[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Removal of Humic Substances by Membrane Processes

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  • 42 Removal of Humic Substances by Membrane Processes

    Hallvard degaard and Thor Thorsen

    Norwegian Institute of Technology, Division of Hydraulic and Sanitary Engineering, N-7034 Trondheim-NTH, Norway

    Both laboratory and pilot-plant experiments have been carried out to evaluate the use of membrane processes for the removal of humic substances. These processes are competitive for small waterworks with high raw-water color. Cellulose acetate membranes with a mo-lecular weight (MW) cutoff of 800-1000 may be used favorably at a pressure of 7-10 bars. The capacity of the membrane will be reduced, even when optimal membrane washing is performed. The washing solution should be citric acid, sodium citrate, and sodium alkylaryl sulfonate in the proportions given in this chapter. The long-term capacity of the spiral-wound cellulose acetate membrane was found to be 25 L/m2h at optimal membrane washing. The lifetime of a membrane at this capacity is estimated as 4 years.

    JVEMBRANE FILTRATION HAS NOT BEEN USED yet in full-scale waterworks for the prime objective of removing humic substances. Membrane filtration is well-known, however, from the analytical practice of fractionating humic substances. In existing drinking-water plants that use reverse osmosis, humic substances in raw water are often considered a nuisance because of their tendency to clog the membranes. These plants are not specifically designed for removing humic substances.

    This chapter summarizes our findings with respect to the use of mem-brane^processes in a research program on the removal of humic substances at small Norwegian waterworks (I). (Chapter 45 summarizes our findings with the use of macroporous anionic resins.) The philosophy behind our research was that, because humic molecules are so big, the use of open

    0065-2393/89/0219-0769$06.00/0 1989 American Chemical Society

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 770 AQUATIC HUMIC SUBSTANCES

    membranes and low pressures might make this traditionally expensive water-treatment method economically competitive with traditional humic-sub-stance removal techniques. Our aim was therefore to evaluate this process, to recommend operational guidelines, and to give design criteria for typical Norwegian surface waters, which are high in color but low in turbidity.

    Several experiments have been performed, both short-term laboratory-scale and long-term pilot-scale (2-5). Only the major experiences wil l be given here. Because the results of the laboratory-scale experiments are presented in more detail elsewhere (3), this chapter wil l concentrate primarily on the long-term experiments.

    Laboratory-Scale Experiments

    Experimental Methods. The raw water used in the laboratory-scale experiments typically had a raw-water color of60-70 mg of Pt/L, permanganate number of 6-8 mg of 0 2 / L , conductivity of 45-55 8/, and iron concentration of 0.08-0.12 mg/L. It was soft humic water of a type commonly found in Norway.

    The experiments were performed in laboratory reverse osmosis units as shown in Table I. Several cellulose acetate membranes were tested with respect to treatment efficiency in terms of color, permanganate number, conductivity, and specific flux (capacity per bar of operating pressure).

    Discussion. Results from the laboratory-scale experiments are summarized in Table II. A great deal of color could be removed (>80%) even with very open membranes ( M W cutoff = 3000). In order to have the same land of removal of organic matter in terms of permanganate number, membranes that were less open had to be used.

    Table II shows that treatment efficiency increased and specific flux decreased when the M W cutoff was lowered. However, systematic relationships between the parameters could not be derived from the data. For a given membrane, the pressure did not seem to have any impact on treatment efficiency. Moreover, the membrane flux was not significantly influenced by the raw-water humic-substance concentration.

    When both treatment efficiency and flux were taken into consideration, it was concluded that low humic-substance concentration (5 mg of Pt /L) in treated water could be achieved with cellulose acetate membranes with a M W cutoff in the range of 500-2000 operated at a pressure of 7-15 bars.

    Table I. Laboratory Units Used in Laboratory Experiments Membrane

    Manufacturer Surface

    Manufacturer Type Area (m2) Pressure (bar) Module Type D D S 20-laboratory 0.36 0-80 plate and frame Osmonics 519-SB 0.48 0-15 spiral PCI B R D M K 2 0.10 0-80 tubular

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • Tab

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 772 AQUATIC HUMIC SUBSTANCES

    Long-Term Pilot-Scale Experiments

    The laboratory-scale investigations gave indications of a considerable flux drop after some time of operation. The flux could, to a certain extent, be regained by washing the membranes, but a residual flux drop of about 20% during 170 h of operation was experienced even when the membranes were washed according to manufacturers* specifications.

    Because of the promising treatment results obtained during the laboratory-scale investigation, we built a comprehensive pilot plant to study the process further, especially the long-term effects of membrane washing and flux reduction.

    Experimental Methods. The pilot plant shown in Figure 1 actually consisted of three separate plants: A, B, and C. Plant A was equipped for recirculation of the concentrate and allowed additions in the recirculation tank. This plant was primarily used to evaluate the efficiency of bacteria and virus removal. Membranes were frequently changed. Plant was planned for long-term operation and was not changed significantly during the experiments. Experiments for the evaluation of treatment efficiency and flux of various membranes (MW cutoff in the range of 1,000-20,000) were performed in Plant C. The water was pretreated in automatically backflushed bag filters with nominal

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