Anaerobic Digestion of Fractionated and Non-fractionated Sugar Beet Tops

Abstract: Continuous stirred tank reactors (CSTR) and batch reactors were used to examine the difference in methane yields and process stability between fractionated and non-fractionated sugar beet leaves. The hypothesis of this project was that by reducing the amount of juice in the sugar beet leaves the amount of soluble sugars and other easily fermented compounds in the substrate will also decrease, and thereby lead to higher process stability. Batch reactor experiments were performed with inoculum-substrate ratios (ISR) of 2:1 and 1:1, and with non-fractionated (Total Solids, TS: 13.0%), two different solid fractions (TS: 14.9% and 17.6%) and liquid fraction (TS: 7.1%) sugar beet leaves. The methane yields of non-fractionated substrate for ISR 2:1 and 1:1 was 328 (sd 12) mL gVS-1 and 339 (sd 8) mL gVS-1. The methane yields of the solid fraction with TS 14.9% for ISR 2:1 and 1:1 was 302 (sd 14) mL gVS-1 and 306 (sd 9) mL gVS-1. The methane yield of the solid fraction with TS 17.6% with ISR 2:1 was 322 (sd 12) mL gVS-1. The methane yield of the liquid fraction for IRS 2:1 and 1:1 was 303 (sd 16) mL gVS-1 and 330 (sd 20) mL gVS-1. The liquid fraction with the higher organic load (ISR 1:1) showed signs of inhibition which was not present in other substrates with the same ISR. This could be due to higher concentrations of quickly fermented compounds leading to inhibiting levels of volatile fatty acids, indicating that this substrate more easily can cause disturbances at higher organic loading rates than the other substrates. The CSTR experiments took place over a period of 80 days and substrates used were non-fractionated (TS: 13.0%) and solid fraction (TS: 14.9%). The methane yields for the two reactors with non-fractionated substrate leaves were 245 and 238 mL gVS-1. The methane yields for the two reactors with fractionated substrate leaves were 205 and 224 mL gVS-1. Process disturbances caused by foam production was common, particularly in reactors with solid fraction. Results from the continuous experiments do not indicate that methane yields or process stability improved for the reactors fed with fractionated substrate, but rather the opposite. However, as the continuous experiments only went on for 80 days and the variations between duplicates were large the results are not conclusive.