Integration of Nanofiltration Hollow Fiber Membranes with Coagulation

Aug 10, 2018 - A positively charged nanofiltration (NF) hollow fiber membrane with an antifouling feature has been fabricated and scaled up to pilot s...
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Integration of Nanofiltration Hollow Fiber Membranes with Coagulation−Flocculation to Treat Colored Wastewater from a Dyestuff Manufacturer: A Pilot-Scale Study Can-Zeng Liang, Shi-Peng Sun,* Bai-Wang Zhao, and Tai-Shung Chung* Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 S Supporting Information *

ABSTRACT: A positively charged nanofiltration (NF) hollow fiber membrane with an antifouling feature has been fabricated and scaled up to pilot scale to treat colored wastewater from a dyestuff manufacturer with the aid of the coagulation−flocculation (CF) process. An effective CF formulation, i.e., iron chloride/anionic polyacrylamide (IC/APAM) = 800/100 ppm, has been identified to treat the NF concentrate stream. The challenges of membrane fouling and processing concentrate streams can be effectively managed by the integrated NF−CF process. The NF process is able to achieve 82% and 67% removal of color and chemical oxygen demand (COD) from the colored wastewater, respectively; while the CF process can remove 91% of color and 81% of COD from the NF concentrate stream. NF and CF were found to complement each other. The NF process alone always generates a concentrate stream, but it can effectively reduce the wastewater volume. The CF processes are particularly favorable for treating small volume concentrate streams. These advantages make the NF−CF process propitious for users located in landor space-limited places and countries.

1. INTRODUCTION Colored wastewaters, generated during the manufacture and applications of dyestuffs, are harmful and aesthetically objectionable because many dyes are carcinogenic and the presence of dyes, even at very low concentrations, may exhibit strong colors.1−3 Most dyes are synthesized in reactors and then blended with other additives to form the final products.4 It is estimated that more than 100 000 synthetic dyes and over 700 000 tons of dyestuff are produced annually,5,6 with 1−2% dyes lost during production.7 The wastewaters generated by dye plants normally consist of suspended solids, oil and grease, raw materials, dye intermediates, dyes, and other auxiliary chemicals.8,9 These constituents result in high turbidity (NTU), total suspended solid (TSS), total dissolved solid (TDS), chemical oxygen demand (COD), biological oxygen demand (BOD), and intense color.8 This kind of wastewater not only influences the photosynthetic activity of aquatic life because of the reduced light penetration, but also induces severe health problems in humans. Therefore, if the dye effluents are discharged without effective treatment, they may cause deleterious consequences to the environment and people.3 More and more stringent environmental legislation10,11 has compelled dye producers and users to handle their wastewater in order to comply with the environmental imperatives.3 The major concerns in treating the colored wastewater are to remove COD, and particularly, the color or dyes.12−14 However, this kind of wastewater generally is difficult to treat owing to the recalcitrant nature of dye molecules. Most of them are complex organic compounds with various numbers of aromatic rings.7,13 Methods to treat such wastewater include chemical oxidation, biodegradation, coagulation−flocculation (CF), adsorption, ion-exchange and membrane filtration, etc. Recently, Dasgupta et al.3 and Brillas et al.15 have provided comprehensive reviews on such technologies and methods; © 2015 American Chemical Society

however, most of these methods are in lab-scale and either expensive or inadequate or both.2,3,15 In industry, chlorine14 and NaOCl9 are used for decolorization, but harmful byproducts, e.g. chloroanilines, chlorophenols, and chloronitrobenzenes, are formed.14 So far, it is still a great challenge for industries to find an effective and low-cost technical solution to treat colored wastewater and to meet strict environmental regulations.9 Nanofiltration (NF), a pressure-driven membrane separation technology, has been demonstrated as a viable solution for removing dyes from aqueous solutions.16−22 Compared to ultrafiltration (UF) and reverse osmosis (RO) processes, NF has been increasingly adopted because of its high rejection, relatively low operating pressure, and low cost.19,23−27 Nevertheless, it is unanimous that membrane technologies, including NF, inevitably suffer from two technical bottlenecks. One is the flux decline owing to membrane fouling. The other is the generation of concentrate streams.28 The management of the membrane concentrate streams poses an arduous challenge for real applications. For the dye-containing wastewater treatment, the reuse of the concentrate is almost impossible. Therefore, it must be further treated before being discharged.3 According to the literature, the concentrate can be treated by ozonation, advanced oxidation, and wet oxidation, but they are quite energy and cost intensive.29 Direct incineration or integration of membrane distillation (MD) and incineration have also been considered. However, incineration and MD are not cheap. Basically, incineration costs about US$ 35−78/t, while MD dictates 70−90% of the benefit/cost ratio in the whole Received: Revised: Accepted: Published: 11159

