Improving the Biogas Production and Methane Yield in a UASB Reactor with the Addition of Sulfate

Sulfate is of great importance in anaerobic digestion as its addition can help control the microbial community. In this study, the effect of sulfuric acid addition on the performance of a UASB reactor fed with hydrolyzed starch was investigated. The total organic carbon (TOC), Fe, SO42- removal and methane production were monitored under various chemical oxygen demand (COD) to SO42- ratios, hydraulic retention times (HRTs) and organic loading rates (OLRs). When the HRT was 16 h, and the OLR was equal to 2 g COD/L-UASB-day, methane production was 0.24 L CH4/L-UASB-day. After the addition of sulfuric acid, TOC removal reached 95%, and the population of Ruminococcus sp. that utilize soluble carbohydrates increased by 19.7%. Nitrogen removal was less than 25%, while the population of Ignavibacterium sp. accounted for 2.3%. When the HRT, OLR and COD/SO42- ratios were equal to 16 h, 2 g COD/L-UASB-d and 3.72, respectively, methane production was 0.267 L CH4/L-UASB-day. When the OLR increased to 5.94 g, the COD/L-UASB-day and COD/SO42- ratios were equal to 12.5, and methane production was three times higher (0.84 L CH4/L-UASB-day), with a methane content in the produced biogas greater than 70% due to the increased amount of Methanosaeta sp. and direct interspecies electron transfer. Sulfate addition increased the relative abundance of Desulfovibrio sp., accounting for 9.9% and an iron accumulation exceeding 98.0%. This study indicates that appropriate sulfate concentrations in the feed in combination with the presence of iron in the UASB lead to enhanced methane yields.