Removal of humic substances from natural waters by reverse osmosis

Removal of humic substances from natural waters by reverse osmosis

Water Res. Vol. 16. pp. 613 to 620. 1982 Printed in Great Britain 0043-1354/82/050613-08103.00/0 Pergamon Press Ltd REMOVAL OF HUMIC SUBSTANCES FROM...

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Water Res. Vol. 16. pp. 613 to 620. 1982 Printed in Great Britain

0043-1354/82/050613-08103.00/0 Pergamon Press Ltd

REMOVAL OF HUMIC SUBSTANCES FROM NATURAL WATERS BY REVERSE OSMOSIS HALLVARD ODEGAARD and SUPORN KOOTTATEP* Division of Hydraulic and Sanitary Engineering, The University of Trondheim, 7034 Trondheim-NTH, Norway (Received January 1981) Abstract--An investigation of the potential use of reverse osmosis for the removal of humic substances in order to remove colour and haloform precursors in small waterworks has been carried out, using three different laboratory scale reverse osmosis units and several different membranes. Membrane pore size was found to be the most important factor that influenced both the permeate quality and the product water flux. Pressure was found to have no significant influence on permeate quality, but was linearly related to product water flux. The concentration of humic substances in the influent was not found to affect product water flux but the transport of humics across the membrane was found to be dependent upon influent concentration. For the selected membranes, the removal of humic substances amounted to 80-10090 in terms of colour removal, and 50-999o in terms of permanganate value reduction. The most suitable membranes for the different available units were found to be Osmonits SEPA 89 (permeate flux 251 m- 2 h- t at 15 bars), DDS 865 Ipermeate flux, 1201 m- 2 h- t at 40 bars) and PCI T2A (oermeate flux 901 m- 2 h- t at 20 bars). At suspended solids concentrations higher than 100 mg 1- t of bentonite, product water flux was significantly reduced.

INTRODUCTION In Norway several smaller waterworks utilize raw water with high colour caused by the presence of humic substances. Both for the reason of colour reduction and for the removal of humic substances as haloform precursors, an extensive program for investigating possible methods for humic substance removal in small waterworks are now being carried out at the Technical University in Trondheim. The program includes investigations on direct coagulation/ filtration, granular activated carbon, ion exchange and reverse osmosis. H u m u s is a more or less undefined mixture of organic c o m p o u n d s giving water a brownish coiour and acting as precursors for the formation by clorination of haloforms. The molecular structure of h u m u s has been proposed to be a straight chain with many functional groups (Christman & Ghassemi, 1966). The average carbon, hydrogen and nitrogen contents of h u m u s in water has been found to be a b o u t 43, 5.5 and 1.1~, respectively (Gjessing, 1976). Humic water normally has a rather low pH, and the h u m u s itself is often called humic acid. Changing the pH of humic water will change the colour, and it is reported that artificial reduction of pH will change the molecular weight distribution considerably (Gjessing, 1971). The molecular weight distribution of h u m u s is wide. Reported values vary from 700 to 80,000 (Gjessing, 1966; Orlov et al., 1971). Because of the high molecular weights the authors believed that reverse osmosis, *Present address: Suporn Koottatep, Faculty of Engineering, Chiang Mai University. Chiang Mai, Thailand. 613

with relatively open membranes, which would give a high product water flux, might be a potential process for h u m u s removal. No other reports on the use of reverse osmosis for this particular purpose have been found in the literature, but several authors have used m e m b r a n e s in analytical procedures to characterize humic substances. EXPERIMENTAL

The water used in these experiments was natural humus water from the river Sagelva near Trondheim. The average colour of the water in the period 1970-1979 was 54 mg Pt 1- t and the average COD as permanganate value was 6.87 mg 021-1. It was found in this study (by gel filtration) that the water contained humic substances which consisted of about 7290 of molecular weight above 5000 of which about 3390 of the whole might have a molecular weight of over 50,000. Less than 30~o had a molecular weight below 5000. The experiments were carried out with three different laboratory scale reverse osmosis units: A spiral wound Osminics module, Model OSMO 519-SB. A plate and frame DDS module, Model 20-Lab. A tubular (single tube) PCI module, Model No. BRO MK2. The membranes chosen for this study are shown in Table 1 and Table 2 outlines the experimental program. Figure 1 shows the principle of the experimental design. In the preliminary experiment a concentrate recycle system was adopted. This of course, caused the raw water quality to change with time for each experimental run. Most of the later experiments were performed with both concentrate and permeate recycle after it had been demonstrated by gel filtration studies that water regained its original composition by blending of the two streams.

