Mika Mänttäri

Fouling effects of polysaccharides and humic acid in nanofiltration

Abstract Fouling in nanofiltration was studied using model substances similar to those in the pulp and paper mill waters, such as vanillin, humic acid, locust bean gum, and karaya gum. The parameters studied were the concentration of foulants, pH, cross-flow velocity and membrane hydrophobicity. The substances were nanofiltered alone and in their binary mixtures. The nanofiltration membranes (NTR-7450 and Desal-5 DK) differed by their cut-off values and their material properties, such as hydrophobicity. Various causes of fouling were observed to exist with the model substances studied. The electrostatic attraction markedly influenced the flux of charged substances, but had an insignificant effect on the flux of uncharged substances. Hydrophobic interactions overcame the electrostatic repulsion and caused more fouling of the more hydrophobic membrane. The prediction of filtration properties of the binary mixtures of model substances from the filtration results of individual substances is extremely difficult. Both the interactions between two model substances as well as the interactions between the model substances and the membrane determined the flux and fouling in the filtration of the binary mixtures of the model substances. Humic acid improved both the permeate flux and the pure water flux at neutral pH, but slightly fouled the membranes under acidic conditions. Humic acid might increase both the membrane hydrophilicity and its negative charge when being adsorbed by hydrophobic attraction.

Critical flux in NF of high molar mass polysaccharides and effluents from the paper industry

Abstract High molar mass polysaccharides (locust bean gum and karaya gum) and effluents from a mechanical pulp mill and a paper mill were nanofiltered with commercially available nanofiltration (NF) membranes. The effect of the filtration conditions on the flux (critical flux), retention, and the fouling of the membranes was studied. The experiments were conducted by increasing and decreasing the pressure and measuring the flux thus obtained. The critical flux was observed to increase with increasing flow velocity and decreasing concentration. An increase in pH increased the electrostatic repulsion between the membrane and the dissociated (charged) components in the paper mill effluents. As a result, a higher critical flux was obtained and also the retentions of the charged substances improved. Only a weak form of the critical flux was observed with the mill effluents. The permeate flux deviated from the pure water flux even at the lowest pressure, but increased linearly with pressure until the weak form of the critical flux was exceeded. The small decrease in flux immediately after filtration was started was probably caused by the plugging of the free spaces in the membranes or by the adsorption of foulants onto the membrane surface. In the filtrations with the high molar mass polysaccharides, a strong form of the critical flux as well as a weak form were observed. The significant irreversible fouling of the most hydrophobic membrane was due to adsorption of the model substances by hydrophobic interaction. A precleaning of the membranes with an alkaline cleaning agent improved the pure water fluxes by up to 30%, but it had only a small effect on the critical or the limiting flux. The pure water fluxes of precleaned membranes after filtration were still higher than the pure water fluxes of the untreated membranes before filtration.

Effect of temperature and membrane pre-treatment by pressure on the filtration properties of nanofiltration membranes

Abstract The effect of temperature on the retention in nanofiltration of model substances (glucose) and substances in industrial streams has been studied in the temperature interval 25°C to 65°C. An increase in temperature decreased the retention until a critical temperature of the membrane was exceeded. After that temperature the flux even decreased and the retention increased. The retention of uncharged substances normally decreased due to alkaline cleaning and the flux of most membranes increased. In the filtration of mechanical pulp mill effluents an increase of temperature decreased the retention of organic substances but did not affect the retention of conductivity. Only few nanofiltration membranes withstood a temperature of 65°C without a significant change in the filtration efficiency. A membrane pre-treatment, such as pressurization with pure water affected dramatically the flux and the flux reduction and somewhat the retention. Experiments made with incompletely wetted membranes underestimate fouling and the retention of the membrane. Most nanofiltration membranes seemed to be almost completely wetted at a pressure of 25 bar.

Natural Organic Matter Removal from Drinking Water by Membrane Technology

Aquatic natural organic matter (NOM) is a heterogeneous mixture of biopolymers and their degradation products that cause harmful by-products during drinking water production. The great variability in NOM composition makes it difficult to completely remove from drinking water by any single technique. The current article reviews the NOM removal by micro-, ultra- and nanofiltration and by hybrid processes combining membrane techniques with other unit processes: coagulation, adsorption, and oxidation, and by membrane bioreactors.

Influence of filtration conditions on the performance of NF membranes in the filtration of paper mill total effluent

Abstract Nanofiltration membranes were studied and their suitability for treatment of paper mill total effluent was evaluated. The membrane flux, its decline and retentions of various components were studied on a laboratory scale using a flat sheet module under different operation conditions. The results showed that during short filtrations temperature, pressure and flow velocity had minor effects on the retention of the measured components. Only the retention of monovalent ions (chloride) was significantly dependent on the flow velocity and the ionic concentration on the surface of the membrane. With the Desal-5 membrane, however, temperature, pressure and flow velocity were found to have a considerable effect on the permeate flux. The critical flux of the membranes was dependent on the flow velocity. The higher the flow velocity, the higher the pressure that could be used without a decrease in permeate flux. pH had a considerable effect on the ability of the membranes to retain various species. Increasing the pH of the solutions improved retention ability of the membranes. An increase in salt concentration (Na2SO4 and NaCl) decreased the flux and led to a decrease in the retentions of COD and chloride. With the RO membrane (TFC ULP) changes in salt concentration did not significantly alter the retentions. Adjustment of the filtration conditions is one way to improve the efficiency of NF membranes. Appropriate pretreatment of the waste water can also improve the flux and filtering efficiency. The permeates from the NF membranes were adequately clean to be reused, for example, as shower waters.

