Removal of Humic Substances by Membrane ... - ACS Publications

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Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch042

Removal of Humic Substances by Membrane Processes Hallvard Ødegaard 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 molecular 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/m ·h at optimal membrane washing. The lifetime of a membrane at this capacity is estimated as 4 years. 2

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 membrane^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

JVÎEMBRANE

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

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


770

AQUATIC H U M I C SUBSTANCES

membranes and low pressures might make this traditionally expensive watertreatment method economically competitive with traditional humic-substance 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 laboratoryscale and long-term pilot-scale (2-5). Only the major experiences will be given here. Because the results of the laboratory-scale experiments are pre sented in more detail elsewhere (3), this chapter will concentrate primarily on the long-term experiments.

Laboratory-Scale Experiments Experimental Methods. The raw water used in the laboratory-scale ex periments typically had a raw-water color of60-70 mg of Pt/L, permanganate number of 6-8 mg of 0 / L , conductivity of 45-55 μ 8 / α η , and iron concentration of 0.080.12 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). 2

Discussion. Results from the laboratory-scale experiments are sum marized 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, mem branes that were less open had to be used. Table II shows that treatment efficiency increased and specific flux de creased 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 d i d 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

Manufacturer DDS

Type 20-laboratory

Osmonics PCI

519-SB BRD M K 2

Membrane Surface Area (m ) Pressure (bar) 2

0.36 0.48 0.10

0-80 0-15 0-80

Module Type plate and frame spiral tubular



SOURCE: Reproduced with permission from réf. 1. Copyright 1986 Pergamon.

2

Table II. Treatment Efficiency and Specific Capacity of 13 Membranes Tested in Laboratory-Scale Experiments Operating Pressure (bar) Molecufor Treatment Efficiency (%) Recommended Capacity Weight (L/m -hbar) Experimental Value Color Permanganate Conductivity Cutoff Membrane — — — — 3.6 80 20,000 SEPA-20KCA 2,000 .— — 14 3.6 15 83 SEPA-OPS 17 7 7-15 15 60 85 1,000 SEPA-OCA 80 65 2.2 11 21 98 600 SEPA-50CA 90 82 1.6 11-15 43 100 400 SEPA-89CA 7-15 60 85 98 1.0 97 200 SEPA-97CA 13 21 5-10 10 DDS-600 58 70 20,000 18 10 10-20 20 DDS-800 66 75 6,000 60 3.1 10-50 40 90 95 500 DDS-865 2.1 10-50 50 78 100 96 500 DDS-870 6.3 5-10 10 23 67 80 3,000 PCI-T4A 44 4.3 10-20 25 100 95 800 PCI-T2A 0.9 10-40 80 92 97 100 300 PCI-T2/15N


772



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 labo ratory-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 con sisted 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 Β 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 light-openings in the range of 1-20 μπι. This chapter concentrates on the results of the long-term experiments in Plant B, where membranes with MW cutoffs of800 and 1000 were installed (OSMONICS, 25 CA and OCA). Discussion. Soon after start-up the capacity of the continuously op erated plant sank more rapidly than in the laboratory experiments, and the pressure loss through the plant increased. Both of these changes are indi cations of membrane fouling (film formation). The thickness of the film was calculated on the basis of flow channel geometry and measured pressure loss. The basis for calculation, data on pressure drop versus bulk flow through one module, was supplied by the membrane manufacturer. The channel height between the membranes and the dimensions of the turbulence pro moter wire were known. According to the laws of fluid flow, pressure drop will increase when the channel height decreases because of fouling, and the linear velocity will thus increase. The film thickness model based on these facts included total pressure drop through the whole plant, feed flow, con centrate flow, and permeate flow from each module in a series of 10 modules. By assuming constant layer thickness in each module, a mean layer thickness could be calculated because the mean flow in each module and total pressure drop were known. In spite of washing procedures as recommended by the membrane manufacturer, the calculated thickness of the film increased almost linearly with time, up to about 100 μιη after about 1900 h of operation (see Figure 2).



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Figure 1. Sketch of membrane process pilot plant.

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42.

0 D E G A A R D & THORSEN

Removal of HS by Membrane Processes

At this point some membranes were replaced, and the film formed on the membrane was investigated. The thickness of the film was indeed of the same magnitude as calculated. The film was found to be soft, dark brown, and loosely connected to the surface of the membrane. Several experiments with a broad range of washing solutions were performed with sheets of fouled membranes. Citrate was effective in terms of regaining capacity, but citrate alone could not prevent film formation over a longer period. An anionic detergent and an optimal p H were needed in addition to citrate. The most effective detergent was found to be sodium lauryl sulfate. This detergent is, however, not chemically stable at the optimal p H for routine wash (pH 3.4-3.7). A n alternative detergent, stable at this p H , is sodium alkylaryl sulfonate. Routine wash performed with only one washing solution leaves a resid ual capacity reduction. Therefore, two different washing solutions are rec ommended. The primary solution for routine wash should maintain stable short-term capacity, and the secondary solution should be used only now and then to maintain acceptable long-term capacity. We recommend the washing routine shown in Table III. When we implemented the washing procedure (at 2000 h), the mem brane film formation problem improved considerably. After 4500 h of op eration, the film thickness was only about 10 μιη. A certain capacity reduction was still experienced, though much smaller. It was concluded that the flux reduction experienced in membrane filtration of humic substances could be divided into two categories. Tem porary flux reduction is recoverable by use of a proper washing routine. Permanent flux reduction, primarily as a result of a different sort of scaling, is not removable by washing. However, it might also be due to some mem brane compaction. Figure 3, based on the total experience from the experiments, shows the mean capacity for properly washed cellulose acetate membranes at 10 bars of operating pressure. The greatest flux reduction is experienced during the first 2 years of the membrane life, and very limited capacity reduction occurs during the next 2 years. Reduction in long-term flux is heavily dependent upon the absolute flux (L/m -h). Therefore, membranes with different initial fluxes gradually ap proach each other in terms of flux. Figure 3 also shows that temperature influences the capacity and in dicates that capacity may be highest during summer. Likewise, capacity will be high after membrane replacement. In order to obtain smooth operation, 25% of the membranes should be replaced every year. The replacement of membranes should be carried out in a period of falling raw-water temperature (during autumn). When the variation in temperature from 2 to 14 °C is taken into account, the design capacity for a plant treating water from the source 2


