Luisa Vera

Enhancing microfiltration through an inorganic tubular membrane by gas sparging

Abstract A novel technique is tested for reducing tubular mineral membrane fouling by injecting gas into a cross-flow stream. The injected gas is thought to form complex hydrodynamic conditions inside the microfiltration module which increase the wall shear stress, preventing the membrane fouling and enhancing the microfiltration mass transfer. The experimental study was carried out with a ferric hydroxide suspension and a biologically treated wastewater, both of them filtered through a tubular inorganic membrane (Carbosep M14). The sparging led to an increase of the permeate flux with a slug flow structure for the two kinds of suspension. New dimensionless quantities of shear stress number and resistance number were developed by generalized dimensional analysis of steady state flux in sparged and unsparged cross-flow filtration. An unique formalism allowed interpretation of the experimental results both in classical diphasic filtration and with gas sparging. The variation in the dimensionless numbers demonstrated the benefit of gas sparging.

Dimensionless numbers for the steady-state flux of cross-flow microfiltration and ultrafiltration with gas sparging

Abstract The steady-state flux, observed for gas-sparged microfiltration or ultrafiltration through inorganic composite membranes, could be expressed using two dimensionless numbers. The generalised shear stress number N S ′, with introduction of an equivalent fluid density, compares the shear stress against the membrane wall to the driving pressure, while the resistance number N f compares the convective cross-flow transport to the driven transport through a layer, whose resistance is the sum of all the resistances induced by the different processes which limit the mass transport. Experimental data, obtained in ultrafiltration of dextran solutions and microfiltration of ferric hydroxide suspensions and secondary treated wastewater, were re-calculated in terms of these dimensionless groups. Straight lines were plotted; their slope is a decreasing function of the gas–liquid velocity ratio when particle deposition or polarisation limited the mass transport. A negative slope and a positive intersection with the N S ′-axis means that the induced resistance can completely be eliminated with gas sparging. A straight line of negative slope followed by a plateau means that an irreversible fouling is superimposed to a reversible phenomenon in the hydrodynamic conditions of the run. A positive slope means that the flux reaches a plateau when cross-flow is increased. Gas sparging allows then decreasing the slope and reaching negative values.

Dimensional analysis of steady state flux for microfiltration and ultrafiltration membranes

Dimensional analysis of the mass, length and time shows that the steady state flux observed for microfiltration or ultrafiltration through inorganic composite membrane can be expressed using two dimensionless numbers. The shear stress number NS compares the shear stress against the membrane wall to the driving pressure, while the resistance number Nf compares the convective cross-flow transport to the drived transport through a layer, whose resistance is the sum of all the resistances induced by the different processes which limit the mass transport. Experimental data obtained in ultrafiltration of hydrocarbon emulsions and microfiltration of methanogenic bacteria suspensions and secondary treated wastewater were recalculated in terms of these dimensionless groups. Straight lines were plotted whose slope depends solely on the suspension and the membrane and not on the solute concentration. A negative slope and a positive intersection with the NS axis means that a cake layer or a polarization layer can be completely eliminated at a critical cross-flow velocity; this was the case for an inorganic particles suspension and for the methanogenic suspension. A straight line of negative slope followed by a plateau means that an irreversible fouling is superimposed to the reversible phenomenon; this was observed for a secondary treated wastewater. A positive slope means that fouling predominates; this was observed with hydrocarbon emulsions.

Nitrification in a hollow-fibre membrane bioreactor

Partial nitrification is required before transporting secondary treated wastewater by pipe, which is common in most schemes of wastewater reclamation for irrigation. Sulfide, generated in the anaerobic conditions of the pipe, induces toxicity, corrosion and unpleasant odours. However, a low nitrate concentration (5 mg/l) enables inhibiting sulfide production. A pilot-scale hollow-fibre membrane bioreactor was tested for partial or total nitrification. Aeration in the membrane module appeared to be very important to avoid fouling of the micro-ultrafiltration membrane. This study enabled the identification of hydraulic conditions required to achieve partial or total nitrification of a real effluent. Moreover, the permeate did not contain TSS, while its organic content was very low. The results obtained can be used for implementing nitrification membrane reactors at full scale.

Influence of biologically treated wastewater quality on filtration through a hollow-fibre membrane

Abstract In Tenerife, Canary Islands, a complex infrastructure exists, whose aim is the reuse of wastewater for crop irrigation. For this reason, techniques of advanced treatment, such as micro-ultrafiltration, have been developed. A laboratory-scale study was carried out to evaluate the feasibility of a combined process of coagulation-flocculation with filtration through hollow-fibre membranes as tertiary treatment of the effluent of a conventional activated sludge plant. The Aluminium Polychloride (AlP) was tested as coagulant-flocculant and a hollow-fibre membrane module of rated pore diameter of 0.03 μm and filtering surface of 0.093 m2, supplied by Zenon Environmental, was used. The water resulting of the flocculation step was filtered by the hollow-fibre. The experiments were carried out in absence of air, without permeate backwash and without maintenance cleaning. In this study, concentrations of 5, 10, 20 and 30 mg/l of AlP and permeate flow-rates of 4.5, 5.9 and 8.5 l/h have been tested. The evolution of membrane fouling, expressed as the variation of transmembrane pressure, shows the clear advantage of the feedwater pretreatment. From the economical and technical point of view, the optimal concentration is 10 mg/l of AlP.