Comparison of nanofiltration and tight ultrafiltration membranes in the filtration of paper mill process water

Several nanofiltration and tight ultrafiltration membranes were tested by mill-site filtrations using a DSS-Labstak M20-filter. Retention, flux and fouling were analysed. Membranes were also characterised by measuring contact angles and pure water permeability. These data were used to explain the experimental results with effluents and to choose the possible membranes for the paper mill process waters. Comparison of different membranes in the constant pressure filtrations showed that usually the membranes with high pure water permeability or contact angle had worse fouling (fouling that had to be removed by alkaline cleaning). In addition, the fouling was stronger in filtrations made at acidic pH than at neutral conditions. The organic substances were retained significantly better at neutral pH than at acidic pH. Generally, when the hydrophilicity of the membrane increased the effect of pH on the retention properties increased as well. However, an individual membrane may deviate remarkably from the general trends. The results showed that some commercially available membranes have both high permeability and good retention properties at the same time.

Recovery of galactoglucomannan from wood hydrolysate using regenerated cellulose ultrafiltration membranes.

Hemicelluloses show promise as a renewable source of raw material for various industrial processes. In this study, galactoglucomannan was recovered from pressurized hot water extract of spruce-sawdust in two steps using hydrophilic regenerated cellulose ultrafiltration membranes having different molecular weight cut-off values. The first step was concentration of galactoglucomannan (GGM) by ultrafiltration using a flat sheet unit and the second step was purification of the retained galactoglucomannan by diafiltration using reverse osmosis filtered water. The highest GGM retention (88%), purity (63%) and recovery (70%) were achieved with the UC005 membrane (cut-off value 5-kDa) at a volume reduction (VR%) of 86%. The UC010 and UC030 membranes (cut-off values 10- and 30-kDa, respectively) partly separated xylan from GGM. Generally, diafiltration did not improve the purity of the GGM due to overlapping of the GGM and lignin molar mass distributions and the fact that most of free low molar mass lignin had already been removed in the concentration filtration step. However, by diafiltration, partial removal of xylan and complete removal of monosaccharides from the GGM rich concentrate was achieved.

Evaluation of nanofiltration membranes for filtration of paper mill total effluent

Abstract In the pulp and paper industry, membrane technology can be applied both in recycling of the valuable materials within the process, and in pollution control. A major challenge for membrane technology is purification of process water, so that it can be reused, for example, as shower water. The study is part of a project to develop a single-stage nanofiltration process for recirculation of paper mill water, and to reuse this water as shower water. Different membranes were used to treat effluent, which consisted of water from a thermomechanical pulp plant and paper machine. In the ultrafiltration experiments it was seen that the resulting permeates were practically free from highly conjugated lignin residuals, anionic trash and turbidity. The reductions in total carbon, chemical oxygen demand and inorganic matter were between 50% and 60%. Even better reductions were achieved from experiments with nanofiltration membranes. It was seen that the resulting permeates were totally free from turbidity, colour and anionic trash. Reductions of more than 80% were measured for chemical oxygen demand, total carbon and inorganic matter. Chloride ions were not well retained by NF membranes. The permeabilities of NF membranes were between 3 and 13 I/(M2h bar), depending on the piece of membrane and the pretreatment of the membrane. The use of parallel spacers greatly decreased the reduction in the permeate flux during the filtration. Reductions in chloride ions were better using RO membranes, as expected. The reductions in chemical oxygen demand, total carbon and sugar were above 95% for filtration using RO membranes. The permeabilities of these membranes were below 2.5 l/(M2h bar). Thus they are not suitable for industrial-scale filtration. The best membranes for treatment of the paper mill effluent used here were the Desal-5 and NF45 membranes. These had a low flux reduction of the pure water flux, stable permeate flux and good permeabilities. The reductions in the measured parameters were also acceptable.

Enhanced membrane filtration of wood hydrolysates for hemicelluloses recovery by pretreatment with polymeric adsorbents

In this study adsorption of foulants from birch and pine/eucalyptus wood hydrolysates on two polymeric adsorbents was studied aiming to reduce the membrane fouling. The effect of the pretreatment of hydrolysate on polyethersulphone membrane performance was studied in dead-end filtration experiments. Adsorption pretreatment improved significantly filtration capacity and decreased membrane fouling. Especially high-molecular weight lignin was efficiently removed. A multistep adsorption pretreatment was found to reduce the amount of adsorbent required. While large adsorbent amount was shown to increase flux in filtration, it was found also to cause significant hemicellulose losses.

Production and recovery of monosaccharides from lignocellulose hot water extracts in a pulp mill biorefinery.

Processing of hemicelluloses obtained with pressurized hot water extraction (PHWE) from Scots pine to monosaccharides and other chemicals was investigated experimentally. A process scheme consisting of ultrafiltration, acid hydrolysis, and chromatographic separation was proposed and evaluated. A two-stage ultrafiltration was found necessary for efficient fractionation of the wood extract. It was shown that the monosaccharides can be released from a concentrated hemicellulose fraction with sulfuric acid hydrolysis without a significant loss of yield due to decomposition of monosaccharides. Acid hydrolysate was successfully fractionated with ion exchange chromatography and the hydrolysis acid was recovered for reuse. The product fractions obtained include polyphenols and high molar mass hemicelluloses (from UF stage 1), arabinose (from UF stage 2), as well as acetic acid and a mixture of monosaccharides (xylose, galactose, mannose, glucose) from chromatography.