Influence of inoculum to substrate ratio and substrates mixing ratio on biogas production from the anaerobic co-digestion of Phragmites australis and food waste

Abstract

Abstract This study focused on determining the effect of the inoculum to substrate ratio (ISR) on biogas production efficiency from the anaerobic co-digestion of two substrates: simulated food waste (SFW) and common reeds (Phragmites australis) that were ground and pre-treated using sodium hydroxide at a concentration of 2% to increase access to their cellulose. It also studied the role of different mixing ratios of the two substrates in improving the stability of the digestion process and increasing biogas production. A series of batch tests were carried out under mesophilic conditions using three ratios of ISR: 1:4, 1:2, and 1:1, and five substrate mixing ratios (SFW: pre-treated P. australis): 25:75, 50:50, 75:25, 100:0, and 0:100. The results showed low biogas production at the ISR 1:4, and the reactors suffered from acidification and instability at the different substrates mixing ratios. While the biogas production increased at an ISR of 1:2, where the reactors with the substrate mixing ratio of 75:25 presented the highest biogas production, and the digestion process was stable. However, the reactors with substrate mixing ratios of 50:50, 75:25 and 100:0 suffered from acidification effects at this ISR. In contrast, at ISR of 1:1, the reactors did not expose to acidification inhibition at all the substrates mixing ratios, and the highest biogas production was found at SFW: pre-treated P. australis mixing ratios of 75:25 and 100:0.