July 14, 2015 October 10, 2015 October 21, 2015 October 21, 2015 DOI: 10.1021/acs.iecr.5b03193 Ind. Eng. Chem. Res. 2015, 54, 11159−11166

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Industrial & Engineering Chemistry Research Table 1. Properties of Coagulants and Flocculants Used in This Work name of coagulant or flocculant aluminum sulfate (octadecahydrate) aluminum potassium sulfate (dodecahydrate) calcium oxide iron(III) chloride (anhydrous) iron(III) sulfate (pentahydrate) magnesium chloride (anhydrous) polyaluminum chloride anionic polyacrylamide cationic polyacrylamide cyanoguanidine polydiallyldimethyl ammonium chloride (cationic, dissolved in water) a

code name

molecular formula

molecular weight (g/mol)

purity

AS APS CO IC IS MC PAC APAM CPAM CYGU PDDA

Al2(SO4)3·18H2O AlK(SO4)2·12H2O CaO FeCl3 Fe2(SO4)3·5H2O MgCl2 Aln(OH)mCl(3n‑m) (0 < m < 3n) (CH2−CH−CONH2)ma (CH2−CH−CONH2)ma NH2C(NH)NHCN (C8H16ClN)n

666 474 56 162 490 95 ≥115b 800−2500 million 800−1000 million 84 200 000−350 000

extra pure 99.5% ≥98% 98% 97% pure ≥30% (as Al2O3) ≥90% ≥90% ≥99% 20 wt %

Obtained from the specification of the products. bCalculated by assuming m = n = 1 for PAC.

Figure 1. Schematic illustrations of (a) cross-linking process and (b) nanofiltration process.

treatment system.3,19,30 So far, CF has been the widely used technology for dyes and color removal because of its low capital cost and simple operation. Nevertheless, it is always challenging to select appropriate coagulant and flocculant because of the large number of existing and emerging dyes that have complex structures.31−36 In order to overcome the aforementioned challenges, recently we have developed an antifouling positively charged NF hollow fiber membrane by coating a positively charged polyethylenimine (PEI) layer on the outer surface of a negatively charged polyamideimide (PAI) substrate for the removal of dyes.23,37 Our previous report demonstrated the effectiveness of treating dye wastewaters by the combined use of the novel NF membrane and the CF process.18 The NF membrane exhibited excellent dye rejections while the CF process with selected coagulants and flocculants can treat highly concentrated dye wastewater. This lab-scale success has inspired us to scale up and optimize the NF−CF system for real wastewater treatment. Therefore, the objective of this work is to integrate the NF process with the CF process in a pilot scale to remove color and COD from wastewater effluents of a local dyestuff manufacturer. The wastewater was treated by the NF process, and subsequently the NF concentrate was handled via a CF process.

The two paramount parameters, color and COD, were measured to evaluate the performance of the integrated NF− CF process. A simple but effective strategy for the selection and optimization of suitable coagulant and flocculant is presented. Concurrently, mechanisms of the NF and CF processes are elucidated. This work may shed light on developing a practical membrane-based technique for the treatment of industrial colored wastewaters.

2. EXPERIMENTS AND METHODS 2.1. Chemicals and Materials. The colored wastewater was sampled from a dye manufacturer. In appearance, the wastewater sample looks dark purple and has about 760 ppm of COD, pH ∼7, conductivity of ∼1354 μS/cm, and TDS of ∼1118 ppm. The 11 chemicals in Table 1 are used as coagulants and flocculants for study. For easy reading, unless state otherwise, the corresponding code names of these coagulants and flocculants are used hereafter. Deionized water (DI water, Milli-Q water) was used for dilution. 2.2. Fabrication of the Pilot-Scale Nanofiltration (NF) Module. The hollow fiber membrane substrates made of polyamideimide (PAI) were produced through a dry-jet wet spinning process in our laboratory; the dope formulation is 11160

