Ann-Sofi Jönsson

The influence of nonionic surfactants on hydrophobic and hydrophilic ultrafiltration membranes

Abstract The influence of different types of surfactants on ultrafiltration membranes has been investigated. A basic approach to membrane-surfactant interactions is also presented. A nonionic (Triton X- 100), two anionic (potassium oleate and sodium dodecylbenzenesulphonate) and a cationic (hexadecyltrimethylammonium bromide) surfactant were investigated. The effects on commercial membranes of polysulphone, poly (vinylidene fluoride) and cellulose acetate were studied. The flux reductions of the hydrophobic membranes were found to be much more pronounced than the flux variations of the hydrophilic membranes. Both the material and the molecular weight cut-off were found to influence the performance of the hydrophobic membranes. A drastic influence of hydrophobic impurities was demonstrated. The retention of the nonionic surfactant was low at concentrations below the critical micelle concentration. The retention of the ionic surfactants was quite high, even at low concentrations.

Membrane distillation - A theoretical study of evaporation through microporous membranes

Abstract Membrane distillation is a process in which, for example, water in a heated salt solution, evaporates through a porous membrane. The vapour condenses on a coolant surface on the other side of the membrane. The two liquid streams, the salt solution and the condensate, are separated by a highly porous hydrophobic membrane. Surface tension forces withhold liquids from the pores and prevent contact between the two liquids. The temperature difference, causing a corresponding vapour pressure difference across the membrane, provides the driving force of the membrane distillation process. Evaporation will occur at the solution surface if the vapour pressure on the solution side is greater than the vapour pressure at the condensate surface. Vapours then diffuse through the pores to the cooler surface, where they condense. The dependence of mass and heat transport upon different process and membrane parameters involved in membrane distillation has been investigated theoretically.

Fouling of electrodialysis membranes by organic substances

In this investigation, the influence of various kinds of organic solutes on the fouling of an anion and a cation selective ED membrane was studied. Fouling by adsorption of organic matter onto the membrane was measured as an increase in the membrane resistance with time. Experiments were performed with a fatty acid (octanoic acid), two anionic surfactants (sodium octanoate and sodium dodecylbenzene sulphonate) and an alkaline bleach plant filtrate from a sulphate pulp mill. A marked difference was observed between the increase in the membrane resistance of the anion selective membrane and that of the cation selective membrane. The cation selective membrane was slightly fouled by the bleach plant filtrate, but was only marginally affected by the other organic solutes. The anion selective membrane, on the other hand, was markedly fouled by all solutes.

Nanofiltration of salt solutions and landfill leachate

Nanofiltration (NF) has attracted increasing attention during recent years due to the development of new applications. The advantage of NF compared with reverse osmosis (RO) and ultrafiltration (UF) is that it is possible to separate organic substances from some types of salts. It is, furthermore, also possible to separate different kinds of salts due to the negatively charged groups on the membrane. The retention of sulphate salts by the membranes used in this investigation was 88-96%, while the retention of chloride salts was only 12-47% at low salt concentrations. This difference in retention is due to the charge density of the anion. The retention of NaCl was found to be strongly related to the concentration. The retention decreased from 45% to 7% when the concentration was increased from 0.05 M to 1 M. Although the repulsion of the anion mainly determines the retention of salt solutions, it was observed that the cation can affect the retention, especially in salt solutions with a high concentration of monovalent anions. It was found that the retention of divalent cations was three times higher than that of monovalent cations in this type of salt solution. In chloride solutions with mixed cations, it was shown that a monovalent cation passed through the membrane preferentially to the divalent cations, to such an extent that the retention became negative. NF was utilized to treat a landfill leachate with an extremely high salt content from a waste cell containing mainly ash because of the good separation of cations. Most of the heavy metals, which are multivalent cations, are rejected while the monovalent cations, which are rather harmless substances, pass through the membrane. The retention of, for example, cadmium, zinc, lead and chromium was found to be higher than 70%, while the retention of potassium and sodium was less than 10%. Since the transmembrane osmotic pressure was low, due to the low retention of the monovalent ions, the flux was several times higher than for RO membranes. The flux of the leachate, with a conductivity of 6800 mS/m, was above 50 l/m(2)h at 3 MPa and 25 degrees C. (Less)

Influence of the membrane material on the adsorptive fouling of ultrafiltration membranes

The hydrophilic/hydrophobic properties of the membrane material have an important influence on the membrane flux reduction when treating solutions containing hydrophobic solutes. Different methods of measuring the hydrophobic properties of membranes have been evaluated; measurement of the contact angle being the most commonly used method. In this investigation the flux reduction of eight membranes of approximately the same cut-off, but of different materials, was determined and related to the contact angle. A low-molecular weight hydrophobic solute (octanoic acid) was used as foulant. The flux of the hydrophilic membrane was only marginally reduced, whereas the flux reduction of hydrophobic membranes was significant. Membranes of polyvinylidene fluoride had the lowest flux reduction of the hydrophobic membranes. Differences between membranes of the same material (polyethersulfone), but from different manufacturers, were observed. No relation was found between the flux reduction and the contact angle of the membrane, the initial pure water flux or the retention of a hydrophilic macromolecule (PEG20). The pure water flux of two of the hydrophobic membranes increased significantly during the experiments, whereas the pure water flux of the other hydrophobic membranes decreased. A membrane of polyaramide, with a contact angle of the same order of magnitude as the other hydrophobic membranes, behaved as a hydrophilic membrane, with no influence on the flux at concentrations below the solubility limit of the hydrophobic solute, but experienced the most severe flux decline of all the membranes included in the investigation at the solubility limit. Two modified membranes were included in the investigation. The flux reduction of these membranes was not significantly lower than for the corresponding unmodified membranes of the same materials.

