Abstract
Anaerobic digestion is a widely applied technology for sewage sludge treatment. Hydrogen and CO(2) are important degradation products, which serve as substrates for both hydrogenotrophic methanogenesis and chemolithotrophic acetogenesis. In order to understand the competition between these processes for H(2)/CO(2), sludge samples were incubated under H(2)/CO(2) headspace at different temperatures, and analyzed with respect to turnover of H(2), CO(2), CH(4) and acetate including their δ(13)C values. At 15°C, (13)C-depleted acetate (δ(13)C of -41 to -43‰) and transient acetate accumulation were observed under H(2)/CO(2), and CH(4) accumulated with δ(13)C values increasing from -53 to -33‰. The copy numbers of the fhs gene, which is characteristic for acetogenic bacteria, were at 15°C one order of magnitude higher in the H(2)/CO(2) incubations than the N(2) control. At 30°C, however, acetate did not accumulate in the H(2)/CO(2) incubation and the δ(13)C of CH(4) was very low (-100 to -77‰). At 50°C, isotopically enriched acetate was transiently formed and subsequently consumed followed by the production of (13)C-depleted CH(4). Collectively, the results indicate a high contribution of chemolithotrophic acetogenesis to H(2)/CO(2) utilization at 15°C and 50°C, while H(2)/CO(2) was mainly consumed by hydrogenotrophic methanogenesis at 30°C. Fermentative production and methanogenic consumption of acetate were active at 50°C.