614

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lable t Membraneste~,tcd in thls,,tudx Description by mailufactttret Molecular ',,,eight Manufact. Membrane cut off Osmonics

SEPA-0 SE PA-89 SEPA-97

DDS

6(X) 800 865 870

PCI

T4A

I(XX) 4tX) 21111

",;Na('l rejection

Ma~. oper pressure (barsl

Permeate flu:, (Ira : h t) at press, bat

0 111 85 90 94 97

15 30 6/1

100 340 15 34 45 3(1125 (3 17 25 311

* -t 311 55

10 2O 411 50

150.5 8(1 5 90 30 120 ('1 85 4()

89 94

It) 25 80

42- 55. I0 25.-40. 10 115 C} 35 4540

20,(XX) 60(X) 5(X) 5(X)

T2A

,"

T2..I5W

*% Permeability of lactose: 98,,,. tO,.oPermeability of lactose: 95'~,,. ~50,°,, Rejection of 6000 molecular weight. ~97",, Rejection of 6000 molecular weight. The different modes of running the experiments are shown in Table 2, and are also referred to in the proceeding text. The methods of analysis were as follows: pH by electrometric method using lonanalyscr Orion research specific ion meter model 404: colour by photometric method at 430 nm with distilled water as blank. (A standard colour was prepared from a solution of K2PtCI6 (potassium chloroplatinate) and calibration was carried out using an EEL filterphotometer with filter 601 and 10 cm cells); KMnO4-value by oxidation with potassium permanganate according to Norwegian Standard 4722: iron by a colorimetric method using l.lO-Phenanthroline method of Hach DR-EL "'Direct Reading": conductivity by conductivity meter type CDM 2d; for the gel filtration study approx. 1 2 ml of concentrated humus samples were eluted separately through two columns containing Sephadex G-25 and G-75 gel. exclud-

ing respectively molecules of molecular weight greater than 5000 and 50,000. The u.v.-instrument used in the experiments was an LKB-UVICORD type 4701A.

RFSULTS AND DISCUSSION

Factors affecting treatment efficiency and treated water

.~IIX M e m b r a n e pore size was expected to be one of the major factors influencing rejection of solutes and permeation of solvents. The driving force (pressure) might also affect the penetration of solute through the membrane. According to Gjessing (1971) pH might change the molecular size of humic substances, hence influencing their rejection.

Table 2. Experimental program Experiment Preliminary experiments efficiency and treated water flux Membrane investigation (a) Removal efficiency (b} Membrane performance (c) Effect of R.O. on molecular weight-distribution (b) Solute transport Operational investigation (a) Effect of SS on membrane clogging Varying SS Constant SS (b) Effect of long period of running

Experimental design Osmonics unit. SEPA-0 and SEPA-97 membranes Concentrate recycle All three R.O.-units Different membranes Concentrate and permeate recycle Concentrate recycle PCI-unit, T2A membrane Concentrate recycle Concent. and permeate recycle Continuous feeding, continuous concent, recycle

Removal of humic substances from natural waters by reverse osmosis

615

EXPERIMENT S.,qvlPLING

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I II

I

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I , ANALYSIS

Fig. 1. Experimental diagram. Some preliminary experiments were carried out using the Osmonics module with membranes SEPA-0 and SEPA-97 in order to investigate the possible influence of pH, pressure and membrane pore size on the removal of colour, permanganate number, conductivity and on product water (permeate) flux. The experimental set up was designed as a recycle system, concentrate being returned to the raw water container. The units were operated at 7.5 and 15 bars. These experiments were designed as a 23 complete experimental design with three main factors to be studied. The statistical treatment of the experimental results gave the conclusions shown in Table 3. pH in the range studied (3.5-7.0) had no significant effect on the removal of humic substances or of dissolved salts or on permeate flux. Pressure did not have any significant effect on the removal of humic substances, but it significantly affected the permeate flux and dissolved salt rejection. These results indicate that pressure plays a more important role in removing dissolved salts than in removing humic substances. Both pressure and membrane pore size had significant effects on permeate flux, but the effect of pore size is much higher than that of pressure. Thus, it is

important to choose a suitable membrane for removal of humic substances with the highest flux regardless of the effect of pressure.