775

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. 0

Table III. Recommended Washing Routine Composition, Solution Wash Frequency Washing Solution Concentration (%) Primary wash 2-7 times 2-3 parts citric acid 0.3-1 per week 1 part sodium citrate 2 parts sodium alkylaryl sulfonate Secondary wash 5-50 times 2 parts sodium citrate 0.5-1 per year 1 part sodium lauryl sulfate (seasonal variation) Temperature of washing solution: 25-30 °C. SOURCE: Reproduced with permission from réf. 1. Copyright 1986 Pergamon.


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Duration of Wash (h) 1-2

42.

0 D E G A A R D & THORSEN

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s Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch042

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1000 hrs. Figure 3. Mean membrane capacity at 10 bar for cellulose acetate membranes with a MW cutoff of 800-1000 separating humic substances at 2 and 14 °C. (Reproduced with permission from réf. 1. Copyright 1986 Pergamon.) 2

used in our experiments would be 24-26 L / m - h , with 3-4 years of mem brane life. On the basis of our experiences, we recommend that a reverse osmosis plant for the removal of humic substances be designed according to Fig ure 4. Specifications for the plant would be according to Table IV, and for the washing routine according to Tables III and V.

Costs Estimating the cost of reverse osmosis for the removal of humic substances is very difficult. Because the cost will vary from one country to the other, we shall primarily estimate the cost of this process for Norwegian conditions, relative to other alternative processes. In Figure 5 the investment costs are shown in Norwegian kroner [$1 (U.S.) = 6.65 Nkr] for three levels of raw water color and three levels of plant size. Figure 6 illustrates the corresponding unit cost (Nkr/m ) (6). Reverse osmosis may be economically competitive at very small plants and high raw-water colors. Membrane processes are particularly attractive at very high colors, because neither the investment cost nor the operating 3



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H a s h i n g system

RO-plant

Out

Haste

Figure 4. Recommendedflowsheet of a membranefiltrationplant for the removal of humic substances. (Reproduced with permission from réf. 1. Copyright 1986 Pergamon.)

In



42.

0DEGAARD & THORSEN


Table IV. Specifications of a Membrane Filtration of Humic Substances Component Prefilter Module Membrane Molecular weight cutoff Pressure Membrane replacement Washing solution Temperature of washing solution SOURCE: Reproduced with permission from réf. 1. Copyright

Plant for the Removal _____ Measure Sieve opening: 20-50 μιη Spiral wound Cellulose acetate 800-1000 7-10 bar 25% annually According to Table III 25-30 °C 1986 Pergamon.

Table V Washing Routine for the Primary Wash Pump/Valve in Operation Duration (min) P2, P3, KV2, PV1 4 6 X W PV2, KV3 P2, P3, KV3, PV2 6x5° P2, P3, KV2, PV1 4 PI, P3, KV2, PV1 10 P4, NV1, PV3 1 Data are according to theflowsheet in Figure 4.

Sequence Pumping in Pause Circulation Pumping out Flushing Afterfilling NOTE:

779

"Repeated 4-8 times. SOURCE: Reproduced with permission from réf. 1. Copyright 1986 Pergamon.

cost is very dependent upon raw-water concentration, which is the case for the other processes.

Summary Membrane filtration is an interesting alternative for the removal of humic substances in small waterworks when the raw-water concentration is very high. Cellulose acetate membranes with a M W cutoff of 800-1000 may be used effectively at an operating pressure of 7-10 bars. The film forming on the membrane consists of a loosely connected layer readily removable by proper washing and a nonremovable compressible layer that will cause ca pacity reduction even with optimal membrane washing. At the design capacity and washing routine recommended here, 25% of the membranes in a plant should be replaced every year. The washing solution should be made up of citric acid, sodium citrate, and sodium alkylaryl sulfonate in the proportions recommended in this chapter. When the wash ing routine and replacement follow this schedule, the long-term design capacity of the membranes can be set at 25 L / m - h . 2



20

40

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80

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0

20

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•Ζ

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Raw water colour, mgPt/1

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40

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Figure 6. Unit cost of reverse osmosis (RO) as compared to conventional treatment (CT) and ion exchange (IE) for the removal of humic substances.

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782



References 1. Ødegaard, H.; Brattebø, H.; Eikebrokk, B.; Thorsen, T. Water Supply 1986, 4, 129-158. 2. Koottatep, S. Dr. ing. Dissertation, Norwegian Institute of Technology, 1979. 3. Ødegaard, H.; Koottatep, S. Water Res. 1982, 16, 613-620. 4. Thorsen, T. SINTEF report STF 21 A 84071. Trondheim, Norway, 1984 (in Norwegian). 5. Thorsen, T. SINTEF report STF 21 A 84094. Trondheim, Norway, 1984 (in Norwegian). 6. Hem, L. J. SINTEF report STF 60 A 86161. Norwegian Institute of Technology, 1986 (in Norwegian). RECEIVED

for review July 24, 1987. A C C E P T E D for publication February 11, 1988.


Removal of Humic Substances by Membrane ... - ACS Publications

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