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Ultrafiltration and reverse osmosis treatment of an acid stage wastewater
•
Pergamon
Waf. Sci. Tech. Vol. 35, No. 2-3, pp. 155-161, 1997. Copyright © 1997 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain.
PH: S0273-1223(96)00927-4
0273-1223/97 $17'00 + 0'00
ULTRAFILTRATION AND REVERSE OSMOSIS TREATMENT OF AN ACID STAGE WASTEWATER R. A. Sierka*, S. P. Cooper** and P. S. Pagoria*** * Department of Chemical and Environmental Engineering, University ofArizona, CE Building, Room 306, Tucson, Arizona 857 J6, USA ** Weyerhaeuser Canada Ltd., P.O. Bag 1020, Grande Prairie, Alberta, Canada, T8V 3A9 *** Weyerhaeuser Technology Center, Tacoma, Washington 98477, USA ABSTRACT The Weyerhaeuser Grande Prairie pulp mill produces 300,000 tons per year of bleached kraft for the tissue and specialty grade paper markets. Alberta Environmental Protection has given the mill stringent limits for Color (90 kilograms/admt) BOD (3 kg/admt), TSS (5 kg/admt), and AOX (1.5 kg/admt). Several technologies and combinations are being studied to ensure that the mill meets future requirements to minimize environmental impact including: oxygen delignification, ozone treatment, chemical coagulation, and membrane separation. The specific research objectives were to: (I) employ ultrafiltration techniques to separate the Do wastewater into various molecular size fractions and to characterize each in terms of total organic carbon (TOC) and color, (2) to quantify the ability of a UF membrane with a molecular weight cut• off of 8,000. Daltons (D) to reject TOC and color at three temperatures (20°, 30° and 40°C) and three wastewater pH levels (2.4, 5.3 and 7.0), and (3) to quantify the effect of 5 Ilm cartridge filtration and UF pretreatment on the flux and rejection characteristics of a reverse osmosis (RO) membrane. The following summarizes the results of this research. 1) Chemical characterization after separation showed that 59% of the TOC is comprised of molecules with a molecular size of less than 1,000 D but, only 20% of the color is due to these molecules. 2) Increasing the processing temperature in the range 20 to 40°C positively impacted permeate flux rate, however, water quality was not significantly affected. 3) UF processing at pH 7.0 above the pKa (5.3), increased the permeation rate but at a wastewater pH below (2.4), the converse was true. 4) Pretreatment by either 5 Ilm filtration or UF followed by RO yielded a permeate equal in quality, however, permeate flux rates were higher with UF treatment. © 1997 IAWQ. Published by Elsevier Science Ltd.
KEYWORDS Acid wastewater; color; reverse osmosis; total organic carbon; ultrafiltration. INTRODUCTION The Weyerhaeuser Grande Prairie pulp mill is located 500 kilometres NW of Edmonton in Alberta, Canada, on the Wapiti River, a small tributary of the Smoky and Peace River systems. The mill, built in 1973, produces 300,000 tons per year of bleached kraft for the tissue, towel, and specialty grade paper markets. The pulp is highly valued for its unrefined tensile strength and uniformity. Alberta Environmental Protection has given the mill stringent limits for Color (90 kilograms/admt) Biochemical Oxygen Demand (BOD) (3 kgladmt), Total Suspended Solids (TSS) (5 kg/admt), and Adsorbable Organic Halide (AOX) (1.5 kgladmt). 155
R. A. SIERKA et at.
156
Weyerhaeuser anticipates even tighter limits in these conventional parameters in the near future as well as new targets for manganese, and nutrients to meet Alberta Surface Water Quality Guidelines (ASWQG) and possibly Chemical Oxygen Demand (COD) to match EPA Cluster Rule limits. Several technologies and combinations are being studied to ensure that the mill meets future requirements to minimize environmental impact including: oxygen delignification, ozone treatment, chemical coagulation, and membrane separation. Membrane treatment of bleach plant effluents has several potential advantages for Grande Prairie, a recovery boiler limited mill, including the possibility of lower capital, compatibility with high-yield (through additives) kraft pulping technologies, no impact on the quality of its preferred product, minimal waste streams, and a good fit with other technologies leading to a bleached effluent free mill. Pilot plant research on membrane treatment for color removal was carried out in Grande Prairie as early as 1985 when the mill was still using chlorine bleaching. The performance of new commercially available membranes has been studied over the past few years. Currently the Grande Prairie bleach plant is <1 5-stage EC DoEopDED sequence producing a 90-ISO brightness pulp from a 30 kappa continuous digester on a diet of 50:50 spruce to pine chips. Effluent filtrate from the last three stages is essentially recycled. The E op filtrate is sewered, mixed with other alkaline effluents and clarified. The Do filtrate is also sewered and then combined