Effect of NaCl on thermophilic (55°C) methanol degradation in sulfate reducing granular sludge reactors

The effect of NaCl on thermophilic (55°C) methanol conversion in the presence of excess of sulfate (COD/SO42=0.5) was investigated in two 6.5 L lab-scale upflow anaerobic sludge bed reactors inoculated with granular sludge previously not adapted to NaCl. Methanol was almost completely used for sulfate reduction in the absence of NaCl when operating at an organic loading rate of 5 g COD L1 day1 and a hydraulic retention time of 10 h. The almost fully sulfidogenic sludge consisted of both granules and flocs developed after approximately 100 days in both reactors. Sulfate reducing bacteria (SRB) outcompeted methane producing archaea (MPA) for methanol, but acetate represented a side-product, accounting for maximal 25% of the total COD converted. Either MPA or SRB did not use acetate as substrate in activity tests. High NaCl concentrations (25 g L1) completely inhibited methanol degradation, whereas low salt concentrations (2.5 g NaCl L1) provoked considerable changes in the metabolic fate of methanol. The MPA were most sensitive towards the NaCl shock (25 g L1). In contrast, the addition of 2.5 g L1 of NaCl stimulated MPA and homoacetogenic bacteria.