Sodium Inhibition of Thermophilic Methanogens

This study investigated the sodium inhibition of methanogens using two thermophilic $(55°C)$ anaerobic sequencing batch reactors (ASBRs). The ASBRs were operated at a chemical oxygen demand (COD) loading of 4 g/L/day and a hydraulic retention time of 3 days. To evaluate the chronic toxicity of sodium to methanogens, the biomass in one of the ASBRs was acclimated to increasing sodium concentrations of 4.1, 7.1, and 12.0 g/L while the feed to the second ASBR was not supplemented with any additional sodium. The methanogenic activity (mL ${\mathrm{CH}}_4/g$ volatile suspended solids/day) decreased by nearly 44% at an acclimation concentration of 12.0 g ${\mathrm{Na}}^{+}/L\text{,}$ but the COD removal efficiency and methane production did not vary appreciably at the different acclimation concentrations studied. The acute toxicity of sodium to methanogens was determined by a series of batch anaerobic toxicity assays (ATAs). The biomass acclimated to different concentrations of sodium was collected from the ASBRs and used as inocula for the batch tests, and the sodium concentration was varied up to 17.7 g/L. The methanogens in the biomass acclimated to 0, 4.1, 7.1, and 12.0 g ${\mathrm{Na}}^{+}/L$ were completely inhibited (100% inhibition) at predicted sodium concentrations of 10.6, 12.7, 18.0, and 22.8 g/L, respectively. To simulate the results of batch ATA in the ASBR, 7-day feeding with sodium concentrations in the influent measuring 6.2, 10.6, and 16.0 g/L were introduced into the reactor. Among each feeding, the reactor was operated with no additional sodium in the feed with 2–3 week intervals. Even though the methanogenic activity was not significantly affected at 6.2 and 10.6 g/L of sodium, there was a deterioration in methanogenic activity at 16.0 g/L dosage of sodium.