with the other clarified effluents and treated in a 16-day aerated stabilization basin. Although membrane treatment study work has been carried out on several effluent streams from the mill, the focus of this paper is on color removal from the Do filtrate. The specific research objectives were to: (1) employ ultrafiltration techniques to separate the Do wastewater into various molecular size fractions and to characterize each in terms of total organic carbon (TOC) and color, (2) to quantify the ability of a UF membrane with a molecular weight cut-off of 8,000 Daltons (D) to reject TOC and color at three temperatures (20°, 30° and 40°C) and three wastewater pH levels (2.4, 5.3 and 7.0) and (3) to quantify the effect of 5 Jlm cartridge filtration and UF pretreatment on the flux and rejection characteristics of a reverse osmosis (RO) membrane. Recovery is the cumulative membrane permeate produced from the initial volume of feed. METHODS AND MATERIALS Wastewater The wastewater employed for this project was obtained as a single, 15-gallon sample of Do' It was stored at 4°C until used. Molecular weight distributions Samples were filtered through a 0.45 Jlm Millipore filter. The organic contents of the filtrates are, therefore by definition, only dissolved organic materials. Filtrates were processed through Amicon Stirred Ultrafiltration Cells (Model 8200), with membranes having the following molecular weight cut-offs (MWCO): YC05 - 500 Daltons (D), YMI - 1,000 D, YM3 - 3,000 D, and YMIO - 10,000 D. Membrane processing Membrane studies were conducted using the Osmonics SEPA CF membrane cell apparatus (Osmonics, Inc., Minnetonka, MN). Commercially available flat sheet membranes were employed with an active filtration area of 138 cm2 . Milli-Q (ultra-pure) water was circulated through the membrane for 30 minutes establishing the pure water flux. With effluents, the process was operated in a total recycle mode for the first 30 minutes, that is, the concentrate and permeate were recirculated to the feed tank. Then, the permeate was recovered and the concentrate recycled to the feed tank. Permeate flux measurements were made and samples extracted for analysis for each 10% recovery.
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RESULTS AND DISCUSSION Chemical characterizations Summarized in Table 1 are the permeate flux, TOC and color data from the UF characterizations of Do' Milli-Q water permeation rates were within manufacturers specifications. Comparing the Milli-Q water and Do flux rates indicated that foul ants were present in the wastewater which reduced permeate productivity by 8% for the YMlO membrane to 61 % for the YC05 membrane. Table 1. Ultrafiltration characterization of Do wastewater Membrane Pure water flux Q.../m hr) Do flux Q.../m2 hr)
YC05 31.2 12.1
Feed Permeate Concentrate
792.5 281.6 1738.8
Feed Permeate Concentrate
1700 107 3066
2
Note:
YMI YM3 23.3 26.5 21.4 25.7 TOC(mg/L) 792.5 792.5 465.1 545.8 1548.0 1158.0 Color (pCCU) 1700 1700 334 835 2221 2972
YMlO 119.0 109.6 792.5 634.4 1033.0 1700 1145 1876
Analytical measurements were made on composited samples at 60% recovery. Platinum. Cobalt Color Units (pCCU).
The permeate TOC increased from 281.6 mglL to 634.4 mglL as the molecular weight cut-offs (MWCO) of the membrane employed increased from 500 D to 10,000 D. These results were utilized to calculate the distribution of TOC in the size fractions indicated in Table 2. These data show that the majority (58.68%) of dissolved organic matter has a molecular size of less than 1,000 D. Table 2. Percent distribution of TOC and color for Do wastewater Molecular size range 1ססoo
%TOC 35.53 23.15 10.18 11.18 19.95
% Color 6.29 13.59 29.21 18.26 32.66
* P = Parameter = TOC or Color. Do feed TOC = 792.5 mglL. Do feed color = 1700 PCCU The permeate colors from the various UF membranes likewise varied in direct proportion to the MWCO. The distribution of chromophores (Table 2) indicated that a minority percentage of color in Do is due to small sized molecules «1,000 D). Most of the color (50.92%) is produced by organics with a molecular size (>3,000 D) molecules. Membrane treatment This initial investigation centered on the performance of one commercially available UF membrane (NTR• 7410) obtained from the Hydranautics Company which had a molecular weight cut-off of 8,000 D for
