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Integrated membrane process for wastewater treatment from production of instant tea powders
Desalination 355 (2015) 147–154
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Integrated membrane process for wastewater treatment from production of instant tea powders Bin Chen a, Xiaohui Xiong a, Zhong Yao a, Na Yin b, Ze-Xian Low c, Zhaoxiang Zhong a,⁎ a b c
College of food science and light industry, Nanjing University of Technology, National Engineering Research Center for Special Separation Membrane, Nanjing 210009, PR China Applied Chemistry & Environmental Engineering faculty, Bengbu College, Bengbu 233030, PR China Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia
H I G H L I G H T S • Design an integrated process to treat wastewater from the production of instant tea powders • Optimize operating parameters and analyze separation mechanism to improve the water quality • Propose an effective cleaning method with a flux recovery ratio higher than 99%
a r t i c l e
i n f o
Article history: Received 16 June 2014 Received in revised form 6 October 2014 Accepted 21 October 2014 Available online xxxx Keywords: Integrated membrane process Instant tea wastewater Fouling mechanism
a b s t r a c t This study investigated an integrated process to treat the wastewater from an instant tea powder factory. The wastewater was pretreated by combination of coagulation–flocculation. Polyaluminum ferric chloride (PAFC) was used as the coagulating agent. The optimum operating conditions with respect to contaminant removal were determined to be pH 5, 20 °C and 800 mg/L of PAFC, with polyacrylamide (PAM) as coagulant aid at a dosage of 20 mg/L. The discharged water from the coagulation–flocculation was treated with ceramic UF membranes. It was found that interactions between the molecules of PAFC and organic foulants reduced the membrane fouling and improved the removal efficiencies effectively. The permeate flux of UF membrane was almost three times higher than typical flux obtained without pretreatment. In the subsequent NF process, the removal efficiencies of turbidity, COD and TOC were all higher than 99.9%, and the TSS wasn't detected in the permeate. The effects of cleaning methods on permeate flux were also investigated. It was found out that 0.5 wt.% NaClO flushing produced the highest flux recovery ratio of 99% and showed good reproducibility. This study showed the feasibility of applying coagulation–flocculation, UF and NF in the treatment of instant tea powder factory wastewater. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Tea is the second most commonly consumed drink in the world. It is a popular, widely accepted, pleasant, economical and safe drink that is enjoyed every day by hundreds of millions of people across all continents [1]. As one type of tea products, instant tea is consumed regularly around the world and therefore constitutes an important pillar in human consumption. Its popularity is due mostly to its availability of varieties of tastes. Moreover, instant tea is also consumed for health reasons because it contains large quantity of tea polyphenols, saccharide, protein, caffeine, vitamins, inorganic elements and trace elements such as physiological active element [2,3]. Nowadays instant tea powder is manufactured from processed leaves, tea wastes or fermented leaves via digestion, followed by disc separation and vacuum concentration ⁎ Corresponding author. E-mail address: [email protected] (Z. Zhong).
http://dx.doi.org/10.1016/j.desal.2014.10.029 0011-9164/© 2014 Elsevier B.V. All rights reserved.
before drying to form powder. All the equipments from the production process of instant tea powder need to be washed by water, so the wastewater produced may contain substances such as salt, alkali, acids, organic additives, dissolved solids and suspended solids. Therefore, more instant tea factories are under increasing pressure to reduce the environmental impact of their wastewater streams. Wastewater must be properly treated to comply with the national standards for discharge into surface water or reuse for factory application [4]. Few researches focus on the treatment of instant tea wastewater. The mechanism of such wastewater is assumed to be similar to that of organic wastewater, because the main components are also organics. Several common techniques are used in the treatment of organic wastewater, such as coagulation, flocculation, conventional biological treatment and advanced oxidation processes [5–7]. Low efficiency, high operating cost, prone to corrosion and recontamination are the general disadvantages of these methods. For example, conventional biological treatment requires a long reaction time and has a large footprint [8]. Additionally, a high
