Jutta Nuortila-Jokinen

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.

Comparison of membrane separation processes in the internal purification of paper mill water

Abstract In recent years the paper industry has been studying different technologies to reduce fresh water consumption in the mills. In this study micro-, ultra- and nanofiltration of some paper mill waters both on a laboratory and a pilot scale were compared. The performances of different membranes and modules are reported and some general conclusions are given. The performance of the membranes was evaluated by determining the reduction of certain measurable parameters in the feed solution such as chemical oxygen demand (COD), total organic carbon (TOC), total solids (TS), lignin, sugar, anionic trash (CD), ions, conductivity, turbidity, silt density index (SDI) and brightness decreasing substances. The tested tubular 0.4 μm microfiltration membrane was not suitable as a pretreatment step to nanofiltration. In ultrafiltration the tested cross rotational (CR) filter using flat sheet membranes gave significantly higher fluxes than the tested polymeric or ceramic tubular membranes. It was shown that the relatively low cut-off (30 kD) hydrophilic C30G membrane made from regenerated cellulose had higher fluxes both at neutral and acidic pH. Also the long time behaviour (duration four or six days) of the C30G was very good. It was also shown that the pH of the original feed, the experimental set-up and the membrane cut-off affected fouling the most in ultrafiltration. In nanofiltration the membrane structure and feed pH affected the results the most. Very good results were obtained by all the tested nanofiltration membranes. It was shown that fluxes similar to microfiltration are obtained by tubular ultrafiltration membranes and that the reductions were better. Moreover, the ultrafiltration process was more reliable than microfiltration as a pretreatment for nanofiltration because the obtained silt density index was always below the value 5 with ultrafiltration. However, it was also shown that in nanofiltration better reductions were obtained if no pretreatment was done, if it was not required by the module configuration.

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.

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.

Influence of shear on flux in membrane filtration of integrated pulp and paper mill circulation water

Abstract Different ultrafilters and membranes were tested with ground wood mill circulation water from an integrated pulp and paper mill. Both the CR 200/1 (cross rotational, CR-filter™) and the VSEP® L (vibratory shear enhanced processing) laboratory modules and the CR 1000/10 pilot plant module were investigated. In the pilot plant filtration the effects of the rotor speed, volume reduction factor and pressure were tested. According to the measurements, the lower the pressure and the higher the rotational frequency (CR 200/1) or the vibration amplitude (VSEP L), the less fouling occurred. For the CR-modules the best short time fluxes were obtained with PA 50H and Desal JX membranes, and for the VSEP-module with the PA 50H and C 30F membranes. The C 30F, PA 50H and PES 50H membranes were chosen for the pilot plant filtrations. In the pilot tests at the mill it was observed that the hydrophilic C 30F membranes fouled very little and had fluxes close to 400 l/(m2h). The more hydrophobic PA 50H and PES 50H membranes were fouled badly, and cleaning only restored flux for a short time. A concentration factor between 10 and 30 did not influence the flux very much, but inside the equipment the membranes handling the highest concentration had a lower flux. Also in the mill site experiments it was shown that the flux should be kept as close to the critical flux as possible and that this could be achieved with a high cross-flow velocity and the lowest pressure possible for the equipment.

Tailoring an economical membrane process for internal purification in the paper industry

