Long-term adaptation of methanol-fed thermophilic (55 °C) sulfate-reducing reactors to NaCl

Abstract

A laboratory-scale upflow anaerobic sludge bed (UASB) reactor was operated during 273 days at increasing NaCl concentrations (0.5–12.5 g NaCl l⁻¹) to assess whether the stepwise addition of the salt NaCl results in the acclimation of that sludge. The 6.5-l thermophilic (55 °C), sulfidogenic [a chemical oxygen demand (COD) to SO₄ ²⁻ ratio of 0.5] UASB reactor operated at an organic loading rate of 5 g COD l⁻¹ day⁻¹, a hydraulic retention time of 10 h and was fed with methanol as the sole electron donor. The results show that the adaptation of the thermophilic, sulfidogenic methanol-degrading biomass to a high osmolarity environment is unlikely to occur. Sulfide was the main mineralization product from methanol degradation, regardless of the NaCl concentration added to the influent. However, sulfide production in the reactor steadily decreased after the addition of 7.5 g NaCl l⁻¹, whereas acetate production was stimulated at that influent NaCl concentration. Batch tests performed with sludge harvested from the UASB reactor when operating at different influent salinities confirmed that acetate is the main metabolic product at NaCl concentrations higher than 12.5 g l⁻¹. The apparent order of NaCl toxicity towards the different trophic groups was found to be: sulfate-reducing bacteria > methane-producing archaea > acetogenic bacteria.