The effect of temperature on the performance of thermophilic uasb reactors

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Abstract
The performance of the thermophilic UASB (upflow anaerobic sludge bed) process was investigated at two different temperatures, 55°C and 65°C, using a pair of identical laboratory‐scale reactors. The overall duration of the experiment was divided into two phases. In the first phase (Phase 1), the two reactors were fed on acetate for 200 days. After introducing 12kg TOC m‐3 d‐1 of organic loading rate on Run 1 (at 55°C) on the 127th day, the removal efficiency dropped significantly from 90 to 30% due to a severe sludge washout. Even when the TOC loading rate was reduced to 5–6 kg TOC m‐3 d‐1 there was no sign of a recovery in the process performance. On the other hand, the TOC loading imposed on Run 2 (at 65°C) hovered between 2 and 4 kg TOC m ‐3 d‐1 during Phase 1. Phase 2 experiments were carried out by replacing the acetate feed with a mixture of acetate and sucrose. In Phase 2, effluent recirculation was initiated for the reactors on and after the 50th day. With this recirculation, a significant improvement in process performance was observed. Run 1 eventually reached 34.4 kg TOC nr3 d‘1 (corresponding to 87 kg COD nr3 d‐1) with a TOC reduction of 91%, whereas Run 2 accommodated 16.3 kg TOC m‐3 d‐1. Thus, operation at 65°C was unfavourable compared with operation at 55°C. The good performance of these thermophilic UASB reactors fed with carbohydrate‐containing waste was a result of the formation of granulated sludge. This suggests that the prolification of fermentative acid‐forming bacteria is essential for granulation.