Effect of specific gas loading rate on thermophilic (55°C) acidifying (pH 6) and sulfate reducing granular sludge reactors

The effect of the specific gas loading rate on the acidifying, sulfate reducing and sulfur removal capacity of thermophilic (55°C; pH 6.0) granular sludge bed reactors treating partly acidified wastewater was investigated. A comparison was made between a regular UASB reactor and a UASB reactor continuously sparged with N2 at a specific gas loading rate of 30 m3 m−2 d−1. Both UASB reactors (upflow velocity 1.0 m h−1, hydraulic retention time about 5 h) were fed a synthetic wastewater containing starch, sucrose, lactate, propionate and acetate and a low sulfate concentration (COD/SO42− ratio of 10) at volumetric organic loading rates (OLR) ranging from 4.0 to 49.8 gCOD l−1 reactor d−1. Immediately after imposing an OLR of 25 gCOD l−1 reactor d−1, the acidification and sulfate reduction efficiency dropped to 80% and 30%, respectively, in the UASB reactor. Both efficiencies recovered slowly to 100% during the course of the experiment. In the N2 sparged reactor, both the acidification and sulfate reduction efficiency remained 100% following the OLR increase to 25 gCOD l−1 reactor d−1. However, the sulfate reduction efficiency gradually decreased to about 20% at the end of the experiment. The biogas (CO2 and CH4) production rate in the UASB was very low, i.e. <3 l biogas l−1 reactor day−1, resulting in negligible amounts (<20%) of H2S stripped from the reactor liquid. The total H2S concentration of the N2 sparged UASB reactor effluent was always below 25 mg l−1, but incomplete sulfate reduction kept the maximal H2S stripping efficiency below 70%. [Copyright &y& Elsevier]