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uncharged molecules. The principle variables studied were the effect of wastewater temperature and pH. NTR-7410 is negatively charged, sulfonated polyether sulfone membrane. Additional studies were carried out with a reverse osmosis (RO) membrane (TFC-LP), a thin film (polyamide) composite (TFC), a product of the Fluid Systems Corporation. This membrane was characterized by a 99.6% chloride ion rejection. The effect of pretreatment, either 5 Jlm cartridge filtration or UF, was the focus of this portion of the research. Ultrafiltration performance The data listed in Table 3 represent the results from the experiments which were used to quantify the effect of Do temperature on NTR-7410 performance. As expected the permeate flux increased with operating temperature throughout the entire range of recoveries. This response is attributed to the decrease in wastewater viscosity. These findings are consistent with those of Elefsiniotis et al. (1995). They found UF membrane (10,000 D, MWCO) flux ratios for 30 and 40°C compared to 20°C were 1.07:1 and 1.45:1, respectively. In this research, flux ratios were 1.44: 1 and 1.84: 1, when the average permeation rates are compared for the same temperature range. Garner (1991) reported that processing temperature plays a major role in the fouling potential of wastewater on membranes. The rejection ofTDS by NTR-7410 for the composited permeate was not temperature sensitive. Overall, the permeate contained approximately 200 mglL less TDS than the Do feedwater. However, permeate TDS concentration increased with recovery. The TOe's of the composited permeates at 90% recovery were 547.4 mglL, 626 mglL and 615.8 mglL respectively for Do processed at 20,30 and 40°C. In general, the higher the temperature the more organic matter was able to diffuse through the membrane. Table 3. The effect of temperature on ultrafiltration membrane (NTR 7410) performance % Recovery
Perm. TDS Cone. TDS (mglL) (mglL) 20 0 e
0 50 80 Composited Permeate Concentrate
3000 2850 3200 2800 3900
3350 3900
30
0 50 90
Composited Permeate Concentrate
0 50 90 Composited Permeate Concentrate
3000 2700 3250 2800 3900
0
TOC (mglL)
Color (PCCU)
Flux (L/m2-hr)
825 1750.00 548.2 568.97 589.4 1249.80 547.40 624.14 2980.00 8520.69
125.77 74.53 56.80
825 614 746.6 626.60 4454
1750.00 555.17 1310.34 593.10 8537.93
208.33 111.25 59.80
825 567.8 779.4 615.80 3278
1750.00 672.41 965.50 700.50 8537.93
217.51 125.05 84.36
e
3450 3900
40 0 e
3000 2890 3275 2850 4000
Pressure =552 kPa, cross-flow velocity
3390 4000
=3.22 mis, feed pH =2.2, batch =4L, 5 Jlm pretreatment
Color measurements were made in accordance with Standard H.5 (Canadian Pulp and Paper Association, 1991), the procedure accepted by the pulp and paper industry which requires that the sample must be
159
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adjusted to the range 7.5 ± 0.1 pH range prior to an absorbance measurement at 465 nm. The values for the composited sample shown in Table 3 are considered to be essentially the same. Essentially, 63% of the Do color was removed by UF treatment. This correlates well with the characterization data (Table 2) which identified 50% of the color in this wastewater had an approximate molecular size greater than 3,000 D. Jonsson (1987) showed that color removal by membranes was directly related to MWCO. The effect of wastewater pH was evaluated and the data reported in Table 4. Based on information gleaned from the titration of Do, the initial pKa was established as 5.3 and therefore chosen for study together with a wastewater pH above (7.0) and below (2.4) this value. Table 4. The effect of pH on ultrafiltration (NTR 7410) membrane performance % Recovery
Temp.
rC) 0 50 90 Composite Permeate Concentrate
0 50 90 Composite Permeate Concentrate
0 60 90 Composite Permeate Concentrate
40 37 40
pH = 2.05 pH = 1.94
40 38 40
pH = 5.16 pH = 5.43
40 39 pH = 6.65 pH = 7.37
Perm Cone TDS TDS (mglL) (mg/L) Feed pH 3000 2890 3275 2850
3390 4000
Flux
(mglL)
(pCCD)
(L/m2-hr)
825 568 779 615.80
1750.00 672.41 965.50 700.50
217.51 125.05 84.36
3278
8537.93
825 379 533 384
1750.00 296.55 641.38 379.31
2432
5696.55
825 454 594 449.40
1750.00 282.76 382.76 265.52
3394.00
3734.48
= 5.30 2150 2300
2300 Feed pH 2300 2150 2500 2200
Color
= 2.40
4000 Feed pH 2000 1800 2250 2000
TOC
= 7.00 2600 3000
3000
Pressure = 552 kPa, temperature = 40°C, velocity pretreatment pH adjusted using NaOH solution
279.69 256.28 128.58
370.17 280.50 150.79
= 3.22 mis, batch = 4 L, 5 JoLm
The data indicate an increase in permeation rates with an increase in solution pH. Examination of the membrane surface at the termination of the pH 7.0 experiment showed the formation of a precipitate. It is speculated that the precipitate may have sorbed dissolved foulants thereby enhancing permeation rates. For all three runs permeate flux decreased as recovery increased (Table 4). Jonsson and Petersson (1989) in their research employed polysulfone membranes and found permeation rate increased with pH increases. The rejection of color producing molecules in our research, likewise followed this pattern. For membrane runs conducted at pH 2.4, 5.3 and 7.0 the composited permeates registered colors, reported in PCCU units of 70 I, 379 and 266. Apparently, negatively charged organic molecules are repelled by the negatively charged membrane surface thus reducing the fouling potential in the system and thereby producing higher fluxes and greater color rejections by the membrane.
