Characterization of a new continuous gas-mixing sulfidogenic anaerobic bioreactor: Hydrodynamics and sludge granulation.

Continuous gas recirculation (CGR) was demonstrated in this study to be an effective method to mitigate the persistent problem of sludge flotation in high-rate sulfate-reducing upflow sludge bed (SRUSB) reactors that do not produce much gas. The effects of hydraulic- and CGR-mixing on the mixing regime of the SRUSB reactors were investigated over a period of 45 d at the average shear rates of 0.9, 1.5, 2.7, 4.2 and 7.2 s (Phase I). CGR-mixing at 4.2 s resulted in the smallest reactor short-circuiting flow of 1.3 ± 0.1% and the smallest dead zone volume of 0.2 ± 0.01% at a lower power consumption (0.0007 W) than hydraulic mixing. In Phase II, the SRUSB reactor with CGR-mixing at 4.2 s was re-inoculated and operated for 150 days. Within the first 65 days, the sludge transformed into micro-granules (300-350 μm) with a high sulfate-reducing bacteria (SRB) activity (0.62 ± 0.05 g COD/(g MLVSS·day)), a low sludge flotation potential (<20%) and a high settleability (SVI/SVI < 1.3). These results are attributed to the following sludge properties: (i) a low ratio of loosely-bound to tightly-bound extracellular polymeric substances (0.06-0.1), (ii) weakly adhesive surface properties as demonstrated by a strongly negative zeta potential (-23 ± 2 mV), a low hydrophobicity (37 ± 3%) and a low viscosity (0.7 ± 0.1 mPa s), and (iii) small size granules resulting in strong mass transfer (sulfate and COD penetration into the granule core) and a homogeneous structure (SRB detected throughout the granule).