Remot, al efficiency and membrane performance Experiments were then designed in order to find which of the available membranes would be most suitable for the removal of humic substances. Such membranes would have to give a permeate quality that would meet the Norwegian water quality criteria for colour (5 mg Pt 1-1) and permanganate number (10 mg KMnO4 I-1) and at the same time give a high product water flux. The experiments were designed with both concentrate and permeate recycle in order to keep constant raw water concentration and composition. Different flow rates and pressures were applied. Since the permeate quality was essentially independent upon pressure, as shown in the previous section, the removal efficiencies for permanganate number are given as mean values over the pressure range tested in Table 4. It is demonstrated that all the manufacturers had units and membranes that were able to meet the requirements regarding permeate water quality. The

Table 3. Effect of various factors on rejection and flux Factors pH Pressure Membrane

Removal of colour

Removal of perman, no.

Removal of conductivity

Permeate flux

NS NS

NS NS

NS $9~

NS S9s

599

599

$99

S99

Note: NS = not significant; S,~0 = significant at 90°.,, confidence limit.

616

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lablc 4. Removal efliciencie,,, v,uh the different membrane-, Does permeate [)¢rmiln~al);,lle

tit.).

111¢t~[ N o r w .

..................... mgOl ~ m permeate "., Removal

:~.:.ltcr

Manufact.

Membrane

Prcs>urc range bar

Osmonics

SEPA-0 SEPA-89 SEPA-97

7.5 15 7.5 15 7.5 15

4.37 2.55 0 74

52.0 80.2 97.5

No Ycs Yc',

No Yc~ Yes

DDS

600 800 865 870

5 10 10 20 10-.50 10--50

3.29 2.85 0.68 0.50

57.5 65.3 91.6 95.2

No No Yes Yes

Ye~, Yes Yes Yes

PCI

T4A T2A T2.I5W

5- 10 10 40 10 40

4.67 0.49 0.32

67.3 94.7 96.7

No Yes Yes

No Yes Yes

membranes which satisfied both the colour and permanganate value requirement were SEPA 89 and SEPA 97 for the Osmonics module, no. 865 and 870 for the DDS module and T2/15W and T2A for the PCI module. The ones that did not satisfy the permeate quality requirement are typical ultrafiltration membranes. The most suitable membranes would be the ones that would give the highest permeate flux and still meet the quality requirement. However, it is rather difficult to compare the membranes in this respect because the limitations of the equipment caused the experiments to be performed at different pressures for the different units. The permeate flux was found to increase linearly with pressure, as would be expected from the theoretical considerations. An example of this is given for the PCI and D D S membranes in Figs 2 and 3. Table 5 shows the membrane transport performance as given by water flux divided by pressure for the different membranes (slope of lines in Figs 2 and 3). It was concluded from this, that the most favourable

qualit) stand ................ Colour COD

membranes in terms of efficiency and product water flux for the removal of humic substances of the ones available in this experiment were PCI T2A and DDS 865. When an actual plant is to be designed one has, however, to take into account an economical evaluation since the units have different capital costs, different energy consumption and different membrane changing costs. On the PCI T2A membrane intensive experiments on the flux flow rate were performed using a variety of raw water qualities (increasing humus concentration). It was found that the concentration of humic substances in the raw water did not affect water flux, which means that the osmotic pressure of the humus in water has no significant effect on the transport mechanism.

Effect of reverse osmosis on molecular weight distribution The first part of the experiment was done with both concentrate and permeate recycle as in the previous

DD$ UNIT

2GO

150

0 0

I0

29

3O

40

.~

Ptwe

Fig. 2. Relationship between product flux and pressure for the DDS membranes (temp. range 15-20'-C).

Removal of humic substances from natural waters by reverse osmosis

617

PCI UNIT

~.

LZ'

I00

80

40

0

1 20

10

30 Pr~

40 bar

Fig. 3. Relationship between product flux and pressure for the PCI membranes (temp. range 10-15"C).

experiment. At the end of each run water samples from the raw water tank was collected and concentrated by evaporation at low temperature (30°C) and low pressure (10 mmHg). The samples were concentrated down to about one-twentieth of their volume, and their molecular weight distributions were studied using gel filtration methods (Sephadex). The gel filtration studies showed that the raw water used in this study contained humic substances which consisted of about 72% of molecular weight over 5000 of which 33% of the whole had molecular weight of over 50,000. Less than 30% had molecular weight below 5000. No significant change in Sephadex molecular weight distribution was found when the process was subjected to different pressures. In order to study how molecular weight distribution was affected by concentration, a gel filtration study of the concentrate in a concentrate recycle system was performed. The elution curves of the concen-

trate at different times in each run was compared with the elution curve of the original raw water. Because of the recycle system, the concentration and composition of the concentrate varied with time. The results show that the molecular weight distribution changes with operating concentration. It can be postulated that at low operating concentrations, the small molecular weight fraction diffuses through a membrane more readily to a greater extent than the larger molecular weight fraction. When the operating concentration increases, however, the concentration gradient of the larger molecular weight fraction across the membrane becomes greater, and also the larger fraction then tend to pass through the membrane. For a concentrate recycle system in practice there will probably exist a maximum recycle ratio where the concentration of the raw water is low enough to obtain the desired permeate quality and at the same time give the maximum possible yield of product water (permeate).

