Conversion of volatile fatty acids in an extreme thermophilic (76–80°C) upflow anaerobic sludgeblanket reactor

The removal of volatile fatty acids (VFA) in a 76–80°C upflow anaerobic sludge-blanket (UASB) reactor was studied. Also evaluated were the effects of temperature (35–80°C) on the achievable residual VFA concentration of the 80°C UASB effluent and the effects of initial acetate concentration on the initial methane production rate (VCH4) of the 80°C UASB sludges in bioassays. The acetate and propionate removals were temperature dependent. In the reactor, the maximum acetate removal efficiency at 76 and 80°C was about 65 and 46%, respectively. Propionate removal was not obtained at 80°C; however, at 76°C it was about 10%. At 76°C, doubling the organic load from 1.5 to 3.2 gCOD l−1 d−1 decreased VFA removal but had little or no effect on the removal rate. In the bioassays with the 80°C UASB effluent, little or no residual VFA was removed at 80°C, while acetate was completely removed at 35 and 55°C and propionate removal was significant only at 55°C. The highest initial VCH4 of the 80°C UASB sludge was 0.019 h−1 at an acetate concentration of 3130 mgCOD l−1. Higher initial acetate concentrations decreased the VCH4. The results suggest that the upper temperature limit for acetate conversion is at least 80°C.