A rumen based anaerobic digestion approach for lignocellulosic biomass using barley straw as feedstock

Abstract

The use of rumen-based anaerobic digestion (RUBAD) has been shown to have a high rate of biogas production and can lead to a lower retention time to digest lignocellulose biomass. Albeit the use of rumen fluid in anaerobic digestion is proven to increase the rate of digestion, there is a lack of literature focused on finding the optimal organic loading of barley straw biomass when using rumen fluid as an inoculum. Therefore, this study aimed to use rumen fluid as an inoculum to test different organic loadings (OLs) of barley straw on biogas production in batch mode and using a locally designed semi-continuous rumen digester. Batch digestions in 120 mL reactor volume were carried out in triplicates to test the effect of different concentrations of barley straw ranging between 2.03 gVS/L and 24.41 gVS/L. The locally designed digester functioned like the rumen simulation technique (RUSITEC) system with a total volume of 2 L inoculum. It was fed daily with 20 g (wet weight) barley straw contained in a nylon bag (20 µm pore size) to determine the daily biogas production. Results from the batch experiment showed the biogas yield increased with an increase in organic loading (OL). Most of the OLs have a biogas yield between 250 mL/gVSadded and 270 mL/gVSadded with a methane content of 22%. The OL of 16.24 gVS/L had the highest biogas yield of 269 ± 2 NmL/gVSadded while a further increase in OL to 24.41 gVS/L led to a 19% decrease in biogas yield (226 NmL/gVS added). The designed rumen reactor had a daily biogas production of 12.63 ml/gVS/day and had a stable pH between 6.8 and 7.4 for 15 days. However, only 37.74% and 48.40% of the total solids were digested after two days of solid retention time. More than half of the barley straw feedstock remained undigested, and therefore, would require a longer retention time to complete the digestion. This study demonstrates the importance of finding the optimal organic load for different feedstocks and led to a 43.78% increase in biogas yield under the same conditions. The use of rumen fluid as inoculum can significantly lower the volume of digesters required and lower the retention times for biogas production vital to the anaerobic digestion industry.