Table 5. Membrane performance for water transport

Membrane

Pressure range (bar)

Membrane transport performance (1 m - z h - 1 bar- ' )

Osmonics SEPA-0 Osmonics SEPA-89 Osmonics SEPA-97

7.5-15 7.5-15 7.5-15

DDS DDS DDS DDS

5-15 5-20 10-30 10-40

21.15 10.0 3.13 2.12

No No Yes Yes

5-25 5-15 10-40

0.91 4.32 0.91

Yes Yes No

600 800 865 870

PCI T4A PCI T2A PCI T2/15W

*Based on very few data.

6.26* 1.65" 0.98*

Does permeate meet quality crit. for humic subst, remov. No Yes Yes

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30

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I

1

100

150

2(I)

'

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Fig. 4. Permanganate value relationship between permeate and concentrate for PCI unit.

Solute tran.sport Solute transport through the membrane is another important factor regarding the potential use of reverse osmosis for humic substance removal. Contrary to water transport performance, one wants as low solute transport through the membrane as possible. These experiments were deliberately performed with concentrate recycle such that the concentration of humus in the raw water would increase with time. When the concentration of solute in the permeate was plotted against that of the concentrate (which with the actual experimental design would virtually be equal to that of the raw water) two typical patterns were found as typified by the plots for the membrane PC1 T4A and PCI T2A in Fig. 4. The pattern for PCI T4A, with a linear relationship between permeate concentration and concentrate (or raw water) concentration, is in agreement with what

o~le would expect based on a solution-diffusion consideration. The second pattern (as typified for PCI T2A) shows constant permeate quality at low concentrate consideration values while it increases linearly with concentrate concentration at higher concentrations. This might be explained by the fact that since the experiments were designed as a concentrate recycling system, the increasing concentration will increase the diffusivity driving force of the less permeate permeable fraction of the humic substances through the membrane.

E~'ect of .suspended solids on membrane elog,qin# Experiments on the PCI module with membrane T2A were carried out to study the effect of suspended solids on membrane clogging. Bentonite was used as artificial SS ranging from I to 3000 m g l - t . The ex-

80

~mn''Ik"~---.~ X

~

&

20

lIME, kiln

Fig. 5. Flux variation with time. PCI T2A, 20 bars; SS 132.9 mg 1-;.

Removal of humic substances from natural waters by reverse osmosis

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100

1

1

-r.

1

~. _a II0

I 60

\

~<~1" i\, I -

60

50

i

I

100

150

200

TIME H. INDICATES MEMBRANE WASHING BY DETERGENT

Fig. 6. Flux variation with time of running showing the membrane clearning effect. PCI T2A: 20 bars.

periments were run as a concentrate recycle system in order to increase influent concentration with time. The results were difficult to interpret because different runs with intermediate membrane washing made it difficult to state whether the results were governed by the effect of washing or the effect of fouling. It was concluded, however, that the maximum suspended solids content in the raw water that did not affect product water flux was about 100 mg 1- t of bentonite at 10 bars operating pressure. One experiment was performed with both concentrate and permeate recycle in order to keep the influent SS-concentration constant. As shown in Fig. 5, the product water flux decreased by 50°, during 3--4h of running in the presence of about

130 mg SS 1-~ as bentonite. The results indicate that the higher the operating pressure the more pronounced was the effect of suspended solids.

Effect of long period of running In practical R.O.-operation. membrane washing and cleaning is very important. Different methods and detergents used for membrane cleaning have been tried by the manufacturers. These experiments were performed using the PCl-unit with membrane T2A run at 20 bars pressure for 12 h each day for 14 days. Two methods of membrane washing were used. one using only water and another using an enzyme detergent recommended by the PCI manufacturer in combination with water washing.

100 MAXIMUM OBSERVED FLUX

,,MEAN LINE Of AVERAGE DAILY FLUX 60 .

.

.

.

.

