Extreme thermophilic (70°C), VFA-fed UASB reactor: performance, temperature response, load potential and comparison with 35 and 55°C UASB reactors

The paper evaluates the reactor performance, load potential and temperature response of a 70°C, VFAs-fed UASB reactor, seeded with mesophilic granular sludge. Batch experiments were, in addition, conducted to assess the effect of temperature on the achievable residual VFAs in the 70°C effluent. The performance of similarly-fed and seeded 35 and 55°C UASB reactors was also tested. At a short HRT (2–3 h) and a moderate VLR of 12–20 g COD l−1 d−1, the 70°C UASB achieved 66–74% VFAs removal (acetate and butyrate 84–90%, propionate <20%), while the 35 and 55°C UASB reactors removed 98–99% of VFAs at a VLR of 34–35 g COD l−1 d−1 and an HRT of about 1.1 h. The doubling of the OLR, by halving the sludge beds of the 35, 55 and 70°C UASB reactors, to about 3.68, 3.54 and 2.6 g COD g VS−1, respectively, had no significant effect on the overall VFAs removal in these reactors. Temperature shifts (70→55→70°C) in the 70°C UASB led to a higher VFAs removal at 55°C (mainly propionate, 46%). In the batch experiments, 92–94 of the 70°C effluent residual VFAs were removed at 35 and 55°C and 72% of acetate and <7% of VFAs were removed at 70 and 80°C, respectively. The 70°C UASB reactor survived starvation at 20°C for ca. a month and recovered fast after temperature was reset to 70°C.