RESUMEN
The impact of the application of mechanically-imposed shear on the propensity for fouling and clogging (or "sludging"-the agglomeration of sludge solids in the membrane channel) of an immersed flat sheet (iFS) membrane bioreactor (MBR) was studied. The bench-scale test cell used contained a single flat sheet fitted with a crank and motor to allow the membrane to be oscillated (or reciprocated) vertically at a low rate (20 RPM). The membrane was challenged with sludge samples from a local MBR installation treating petroleum industry effluent, the sludge having previously been demonstrated as having a high sludging propensity. Sludging was measured by direct visual observation of membrane surface occlusion by the agglomerated solids, with fouling being notionally represented by the rate of transmembrane pressure increase. Results demonstrated membrane reciprocation to have a more beneficial impact on sludging amelioration than on suppressing fouling. Compared with the stationary membrane, sludging was reduced by an average of 45% compared with only 13% for fouling suppression at the reference flux of 15 L·m-2·h-1 applied. The specific energy demand of the mechanical shear application was calculated as being around 0.0081 kWh·m-3, significantly lower than values reported from a recent pilot scale study on a reciprocated immersed hollow fibre MBR. Whilst results appear promising in terms of energy efficiency, it is likely that the mechanical complexity of applying membrane movement would limit the practical application to low flows, and a correspondingly small number of membrane modules.
RESUMEN
We present a comprehensive study of cross-flow ultrafiltration (UF) of charge-stabilized suspensions, under low-salinity conditions of electrostatically strongly repelling colloidal particles. The axially varying permeate flux, near-membrane concentration-polarization (CP) layer and osmotic pressure profiles are calculated using a macroscopic diffusion-advection boundary layer method, and are compared with filtration experiments on aqueous suspensions of charge-stabilized silica particles. The theoretical description based on the one-component macroion fluid model (OCM) accounts for the strong influence of surface-released counterions on the renormalized colloid charge and suspension osmotic compressibility, and for the influence of the colloidal hydrodynamic interactions and electric double layer repulsion on the concentration-dependent suspension viscosity η, and collective diffusion coefficient Dc. A strong electro-hydrodynamic enhancement of Dc and η, and likewise of the osmotic pressure, is predicted theoretically, as compared with their values for a hard-sphere suspension. We also point to the failure of generalized Stokes-Einstein relations describing reciprocal relations between Dc and η. According to our filtration model, Dc is of dominant influence, giving rise to an only weakly developed CP layer having practically no effect on the permeate flux. This prediction is quantitatively confirmed by our UF measurements of the permeate flux using an aqueous suspension of charged silica spheres as the feed system. The experimentally detected fouling for the largest considered transmembrane pressure values is shown not to be due to filter cake formation by crystallization or vitrification.
RESUMEN
We report a novel tubular electrochemical cell which is operated in a cyclic adsorption - electro-Fenton process and by this means overcomes the drawbacks of the traditional electro-Fenton process. A microtube made only of multi-walled carbon nanotubes (MWCNT) functions as a gas diffusion electrode (GDE) and highly porous adsorber. In the process, the pollutants were first removed electroless from the wastewater by adsorption on the MWCNT-GDE. Subsequently, the pollutants are electrochemically degraded in a defined volume of electrolyte solution using the electro-Fenton process. Oxygen was supplied into the lumen of the saturated microtubular GDE which was surrounded by a cylindrical anode made of Ti-felt coated with Pt/IrO2 catalysts. For the proof of concept the model pollutant Acid Red 14 (AR14), an azo dye, was used. The decomposition of AR14 was studied at different applied current densities and initial concentrations of ferrous iron in the electrolyte solution. At optimal conditions, complete regeneration of the adsorption capacity of the MWCNT-GDE, complete decolorization and TOC and COD removal rates of 50% and 70% were achieved, respectively. The MWCNT-GDE is regenerated and again available for adsorption. This approach allows water treatment independent of its composition, thus does not require any addition of chemicals to the wastewater.