R. A. SIERKA et al.
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Overall TDS and TOC rejections were maximized when Do was processed at a pH of 5.3. For.TO~, 25%, 54% and 46% rejections were obtained when the Do pH was respectively 2.4, 5.3 and 7.0. LikewIse, the TDS of the permeates were 2,850 mgIL, 2,000 mgIL and 2200 mglL respectively for wastewater pH's of 2.4, 5.3 and 7.0. Both TDS and TOC increased as a function of permeate recovery. Reverse osmosis performance Acid state (D o) filtrates are difficult wastewaters to treat in membrane processes and manifest this. behavior particularly in terms of lost permeate productivity. Pretreatment of Do would appear to be an Important consideration before RO processing would be considered. In this research two types of pretreatment were studied: (1) 5-J.lm cartridge filtration and (2) UF filtration of Do. The filtrates from both these operations were processed by the TFC-LP after NaOH addition to raise the pH to 7.0. Listed in Table 5 are the results of these experiments. Table 5. The effect of pretreatment on reverse osmosis performance % Recovery
Temp.
rC) 0 50 80 Composite Permeate Concentrate 0 50 80 Composite Permeate Concentrate Pressure
40 42 pH = 6.65 pH
= 7.37
39 42 pH = 6.44 pH
= 6.47
Cone. TDS
TOC
Color
Flux
(mglL)
(mglL)
(PCCU)
(L/m2-hr)
5 J'm pretreatment 825 2250 76.25 4125 330 122.6 1ססoo 1200 74.41 450
1750.00 10.34 24.14 24.14
25.53 18.28
Perm. TDS (mglL)
1ססoo
4986.00
UF pretreatment 555.4 1850 73.75 3700 250 121.5 900 8000 70.68 360 8000
3219.00
10.71
3641.38 1231.00 13.79 13.79 13.79
34.60 24.82 16.27
2741.38
= 1104 kPa, temperature = 40°C, feed pH = 7.0, bateh = 4 L
The permeate flux rates for the 5-J.lm and UF treated Do declined from 25.5 L1m 2-hr to 10.7 L1m2-hr and 34.6 Llm2-hr to 16.34 Llm2-hr respectively over the course of recovery of 80% of the permeate. With pressure limitations imposed by the pump, much of the net driving force was lost due to the increased osmotic pressure of the concentrate. Also, it should be noted that UF pretreatment provided a lower TDS feedwater, 1850 mglL versus 2250 mglL, which was the cause for the higher initial flux rate for this pretreatment. Finally, the removal of organic matter by UF pretreatment produced a feedwater with a TOe of 555 mgIL compared to 825 mglL for the 5-J.lm filtrate. Permeate quality in terms of TDS was very similar for the UF and 5-J.lm pretreated Do yielding composited permeates with 360 mgIL and 450 mglL respectively. This small quality difference is ascribed to influent concentrations of TDS from the two differently pretreated streams. RO treatment was effective in removing TOC from Do. The composited permeates from both pretreatments were nearly the same; 71 mgIL for the UF pretreatment and 74 mgIL for the 5-J.lm treated Do. Since 35.5% of the original TOC had a molecular size of <500 D, it can be assumed that some of these compounds would not be removed by pretreatment and would transport through the RO membrane. Color removal was
Ultrafiltration and reverse osmosis treatment
161
virtually complete (99%) after RO treatment. The characterizations previously discussed would predict this response. CONCLUSIONS The following summarizes the research on the chemical characterization, UF and RO membrane processing of Do' 1.
Chemical characterization after separation showed that 59% of the TOC comprises molecules with a molecular size of less than l,OOO D but, only 20% of the color is due to these molecules.
2.
Increasing the processing temperature in the range 20 to 40°C positively impacted the UF permeate flux rate, however, water quality was not significantly affected. Overall membrane rejections were: 5% to 7.3% for TDS, 28% for TOC and 73% for color.
3.
UF processing at pH 7.0 above the pKa (5.3) increased the permeation rate but at a wastewater pH below (2.4), the converse was true.
4.
Pretreatment by either 5 Jlm filtration or UF followed by RO yielded a permeate approximately equal in quality, however, permeate flux rates were higher with UF treatment, but the same percentage loss of permeate flux was noted at 90%. ACKNOWLEDGMENTS
The authors wish to thank graduate students Troy A. Bontrager and Sunil Kommineni for their diligent work in the laboratory. Also, we wish to recognize Nina Welch for the production of this manuscript. Finally, we acknowledge the support of the Weyerhaeuser Company. REFERENCES Canadian Pulp and Paper Association, Technical Section (1991). Standard B.5, Approved Method, Colour of Pulp Mill Effluents. Elefsiniotis, P., Hall, E. R. and Johnson, R. M. (1995). Contaminant removal from recirculated white water by ultrafiltration and/or biological treatment. 1995 International Environmental Conference Proceedings, TAPPI Conference, Atlanta, Georgia, 7-10 May 1995,861-865. Garner, J. W. (1991). Treatment technologies emerging to meet organochoride removal needs, Pulp and Paper, 64(10), 55. Jonsson, A. S. (1987). Ultrafiltration of bleach plant effluent. Nordic Pulp and Paper Research Journal, 1, 23-29. Jonsson, A. S. and Petersson, E. (1989). Treatment of C-stage and E-stage Bleach Plant Effluent by UFo Nordic Pulp and Paper Research Journal, #3, p. 184-189, (124).