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The chemical oxygen demand of the wastewater was measured using an ultraviolet−visible (UV−vis) spectrophotometer (Pharo 300, Merck) with the aid of COD test kits (Spectroquant, Merck). The removal of COD (RCOD) was estimated by the following equation:

PAI/ethylene glycol (EG)/N-methyl-2-pyrrolidinone (NMP) = 21.5/13.5/65.0 wt %; the as-spun membranes have inner and outer diameters of around 280 and 430 mm, respectively. More details about this membrane can be found elsewhere.18,23 The module was made of PVC pipe with 2 in. inside diameter, 1 m in length, 40% potting density. As shown in Figure 1a (the real setup is presented in the Supporting Information, Figure SI1), once the membrane substrates were housed in the module, the cross-linking solution (0.5 wt % PEI (60 000 g/mol) solution, PEI was dissolved in a 1:1 mixture of isopropanol and DI water)) was pumped into the module through the shell side of the hollow fiber at 50 °C for 1 h. Thereafter the hollow fibers were cross-linked and the positively charged NF membranes were formed. The effective membrane area is ∼4.6 m2. 2.3. NF Experiments. The NF experiments were carried out in a pilot-scale NF setup, as shown in Figure 1b (Figure SI1 presents the real setup). The wastewater was circulated at the shell side of the hollow fiber module at a flow rate of 5 ± 0.5 L/ min at 5.0 ± 0.2 bar and 41 ± 1 °C. The NF permeate flowed out from the lumen side of the hollow fibers. The procedures of the NF tests include three steps: First, the initial pure water flux (PWF) was determined using tap water (conductivity, 182 μS/ cm). Second, 45 L of the colored wastewater were used as the feed solution for NF filtration. In this stage, initial experiments were executed under a total recirculation mode (i.e., both the retentate and permeate were recycled into the feed tank) to study the long-term stability of the system. After that, the NF filtration was run in a concentrate mode (i.e., the permeate was collected and not returned to the feed tank, but the retentate was circulated), which leads to the augmentation of feed concentration. The volume reduction factor (VRF) was then determined: VRF = Vi/Vr, where Vi and Vr are the initial feed volume and the retentate volume, respectively. Finally, the NF system was cleaned thoroughly with tap water until no color was observed in the retentate. Then the final pure water flux was tested. Through the whole NF experiments, accordingly, the samples of permeate and retentate were collected regularly and kept for analyses. 2.4. CF Treatment of the NF Concentrate Stream. To screen out the suitable coagulant and flocculant and subsequently determine their optimal dosages, a jar-test technology was used to execute coagulation and flocculation experiments, where a certain amount of coagulant or flocculant was put into a 100.0 g NF concentrate in a 250 mL glass beaker and stirred using a jar-test apparatus (VELP Scientifica, JLT4, Flocculation Tester) at 200 rpm for 2 min. If two chemicals (coagulant and flocculant) were used together, the coagulant was mixed with the wastewater first and stirred at 200 rpm for 2 min. Then the flocculant was added and the mixture was stirred at 200 rpm for 2 min. In all CF experiments, after a certain time of settling, the supernatant solution was withdrawn with or without filtering (Millex filter; pore size, 0.22 μm) then kept for further analyses. The rest of the NF concentrate was treated by the obtained optimal CF formulation. 2.5. Physicochemical Analyses. The permeate flux (J, L m−2 h−1) was measured by the following equation: J=

Q A×t

R COD =

(C i − C t ) × 100% Ci

(2)

where Ci and Ct represent the COD concentrations in initial wastewater (the original/retentate) and treated wastewater (NF permeate/CF treated solution), respectively. The relative color concentration or intensity can be determined by a UV−vis integral standard dilution method (detailed reasons and descriptions are given in the Supporting Information, Figure SI2−4). The integral of the UV−vis absorbance spectrum was measured using a UV−vis spectrophotometer (the same as aforementioned). A linear correlation between UV−vis integral (integrated range, 350− 650 nm) and the dilution factor was observed (as can be seen in Figure SI 3 and 4). Therefore, based on the UV−vis integral, the color removal (RC, %) by coagulation−flocculation was calculated through the following equation: RC =

