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Quantitative Influences of Butyrate or Propionate on Thermophilic Production of Methane from Biomass

Sodium butyrate and sodium propionate were continuously infused into separate 4-liter thermophilic digesters. These digesters were operated at 55°C, had a retention time of 20 days, and had a pH of 7.8. Infusion rates were started at 10 mM day−1 and were increased incrementally when new stable external organic acid pool sizes and new stable gas production rates were observed. Stable conditions were obtained in both digesters at an infusion rate of 15 mM day−1, with methanogenesis elevated over that of control digesters. Calculations based on expected CH4 at this infusion rate and measured CH4 production in the treated and control digesters, however, showed an approximately 25% inhibition of methanogenesis in both digesters. A digester infused with sodium chloride showed little or no inhibition at this infusion rate, but was totally inhibited when its infusion rate was increased to 20 mM day−1, and cumulative added NaCl reached 0.38 M. The butyrate and propionate-amended digesters tolerated addition rates of 20 mM day−1, but both failed when they were increased to 25 mM day−1. These results indicate that the thermophilic digesters could function stably at higher external pool sizes of butyrate or propionate than routinely observed.