[advances in chemistry] aquatic humic substances volume 219 (influence on fate and treatment of...

42 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/m2·h at optimal membrane washing. The lifetime of a membrane at this capacity is estimated as 4 years.

JVÎEMBRANE 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 μ8/αη, 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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Tab

le I

I. T

reat

men

t E

ffici

ency

and

Spe

cific

Cap

acity

of

13 M

embr

anes

Tes

ted

in L

abor

ator

y-Sc

ale

Exp

erim

ents

Mem

bran

e

Mol

ecuf

or

Wei

ght

Cut

off

Col

or T

reat

men

t E

ffic

ienc

y (%

) P

erm

anga

nat

e C

ondu

ctiv

ity C

apac

ity

(L/m

2-hba

r)

Ope

rati

ng

Pre

ssu

re (

bar)

R

ecom

men

ded

Exp

erim

enta

l V

alu

e SE

PA-2

0KC

A

20,0

00

80

—

—

—

3.6

—

SEPA

-OPS

2,

000

83

.—

—

14

3.6

15

SEPA

-OC

A

1,00

0 85

60

17

7

7-15

15

SE

PA-5

0CA

60

0 98

80

65

2.

2 11

21

SE

PA-8

9CA

40

0 10

0 90

82

1.

6 11

-15

43

SEPA

-97C

A

200

97

98

85

1.0

7-15

60

D

DS-

600

20,0

00

70

58

13

21

5-10

10

D

DS-

800

6,00

0 75

66

18

10

10

-20

20

DD

S-86

5 50

0 95

90

60

3.

1 10

-50

40

DD

S-87

0 50

0 10

0 96

78

2.

1 10

-50

50

PCI-

T4A

3,

000

80

67

23

6.3

5-10

10

PC

I-T

2A

800

100

95

44

4.3

10-2

0 25

PC

I-T

2/15

N

300

100

97

92

0.9

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0 80

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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 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 operated plant sank more rapidly than in the laboratory experiments, and the pressure loss through the plant increased. Both of these changes are indications 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 promoter 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 wil l thus increase. The film thickness model based on these facts included total pressure drop through the whole plant, feed flow, concentrate 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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42. 0 D E G A A R D & THORSEN Removal of HS by Membrane Processes 775

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). An alternative detergent, stable at this p H , is sodium alkylaryl sulfonate.

Routine wash performed with only one washing solution leaves a residual capacity reduction. Therefore, two different washing solutions are recommended. 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 membrane film formation problem improved considerably. After 4500 h of operation, 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. Temporary 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 membrane 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 2-h). Therefore, membranes with different initial fluxes gradually approach each other in terms of flux.

Figure 3 also shows that temperature influences the capacity and indicates that capacity may be highest during summer. Likewise, capacity wil l 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

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Tabl

e II

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Was

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42. 0 D E G A A R D & THORSEN Removal of HS by Membrane Processes 777

•

\ 1 1 1 1

i

V\ 1 J 1 1 1 1

MWC = 800 MWC = 1000

1 1 1 1

s 1 ;

1 ~~ v

! 1 ~~ v τ i^c 1 1 1 1 1

ί

1 1 1 1 I

ί

0 5 10 15 20 25

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.)

used in our experiments would be 24-26 L / m 2 - h , with 3-4 years of membrane life.

On the basis of our experiences, we recommend that a reverse osmosis plant for the removal of humic substances be designed according to Figure 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 wil l 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 3) (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

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42. 0DEGAARD & THORSEN Removal of HS by Membrane Processes 779

Table IV. Specifications of a Membrane Filtration Plant for the Removal of Humic Substances _____

Component Measure Prefilter Sieve opening: 20-50 μιη Module Spiral wound Membrane Cellulose acetate Molecular weight cutoff 800-1000 Pressure 7-10 bar Membrane replacement 25% annually Washing solution According to Table III Temperature of washing solution 25-30 °C SOURCE: Reproduced with permission from réf. 1. Copyright 1986 Pergamon.

Table V Washing Routine for the Primary Wash Sequence Duration (min) Pump/Valve in Operation Pumping in 4 P2, P3, KV2, PV1 Pause 6 X W PV2, KV3 Circulation 6 x 5 ° P2, P3, KV3, PV2 Pumping out 4 P2, P3, KV2, PV1 Flushing 10 PI, P3, KV2, PV1 Afterfilling 1 P4, NV1, PV3 NOTE: Data are according to the flow sheet in Figure 4. "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 wil l cause capacity 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 washing routine and replacement follow this schedule, the long-term design capacity of the membranes can be set at 25 L / m 2 - h .

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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. ACCEPTED for publication February 11, 1988.

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[advances in chemistry] aquatic humic substances volume 219 (influence on fate and treatment of...

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