Jean-Christophe Rouch

Impact of coagulation conditions on the in-line coagulation/UF process for drinking water production☆

An in-line coagulation (without settling)/UF process has been studied to improve membrane performance and water quality for surface water treatment. Using coagulation before UF increases permeate quality; the extent of dissolved organic matter removal is controlled by the coagulation step. Efficient coagulation conditions for a coagulation/settling process can be applied for the in-line coagulation/UF process and membrane fouling is reduced. Floc cake resistance is lower than resistance due to the unsettled floc and the uncoagulated organics. For an inside-out hollow-fiber system, the impact of coagulation dose depends on filtration mode: for instance, in cross-flow with a feed-and-bleed configuration, a reduction of coagulant dose induces an increase of the mass transfer resistance even in quasi-stable hydrodynamic operating conditions.

Mass transfer improvement by secondary flows: Dean vortices in coiled tubular membranes

A helically wound hollow-fibre (or tubular) membrane module was studied in an oxygenation operation with water flowing in the laminar regime inside the tube. Data are compared with a conventional module where straight hollow-fibre membranes are in parallel alignment. In the former design, the results are analyzed taking into account the formation of secondary flow (Dean vortices) and a new mass transfer correlation is presented. In the latter design, results are consistent with the Leveque equation. It is shown that the presence of vortices gives better performance in terms of oxygen transfer. Improvement factors were in the range of 2 to 4.

From ultrafiltration to nanofiltration hollow fiber membranes: a continuous UV-photografting process☆

A new way to prepare nanofiltration membranes, consisting of in-line external modification of the skin of a polysulfone ultrafiltration hollow fibre skin, is described. The paper presents a continuous process (dip-coating followed by photografting) and the influence of the operating conditions on the membrane characteristics. This study is focused on a simplified model based on the evaluation of the two sources of monomer available for grafting: one is monomer contained in the thin film drawn from the dip bath, second is contained in the pores of the membrane. The results show that two extreme types of modification can be produced, depending on the experimental conditions: a high rate coupled with a short dip contact time leads essentially to an external grafting whereas a low rate coupled with a long dip residence time leads essentially to an “in pore” grafting.

Use of air sparging to improve backwash efficiency in hollow-fiber modules

The use of air in backwash of hollow-fiber modules was investigated experimentally from bench to full scale. Modules operated in a dead-end and outside-in mode: they were fouled by either a bentonite suspension or a raw river water and then backwashed in presence of air. The air was injected into the retentate compartment either in combination with a reversed permeate flux or together with feed water after a brief permeate back flow. Results indicate that the cake layer is instantaneously lifted off by the reversed permeate flux and is concentrated in the free volume of the module. To remove it from the module and recover the feed concentration, this volume has to be rinsed with a volume at least three times as big. The air, by its piston-like action, improves material removal and reduces the volume of concentrated foulant to be flushed. So the backwash time is reduced and its efficiency is improved. An optimum air flow rate can be found that is independent of the water flow rate used to flush the module free-volume.

Dead-end ultrafiltration in hollow fiber modules: Module design and process simulation

Abstract Ultrafiltration in a hollow-fiber module operating with outside-in and dead-end flow at a constant flow rate was simulated using a model that takes into account the longitudinal pressure drops inside the fibers and within the fiber bundle. The model considers both the filtration phase during which the membrane is fouled by the formation of a filter cake and the backwash phase in which it is cleaned, so as to predict the net rate of production of the module during an operating cycle. The results show that there is a combination of packing density and fiber diameter that gives a maximum net flow rate. Furthermore, this model allows the influence of operating conditions and feed properties on the module performance to be estimated. This can be used to determine how operating parameters must be modified when there is a change in the feed properties.