Pergamon
Waf. Sci. Tech. Vol. 35, No. 2-3, pp. 155-161, 1997. Copyright © 1997 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain.
PH: S0273-1223(96)00927-4
0273-1223/97 $17'00 + 0'00
ULTRAFILTRATION AND REVERSE OSMOSIS TREATMENT OF AN ACID STAGE WASTEWATER R. A. Sierka*, S. P. Cooper** and P. S. Pagoria*** * Department of Chemical and Environmental Engineering, University ofArizona, CE Building, Room 306, Tucson, Arizona 857 J6, USA ** Weyerhaeuser Canada Ltd., P.O. Bag 1020, Grande Prairie, Alberta, Canada, T8V 3A9 *** Weyerhaeuser Technology Center, Tacoma, Washington 98477, USA ABSTRACT The Weyerhaeuser Grande Prairie pulp mill produces 300,000 tons per year of bleached kraft for the tissue and specialty grade paper markets. Alberta Environmental Protection has given the mill stringent limits for Color (90 kilograms/admt) BOD (3 kg/admt), TSS (5 kg/admt), and AOX (1.5 kg/admt). Several technologies and combinations are being studied to ensure that the mill meets future requirements to minimize environmental impact including: oxygen delignification, ozone treatment, chemical coagulation, and membrane separation. The specific research objectives were to: (I) employ ultrafiltration techniques to separate the Do wastewater into various molecular size fractions and to characterize each in terms of total organic carbon (TOC) and color, (2) to quantify the ability of a UF membrane with a molecular weight cut• off of 8,000. Daltons (D) to reject TOC and color at three temperatures (20°, 30° and 40°C) and three wastewater pH levels (2.4, 5.3 and 7.0), and (3) to quantify the effect of 5 Ilm cartridge filtration and UF pretreatment on the flux and rejection characteristics of a reverse osmosis (RO) membrane. The following summarizes the results of this research. 1) Chemical characterization after separation showed that 59% of the TOC is comprised of molecules with a molecular size of less than 1,000 D but, only 20% of the color is due to these molecules. 2) Increasing the processing temperature in the range 20 to 40°C positively impacted permeate flux rate, however, water quality was not significantly affected. 3) UF processing at pH 7.0 above the pKa (5.3), increased the permeation rate but at a wastewater pH below (2.4), the converse was true. 4) Pretreatment by either 5 Ilm filtration or UF followed by RO yielded a permeate equal in quality, however, permeate flux rates were higher with UF treatment. © 1997 IAWQ. Published by Elsevier Science Ltd.
KEYWORDS Acid wastewater; color; reverse osmosis; total organic carbon; ultrafiltration. INTRODUCTION The Weyerhaeuser Grande Prairie pulp mill is located 500 kilometres NW of Edmonton in Alberta, Canada, on the Wapiti River, a small tributary of the Smoky and Peace River systems. The mill, built in 1973, produces 300,000 tons per year of bleached kraft for the tissue, towel, and specialty grade paper markets. The pulp is highly valued for its unrefined tensile strength and uniformity. Alberta Environmental Protection has given the mill stringent limits for Color (90 kilograms/admt) Biochemical Oxygen Demand (BOD) (3 kgladmt), Total Suspended Solids (TSS) (5 kg/admt), and Adsorbable Organic Halide (AOX) (1.5 kgladmt). 155
R. A. SIERKA et at.
156
Weyerhaeuser anticipates even tighter limits in these conventional parameters in the near future as well as new targets for manganese, and nutrients to meet Alberta Surface Water Quality Guidelines (ASWQG) and possibly Chemical Oxygen Demand (COD) to match EPA Cluster Rule limits. Several technologies and combinations are being studied to ensure that the mill meets future requirements to minimize environmental impact including: oxygen delignification, ozone treatment, chemical coagulation, and membrane separation. Membrane treatment of bleach plant effluents has several potential advantages for Grande Prairie, a recovery boiler limited mill, including the possibility of lower capital, compatibility with high-yield (through additives) kraft pulping technologies, no impact on the quality of its preferred product, minimal waste streams, and a good fit with other technologies leading to a bleached effluent free mill. Pilot plant research on membrane treatment for color removal was carried out in Grande Prairie as early as 1985 when the mill was still using chlorine bleaching. The performance of new commercially available membranes has been studied over the past few years. Currently the Grande Prairie bleach plant is <1 5-stage EC DoEopDED sequence producing a 90-ISO brightness pulp from a 30 kappa continuous digester on a