X

40

20

~

50

100

150

"

t 206

TIME H l

INDICATES MEMBRANE WASHING BY DETERGENT

Fig. 7. Diagram showing average daily flux with respect to time. PCI T2A; 20 bars•

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l"igurc 6 s h o ~ s the llux ~:ariation with time denlon,,trating the m e m b r a n e cleaning etTcct. The difference between m e m b r a n e cleaning bv ~atcr and b~ detergent was that ~hile both cleaning methods irnpro~cd the initial flux of thei~ext day's operation ot~l3 dctclgent cleaning improved the dail'v a'.eragc flux. l"igure6 shows that the improvement of Itux after water cleaning occurred only at the beginning of a run. After that, in a few hours, the flux decreased rapidly to follow the trend of the previous das. Detergent cleaning gave a greater improvement of permeate flux than water cleaning did, as can be seen from the plot of average daily flux with respect to time IFig. 7L The average flux after detergent washing was higher than that after water washing. The mean line for a~erage daily flux that is drawn on the figure indicates a reduction of the average flux during the whole experiment. The mean line in the figure shows the washing effect of the two methods combined. Economic ecaluafion An economic evaluation of the RO.-process for the removal of humic substances was made based on the results from these studies, on capital cost, period cost and on operating cost. The economic evaluation is based on a life expectancy of 15 years, an interest rate of 10",., a 2 )ears m e m b r a n e life at a cost of 300 Nkr m- 2 t60 US $ m--'), maintenance cost of 2". of investment per year. labour cost based on three working hours a day at a wage of 60 Nkr h-~ (12 I,;S $ h ~), energy consumption of 0.08 k W h . m -' at a cost of 0 . 1 5 N k r k W ~ h - ~ 1 0 . 0 3 U S S k W ~ h t ) a n d a detergent cost of I",, of investment per year. Buildings costs to house the plant are not included. From this it was estimated that the unit T2A m e m b r a n e would range from 1.25 N k r m .3 (0.25 U S $ m '~) for a 2 0 0 m 3 h - ' - p l a n t . to 2 . 3 N k r m '~ (0.46 US $ m 31 for a 10m 3 h l-plant, each with 7000 operating hours per year. The corresponding costs for a DDS plant with No. 865 m e m b r a n e was found to be 0 . 9 0 N k r m ~ I 0 . 1 8 U S S m -~)and 2 . 0 N k r m --~ (0.40 US $ m - ~I. It is suggested that these estimates are on the pessimistic side. as the evaluation was based on the most expensive units (tubular and plate and

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Ih~lt ,L che;tpcr unit cost m~ghl b~. achieved tlMng >;pilal \~.Otllld or ho[[t~\ ~. librc modtlic-,

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h h,~Lb,been dcnlon,'.,Iralt.'d thztt tc~.ClS~.' OslllO~,1~, i-., ..1i/ interesting ahernati~e for the removal of humw ,,ubstances from surface ~ater,, in small ~atcr work,,. It i~, inalm)rtant to run pilot plant tests in order to choose the mose suitable membrane. M e m b r a n e pore si/c signilicantly affects both permeate q u a l i b zttld product water (lux. Since pressure has no significant effect on permeate quality but does significantly affect the product water llux. it is important to lind the membranes that gi',.e the highest water transport performance (Ira -'h t bar ~ } a n d a t the samc time meet the permeate quality requirement. In these experiments the most favourable membranes for the three different units tested were Osmonits SEPA-89 t 2 5 1 m -'h ~ at 15bars). DDS I.;65 (1201m " h i at 4 0 b a r s l a n d P C I T 2 A I 9 0 1 m ' h at 20 barsl. Presst, re did not have any effect on the molecular weight distribution of blended concentrate and permeate as compared to the raw water. At suspended solids concentrations lower than about 100 m g l ~ the product water flux was significantly affected b ) suspended solids in a tubular module at 10 bars operating pressure. REFERENCES

Christman R. F, & Ghassemi M. 119661 Chemical nature of organic colour in water. JA rt,'14~ 58, 723--741. Gjessing E. T. (1966J Humic substances in natural water: method for separation and characterization. Proceedings for the IBP-Symposium, pp. 191-201. Amsterdam. Gjessing E. T. (19711 Effects on pH on tiltration of aquatic humus using gels and membranes. Schwa. Zh. ttvdrol, 33, 592 600. Gjessing E. T. (19761 Ph.wical apld ('heroical ('haracteri.~tic~ of .4quatic Humus. Ann Arbor Science. Ann Arbor. MI. Orlov D. S., Ammasova Ya. M., Glebova G. J.. Gorschko',a Yc. 1., II'in N. P. & Kolesniko', M P. 119711 Molecular weights, sizes and tiguration of humic acid particles. Sorter Soil Sci. 673 687.