(Ib − Ia) × 100% Ib

(3)

where Ib is the UV−vis integral value of the wastewater before CF treatment and Ia is the UV−vis integral value of solution after CF treatment. The color removal (RC, %) by the NF filtration was determined by the following equation: RC =

(If − Ip) If

× 100%

(4)

where If and Ip are the UV−vis integral values of the feed and the permeate solutions, respectively. It should be noted that the NF concentrate/retentate needs to be diluted for accuracy. The normalized efficiency of coagulant and flocculant (Ec) can be defined as

Ec =

RC mc

(5)

where RC is the color removal (%) by CF and mc is the weight of the coagulant−locculant applied for CF treatment. The particle size of the CF settlement was evaluated via a laser diffraction particle size analyzer (Beckman Coulter, LS 13 320; analysis range, 0.04−2000 μm). The pH of solution was measured by a pH meter (pH/ion S220, Mettler Toledo). The conductivity was determined via a conductivity meter (SCHOTT instruments, Lab960). The osmotic pressure was measured by an osmometer (Advanced Instruments, Model 3250 Osmometer). The total dissolved solids (TDS) value was determined by a portable TDS tester (Hanna Instruments, DiST1 TDS Tester, HI98301).

3. RESULTS AND DISCUSSION 3.1. Treatment of Colored Wastewater by the NF Process. 3.1.1. NF in the Total Recirculation Mode. Before the wastewater was treated, an initial pure water flux (PWF) of 9.8 L m−2 h−1 was obtained using tap water as the feed. To remove coarse particles prior to the NF process, the raw wastewater was prefiltrated by a polypropylene (PP) filter bag

(1)

where Q is the volume (L) of permeate collected during the sampling time t (hour) and A is the effective membrane area (m2). 11161

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Industrial & Engineering Chemistry Research (pore size, ∼ 200 μm). A total recirculation mode, in which the feed concentration was kept constant, was run for accumulative 120 h (10 h/day) to investigate the long-term stability of the NF system. Figure 2 shows the flux variation and color removal

Figure 3. Effects of volume reduction factor (VRF) on the permeate flux and the feed’s osmotic pressure.

mainly governed by the increase of osmotic pressure because the osmotic pressure increase reduces the effective driving force across the membrane. Because the osmotic pressure of the feed solution is relatively small and it increases slowly, these cause the permeate flux to drop slowly. As depicted in Figures 4 and 5, both color intensity and COD concentration of the feed increase linearly with an

Figure 2. Long-term testing performance of the NF system.

as a function of time with the following observations: First, a color removal of about 95% is achieved and remains almost unchanged over the entire testing duration. In addition, the average COD removal is 81 ± 2% (not shown in the figure) based on COD measurements from the initial, middle, and final permeate samples. The consistent and effective removal of color and COD indicate that the newly developed positively charged NF hollow fiber membrane is stable in the colored wastewater environment. Second, the permeate flux decreases from ∼5 to ∼4 L m−2 h−1, and the trend is stable after 100 h. The flux decline of approximately 20% is mainly caused by the adsorption of dissolved inorganic and organic components, colloids, bacteria, and suspended solids on the membrane and the consequent pore blocking.38,39 It is worth noting that it takes a long time (about 100 h) for the permeate flux to reach a stable stage, which implies a slow and less severe fouling process. The lower fouling phenomenon might arise from the following three reasons: (1) The membrane surface is not only positively charged but also hydrophilic;23,37 thus, it is not easy for hydrophobic foulants to stick on it. (2) The rapid flow of wastewater along the shell side of the hollow fiber module creates high shear, which minimizes the formation of fouling cake and biofilm. (3) The ability of dyes and colorants to attach or color the membrane has been significantly reduced because of the high heterogeneity of the wastewater. 3.1.2. NF in the Concentrate Mode. In the real application scenario, the permeate is collected while the retentate is recycled to the feed tank. In a batch NF treatment process, the concentrate mode leads to a reduction in feed volume and an increase in feed concentration. The volume reduction factor (VRF) is one of the most important operational parameters in the concentrate mode.19 Figures 3−5 show the variations of permeate flux, color intensity, and COD as a function of VRF after long-term tests under the concentrate mode. As presented in Figure 3, the permeate flux drops rapidly in the very beginning and then decreases slowly. This phenomenon might be ascribed to the following facts: (1) The increase in feed concentration renders further adsorption and pore blocking on and in the membrane. The additional adsorption and pore blocking are likely the dominant factors that result in a sharp drop in the initial permeate flux because the corresponding osmotic pressure has almost no change. (2) After the sharp drop, the sites and pores available for adsorption and pore blocking tend to be saturated. Consequently the flux decline is

Figure 4. Effects of volume reduction factor (VRF) on color removal (color removal was based on the corresponding retentate).