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content of non-reacted chemicals is consumed in the wastewater treatment [9]. In the last decade, membrane processes have attracted lots of attention over conventional separation processes, due to their high efficiency, high quality product, ease of operation, low footprint and low energy requirement comparing with biological and chemical treatment methodologies. Membrane technology (especially UF and NF) has often been considered as a promising method for water reuse and wastewater clean-up. The UF membrane is useful for the removal of suspended solids, organic components and microorganisms larger than its pore size. Also, UF acts as a disinfection barrier by removing bacteria as well as viruses; however, UF does not remove low molecular weight and soluble compounds. Organic compounds with low molecular weight can be removed by using NF membranes. Membrane technology has been applied in a wide variety of fields, such as pharmaceutical industry [10], dairy and food technology [11], pulp and paper industry [12], textile industry [13] and a wide diversity of other new applications [14]. However, membrane fouling, which will remarkably deteriorate membrane permeability and consequently result in high energy consumption, has become a major obstacle to further the application of UF technology in wastewater treatment [15]. The dissolved organics on the membrane surface are considered to be the most important foulants in the processes of microfiltration (MF) and UF [16]. Moreover, the fouling results in deterioration of membrane performance, which increases the use of chemical cleaning agents and energy consumptions, ultimately increasing the maintenance and operation costs [17]. Membrane fouling is caused by the interactions of membrane and foulant, and also by foulant–foulant interactions which results in flux decline [18]. The internal pore blocking by the retained molecules may lead to a sharp and rapid flux decline. The further flux decline may be attributed to the formation and subsequent growth of the gel and cake layer, which come from the deposition and accumulation of rejected particles [19]. The common main foulants include dissolved organic or inorganic components, colloidal matter, soluble microbial products, bacteria and suspended solids [20]. In the case of instant tea wastewater treatment, organic fouling is a significant issue. To a large extent, the size of organic matter, which falls in the lower region of the organic matter size distribution, is smaller than the pore size of UF membrane. Thus, the dissolved and colloidal organic matter may permeate through the membrane and cause solute adsorption onto the pore walls. Consequently, pre-treatment has been used to alleviate the membrane fouling, such as pre-filtration [21], oxidation [22], adsorption [23], and coagulation/flocculation [24]. Because of the low capital cost and excellent performance, coagulation/flocculation has been regarded as one of the most successful pre-treatment [25]. High dissolved organic
components can be pre-removed by proper selection of coagulant/flocculant aids, initial pH, dosage of coagulant and temperature. After pre-treatment via coagulation/flocculation, an integrated membrane process was used in the present study to treat the wastewater from an instant tea factory, and the complex ceramic membrane fouling was also examined. In the treatment of coagulation/flocculation, the optimizations of the parameters such as initial pH, dosage of coagulant, and temperature were studied. The effects of transmembrane pressure (TMP) and cross-flow velocity (CFV) were also studied. Moreover, experimental and instrumental methods were coupled with modeling to analyze the resistances and fouling in the UF process. Based on these results, an effective cleaning method was developed to clean the fouled membranes. 2. Materials and methods 2.1. Experimental setup Fig. 1 is a schematic representation of the experimental set-up, including both UF and NF. In the UF process, the feed wastewater was filtered through the membrane by a centrifugal pump at different pressures. The TMP was monitored with a manometer. In the experimental process, the valves A, C and E were closed and the retentate was recirculated to the feed tank, while the penetrants were collected from valve D. The setup was operated in a continuous or recycling mode, the permeated solution was removed or recycled continuously and the membranes were fouled by the retentate which was recycled to the feed tank in each mode. Four different 19-channel zirconia ceramic UF membranes with average pore size of 0.5, 0.2, 0.05 and 0.02 μm (corresponding pure water flux was 1312.5, 780.4, 604.3 and 201.1 L·m−2·h−1, respectively; 0.1 MPa, 20 °C) were used in this UF process study (Jiuwu High-Tech Co. Ltd., Jiangsu, China). The membrane area for mass transfer is 0.11 m2 with channel diameter of 4 mm. Also, the NF set-up had a similar working mode. The NF polyamide composite spiral-wound membrane (NF90-2540; pure water flux was 56.3 L·m−2·h−1; 1.0 MPa, 20 °C) was purchased from Dow Chemical Company (USA). 2.2. Wastewater characteristics and measuring method Turbidity was measured by HACH turbidity meter (2100N), and COD was determined by the fast digestion spectrophotometric method followed by the Chinese National Method [26]. Total organic carbon (TOC) was measured by Shimadzu (ASI-V). After a continuous mode, the fouled ceramic membrane was broken into small samples and
10 P
4 5 P T
14 L
C
1
6
3
2
13
7
P
T
10
11
K
T
I
11
J
D H
B A
12
p
4 8
F E
V-1
9
G P-38
(a)
(b)
Fig. 1. Setup of UF (a) and NF (b) processes: (1) feed tank, (2) centrifugal pump,(3) rotameter, (4) manometer, (5) thermometer, (6) UF membrane, (7) feed tank, (8) centrifugal pump, (9) HP pump, (10) manometer, (11) thermometer, (12) NF membrane, and (13) rotameter.
