Nanofiltration Concentrate Treatment For Increasing The Water Recovery In Drinking Water Production S. Van Geluwe, C. Vinckier, L. Braeken, B. Van der Bruggen * Laboratory of Physical Chemistry and Environmental Technology W. de Croylaan 46, PO Box 02423 B-3001 Leuven ** Laboratory of Molecular Design and Synthesis Celestijnenlaan 200F, PO Box 02404 B-3001 Leuven Keywords: membrane filtration; ozone; electrodialysis Problem sketch Nanofiltration is an effective and reliable method for the combined removal of a broad range of pollutants in surface water. However, fouling of the membrane limits the water recovery for this application to about 80%. As problems with water scarcity are expected to grow worse in the coming decades, even in regions currently considered water-rich, it cannot be tolerated that 20% of the feed water is wasted. Therefore, it is necessary to develop technologies that make the discharge of concentrate streams superfluous. The general concept of this study is to remove specific pollutants in the concentrate stream so that this stream can be returned to the feed side of the membrane without increased membrane fouling. The natural organic matter (NOM) in the concentrate is decomposed by ozone oxidation, if necessary combined with hydrogen peroxide. The salts in the concentrate are removed by electrodialysis. In this way, a closed cycle with a recovery of almost 100% may be obtained, as shown in figure 1.1. The aim of this research is to find the optimal process parameters that make the recycle of the concentrate stream possible. Figure 1.1 Diagram of the proposed process for the treatment of nanofiltration concentrates in order to increase the water recovery of nanofiltration in the drinking water industry. Ozone oxidation Due to their hydrophobic chemical structure, humic acids are often considered as the most severe membrane foulants in nanofiltration of surface water. Concentrated solutions of natural humic acids were oxidized by ozone for ten minutes. A substantial decrease of the chemical oxygen demand (COD) by 40% could be achieved, even at low ozone concentrations in the gas phase (< 20 g O 3/Nm3)(~ 2 g O3/ g COD). The COD was further reduced at higher ozone concentrations or reaction times, but only to a small extent, which could not be justified by the higher treatment costs. A COD reduction of 70-80% is required in order to avoid the accumulation of organic matter in the closed cycle. It was not possible to achieve this requirement with pure ozone oxidation. Ozone reacts selectively with the hydrophobic fraction of the organic matter, which is responsible for severe membrane fouling, and could reduce the hydrophobic COD by 70% at low ozone doses. These observations are explained by the fact that ozone preferentially oxidizes electrophilic aromatic groups to oxygenated functional groups, such as aldehydic, ketonic and especially carboxylic groups. These saturated compounds react typically very inefficiently with ozone, so they are not further mineralized into carbon dioxide and water. The oxidation of these reaction products rather than the primary organic matter dictates the required oxidants doses and treatment cost. The mineralization of the carboxylic reaction products was enhanced by the addition of hydrogen peroxide to the ozonated solution. Mineralization could be further improved but this improvement was limited, as shown by the data in table 1.1. Futher optimisation of the hydrogen peroxide dose, pH and reaction time is necessary to make this process economically attractive. Table 1.1 Procentual UV absoption (UVA)(280 nm) removal during ozone oxidation (ozone gas phase concentration: 18 g O3/Nm3, pH: 7-8, initial UVA: 0.932, alkalinity: 420 mg/L NaHCO3). After ten minutes, H2O2 was continuously added (molar ratio of H2O2 to O3 dose was 0.72) Reaction time (min) % UVA removal Ozonation Perozonation 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0% 20% 33% 43% 47% 50% 56% 57% 60% 61% 62% 62% 64% 64% 65% 67% Electrodialysis Electrodialysis has to be able to reduce the concentration of the main ions (Na +, Ca2+, Mg2+, Cl-, SO42-, NO3-) in the nanofiltration concentrate sufficiently in order to prevent salt accumulation in the closed cycle. Due to their high rejection by nanofiltration membranes, divalent ions have a high concentration in the membrane concentrate and consequently require a high removal degree in the electrodialysis step. Batch-scale experiments were conducted with the Fumasep-membranes FTCM and FTAM of Fumatech. Experiments with single salt solutions proved that these membranes are aselective. The desalination of salt mixtures with a typical composition of nanofiltration concentrates, showed unexpected but very valuable results. The permselectivity of SO42- was higher compared to Cl- or NO3-, although steric hindrance typically obstructs the transport of SO42- ions through ion selective membranes. Thus, the characteristics of the Fumasep membranes, especially the high crosslinking density, were effective for changing the permselectivity between monovalent and divalent ions, which makes these membranes attractive for this application. Significance and impact Combinations of chemical oxidation and membrane filtration are very promising to tackle problems encountered in both technologies. For instance, ozone oxidation can mitigate membrane fouling by natural organic matter and membranes are important to resolve catalyst recovery problems in heterogeneous photocatalysis. Research about hybrid oxidation/membrane systems is necessary to reduce the cost of both technologies and make them more attractive in drinking water and wastewater industry. For electrodialysis, the desalination of other salts than NaCl is hardly investigated. Although the presence of other ions in the feed solution has a large impact on the separation efficiency, the effect of a mixture of monovalent and multivalent ions on the performance of electrodialysis is unknown.