Model Modification Of Anaerobic Digestion Process Of Silage Corn Straw

Silage corn straw（SCS）is a critical substrate to ensure the continuous operation of anaerobic digestion（AD）,which solves the decay and preservation problems of fresh straw and has received more attention for a long time.However,the lactic acid concentration and the total solids（TS）content will affect the operational stability of AD for SCS.The construction of a mathematical model for AD can not only reflect the main characteristics of the process but also simulate the system’s operation under different conditions and simultaneously reduce experimental costs and risks.The anaerobic digestion model No 1（ADM1）concentrates the important research results in mathematical modelling and process simulation in the field of AD,which is widely recognized internationally,and its scope of application has been extended from wastewater to solid waste.In addition,the effect of lactic acid on AD remains divergent.Therefore,the research firstly determined the effects of lactic acid on AD.Then,the ADM1-SCS model suitable for the AD of SCS was established by modifying the ADM1 model.Finally,the modified model was validated and evaluated under continuous conditions and co-digestion patterns.（1）Firstly,this study investigated the effect of lactic acid on AD using glucose as a simulation system.Results showed that the cumulative methane yields of glucose decreased by 20.3%-30.7%with lactic acid addition of 0.5-4g/L.The organic acid content in the anaerobic digestion system indicated that lactic acid could rapidly degrade into acetic and propionic acid,in which propionic acid was dominant.The increasing lactic acid concentration would lead to propionate accumulation,gradually decreasing methane-production performance and eventually leading to system collapse.（2）According to the AD characteristics of SCS,the ADM1-SCS model was proposed in the project based on the original ADM1 model.The model modification work included:（1）Lactic acid degradation and related microbial metabolic pathway were added,and the acid-base balance equation was modified;（2）The inhibition function of TS was added to describe the inhibitory effect of TS on the hydrolysis process;（3）The inhibitory function of lactic acid was added to describe the inhibitory effect of lactic acid on the methane production process.（3）The ADM1-SCS model was calibrated based on the experimental results of batch AD of SCS with TS contents of 5%,8%and 11%.After fitting,the simulated results of methane production,VFAs and p H value in the modified model were better than those in the original ADM1 model.Key parameter values modified by sensitivity analysis and parameter estimation were as follows:the inhibition coefficients including K_TS,K_la,K_I,NH3and p H_LL,aawere 1.151 kg COD/m³,46.769 kg COD/m³,0.085 kmol/m³and 4.335,respectively.The first-order kinetic constants k_disand k_hyd,chwere 8.023 and11.801 d^-1,respectively.Monod maximum specific absorption rates of k_m,ac,k_m,proand k_m,H2were 5.569,36.331 and 16.384 d^-1,respectively.The half-saturation constants including K_S,su,K_S,ac,K_S,proand K_S,c4were 0.948,0.421,0.168 and 0.378 kg COD/m³,respectively.The yields of Y_su,Y_pro,Y_c4,Y_h2,Y_aaand Y_fawere 0.094,0.031,0.004,0.002,0.015 and 0.044 COD/COD,respectively.（4）The ADM1-SCS model was verified by the continuous AD experiment of SCS and the continuous anaerobic co-digestion experiment of SCS and cow manure.The simulated results showed that the R²of methane production,TVFA,and p H were 0.85,0.80,and 0.80,respectively.Meanwhile,the RMSE values of all experimental groups were in the range of 0.20 to 0.51,indicating that the simulation results were close to the experimental results.In conclusion,the ADM1-SCS model is a reliable tool for modelling AD of SCS.This paper described the effects of lactic acid on AD and subsequently established the ADM1-SCS model to simulate the AD of SCS after model modification,calibration,verification,and evaluation,which provided important theoretical support for process control and engineering application of AD of SCS.