Aquatic Humic Substances - ACS Publications - American Chemical

45 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.ch045

Removal of Humic Substances by Ion Exchange Hallvard Ødegaard, Helge Brattebø, and Ola Halle Norwegian Institute of Technology, Division of Hydraulic and Sanitary Engineering, N-7034 Trondheim-NTH, Norway

This chapter presents the major results from a research program in which ion exchange has been evaluated as an alternative process for removal of humic substances, especially at small waterworks. It has been demonstrated that ion exchange in a strong-base, anionic, macroporous resin is economically competitive with other alternatives when the raw-water humic-substance concentration is relatively low (50,000 10,000-50,000 1,000-10,000 ), as such, was of minor importance. A t a given filter velocity, the empty-bed contact time will be proportional to the length of the column (I). These three factors (t , v and I) are therefore mutually dependent upon each other. Figure 2 shows results from three different experiments where the contact time was constant, but the column length and filter velocity varied. The figure demonstrates clearly that the empty-bed contact time is the important design parameter, and that a suitable combination of column length and filter velocity should be chosen on the basis of the necessary contact time. The good agreement of the curves in Figure 2 indicates that the sorption kinetics are intraparticle diffusion controlled. The treatment efficiency, (1 - C / C ) * 100%, where C / C is the relative concentration at the sampling point, will consequently be primarily depen0

k

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

45.

0DEGAARD ET AL.

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Removal of Humic Substances by Ion Exchange 819

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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.ch045

1.0

0.5

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Lewatit OC 1035

MMA-1

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ι

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ι 100

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120

MP 500 A

1

1

HO

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180

Run time (hrs) Figure 1. Breakthrough curves for three types of resin at a constant contact time (Co = 15 E/m, ν = 10 m/h, 1 = 0.5 m). (Reproduced with permission from ref 1. Copyright 1986 Pergamon.) C/C

0

0.3

V L=1.2i v=20i/h

•

L=0.6i v=10i/h

0.1 1 20

! 40

1 60 1000

1

1

1

80

100

120 2000

• L=0.3i v= 5i/h 1 1 140 160 hrs Bed volumes

Figure 2. Breakthrough curves for Lewatit MP 500 A at afixedinfluent con centration but at varying column length and filter velocity (C = 16 E/m, tk = 3.6 min). 0


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AQUATIC HUMIC SUBSTANCES

dent upon empty-bed contact time, as shown in Figure 3. However, treat ment efficiency will decrease with increasing raw-water concentration. The effluent standard is normally given at a certain level; therefore, the treatment efficiency must be increased when raw-water concentration increases. As a consequence, necessary empty-bed contact time, and therefore investment costs, will increase considerably with increasing raw-water concentration.


D i s c u s s i o n . A n empirical breakthrough model was developed on the basis of the single-bed data:

l

£

= ko · Cji · φ · t

(5)

0

where C is influent concentration (E/m), t is empty-bed contact time (min), and t is run time between regenerations (h). The model gave a squared correlation coefficient, R = 0.9281, for the following k values: k = 0.0396, k = 0.4561, k = -0.6721, and k = 0.3739. In Figure 4 this model has been used to illustrate the relationship between the influent concentration, the contact time, and the run time, if one has to satisfy a water quality standard of 5.0 E/m, which corresponds to about 15 mg of P t / L and 1.3 mg of T O C / L in this water. 0

k

2

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l

2

3

T r e a t i e n t e f f i c i e n c y (%) 100 ι

50

0 0

5

10 tj< (iin)

Figure 3. Treatment efficiency versus empty-bed contact time at two influent concentrations. Time of operation was equivalent to 1000filter-bedvolumes.


45.

0DEGAARD ET AL.

Removal of Humic Substances by Ion Exchange 821 C =35 E/i 0

30 E/i

25 E / i

20 Ε/·


15 E/« 0

10 E/i 0

50

100

150

200

t (hrs) Figure 4. Modeled refotionship among the three important parameters for process design when a water quality standard of 5.0 E/m has to be satisfied. (Reproduced with permission from refi 1. Copyright 1986 Pergamon.) Figure 4 clearly illustrates that the run time between regenerations is heavily influenced by the raw-water concentration. A high raw-water con centration requires a long contact time (high investment cost) or frequent regeneration (high operation cost). We estimate a raw-water color of about 50 mg of P t / L (about 15 £ / m ) to be an upper economical limit for the use of ion exchange for humic-substance removal.