Figure 5. Effects of volume reduction factor (VRF) on COD (COD removal was based on the corresponding retentate).

increment in VRF, while the color and COD of the permeate are also increasing but at a much slower pace. Because the rejection mechanisms in NF are governed mainly by steric and electrostatic repulsion,23,40 a higher feed concentration induces a more severe concentration polarization on the membrane surface.41,42 The effectiveness of the electrostatic repulsion from the electric double layer on the membrane surface was thereby reduced in the presence of a higher solute concentration. As a result, more solutes that contribute to color and COD (e.g., dyes, intermediates, and other dissolved organic molecules) pass through the membrane.19 However, the color removal efficiency drops only slightly and keeps roughly stable at 90% as shown in Figure 4, while the COD 11162

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check whether the coagulation took place, because some coagulants create very fine particles that cannot settle down. The condition “filtered” in Table 2 means that the supernatant was collected and then filtrated through a filter with a pore size of 0.22 μm, then tested. The nonfiltered one means that the supernatant was collected and then tested directly. As shown in Table 2, IC exhibits the best performance in the case of 1000 ppm dosage. It can remove 93% of color without filtration. In terms of color removal, there is only 0.5% difference between the filtered and nonfiltered ones, which means that the NF concentrated wastewater was coagulated by IC with effective settling. However, when the dosage of IC is increased to 2000 ppm, the color removal rate declines by about 10% because of over dosage. The excess IC dissolves and its Fe3+ ions induce brown-yellow colors. For the other 10 chemicals with 1000 ppm dosage, without filtering, their performances are either zero or very poor. However, the color removal rates of PAC, AS, and IS can significantly increase to about 80% after filtration. This indicates that the three chemicals, PAC, AS, and IS, are effective but they form fine particles that are too small to settle down and suspend in solution. The effectiveness of coagulation changes remarkably when the dosage is increased to 2000 ppm. Weak electrolytes such as CO, MC, CPAM, APAM, PDDA, and CYGU are either poor or ineffective in removing color at both low and high dosages, while strong electrolytes such as PAC, AS, APS, and IS have better effectiveness when their dosages are increased. Because charge neutralization is likely the mechanism of coagulation,18 the aqueous mixture system can be easily neutralized by strong electrolytes at a high dosage. Apparently, IC is the most suitable coagulant in terms of color removal and efficiency as well as the ratio of color removal (%) to coagulant (mg); therefore, it is selected for further studies. 3.2.2. Optimization of the Coagulant Dosage. The effect of IC dosage on color removal is presented in Figure 7. At low

removal increases from around 75% to 85% as illustrated in Figure 5 because of the robustness of the NF membrane. 3.1.3. Evolution of Permeate Flux. The evolution of permeate flux under different testing modes and durations is illustrated in Figure 6. When the wastewater is filtered in the

Figure 6. Evolution of permeate flux during the NF filtration experiments.

total recirculation mode (i.e., both the retentate and permeate were recycled into the feed tank), the permeate flux at the initial stage (first 24 h) is about half of the initial pure water flux (PWF, 9.8 L m−2 h−1). The permeate flux drops to about 40% of the initial PWF at the stable stage after 100 h testing. The 60% decline in permeate flux is due to the membrane fouling and the increase of osmotic pressure in the feed stream. In contrast, the permeate flux reaches 2.9 L m−2 h−1 when VRF = 5 in the concentrate mode (i.e., the permeate was collected and not returned to the feed tank, but the retentate was circulated). After flushing and cleaning with tap water, the final PWF is about 7.7 L m−2 h−1. This means the recovery rate (i.e., final PWF/initial PWF) can reach around 78% by simple washing. Our previous study has demonstrated that an almost 100% recovery rate can be achieved by chemical (or acid−base) cleaning for this NF membrane.43 3.2. Treatment of the NF Concentrate by Coagulation−Flocculation. In order to obtain the right coagulant and flocculant, the following protocols were performed: first, examine the effectiveness and the performance of each coagulant (inorganic) and flocculant (organic); second, choose the best coagulant and optimize its dosage; and finally, identify the suitable flocculant and optimize the formulation of the combined coagulant−flocculant. Unless stated otherwise, the wastewater used for coagulation−flocculation is referred to the NF concentrate (VRF = 5). 3.2.1. Evaluation of the Effectiveness of Coagulants and Flocculants. All the coagulants and flocculants were evaluated with their dosages in 1000 and 2000 ppm. A sufficient settling time (overnight) was allowed, and the supernatants were then taken for analyses. The effectiveness and performance of each chemical is compiled into Table 2. Filter papers were used to