In this study membrane filtration processes, e.g. micro-, ultra- and nanofiltration, have been considered from an application directed point of view set by the pulp and paper industry. Three different shear-enhanced modules are compared. The modules are the vibration shear enhanced processing (VSEP) unit from New Logic Int., the compact tubular modules from Stork-Friesland and the cross rotational (CR) filter from Raisio Engineering. The paper mill waters studied have been the save-all (disc filter) clarified fractions of white water resulting from either an acidic or a neutral paper making process. A feasible membrane filtration process in the pulp and paper industry means high flux and low fouling tendency. In this study it is shown that optimized operation of the shear-enhanced modules fulfils these requirements. Optimization is done by selecting relatively low pressure and high flow velocity and/or high shear at the membrane/liquid interface. Moreover, the flux can be further increased by using chemical pretreatment. A relatively low dosage (0.5 mg/l) of a fixative (Raifix 07525) was needed to increase flux about 20% and reduce irreversible fouling significantly. It was further seen that ultrafiltration produces permeate that is usually clean enough for recycling purposes in the paper mill. However, if ion content is to be reduced, only nanofiltration qualifies. In addition, ultrafiltration, in most cases, produced higher fluxes than microfiltration because microfiltration membranes are more apt to foul in this application.

Closing pulp and paper mill water circuits with membrane filtration.

Abstract: In this study membrane filtration, ultrafiltration, and nanofiltration alone and as part of hybrid processes are considered as means to purify pulp and paper mill process waters suitable for reuse. Thermophilic ærobic biological treatment, pH adjustment, flocculation, and ozonation were tested as pretreatment methods on pilot or on laboratory scale. The aim was to increase flux and reduce fouling by various pretreatment steps and, thus, increase the competitiveness of the membrane process. The results were also evaluated by comparing the benefits obtained against the costs. It was discovered that benefits could be obtained with all the pretreatments tried. Thermophilic ærobic biology assisted in the removal of organic material and increased flux significantly, but the costs were the highest. The most cost‐effective processes, however, seem to be pH‐adjusted nanofiltration and flocculation nanofiltration hybrid processes, which is understandable because of their significantly lower investment costs compared to, for example, those of biological process. The pH adjustment increased the electrostatic repulsion between negatively charged solutes and membrane, thereby increasing the flux. Flocculation removed the foulants effectively from the feed and it both increased flux and reduced fouling. Yet, many noteworthy benefits were obtained also with ultrafiltration and ozonation. All of the hybrid processes tested could be applied at various points of the water circuit of an integrated pulp and paper mill for purification purposes. The eventual superiority and cost‐effectiveness of the applied process remains to be determined case by case.

Effect of High Filtration Temperature on Regenerated Cellulose Ultrafiltration Membranes

Abstract The hydrophilicity of regenerated cellulose (RC) ultrafiltration (UF) membranes could be utilized in industrial processes even more than today if there was more information on their thermal stability. Therefore, in this study two different RC UF membranes were used in a temperature range from 15 to 70°C in order to evaluate their performance stability at high temperature. The experiments were performed in neutral, alkaline, and acidic conditions. The influence of temperature on pure water permeability, observed Dextran retention, and membrane structure were used as evaluation criteria. According to the results the tested membranes could be used at 70°C in the tested conditions for several days. The results, however, indicated also that the use in acidic and alkaline conditions at 70°C might shorten the operational life of the membranes.

Internal Purification of Thermomechanical Pulp Clear Filtrate with a Combined Biological and Membrane Filtration Method: A Preliminary Study

Abstract In this study a combined anaerobic biological-nanofiltration method was studied as a means of internal purification of a thermomechanical pulp (TMP) plant effluent. A TMP plant clear filtrate was first subjected to a thermophilic anaerobic treatment at 55 or 70°C in an up flow anaerobic sludge blanket reactor and then nanofiltered in a flat-sheet laboratory-scale module. The anaerobic treatment removed 55% of the chemical oxygen demand at 70°C and 65% at 55°C. Sugars were removed both at 55 and 70°C while the low molar mass ligneous material was removed only at 55°C. By nanofiltration the remaining low molar mass ligneous material was removed by about 98–99% and the high molar mass ligneous material by 96–99%. Sugar was removed by 88–98% and chemical oxygen demand by 78–81%. It was also shown that most of the pulp-brightness-decreasing substances had been removed. The permeate flux depended on the sample but was at its best [about 38 L/(m2-h) at 8 bar] for the first hour of filtration for the sam...