Multispecies hydrodynamic dispersion under high concentration gradients.

Recent research suggests that when high concentration gradients (HCG) are present, resulting sharp density differences can cause the dispersive flux relationship to deviate from its classical Fickian form. This paper presents stable, upward, miscible displacement experiments conducted in two different types of porous media for a wide range of concentration differences between resident and displacing fluids. The considered groundwater velocities ranged from advection-dominated transport to velocities where the contribution of molecular diffusion is important, with the corresponding Peclet numbers ranging from 0.2 to 320. In addition to single component displacing fluids, mixtures consisting of multiple solutes were considered. The results of this study provide further evidence that classical Fick's law over-estimates the dispersion coefficient under HCG conditions. The decrease in the apparent dispersion coefficient is shown to be a nonlinear function of both concentration difference and groundwater velocity. This observation is attributed to gravitational effects at the sub-continuum scale which are not directly accounted for in classical variable density advection/dispersion models. Mixture experiments showed that the dispersive behaviors of individual components in a groundwater contaminant mixture are coupled.

Effect of biomass concentration on the performance and modeling of nitrogen removal for membrane bioreactors.

The study investigated the effect of biomass concentration on nitrogen removal in a membrane bioreactor by model evaluation of system performance. The steady state operation of a pilot membrane bioreactor fed with domestic sewage at a sludge age of 74 days and an average biomass concentration of 27,000 mg/L was monitored. The results were evaluated by calibration of a suspended growth model designed for this purpose and compared with those of an earlier experiment on the same system operated at a sludge age of 34 days, with a markedly lower biomass concentration. The membrane bioreactor always sustained a dissolved oxygen concentration of around 2 mg O2/L which could be explained by diffusion limitation of dissoved oxygen from the bulk liquid into the floc. Nitrogen removal was controlled and limited by nitrification which occurred only partially throughout the study. The oxidized nitrogen was always fully removed by means of simultaneous nitrification denitrification reducing the level of nitrate to a very low level so that the anoxic tank in front of the membrane bioreactor was totally useless in terms of nitrogen removal. Comparison of the results of the two experimental runs indicated that increasing the biomass concentration drastically changed the system behavior from denitrification limitation to nitrification limitation due to increased constraints on the mass transfer of dissolved oxygen. The selected model could be successfully calibrated for the model parameters by means of substantially higher oxygen half saturation constants for heterotrophs (K(OH)) and autotrophs (K(OA)) determined as 2.0 mg O2/L and 2.25 mg O2/L, respectively.