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

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: synthetic food waste 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 (synthetic food waste: 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 (21.58±0.00–44.46±0.01 mL/g volatile solid (VS) added), and the reactors suffered from acidification 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 25:75 presented the highest biogas production (82.17±0.62 mL/g VS added), 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 synthetic food waste: pre-treated P. australis mixing ratios of 75:25 and 100:0 (76.15±1.85 and 82.47±1.85 mL/g VS added, respectively).

Stimulating Methane Production from Poultry Manure Digest with Sewage Sludge and Organic Waste by Thermal Pretreatment and Adding Iron or Sodium Hydroxide

“…Theoretically, it should not affect the value of the methane/biogas yield but only the kinetics of the process, including the length of the lag phase (it shortens as the I/S value increases) [36][37][38]. Numerous literature reports, however, demonstrate the significance of the I/S ratio in preventing the process inhibition caused by the accumulation of inhibitors like VFAs, while also impacting the efficacy of the anaerobic digestion process [39]. Each substrate has an optimal characteristic value of the I/S ratio, and its value is a specific compromise between the endogenous production of inoculum biogas, the acceptable lag phase length, and the buffer capacity.…”

“…Above the indicated value, they observed a decrease in biogas production, which they associated with the imbalance of nutrients in the feedstock (fruit and vegetable waste). In turn, Al-Iraqi et al [39], for a feedstock composed of simulated food waste and common reeds, recommend an I/S ratio of 1. Other studies have shown that, for easily biodegradable substrates like household waste, I/S ratios ranging from 5 to 15 are suitable for batch assays without volatile fatty acid accumulation [41].…”