameter K_IMI,indicating the addition of Ca promoted the utilization of acetate and H₂/CO₂by methanogen.The validated model was further applied to assess the contribution of functional enzymes of the acetotrophic and hydrogenotrophic methanogenesis,and it was found that the enzymatic contribution of the two pathways was most balanced（close to 1.0）in the Ca-added reactor.The simulation results were consistent with experimental results on microbial community（Methanosaeta enrichment with relative abundance of 49%-77%）and functional enzyme abundance（EC 1.1.1.95enhanced by 25%-50%）,revealing that the mechanism of Ca alleviating ammonia inhibition is a balance of enhanced functional dehydrogenase and rebuilding protein structure.（4）The gas-liquid mass transfer process was extended to the physicochemical structure,the syngas inhibitor term was added to the biochemical structure,and the syngas biomethanation process was established to extend ADM1 from lab-scale to pilot-scale.ADM1 was developed with laboratory data,and the R²reached 0.86,indicating good simulation results.ADM1 was validated with pilot data,whose R²for methane production,CO and H₂ranged from 0.61-0.89,with possible effect of tar in the actual syngas.The CO kinetic parameters Y_CO(0.025 kg COD biomass·（kg COD substrate）^-1),k_mCO(75 kg COD substrate·（kg COD biomass）^-1),K_sCO(8×10^-6kg COD·m^-3),k_decCO(0.02 d^-1)provided reference values for other model work.ADM1 was further assessed the contribution of the methanogenic pathway and compared to the results from 16S r RNA gene sequencing.The modified and extended ADM1 could describe and predict the syngas biomethanation process from macro and micro perspectives.（5）Ethanol related process and tar kinetic equation was created in ADM1,and the mapping relationship between the synthetic tar（ST）group and the actual tar（RT）group was established under the guidance of the equivalence method.Model calibration and validation were based on ST group data and RT group data,repectively,improving the R²of simulated methane production to between 0.80 and 0.97.The validated ADM1 evaluated COD fluxes and found that the distribution pattern of COD fluxes in the ST group（84%and 16%contribution of glucose COD and tar COD to methane production,respectively）could be applied to the RT group.The tar kinetic parameter K_Itarindicated that low tar concentrations(ST≤150 mg·L^-1,RT≤250 mg·L^-1)promoted methanogenesis and high tar concentrations severely inhibited biochemical processes（especially acetotrophic methanogenesis）.Further links between kinetic parameters and microbial communities were established:the decrease in the maximum specific uptake rate of hydrogen(18%of k_mH2)was smaller than that of acetate(54%of k_mac),and the hydrogen half-saturation rate(K_sH2maintained at 2.5×10^-5kg COD·m^-3)was more stable than the acetate half-saturation rate(K_sacincreased 50%),certifying the low sensitivity of the hydrogenotrophic methanogenesis to tar toxicity reflected by 16S r RNA.Based on the modified and extended ADM1,this study evaluated the liquid–gas–solid three-phase flows,transfer process,carbon flux distribution,biofilm function,enzyme contribution and dynamic evolution of community structure in methanogenic system,and established the linkage between model simulation and community function,model kinetic parameters and experimental process parameters.New developed ADM1 could provide theoretical guidance and technical support for the design,operation,and optimization of the system from macroscopic and microscopic perspectives,and promote the wider application of AD and ADM1.