diet of 50:50 spruce to pine chips. Effluent filtrate from the last three stages is essentially recycled. The E op filtrate is sewered, mixed with other alkaline effluents and clarified. The Do filtrate is also sewered and then combined with the other clarified effluents and treated in a 16-day aerated stabilization basin. Although membrane treatment study work has been carried out on several effluent streams from the mill, the focus of this paper is on color removal from the Do filtrate. The specific research objectives were to: (1) employ ultrafiltration techniques to separate the Do wastewater into various molecular size fractions and to characterize each in terms of total organic carbon (TOC) and color, (2) to quantify the ability of a UF membrane with a molecular weight cut-off of 8,000 Daltons (D) to reject TOC and color at three temperatures (20°, 30° and 40°C) and three wastewater pH levels (2.4, 5.3 and 7.0) and (3) to quantify the effect of 5 Jlm cartridge filtration and UF pretreatment on the flux and rejection characteristics of a reverse osmosis (RO) membrane. Recovery is the cumulative membrane permeate produced from the initial volume of feed. METHODS AND MATERIALS Wastewater The wastewater employed for this project was obtained as a single, 15-gallon sample of Do' It was stored at 4°C until used. Molecular weight distributions Samples were filtered through a 0.45 Jlm Millipore filter. The organic contents of the filtrates are, therefore by definition, only dissolved organic materials. Filtrates were processed through Amicon Stirred Ultrafiltration Cells (Model 8200), with membranes having the following molecular weight cut-offs (MWCO): YC05 - 500 Daltons (D), YMI - 1,000 D, YM3 - 3,000 D, and YMIO - 10,000 D. Membrane processing Membrane studies were conducted using the Osmonics SEPA CF membrane cell apparatus (Osmonics, Inc., Minnetonka, MN). Commercially available flat sheet membranes were employed with an active filtration area of 138 cm2 . Milli-Q (ultra-pure) water was circulated through the membrane for 30 minutes establishing the pure water flux. With effluents, the process was operated in a total recycle mode for the first 30 minutes, that is, the concentrate and permeate were recirculated to the feed tank. Then, the permeate was recovered and the concentrate recycled to the feed tank. Permeate flux measurements were made and samples extracted for analysis for each 10% recovery.
Ultrafiltration and reverse osmosis treatment
157
RESULTS AND DISCUSSION Chemical characterizations Summarized in Table 1 are the permeate flux, TOC and color data from the UF characterizations of Do' Milli-Q water permeation rates were within manufacturers specifications. Comparing the Milli-Q water and Do flux rates indicated that foul ants were present in the wastewater which reduced permeate productivity by 8% for the YMlO membrane to 61 % for the YC05 membrane. Table 1. Ultrafiltration characterization of Do wastewater Membrane Pure water flux Q.../m hr) Do flux Q.../m2 hr)
YC05 31.2 12.1
Feed Permeate Concentrate
792.5 281.6 1738.8
Feed Permeate Concentrate
1700 107 3066
2
Note:
YMI YM3 23.3 26.5 21.4 25.7 TOC(mg/L) 792.5 792.5 465.1 545.8 1548.0 1158.0 Color (pCCU) 1700 1700 334 835 2221 2972
YMlO 119.0 109.6 792.5 634.4 1033.0 1700 1145 1876
Analytical measurements were made on composited samples at 60% recovery. Platinum. Cobalt Color Units (pCCU).
The permeate TOC increased from 281.6 mglL to 634.4 mglL as the molecular weight cut-offs (MWCO) of the membrane employed increased from 500 D to 10,000 D. These results were utilized to calculate the distribution of TOC in the size fractions indicated in Table 2. These data show that the majority (58.68%) of dissolved organic matter has a molecular size of less than 1,000 D. Table 2. Percent distribution of TOC and color for Do wastewater Molecular size range
%TOC 35.53 23.15 10.18 11.18 19.95
% Color 6.29 13.59 29.21 18.26 32.66
* P = Parameter = TOC or Color. Do feed TOC = 792.5 mglL. Do feed color = 1700 PCCU The permeate colors from the various UF membranes likewise varied in direct proportion to the MWCO. The distribution of chromophores (Table 2) indicated that a minority percentage of color in Do is due to small sized molecules «1,000 D). Most of the color (50.92%) is produced by organics with a molecular size (>3,000 D) molecules. Membrane treatment This initial investigation centered on the performance of one commercially available UF membrane (NTR• 7410) obtained from the Hydranautics Company which had a molecular weight cut-off of 8,000 D for
