Effect of mixing mode on the behavior of an ASBBR with immobilized biomass in the treatment of cheese whey

Abstract

A hydrodynamic study of a mechanically stirred anaerobic sequencing batch biofilm reactor (ASBBR) containing immobilized biomass on polyurethane foam was performed with the aim to determine homogeneity of the reactor based on total mixing time. Turbine or helix propellers were used for stirring at rotor speeds of 100, 200, 300 and 500 rpm. Experimental values obtained were fitted to a Boltzmann sigmoid. Homogenization times of the reactor were negligible when compared to the 8-h cycle time for all conditions studied. At low propeller rotations the turbine propeller showed the best performance. For higher rotations total mixing times were similar for both propellers; however the helix propeller had better homogeneity conditions. At a subsequent stage the system was operated in batch mode treating cheese whey at concentrations of 500, 1000 and 2000 mgCOD/L and rotations of 200, 300 and 500 rpm. In these assays the importance of the propeller became evident not only for mixing, but also for substrate flow across the bed containing immobilized biomass. Due to axial flow, the helix propeller offered better mass transfer conditions, evidenced by improved organic matter conversion and lower production of total volatile acids.