Optimization of influencing parameters for dry anaerobic co-digestion of lignocellulosic biomass

Abstract

Anaerobic digestion offers an attractive solution for recovering energy from rice straw (RS) which is a lignocellulosic agricultural residue produced in huge quantities in Asia and Africa. Given the high solids content of this feedstock, high solids anaerobic co-digestion in batch mode is a process that can be applied. In this study, optimal operating conditions for the co-digestion of RS with cow dung (CD) in pure batch reactors and batch reactors with leachate recirculation are assessed. The preliminary experiments carried out in pure batch conditions showed that the initial concentration of RS in the mixture of substrates, i.e., S 0 , (g VS rice straw /kg of mixture) is an important parameter. Only the batch reactors with the lowest S 0 values (29g VS RS /kg) produced biogas after a long lag phase of 14 days. The use of digestate from a previous batch as an inoculum was investigated with S 0 values of 29 and 55 g VS RS /kg. Re-use of the digestate as an inoculum source drastically improved both the initial degradation kinetics and the methane yield measured after 60 days for the S 0 of 29 g VS RS /kg, as lag phase time period almost reached to zero and final methane yield of this reactor was 222 ml/g VS. This indicates a 104 % increase of specific methane yield increase compared to the reactor that only has the same S 0 concentration but the substrate mixture comprises only RS and CD. However, for 55 g VS RS /kg, the degradation kinetics were affected: after two months, 32% of the biodegradable organic matter was not eliminated. Leachate recirculation experiments were conducted in leach-bed reactors (LBRs) with S 0 between 30 and 65 g VS RS /kg, the highest methane yield was recorded at the lowest S 0 value, confirming that in batch mode during high solids anaerobic co-digestion (HSAcoD)

conditions, an initial RS concentration around 30 g VS RS /kg is recommended for industrial applications. Then mathematical modeling was applied to estimate kinetic parameters related to HS-AcoD process using the modified Gompertz model. Results obtained from Batch experiment no.3 (i.e., the three consecutive batches) were considered for the mathematical modelling. Modified Gompertz model very closely predicted the ultimate methane yield (M max

almost 0.99 in each scenario. Degradation kinetics improved drastically with the strategy of re-using digestate, as for the Batch-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 Batch-3 ultimate methane yield was 138 ml/g VS which was a 38% reduction compared to Batch-2, even though digestate was used as the main inoculum source for the both batches. But it was a 27% increase compared to Batch-1 which CD was used as the only inoculum.