Figure 7. Effect of IC dosage on color removal.

dosages, the color removal rate rises with the augment of IC dosage. It reaches a plateau stage of 92% at 800 ppm and then levels off or slightly declines beyond 1000 ppm because of the over dosage. At 500 ppm, the color removal rate is zero without

Table 2. Preliminary Evaluation of the Performances of the Coagulants and Flocculants on Color Removal conditions

color removal (%)

dosage (ppm)

filtered

AS

APS

CO

IC

IS

MC

PAC

APAM

CPAM

CYGU

PDDA

1000

no yes no yes

0 77.6 81.3 90.3

0 26.2 76.4 79.2

1.2 18.9 32.0 52.6

93.0 93.5 82.0 90.7

0 85.7 90.5 94.2

0 1.3 0 12.6

33.5 83.7 90.1 95.0

0 0 0 0

9.1 17.2 27.8 26.0

0 0 0 0

0 32.0 0 71.1

2000

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Industrial & Engineering Chemistry Research filtering the supernatant, whereas it is about 78% after filtering. This implies that the particles were formed by an insufficient IC dosage (e.g, ≤500 ppm) with sizes large than 0.22 μm but they were unable to settle down. Based on the efficiency of the color removal, 800 ppm is the turning point because the filtered and nonfiltered supernatant have almost the same efficiency. A dosage more than 800 ppm will lead to a further reduction in efficiency. Therefore, an IC dosage of 800 ppm is considered as the optimal dosage as a coagulant. 3.2.3. Combination of Coagulant and Flocculant. To accelerate the settlement process, organic polymers are often chosen as flocculants and used together with inorganic coagulants in the coagulation−flocculation process. The coagulation process creates agglomerates or flocs, which can be bridged or linked together by flocculants to form bigger agglomerates that precipitate faster under gravity.44,45 An IC of 800 ppm and an identical amount of flocculants (polymers APAM, CPAM, CYGU, and PDDA) were employed to identify the suitable flcocculant. As depicted in Figure 8, the color

no stickiness. It can not only induce short settlement time but also provide easy filtration. 3.2.4. Optimization of the Flocculant Dosage. The effect of flocculant (APAM) dosage on the CF process was studied by fixing the IC dosage at 800 ppm and varying the amount of APAM. Figure 9 displays that the mean particle size of the

Figure 9. Effect of APAM on settlement particle size.

settlement increases with an increase in APAM dosage. With the aid of APAM, the fine particles are amassed and the mean particle size is significantly increased by approximately an order of magnitude. As a result, the settlement speed is also dramatically enhanced. As displayed in Figure 10, the

Figure 8. Effects of flocculants on color removal and settlement particle size. Figure 10. Effect of APAM on settlement speed.

removals are nearly the same (∼90%) for all cases with or without flocculants; while the settlement particle sizes differ very much. Clearly, IC is responsible for the color removal, while the flocclants are responsible for enlarging the particle size. IC alone can produce particles with a mean size of about 6 μm. If IC is combined with APAM or CPAM, the mean particle sizes become 50−70 times bigger than that of IC alone owing to the huge molecular weights of APAM and CPAM. As shown in Table 1, both have molecular weight in the hundreds of millions. If flocculants with relatively small molecular weights such as CYGU and PDDA are used, they can increase the particle size by only ∼30% to ∼8 μm. Interestingly, the color removal by IC and PDDA can reach about 95%, which is about 5% higher than the others, as shown in Figure 8. This is due to the fact that PDDA possesses a high density of positive charge (Table 1) and itself can remove color (Table 2). During the CF experiments, the dissolved CPAM was very sticky and glued to the stirring impeller and the wall of container. In contrast, APAM was not sticky and was found to be well dissolved and dispersed in the mixture. Therefore, APAM is selected as the right flocculant because it can form very large particles and has