Regeneration Experiments E x p e r i m e n t a l M e t h o d s . The regeneration experiments were performed in the columns previously described. One experiment was carried out to study desorption during regeneration with 3.0 bed volumes. This experiment was done in a 1.5-m-high bed of Lewatit MP 500 A. Treated water samples were collected each third minute and analyzed in a TOC analyzer (Barnstead). In addition, a 0.45-m-high bed of Lewatit MP 500 A was loaded and regenerated in a total of 11 cycles. In these runs we reused 3.0 bed volumes of régénérant. The pore volume of the bed necessarily contained water before regeneration, so the régénérant was diluted somewhat during each regeneration. Thus, we corrected the volume and the strength of the régénérant after each cycle. The experiments were designed to study the extent to which the sorption cycle was affected by the increasing number of previous regenerations, in order to evaluate how many times the régénérant could be reused. Results. Figure 5 shows the desorption during one regeneration with 3.0 bed volumes of solution over 51 min. Figure 6 illustrates the accumulated removal of color from the water during each cycle, as a function of the number of previous regenerations. In these experiments the volume ad-



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Figure 5. Desorption during a normal regeneration. (Reproduced with permission from ref. 15. Copyright 1987 Pergamon.) ^

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The number of previous regenerations

Figure 6. Accumulated color adsorption during each cycle as a function of the number of previous regenerations (C = 50 mg of Pt/L, I = 0.45 m, ν = 30 m/h). 0

justment of the régénérant resulted in 88% as the highest possible recir culation ratio. D i s c u s s i o n . The results in Figure 5 demonstrate that about 95% of the desorption happened in the first 9-24 min of regeneration. This period corresponds to a régénérant volume of about 0.9 bed volumes, which might In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.


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0DEGAARD ET AL.


be led to a separate waste tank; therefore, about 70% of the total régénérant solution of 3 bed volumes could be recirculated without any significant enrichment of humic substances. We accomplished a thorough regeneration of the bed when 3.0 bed volumes of the régénérant was eluted through the bed for 50-60 min. Figure 6 indicates that the maximum reuse of régénérant did not affect the uptake of humic substances significantly during the first seven or eight regenerations. The poor result obtained at two early regenerations could be attributed to a poor regeneration procedure. More runs would probably have given a more marked decrease in humic-substance uptake than the graphed results indicate. The maximum number of reuses with efficient removal of humic sub stances appears to be 8-10. The use of a fresh régénérant after 10 regen erations seems to be the best way to cut the consumption of régénérant chemicals.

Countercurrent Beds-in-Series Experiments The reasoning behind the study of a countercurrent beds-in-series system was 1. The relatively poor sorption kinetics require a rather long bed in order to operate continuously for a longer period. 2. The capacity and the kinetics of the process indicate that the use of a countercurrent system would decrease the regener ation frequency significantly. 3. A design should be developed to be operated with a high degree of automation. E x p e r i m e n t a l M e t h o d s . The beds-in-series system is illustrated in Figure 7. The system consisted of four columns, each 2.0 m high with 4.2-cm i.d. Each column was halffilledwith Lewatit MP 500 A. Prior to the resin column, a sand filter column was used for particle removal. During the 51-day experiments, only three of the columns were operated at the same time, giving a total bed length of 3.0 m. A pipe system containing 12 magnetic valves connected the four bed units. The operation was automated by the use of a microcomputer, which controlled both the valves and the effluent-analyzing equipment. Regeneration and backwashing of each column was, however, carried out manually. The device for analyzing the effluent quality is illustrated in Figure 8. Three magnetic valves were connected to the computer. The apparatus included a peristaltic pump that, on a signal from the computer, started pumping water through a UV monitor (ISCO). Distilled water was pumped for a given time, followed by a standard humic-substance solution with a known concentration, in order for the computer to calculate a standard curve. Then the treated water was pumpedfromthe sampling tank and the concentration was determined in relation to the standard curve. The valves were controlled by computer so that the four columns were operated in sequence. An exhausted column was regenerated while the other three were operating. In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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Water in