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then dried at 80 °C for 8 h. The foulants on the ceramic membrane were scraped and collected, and then dried in the same way. The morphologies of collected foulants were characterized and analyzed by field emission scanning electron microscope (FE-SEM) and energydispersive X-ray spectroscopy (EDX; S-4800, Hitachi, Japan). The foulants on ceramic membranes were also determined by FTIR Avatar 360 (Thermo Nicolet, USA). The instant tea wastewater was obtained from Nanjing Rongdian Foodstuff Co. Ltd. The wastewater was mainly comprised of dissolved organic components and colloidal matter with the following characteristics — COD (5000–40,000 mg/L), turbidity (1000–10,000 NTU) and TOC (3000–20,000 mg/L). The detailed values of wastewater characteristics are shown as follows: turbidity (11,549 NTU), COD (9850 mg/L), pH 7.5 and TOC (5057 mg/L), and the concentration of tea polyphenol (9.39 g/kg) and caffeine (6.52 g/kg). In the pre-treatment step, variable doses of coagulant agent such as polyaluminum ferric chloride (PAFC; 200–1200 mg/L) and flocculant anionic polyacrylamide (PAM; 20 mg/L) were added to wastewater at different pH values which ranged between 4 and 8. The coagulant solutions were added to the wastewater and the mixture was stirred at a rapid speed of 200 rpm for 3 min. Then PAM was added at a low stir speed of 30 rpm for 15 min. After that, the flocs were formed and allowed to settle for 30 min. The optimal conditions of coagulant/flocculant were determined in terms of coagulant and flocculant dosing and pH, considering the COD and turbidity removal. The COD and turbidity removal rate, R, are defined as (C0 − C1) / C0, where C0 stands for the COD and turbidity of raw wastewater and C1 stands for the COD and turbidity of pre-treated wastewater. After each filtration run, different cleaning methods evaluated by the flux recovery ratio, were investigated, including cleaning by pure water, NaOH and HNO3 solution, and NaClO solution. The fouled membranes were cleaned in a recycling mode, which was shown in Fig. 1a. Pump provides the energy for heating. Unless otherwise indicated, the detailed cleaning conditions were at temperature of 55 °C for 45 min at low TMP (0.01 MPa) and high CFV (5 m/s). The fouling degree m = (J0 − J1) / J0 × 100% and the flux recovery ratio FRR = (J2 / J0) × 100% were used to determine the anti-fouling performance of each cleaning method [27], where J0 and J1 refer to the water fluxes of membranes before and after fouling, respectively, and J2 stands for the water flux of cleaned membrane. During the filtration of the wastewater, according to the resistancein-series model [28], the main resistances including both hydraulically reversible resistance and irreversible resistance are determined. The flux of permeate (Jp) follows Eq. (1) JP ¼
ΔP ΔP ΔP ¼ ¼ μ w RT μ w ðRm þ R f Þ μ w ðRm þ Ri þ Rr Þ
ð1Þ
where μw is the permeate viscosity (10−3 Pa·s at 25 °C), ΔP is the transmembrane operating pressure (Pa). RT is the total resistance to solvent permeation, defined as the sum of the membrane resistance (Rm) and the total fouling resistance (Rf). Herein, Rf is provided by the sum of hydraulically irreversible resistance (Ri) and the hydraulically reversible resistance (Rr). Rm was determined from the intrinsic membrane permeability. RT was obtained from the steady-state permeate flux, and the sum of Rm and Ri was obtained from the pure water flux measurements performed after rinsing. Rr was finally determined by RT − Rm − Ri [29]. 3. Result and discussion 3.1. Effects of PAFC dosage, pH and temperature in coagulation/flocculation process To reduce the fouling on the membrane and further improve the efficiency of membrane filtration, the coagulation–flocculation process was used as the pre-treatment. Due to the reduction in the surface
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ionic charge of the organic suspended solids and colloid, the coagulation–flocculation process causes the agglomeration of organic suspended solids and colloidal matter, which form large aggregates (known as flocs). The flocs then contact with each other, further increasing in size. When these flocs become larger, they can be removed through sedimentation. Also the larger size flocs cannot flow easily through the membrane [30]. Fe-salt and Al-salt are the most commonly used coagulants in water treatment. PAFC is developed by partially neutralizing AlCl3 and FeCl3, which has been widely used [31–33]. Thus, PAFC has the advantages of both polyaluminum and polyferric coagulants, which includes low price, large particle size, wide pH range and excellent adsorption ability. Fig. 