R. A. SIERKA et al.
158
uncharged molecules. The principle variables studied were the effect of wastewater temperature and pH. NTR-7410 is negatively charged, sulfonated polyether sulfone membrane. Additional studies were carried out with a reverse osmosis (RO) membrane (TFC-LP), a thin film (polyamide) composite (TFC), a product of the Fluid Systems Corporation. This membrane was characterized by a 99.6% chloride ion rejection. The effect of pretreatment, either 5 Jlm cartridge filtration or UF, was the focus of this portion of the research. Ultrafiltration performance The data listed in Table 3 represent the results from the experiments which were used to quantify the effect of Do temperature on NTR-7410 performance. As expected the permeate flux increased with operating temperature throughout the entire range of recoveries. This response is attributed to the decrease in wastewater viscosity. These findings are consistent with those of Elefsiniotis et al. (1995). They found UF membrane (10,000 D, MWCO) flux ratios for 30 and 40°C compared to 20°C were 1.07:1 and 1.45:1, respectively. In this research, flux ratios were 1.44: 1 and 1.84: 1, when the average permeation rates are compared for the same temperature range. Garner (1991) reported that processing temperature plays a major role in the fouling potential of wastewater on membranes. The rejection ofTDS by NTR-7410 for the composited permeate was not temperature sensitive. Overall, the permeate contained approximately 200 mglL less TDS than the Do feedwater. However, permeate TDS concentration increased with recovery. The TOe's of the composited permeates at 90% recovery were 547.4 mglL, 626 mglL and 615.8 mglL respectively for Do processed at 20,30 and 40°C. In general, the higher the temperature the more organic matter was able to diffuse through the membrane. Table 3. The effect of temperature on ultrafiltration membrane (NTR 7410) performance % Recovery
Perm. TDS Cone. TDS (mglL) (mglL) 20 0 e
0 50 80 Composited Permeate Concentrate
3000 2850 3200 2800 3900
3350 3900
30
0 50 90
Composited Permeate Concentrate
0 50 90 Composited Permeate Concentrate
3000 2700 3250 2800 3900
0
TOC (mglL)
Color (PCCU)
Flux (L/m2-hr)
825 1750.00 548.2 568.97 589.4 1249.80 547.40 624.14 2980.00 8520.69
125.77 74.53 56.80
825 614 746.6 626.60 4454
1750.00 555.17 1310.34 593.10 8537.93
208.33 111.25 59.80
825 567.8 779.4 615.80 3278
1750.00 672.41 965.50 700.50 8537.93
217.51 125.05 84.36
e
3450 3900
40 0 e
3000 2890 3275 2850 4000
Pressure =552 kPa, cross-flow velocity
3390 4000
=3.22 mis, feed pH =2.2, batch =4L, 5 Jlm pretreatment
Color measurements were made in accordance with Standard H.5 (Canadian Pulp and Paper Association, 1991), the procedure accepted by the pulp and paper industry which requires that the sample must be
159
Ultrafiltration and reverse osmosis treatment
adjusted to the range 7.5 ± 0.1 pH range prior to an absorbance measurement at 465 nm. The values for the composited sample shown in Table 3 are considered to be essentially the same. Essentially, 63% of the Do color was removed by UF treatment. This correlates well with the characterization data (Table 2) which identified 50% of the color in this wastewater had an approximate molecular size greater than 3,000 D. Jonsson (1987) showed that color removal by membranes was directly related to MWCO. The effect of wastewater pH was evaluated and the data reported in Table 4. Based on information gleaned from the titration of Do, the initial pKa was established as 5.3 and therefore chosen for study together with a wastewater pH above (7.0) and below (2.4) this value. Table 4. The effect of pH on ultrafiltration (NTR 7410) membrane performance % Recovery
Temp.
rC) 0 50 90 Composite Permeate Concentrate
0 50 90 Composite Permeate Concentrate
0 60 90 Composite Permeate Concentrate
40 37 40
pH = 2.05 pH = 1.94
40 38 40
pH = 5.16 pH = 5.43
40 39 pH = 6.65 pH = 7.37
Perm Cone TDS TDS (mglL) (mg/L) Feed pH 3000 2890 3275 2850
3390 4000
Flux
(mglL)
(pCCD)
(L/m2-hr)
825 568 779 615.80
1750.00 672.41 965.50 700.50
217.51 125.05 84.36
3278
8537.93
825 379 533 384
1750.00 296.55 641.38 379.31
2432
5696.55
825 454 594 449.40
1750.00 282.76 382.76 265.52
3394.00
3734.48
= 5.30 2150 2300
2300 Feed pH 2300 2150 2500 2200
Color
= 2.40
4000 Feed pH 2000 1800 2250 2000
TOC
= 7.00 2600 3000
3000
Pressure = 552 kPa, temperature = 40°C, velocity pretreatment pH adjusted using NaOH solution
279.69 256.28 128.58
370.17 280.50 150.79
= 3.22 mis, batch = 4 L, 5 JoLm
The data indicate an increase in permeation rates with an increase in solution pH. Examination of the membrane surface at the termination of the pH 7.0 experiment showed the formation of a precipitate. It is speculated that the precipitate may have sorbed dissolved foulants thereby enhancing permeation rates. For all three runs permeate flux decreased as recovery increased (Table 4). Jonsson and Petersson (1989) in their research employed polysulfone membranes and found permeation rate increased with pH increases. The rejection of color producing molecules in our research, likewise followed this pattern. For membrane runs conducted at pH 2.4, 5.3 and 7.0 the composited permeates registered colors, reported in PCCU units of 70 I, 379 and 266. Apparently, negatively charged organic molecules are repelled by the negatively charged membrane surface thus reducing the fouling potential in the system and thereby producing higher fluxes and greater color rejections by the membrane.
