Bart Molenberghs

A comparison between polysulfone, zirconia and organo-mineral membranes for use in ultrafiltration

Abstract In this paper, representative polymeric (a PSf/PVP membrane), ceramic (a ZrO 2 membrane) and organo-mineral (a ZrO 2 /PSf membrane) ultrafiltration membranes, all in the tubular configuration, are being compared for their basic membrane properties, and for the typical ultrafiltration application of protein recovery of cheese whey. These three different membranes with a quite similar pore size (the cut-off values for each of the three membranes were comprised between 25 000 and 50 000 Dalton) showed pure water permeability coefficients between 135 and 1250 l/h m 2 bar. The highest pure water flux was found for the organo-mineral membrane, the lowest for the polymeric membrane. By FESEM analysis of the top-surfaces (skin) of both the PSf/PVP and the ZrO 2 /PSf membrane a strong difference in surface-porosity was found. These results were claimed to partially explain the difference in pure water flux. From SEM pictures of the cross-section of the ZrO 2 /PSf membrane it could also be seen that the skin layer thickness is smaller, at these places where particles are present near the skin-surface, compared to the rest of the membrane as well as to the skin of the PSf/PVP membrane. These latter observations were also used to further explain the flux difference between the PSf/PVP and the ZrO 2 /PSf membrane. In application tests (ultrafiltration of a sweet Gouda cheese whey) these three rather different membranes surprisingly showed practically the same gel-layer controlled or plateau fluxes, the same flux stability, and flux/concentration factor behaviour. The protein retention in all the experiments was 99% or more. The permeability coefficient however for this sweet Gouda whey was identical for the PSf/PVP and the ZrO 2 membrane and equal to 50 l/h m 2 bar. On the contrary for the ZrO 2 /PSf organo-mineral membrane a nearly three fold higher permeability coefficient of 135 l/h m 2 bar was found. This property is partly attributed to the much higher surface porosity of the organo-mineral membrane as compared to the polymeric membrane. From this comparison one may conclude that for high fouling applications, the only positive effect upon using membranes with high permeability coefficients is a reduced transmembrane pressure for a given flux. However, for low fouling applications distinct gains in terms of flux can be expected upon using such membranes.

A new method of measuring and presenting the membrane fouling potential

The silt density index (SDI) and modified fouling index (MFI) characterisation methods are well known for the evaluation of membrane fouling potential of dispersed particulate matter (suspended solids, colloids) in a feed. The SDI and MFI methods, however, reduce the overall and very complex fouling phenomena into a one number value, on which the interpretation of the fouling potential of the feed is based. Considering such a one number characteristic, a significant amount of information from the fouling measurement (data) is lost. In this paper a concept is introduced in order to preserve such information and supplement the existing indexes. The proposed method measures, processes and presents data in a specific format. To illustrate the concept, some results are shown from measurements on three types of feed. Future systematic research will also include the measurement on some model feeds with, for example, well characterised dispersions for comparison purposes. However, based on the contents of this paper, a discussion on the method could be initiated.

Continuous Bioethanol Fermentation from Wheat Straw Hydrolysate with High Suspended Solid Content Using an Immersed Flat Sheet Membrane Bioreactor

Finding a technological approach that eases the production of lignocellulosic bioethanol has long been considered as a great industrial challenge. In the current study a membrane bioreactor (MBR) set-up using integrated permeate channel (IPC) membrane panels was used to simultaneously ferment pentose and hexose sugars to ethanol in continuous fermentation of high suspended solid wheat straw hydrolysate. The MBR was optimized to flawlessly operated at high SS concentrations of up to 20% without any significant changes in the permeate flux and transmembrane pressure. By the help of the retained high cell concentration, the yeast cells were capable of tolerating and detoxifying the inhibitory medium and succeeded to co-consume all glucose and up to 83% of xylose in a continuous fermentation mode leading to up to 83% of the theoretical ethanol yield.

The recovery of backwash water from sand filters by ultrafiltration

Abstract Groundwater is still one of the main sources for the production of drinking water. In the preparation of drinking water, the groundwater is first aerated and then filtered through a sand filter in order to remove Fe, Mn, NH4 and methane. After saturation, these sand filters have to be backwashed periodically. In the past, the backwash water was discharged in the sewage either directly or after sedimentation. However, the first pilot trials with ultrafiltration showed already in 1995 that this technique was very promising for the recovery of backwash water. As a result, the first full-scale ultrafiltration installation was built at the drinking water company NRE (Nutsbedrijf Regio Eindhoven, now WOB) in Eindhoven, Holland, in 1997. A Belgian drinking water company started in the beginning of 1998, in collaboration with Vito, with pilot testing on this subject. Long-term experiments were carried out at four different locations, with different water qualities. At one location the backwash water coming directly from the sand filters (bulk fraction) was filtered; at the other locations the supernatant, which is formed after sedimentation in a settler, was used as feed for the UF pilot. As a result of the sedimentation process, the Fe content is lower in the supernatant than in the bulk fraction. The experiments were done with two different membranes: X-Flow and Stork. Both membranes are operating in dead-end mode with a standard backwash procedure in a very reliable and stable way. Special attention was given to the optimization of the chemical cleaning procedure and the permeate quality in respect of colony forming units (CFU). At the last location, a set of short-term experiments was carried out in order to get more insight in the fouling behaviour of the backwash water. Therefore, different operation modes were compared (low-high fluxes, short-long filtration time), keeping the dirt load either constant or variable. These results will also be discussed in this paper.