Régénérant -ο Sanpling point


Hater out

Backwash water in

Régénérant out

Figure 7. The countercurrent beds-in-series system (I-12 are magnetic valves). A pump (Seepex) fed the system with water. This particular type of pump could provide a nearly constant amount of water, independent of the head loss in the system. The hydraulic load was constantly 20.8 L/h, which gave a superficial velocity of 15 m/h and an empty-bed contact time of 12 min. The influent water had a nearly constant color level of 52 mg of Pt/L, with a corresponding UV extinction of 19.5 E/m. When required, the columns were regenerated with 3.0 bed volumes of fresh régénérant. Results. Results from the countercurrent beds-in-series system are given in Figure 9. The operation time of 51 days required six cycles. Despite some variations in each cycle, the breakthrough results from the countercurrent beds-in-series system follow a consistent pattern. In cycles II and IV, however, the cycles were terminated too early because of a wrong determination of the effluent concentration and to higher hydraulic load than expected, respectively. The mean run time is a measure of the required regeneration frequency at the given load. The average run time was 8.5 days for the 51-day exper iment. Considering that two cycles terminated too early, the correct run time could have been higher. Cycles V and V I were run without any kind of operational problems. For these cycles, the mean run time was 11.8 days. These last cycles were clearly the most successful ones, possibly due to a more skilled regeneration as time passed. In conclusion, the experiments showed that it is possible to obtain a cycle length of at least 11 days at the actual loadings. In order to evaluate what may be gained by using a countercurrent beds-in-series system rather than a single-bed system, we can calculate the



Figure 8. The device for effluent quality control (13-15 are magnetic valves).

Destilled water



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Figure 9. Effluent quality from the countercurrent beds-in-series system during six cycles. run time in a single-bed system from the model given earlier. W i t h the actual raw-water quality ( C = 19.5 E/m) and contact time (t = 12 min), the run time would be 342 h (14.3 days) if the treated water standard was 5.0 E / m . The possible run time for the beds-in-series system was found to be 11 days. In this system, however, only one of the three operating beds was regenerated at a time. The corresponding run time for the whole 3-m bed length would be 33 days. By the use of a beds-in-series system, one may therefore obtain a total run time 2.3 times greater than that of a comparable (3-m) single bed. A n increase in run time by a factor of 2.3 gives a corresponding reduction in the consumption of regeneration chemicals and volume of waste solution. The beds-in-series design will thus improve the total exchange process sig nificantly, because the two major disadvantages of the process are the chem ical costs of regeneration and the production of a waste solution. When using both a countercurrent beds-in-series system and régénérant reuse, one may treat 5910 m of water to an effluent quality of 5 E / m with the production of 1 m of waste solution. These numbers are valid for the loadings that were used in this study ( C = 19.5, E / m = 52 mg of P t / L , ν = 15 m / h , t = 12 min). The water-waste production ratio will, of course, increase as the loading decreases. 0

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Full-Scale Experiments This research program has been aimed primarily at the development of reliable treatment methods for small waterworks. A small, but full-scale,


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treatment plant based on the ion-exchange filters of Eurowater A B was therefore operated during the autumn of 1985. Experimental Methods. The plant consisted of three columns in series, with alteration of the sequence of operation for each regeneration. Each column was filled with 300 L of resin (Lewatit MP 500 A), giving a column length of 93 cm. Only one column was regenerated at a time, and the water was allowed to pass through the remaining two columns. Each regeneration lasted for 85 min. The valve scheme of the plant is shown in Figure 10. During October-December 1985, the color of the raw water was in the range of 52-58 mg of Pt/L, the UV extinction 24-27 E/m, and the TOC 6.0-6.5 mg TOC/L. In the full-scale experiments we decided to evaluate the use of a pure NaCl solution for regeneration instead of the alkaline NaOH-NaCl solution used in the previous experiments. We reasoned that a pure NaCl solution would be much easier for the operators of small waterworks to handle. The plant was operated at aflowof 3 m /h, corresponding to an empty-bed contact time of 6 min per column (18 min total). Filter velocity was 9.3 m/h. The treated water was analyzed with respect to color, UV extinction, TOC, and pH. 3

Results. Figure 11 shows the effluent concentration with respect to color. Each operation cycle was very much like the others. It is, however, demonstrated that regeneration every 3-4 days was needed to meet the treated water standard of 15 mg of P t / L . The p H of the treated water was consistently in the 4.9-5.9 range. Discussion. A comparison of the basic data from the laboratory and the full-scale experiments (as in Table IV) shows that although the loading in the full-scale experiments was lower, the run time was also lower. This result may be attributed primarily to the much lower temperature of the water and the poorer regeneration obtained, but may also be due to differ ences between the humic materials used in each of the tests.