R. A. SIERKA et al.
160
Overall TDS and TOC rejections were maximized when Do was processed at a pH of 5.3. For.TO~, 25%, 54% and 46% rejections were obtained when the Do pH was respectively 2.4, 5.3 and 7.0. LikewIse, the TDS of the permeates were 2,850 mgIL, 2,000 mgIL and 2200 mglL respectively for wastewater pH's of 2.4, 5.3 and 7.0. Both TDS and TOC increased as a function of permeate recovery. Reverse osmosis performance Acid state (D o) filtrates are difficult wastewaters to treat in membrane processes and manifest this. behavior particularly in terms of lost permeate productivity. Pretreatment of Do would appear to be an Important consideration before RO processing would be considered. In this research two types of pretreatment were studied: (1) 5-J.lm cartridge filtration and (2) UF filtration of Do. The filtrates from both these operations were processed by the TFC-LP after NaOH addition to raise the pH to 7.0. Listed in Table 5 are the results of these experiments. Table 5. The effect of pretreatment on reverse osmosis performance % Recovery
Temp.
rC) 0 50 80 Composite Permeate Concentrate 0 50 80 Composite Permeate Concentrate Pressure
40 42 pH = 6.65 pH
= 7.37
39 42 pH = 6.44 pH
= 6.47
Cone. TDS
TOC
Color
Flux
(mglL)
(mglL)
(PCCU)
(L/m2-hr)
5 J'm pretreatment 825 2250 76.25 4125 330 122.6 1ססoo 1200 74.41 450
1750.00 10.34 24.14 24.14
25.53 18.28
Perm. TDS (mglL)
1ססoo
4986.00
UF pretreatment 555.4 1850 73.75 3700 250 121.5 900 8000 70.68 360 8000
3219.00
10.71
3641.38 1231.00 13.79 13.79 13.79
34.60 24.82 16.27
2741.38
= 1104 kPa, temperature = 40°C, feed pH = 7.0, bateh = 4 L
The permeate flux rates for the 5-J.lm and UF treated Do declined from 25.5 L1m 2-hr to 10.7 L1m2-hr and 34.6 Llm2-hr to 16.34 Llm2-hr respectively over the course of recovery of 80% of the permeate. With pressure limitations imposed by the pump, much of the net driving force was lost due to the increased osmotic pressure of the concentrate. Also, it should be noted that UF pretreatment provided a lower TDS feedwater, 1850 mglL versus 2250 mglL, which was the cause for the higher initial flux rate for this pretreatment. Finally, the removal of organic matter by UF pretreatment produced a feedwater with a TOe of 555 mgIL compared to 825 mglL for the 5-J.lm filtrate. Permeate quality in terms of TDS was very similar for the UF and 5-J.lm pretreated Do yielding composited permeates with 360 mgIL and 450 mglL respectively. This small quality difference is ascribed to influent concentrations of TDS from the two differently pretreated streams. RO treatment was effective in removing TOC from Do. The composited permeates from both pretreatments were nearly the same; 71 mgIL for the UF pretreatment and 74 mgIL for the 5-J.lm treated Do. Since 35.5% of the original TOC had a molecular size of <500 D, it can be assumed that some of these compounds would not be removed by pretreatment and would transport through the RO membrane. Color removal was
Ultrafiltration and reverse osmosis treatment
161
virtually complete (99%) after RO treatment. The characterizations previously discussed would predict this response. CONCLUSIONS The following summarizes the research on the chemical characterization, UF and RO membrane processing of Do' 1.
Chemical characterization after separation showed that 59% of the TOC comprises molecules with a molecular size of less than l,OOO D but, only 20% of the color is due to these molecules.
2.
Increasing the processing temperature in the range 20 to 40°C positively impacted the UF permeate flux rate, however, water quality was not significantly affected. Overall membrane rejections were: 5% to 7.3% for TDS, 28% for TOC and 73% for color.
3.
UF processing at pH 7.0 above the pKa (5.3) increased the permeation rate but at a wastewater pH below (2.4), the converse was true.
4.
Pretreatment by either 5 Jlm filtration or UF followed by RO yielded a permeate approximately equal in quality, however, permeate flux rates were higher with UF treatment, but the same percentage loss of permeate flux was noted at 90%. ACKNOWLEDGMENTS
The authors wish to thank graduate students Troy A. Bontrager and Sunil Kommineni for their diligent work in the laboratory. Also, we wish to recognize Nina Welch for the production of this manuscript. Finally, we acknowledge the support of the Weyerhaeuser Company. REFERENCES Canadian Pulp and Paper Association, Technical Section (1991). Standard B.5, Approved Method, Colour of Pulp Mill Effluents. Elefsiniotis, P., Hall, E. R. and Johnson, R. M. (1995). Contaminant removal from recirculated white water by ultrafiltration and/or biological treatment. 1995 International Environmental Conference Proceedings, TAPPI Conference, Atlanta, Georgia, 7-10 May 1995,861-865. Garner, J. W. (1991). Treatment technologies emerging to meet organochoride removal needs, Pulp and Paper, 64(10), 55. Jonsson, A. S. (1987). Ultrafiltration of bleach plant effluent. Nordic Pulp and Paper Research Journal, 1, 23-29. Jonsson, A. S. and Petersson, E. (1989). Treatment of C-stage and E-stage Bleach Plant Effluent by UFo Nordic Pulp and Paper Research Journal, #3, p. 184-189, (124).