Potential Pitfalls in Membrane Fouling Evaluation: Merits of Data Representation as Resistance Instead of Flux Decline in Membrane Filtration

The manner in which membrane-fouling experiments are conducted and how fouling performance data are represented have a strong impact on both how the data are interpreted and on the conclusions that may be drawn. We provide a couple of examples to prove that it is possible to obtain misleading conclusions from commonly used representations of fouling data. Although the illustrative example revolves around dead-end ultrafiltration, the underlying principles are applicable to a wider range of membrane processes. When choosing the experimental conditions and how to represent fouling data, there are three main factors that should be considered: (I) the foulant mass is principally related to the filtered volume; (II) the filtration flux can exacerbate fouling effects (e.g., concentration polarization and cake compression); and (III) the practice of normalization, as in dividing by an initial value, disregards the difference in driving force and divides the fouling effect by different numbers. Thus, a bias may occur that favors the experimental condition with the lower filtration flux and the less-permeable membrane. It is recommended to: (I) avoid relative fouling performance indicators, such as relative flux decline (J/J0); (II) use resistance vs. specific volume; and (III) use flux-controlled experiments for fouling performance evaluation.

Investigation of membrane fouling mechanism of intracellular organic matter during ultrafiltration

This study investigated the ultrafiltration (UF) membrane fouling mechanism of intracellular organic matter (IOM) from Chlorella vulgaris (CV) and Microcystis aeruginosa (MA). Both CV- and MA-IOM caused severe membrane fouling during UF; however, there were significant differences in the membrane fouling by these two materials. Neutral hydrophilic (N-HPI) compounds were the organics that caused the most severe membrane fouling during CV-IOM filtration, whereas the MA-IOM membrane fouling was induced by mainly hydrophobic (HPO) organics. From an analysis based on Derjaguin–Landau–Verwey–Overbeek theory, it was found that the interaction energy between the membrane and foulants in the later stage of filtration was the major factor determining the efficiency of filtration for both CV-IOM and MA-IOM. The TPI organics in CV-IOM fouled the membrane to a more severe degree during the initial filtration flux; however, when the membrane surface was covered with CV-IOM foulants, the N-HPI fraction of CV-IOM caused the most severe membrane fouling because its attractive energy with the membrane was the highest. For MA-IOM, regardless of the initial filtration flux or the late stage of filtration, the HPO organics fouled the membrane to the greatest extent. An analysis of modified filtration models revealed that cake layer formation played a more important role than other fouling mechanisms during the filtration of CV-IOM and MA-IOM. This study provides a significant understanding of the membrane fouling mechanism of IOM and is beneficial for developing some strategies for membrane fouling control when treating MA and CV algae-laden waters.

One-Pot Polymerization of Dopamine as an Additive to Enhance Permeability and Antifouling Properties of Polyethersulfone Membrane

This paper reports the fabrication of polyethersulfone membranes via in situ hydrogen peroxide-assisted polymerization of dopamine. The dopamine and hydrogen peroxide were introduced into the dope solution where the polymerization occurred, resulting in a single-step additive formation during membrane fabrication. The effectivity of modification was evaluated through characterizations of the resulting membranes in terms of chemical functional groups, surface morphology, porosity, contact angle, mechanical strength and filtration of humic acid solution. The results confirm that the polydopamine was formed during the dope solution mixing through peroxide-assisted polymerization as proven by the appearance of peaks associated OH and NH groups in the resulting membranes. The presence of polydopamine residual in the membrane matric enhances the pore properties in terms of size and porosity (by a factor of 10), and by lowering the hydrophilicity (from 69° to 53°) which leads to enhanced filtration flux of up to 217 L/m2 h. The presence of the residual polydopamine also enhances membrane surface hydrophilicity which improve the antifouling properties as shown from the flux recovery ratio of > 80%.

Tangential-flow membrane clarification of Malvar (Vitis vinifera L.) wine: incidence on chemical composition and sensorial expression

Malvar white wine (Vitis vinifera L.) was cold settled (CSW) and clarified by tangential-flow membrane filtration (TFMF). A 500 kDa molecular mass cut-off membrane was used. Filtration flux of 49-48 L/hm2 was achieved at transmembrane pressure of 0.7 bar. The treatment produced a completely clarified wine with turbidity of 0.11 NTU, but also a 10.3% loss of proteins, which could be related to the decrease of some flavour compounds. The CSW and the membrane filtered wines (MFW) were assessed by means of their aroma and phenolic composition, as well as their sensory properties. The results showed that the general physicochemical parameters and most of the analysed phenolic compounds were not or slightly (up to 7.6%) affected by the TFMF process. Nevertheless, the treatment produced an important loss of some key aroma compounds: up to 43% of fatty acid and alcohol esters and up to 26% of higher alcohols. Most affected were aroma species with higher molecular masses and lower polarities. Sensory analysis confirmed the global decrease in wine aroma. TFMF treatment produced also an increase of 52% of the wine benzaldehyde content.

Improvement on filterability in the aerobic treatment of carboxymethyl cellulose (CMC) wastewater

CMC is chemically modified from natural cellulose and widely applied in various industries. CMC wastewater consists mainly of sodium glycolate, sodium chloride and water. With extremely high COD and salinity, high concentration CMC wastewater can?t be biologically treated, but with COD and salinity around 15000 mg/L and 30000 mg/L respectively, low concentration CMC wastewater can be aerobically treated. In a CMC factory, the treatment of low concentration wastewater with aerobic MBR was successful except for one serious problem: poor filterability. Two trial solutions: adding micronutrients and applying MBBR were expected to improve the filterability. In the experiment, adding micronutrients was achieved by mixing filtered natural water into the wastewater, rather than dosing chemicals into it. The treatment efficiency of both solutions was close, but adding micronutrients showed distinguished performance in improving filterability, which includes higher filtration flux and slighter membrane fouling. Adding micronutrients also effectively improved the filterability under severe salinity shock.