Effect of staging on volatile fatty acid degradation in a sulfidogenic granular sludge reactor

The use of an upflow staged sludge bed (USSB) reactor for the optimization of acetate removal during treatment of sulfate rich wastewaters was investigated. The USSB treated a VFA mixture (acetate:propionate:butyrate ratio 1:2:2 on COD basis; pH 8) under mesophilic (30°C) and sulfidogenic (COD:SO42− ratio 0.5) conditions. Its performance was compared to that of an upflow anaerobic sludge bed (UASB) reactor operating at the same operational conditions. The hydrodynamic conditions were clearly different in both reactor types. The USSB reactor (number of ideal mixed reactors Nth=10) had a more plug-flow pattern than the UASB reactor (Nth=3). Both reactors reached a specific sludge loading rate of 1.20 gCOD gVSS−1 d−1, corresponding to a volumetric loading rate of 20 and 30 gCOD l−1 d−1 for the UASB and USSB reactor, respectively. At these loading rates, a maximum COD removal of 80% was obtained by both reactors and the average %COD used by sulfate reducing bacteria was 74(±7)% and 80(±9)% for the UASB and USSB reactor, respectively. The UASB and USSB reactor reached a sulfate loading rate of 40 and 60 g SO42− l−1 d−1, respectively, with an average sulfate removal of 45(±19)% and 35(±24)%, respectively. The USSB reactor showed more oscillations in its performance than the UASB reactor, already at lower loading rates. Staging of sulfidogenic treatment in the USSB reactor resulted in development of sludges with different maximum specific microbial activities, but with similar physico-chemical sludge characteristics. Activity of the USSB sludge on a VFA mixture or with acetate as the sole substrate increased from the bottom to the top compartments. Sludge in the top compartments adapted only slowly to acetate, as indicated by the minor increase (13.5% in 138 days) of the activity on acetate and sulfate in those compartments.