Hollow-fibre membrane module design: comparison of different curved geometries with Dean vortices

Abstract The performance of several designs of curved membrane modules with Dean vortices was compared through experiments using a colloidal bentonite suspension and cellulose acetate hollow-fibre ultrafiltration (UF) membranes. The different module geometries were: straight, helically coiled, twisted and sinusoidal, or meander-shaped. The experiments show a remarkable increase in mass transfer in curved modules as compared to conventional straight ones. Comparisons were made for modules equipped with the same hollow fibres and the same Dean number (De) for a given Reynolds number (Re). At the same Dean number, all the curved geometries gave the same limiting permeate flux. A mass transfer correlation relating limiting UF flux with the mean wall shear stress has been obtained.

Flux improvement by Dean vortices: ultrafiltration of colloidal suspensions and macromolecular solutions

Coiled and straight hollow-fibre modules have been built and tested; the permeate flux obtained in ultrafiltration with these two geometries is compared for two feeds: a colloidal bentonite suspension and a dextran solution. In the case of colloidal suspensions, the secondary flows induced by the coiled geometry allow fouling to be reduced and the permeate flux is multiplied by a factor of up to 2. An empirical relationship is proposed to express the limiting flux of permeate as a function of both the velocity and some geometrical parameters of the coiled modules. Analogous results are obtained during the ultrafiltration of dextran. It is also shown that under certain conditions almost no deposit was formed; the permeate flux under these conditions is three times higher for coiled modules than for straight ones. For a given energy expenditure and ultrafiltration process, the gain in permeate flux can reach a factor of 1.8.

Mass transfer improvement in helically wound hollow fibre ultrafiltration modules: Yeast suspensions

Secondary flows known as Dean vortices, that appear in a curved pipe due to centrifugal force, are found to reduce concentration polarisation and fouling and increase membrane permeation rates. The influence of Dean vortices on mass transfer is studied in membrane filtration. The experimental study consists in filtering suspensions of bakers yeast with ultrafiltration hollow fibre membranes, arranged in straight or coiled modules. The effect of different operating parameters is tested: flow velocity, suspension concentration and module geometry. It is found that secondary flow enhances membrane permeation by a factor of up to 5. Moreover, an energy analysis shows that for the same energy consumption, the permeate flux obtained in a coiled module is still far greater than that in a straight module.

The use of Dean vortices in coiled hollow-fibre ultrafiltration membranes for water and wastewater treatment

Mass transfer in ultrafiltration for water and wastewater treatment is improved by Dean vortices. With this secondary flow, which appears in a coiled hollow-fibre module, the shear stress is higher than in a straight module and is a maximum near the external wall of the coiled tube. As a consequence, concentration polarisation and cake deposition are reduced and the limiting flux in ultrafiltration of model fluids (bentonite and yeast suspensions) and activated sludge is improved by up to 5 times. The effect of the hydrodynamic conditions and feed concentration is tested. An energy analysis shows that an improvement in permeate flux is found at a given energy consumption.

Improving PVDF Hollow Fiber Membranes for CO2 Gas Capture

Poly(vinylidene fluoride) (PVDF) hollow fiber membranes were obtained by the phase inversion technique. The influence of internal coagulant viscosity (0.001 to 3 Pa s) and air gap (0.6 to 86.4 cm) on the structure and mechanical resistance of the fibers was studied. A “sponge-like” structure free of macrovoids was obtained by using polyvinyl alcohol (PVA) with N-methyl pyrrolidinone and water as internal coagulant (viscosity 3 Pa s). The effect of the air-gap was studied in order to control the structure and obtain mechanically resistant membranes with tensile strength at break between 2.2 and 54.3 N/mm2 and pure water permeability ranging from 4 to 199 Lh−1m−2bar−1. CO2 permeability of these membranes was measured and found to be in the range of 365 to 53200 NLh−1m−2bar−1. The “Dusty Gas” model (DGM) was used to calculate the pore size of the membranes from CO2 permeability experiments, obtaining pore radius values going from 0.6 to 10.8 µm. Results from modeling were compared with pore sizes observed in SEM images showing that this model can accurately predict pore radius of sponge-like structures; however, pore sizes of membranes presenting sponge-like structures together with finger-like pores were inaccurately predicted by the DGM.