Kinetic Modeling of Dry Anaerobic Co-Digestion of Lignocellulosic Biomass

Anaerobic digestion is a widely used conversion process for recovering energy from bio degradable waste materials. Dry anaerobic co-digestion of lignocellulosic biomass such as rice straw (RS) with cow dung (CD) offers various benefits other than mono digestion or liquid state anaerobic digestion. However dry anaerobic co-digestion of lignocellulosic biomass has restrictions due to low moisture content and retarded mass transfer within the mixture of substrates which contains in the reactor. Three consecutive batch experiments (E1, E-2, and E-3) were conducted by reusing solid digestate of E1 as an inoculum to E-2 and solid digestate of E-2 as an inoculum for E-3 in lab scale reactors using RS and CD as substrates. In E-1, CD was the only inoculum and it acted as a substrate as well. Total solids (TS) content of the substrate mixtures of E-1, E-2 and E-3 were 15%, 16%, and 20% respectively. Then mathematical modeling was applied to estimate kinetic parameters related to dry anaerobic co-digestion process using the modified Gompertz model for the three experiments. Modified Gompertz model very closely predicted the ultimate methane yield (M_max) with R² almost 0.99 in each scenario. Degradation kinetics improved drastically with the strategy of reusing of digestate, as for the E-2 the lag phase period (λ) reduced from 14 days to almost zero. Ultimate methane yield increased by 104% through this approach. Degradation kinetics were negatively affected with the increase of TS% within the substrate mixture even though digestate was reused as an inoculum. In E-3 ultimate methane yield was 138 ml/g volatile solids (VS) which was a 38% reduction compared to E-2, even though digestate was used as the main inoculum source for the both experiments. But it was a 27% increase compared to E-1 which CD was used as the only inoculum.