Practical Considerations Regeneration. The regeneration was not optimal in the full-scale experiments. Only 78% of the humic substances adsorbed were actually washed out during regeneration. This result indicated that regeneration by a pure N a C l solution is not good enough, and that an alkaline salt solution should be preferred (2-3 bed volumes of a 10% N a C l and 2% N a O H solution). Influence of Temperature. < As demonstrated, the temperature ef fect was probably quite significant in the full-scale experiments. The influ ence of temperature was examined in a separate investigation (16). The results based on raw water from three different sources and on temperatures varying from 2 to 20 °C were statistically treated versus the single-bed model



828


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m Figure 13. Unit cost of ion exchange (I.E.) as compared to conventional treatment (CT.) and direct filtration

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45.

0DEGAARD ET AL.


50 mg of P t / L , ion exchange seems to be cheaper than the conventional combination of coagulation, flocculation, sedimentation, and filtration at all the plant sizes examined here. For the smallest plants ion exchange even seems to be economically competitive with direct filtration, especially with respect to the unit cost of treatment.


Summary Ion exchange in a strong-base, anionic, macroporous resin is a viable alter native for the removal of humic substances from surface water, especially for smaller waterworks. In order for the process to be economically com petitive with other alternatives, the raw-water color ought to be lower than 50 mg of P t / L . The empty-bed contact time is the most important design parameter. It should be >8-10 min. The optimum design is that of a countercurrent beds-in-series system, which will cut the consumption of régénérant chem icals and production of waste solution. The régénérant may consist of an alkaline salt solution (10% N a C l and 2% NaOH), and 2-3 bed volumes may be used for each regeneration. A n average régénérant reuse of about 75% will not reduce the sorption behavior of the resin. Water temperature has a significant influence on necessary contact time, which has to be increased about 20% if the temperature is reduced from 15 to 5 °C. The process is economically competitive with alternative, traditional humic-substance removal processes, especially at smaller waterworks, when the raw-water color is relatively low.

References 1. Ødegaard, H.; Brattebø, H.; Eikebrokk, B.; Thorsen, T. WaterSupply1986, 4, 129-158. 2. Kölle, W. Humic acid removal with macroreticular ion-exchange resins at Hannover, Nato-CCMS-Congress, Reston, VA, 1979. 3. Rüffer, H.; Schilling, J. Die Aufbereitung huminsäurehaltigen Oberflächenwassers durch Flockung und Adsorption an makroporösen Ionenaustauschharzen undÜbertragungder Ergebnisse auf ähnliche Wässer; report, Institut für Siedlungswasserwirtschaft der Universität Hannover, 1977. 4. Kölle, W. Erfahrungen bei der Aufbereitung eines reduzierten huminstoffhaltigen Grundwassers im Wasserwerk Fuhrberg der Stadtwerke Hannover AG; Abschlussbericht zu dem BMFT-Forschungs-vorhaben 02 WT 606, Hannover, 1981. 5. Jørgensen, S. E. Water Res. 1979, 13, 1239-1247. 6. Boening, P. H.; Beckmann, D. D.; Snoeyink, V. L. J. Am. WaterWorksAssoc. 1980, 72, 54-59. 7. Anderson, C. T.; Maier, W. J. J. Am. WaterWorksAssoc. 1971, 71, 278-283. 8. Macko, C. A. Ph.D. Dissertation, University of Minnesota, 1980. 9. Baldauf, G.; Woldmann, H.; Klette, J.; Sontheimer, H. GWF Gas Wasserfach: Wasser/Abwasser 1985, 126(H3), 107-114. In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.


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10. Rüffer, H.; Slomka, T. Wasser AbwasserForsch.Prax. 1981, 14(516), 176-180. 11. Slomka, T. Entwicklung weitergehender Abwasserreinigung durch Einsatz von Flockung, Mehrschichtfiltern und makroporösen Adsorberharzen; report, Institut für Siedlungswasserwirtschaft der Universität Hannover, 1982. 12. Halle, O. Dr. ing. Dissertation, Norwegian Institute of Technology, 1983 (in Norwegian). 13. Brattebø, H. Proc. NATO Adv. Study Inst. Conf. "Ion Exchange: Science and Technology", Troia, Portugal, 1985. 14. Halle, O.; Brattebø, H. SINTEF report, STF 60 A 86051. The Norwegian Institute of Technology, 1986 (in Norwegian). 15. Brattebø, H.; Ødegaard, H.; Halle, O. Water Res. 1987, 21(9), 1045-1052. 16. Johansen, H. Diploma Thesis, Norwegian Institute of Technology, 1986 (in Norwegian). 17. 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.


Aquatic Humic Substances - ACS Publications - American Chemical

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