Kinetic modeling of dry anaerobic co-digestion of Lignocellulosic Biomass

Abstract

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 (E- 1, E-2, and E-3) were conducted by reusing solid digestate of E- 1 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 (Mmax) 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.