Fractionation and concentration of kraft black liquor lignin with ultrafiltration

Kraft black liquor lignin is a biofuel that is separated from the cellulose during kraft pulping. Improved technology and energy integration in paper mills have led to an energy surplus at many mills. It is therefore of great interest to extract the lignin from the pulp mill and sell it as fuel to replace fossil fuel in other furnaces. The fractionation of kraft black liquor with a total dry matter content of about 15 wt% has been studied using ultrafiltration. The flux for three polymeric membranes with different cut-offs was investigated as well as their retention of lignin and other process specific substances. The retention of lignin for the three membranes with cut-offs of 4,8 and 20 kDa was 80%, 67% and 45%, respectively. The retention of sulphur and sodium was zero for all three membranes. The purity of the final lignin fuel is of importances as the ash content preferably should be as low as possible. The flux and retention during concentration and diafiltration of the black liquor were therefore studied. The dialfiltration operation was conducted in batch and semicontinuous mode. The lignin purity was 36% in the original kraft black liquor and 78% after semi-continuous dialfiltration.

Treatment of three types of landfill leachate with reverse osmosis

Treatment of landfill leachate has gained increasing attention during recent years, and several commercial RO leachate treatment plants have been installed in, e.g., Germany, the Netherlands and Switzerland. in this investigation the influence on membrane performance when treating new types of landfill leachate was studied. Three different types of landfill leachate were included in this study: leachate from a conventional landfill and two leachates from a new type of landfill. At the new landfill the waste is divided into different categories and deposed in waste cells with separate leachate collecting systems. Leachate from a cell with biodegradable waste and from a cell with special waste, containing mainly ashes, were included in this study. A linear correlation between flux and conductivity was found for leachate both from the conventional landfill and from the biodegradable waste cell. The flux varied, depending on the conductivity of the leachate, between 48 and 3 l/m(2)/h. The reduction of pollutants was high. The reduction of the chemical oxygen demand and NH4-N was more than 98% for leachate from both the conventional landfill and the biodegradable waste, for example. The salt concentration, and thus the osmotic pressure, was very high in the leachate from the cell containing special waste. The flux was therefore too low for RO to be a suitable treatment process for this leachate. (Less)

Influence of pH on the adsorptive fouling of ultrafiltration membranes by fatty acid

Fatty acids are found in many solutions treated in ultrafiltration plants, for example, in dairy products, fermentation broth, oily waste water and bleach plant effluents from pulp mills. The influence of fatty acids on the fouling of membranes is often ignored because their concentration is rather low. However, during recent years the significant influence of fatty acids on the flux reduction of ultrafiltration membranes has attracted much attention. The pore radius of the membrane, the concentration of fatty acids and the pH of the solution all have a profound influence on the flux reduction when treating solutions containing fatty acids. In the work presented in this paper, the influence of pH on the flux reduction of an ultrafiltration membrane made of polyethersulphone was studied. It was found that there was no flux reduction under alkaline conditions, whereas the flux reduction under acidic conditions was severe. In order to elucidate the adsorption process, the adsorption of octanoic acid on a planar, hydrophobized silica surface was studied by means of in situ null ellipsometry. In the ellipsometry study a sharp increase in the amount adsorbed was observed when the concentration of undissociated acid approached the saturation concentration as the pH was decreased. This explains the observed flux reduction under acidic conditions

Ultrafiltration of kraft black liquor with a ceramic membrane

Emissions from the combustion of fossil fuels contribute to the greenhouse effect around the globe. It is therefore of great interest to reduce the use of fossil fuels and revert to some form of biofuel. Lignin, extracted from kraft black liquor, has the potential to become a significant contributor to the biofuel market. A modern pulp mill has an energy surplus that can amount to 7 GJ per tonne of pulp. One way to reduce this surplus is to extract lignin, which is the main source of energy in a kraft pulp mill, and convert it to a usable biofuel, which can be sold on the market. If membrane technology is used to extract lignin from kraft black liquor additional benefits in the overall operation of the mill can be achieved, such as, an improved pulp quality. In this investigation a tubular ceramic membrane manufactured by Orelis, France, with a cut-off of 15,000 Da was used to fractionate kraft black liquor. The experiments were performed at 60, 75 and 90°C at various pressures. The flux and retention during concentration to a volume reduction of 0.9 were studied. The fluxes were 90, 110 and 130 l/m2h at 60, 75 and 90°C and TMP 100 kPa. The lignin retention was about 30%. The retention of monovalent ions was virtually zero, whereas multivalent ions were retained to a high degree, probably due to their association with the organic material. During concentration to a volume reduction of 0.9 at 100 kPa and 90°C, the dry substance increased from 16 wt% to 22 wt% and the lignin concentration from 55 g/1 to 158 g/I. The average flux during the concentration was 90 1/m2h. The flux was 20 l/m2h at the end of the concentration.

Cleaning of ultrafiltration membranes after treatment of oily waste water

The influences of different types of cleaning agents on a polysulphone ultrafiltration (UF) membrane which had been used to treat oily waste water were investigated. The cleaning experiments were performed with samples of polysulphone membranes removed from a commercial plant in which oily waste water is treated. Five different cleaning agents were used and their influences on the fluxes were investigated. The influence on the flux when cleaning with the different cleaning agents in succession was also studied. Deposits on the membrane surface, before and after cleaning, were analysed using different methods. Most of the analyses were carried out using scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) combined with a micro-analysis system permitting quantitative determination of elements. Some analyses were also performed using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). (Less)