settlement speed is very slow without APAM. In fact, it took about 20 h to complete the settlement (not shown in the figure). With the aid of APAM, the settlement speed is swift, and the settling process is almost completed within 10 min. In the case of APAM = 100 ppm, the settlement rate is the highest. Therefore, APAM of 100 ppm is selected as the optimal flocculant dosage and the combination of IC/APAM at 800/100 ppm is identified as the optimal CF formulation to treat the NF concentrate. 3.3. Integration of the NF and CF process. Figure 11 illustrates the process and performance of the integrated NF− CF process for the treatment of colored wastewater. The only waste generated from this integrated process is the CF settlement in the form of solids, which could be disposed in landfill with a cost about 7 times cheaper than incineration.46 As displayed in Figure 11, 82% of color and 67% of COD of the original wastewater stream are removed by the NF process, while the CF process can remove 91% of color and 81% of COD of the NF concentrate stream. The additional bonus of the NF−CF process is that the CF process involves a small volume of liquid (∼20% of the original volume; VRF = 5), 11164

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explanation of adopting UV-integral method to evaluate the color (PDF)

AUTHOR INFORMATION

Corresponding Author

*T.-S.C.: tel., +65 6516 6645; fax, +65 6779 1936; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support provided by National Research Foundation (NRF) of Singapore under its NRF Proof-of-Concept 8th Grant Call (NRF2012NRFPOC001-059) for the project entitled “Development of advanced nanofiltration membranes for high removing rate of dyes in textile wastewater” (NUS Grant R-279-000-389-281) as well as the Singapore-MIT Alliance for Research and Technology (SMART) Centre under its Innovation Grant (ING12045-ENG) for the project entitled “Development of robust high-performance nanofiltration membranes for textile wastewater treatment” (NUS Grant R-279-000-377-592). Special thanks are given to Dr. Li Fuyun, Dr. Ong Yee Kang, and Dr. Yang Liming for their support during the preparation of this work.

Figure 11. Schematic illustration of the NF−CF process and its performance. (1) The removal was based on the original wastewater, and the NF permeate was sampled under the concentrate mode. (2) The removal was based on the final NF concentrate (VRF = 5), and optimal CF formulation (IC/APAM = 800/100 ppm) was applied.

which means that the NF−CF process can be more compact with a smaller footprint. It should be noted that the integrated process is a NF−CF process, not CF−NF. In the NF−CF process, we did the NF process first; as a result, the NF concentrate was inevitably created, and the volume of the NF concentrate stream become one-fifth that of the original wastewater (VRF = 5). This small volume NF concentrate was treated by the CF process, which can benefit from the high concentration and the small volume. In our previous paper,18 which was a lab-scale study, the comparison for permeate flux between NF alone and CF−NF was carried out, and we found that CF−NF performed better than NF alone. In our current study, however, it is not practical to carry out such a comparison.

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4. CONCLUSIONS A NF hollow fiber membrane with a positively charged selective surface has been scaled up to a pilot scale and effectively treated real colored wastewater from a dye manufacturer. An efficient CF optimal formulation, IC/APAM = 800/100 ppm, has been identified through screening, selection, and optimization. The NF process is able to achieve 82% and 67% removal of color and COD from the colored wastewater, respectively; while the CF process can remove 91% of color and 81% of COD from the NF concentrate stream. The slowly declining and recoverable permeate water fluxes indicate that the positively charged NF membrane has antifouling properties. The newly developed NF−CF process has demonstrated that the membrane fouling and NF concentrate can be managed effectively during the treatment of colored wastewater. NF and CF complement each other and render the integrated NF−CF process highly propitious with a small footprint. This is especially desired for users located in land- or space-limited regions and countries.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03193. Figures showing the real pilot-scale setup of cross-linking process and nanofiltration process, UV−vis spectra of the original colored wastewater and the diluted samples, and correlation between UV−vis integral and dilution factor; 11165

DOI: 10.1021/acs.iecr.5b03193 Ind. Eng. Chem. Res. 2015, 54, 11159−11166

Article

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