2 shows the variations of wastewater turbidity and COD at different PAFC concentrations, pH values and temperatures. As plotted in Fig. 2a, the percentage COD reduction of wastewater increases with the PAFC dosage, and the COD reduction efficiency nearly reached a plateau when the dosage was higher than 800 mg/L. This can be explained by the role of aluminum and iron salts as coagulant which can generally be divided into two distinct mechanisms: charge neutralization of negatively charged colloids by cationic hydrolysis products and incorporation of impurities in an amorphous hydroxide precipitate (‘sweep flocculation’) [34]. On the one hand, when the dosage of Fe3+ in PAFC increases, the zeta potential of the wastewater after coagulation would increase [35], illustrating that the positively charged ferric was growing with increasing PAFC dosage, and thus charge neutralization between ferric and negatively charged phenolic hydroxyl group of organic material was enhanced with the increase of PAFC dosage, leading to the increase of COD removal efficiency. On the other hand, the polyaluminum would change to hydroxide precipitating gradually until saturation [36]. From Table 2, the negatively charged suspended solids reacted with PAFC, which settle with other impurities under the action of gravity, which can be reflected from the removal rate of TSS (72.5%). However, when PAFC dosage increased beyond 800 mg/L, there was a little excess of PAFC in wastewater, which could lead to restabilization of colloids, thus decreasing turbidity removal efficiency. The removal of turbidity and COD at 800 mg/L PAFC was also studied at various pH values, as plotted in Fig. 2b. The reduction of COD and turbidity was found to be the most effective at pH 5 as efficiency was reduced beyond pH 5. Studies have suggested that the dominating mechanism for organic matter coagulation–flocculation is dictated by the nature of the hydrolyzed Al and Fe [37]. In alkaline conditions, the hydrolysis of PAFC was enhanced, which resulted in lower positive iron salt [38]. The solubility products of Fe3 + and Al3 + are shown in Eq. (2) h i h i 3þ − 3 ; KSP1 ¼ Fe OH
h i h i 3þ − 3 KSP2 ¼ Al OH
ð2Þ
where the Ksp1 and Ksp2 stand for the solubility constant of the ferric hydroxide and aluminum hydroxide, respectively. Increase in Fe3 + and Al3+ or OH− ion concentration beyond Ksp1 and Ksp2 value will lead to the formation of the hydroxide [39]. Meanwhile, the functional groups (carboxylic or phenolic acids) of natural organic material in alkaline region were deprotonated, which led to the increase of negative charge of the organic material [40]. It was also observed that COD and turbidity removal efficiencies were lower at pH 4 than that at pH 5. This was because the organic wastewater was neutral, while Al3+ in PAFC was positively charged at low pH. When PAFC reacted with organic material in wastewater, small portion of the complexes undergone a charge reversal mechanism (from negative to positive charge) [41], which caused the repulsion between organic material and PAFC. As a result, the removal of the complexes was weakened at pH b5. Moreover, Fig. 2c shows the effect of temperature on the removal of turbidity and COD. Temperature is an important factor which affects the coagulation efficiency. Based on the turbidity and COD reduction, flocs formed at temperature beyond 20 °C were much poorer. It was suggested that the high temperature may result in poor coagulation performance as
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Fig. 2. The effect of PAFC dosage (pH = 5, 20 °C) (a), pH (PAFC dosage = 800 mg/L, 20 °C) (b) and temperature (PAFC dosage = 800 mg/L, pH = 5) (c) of coagulation–flocculation process.
particles move quickly at higher temperature [42]. Also, high temperature influences the hydrolysis of coagulant and solubility of Al and Fe hydroxides which affect the coagulation performance. Moreover, a more important reason is that the coagulation of PAFC becomes
unstable at higher temperature due to faster motion of organic molecules which result in deflocculation. The pretreatment process can be efficiently used for the removal of natural organic matter and to relieve the burden to the following UF treatment.
Fig. 3. Effect of membrane pore size on permeate flux. (a) Raw wastewater permeate flux. (b) Pre-treated wastewater permeate flux (TMP = 0.1 MPa, CFV = 3.0 m/s).
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Table 1 Resistances of four different pore size membranes. Wastewater
Raw wastewater
Membrane pore size/μm
0.5
0.2
0.05
0.02
0.5
Pre-treated wastewater 0.2
0.05
0.02
Rm/RT × 100% Rr/RT × 100% Ri/RT × 100%
7.84 37.79 54.37
12.87 44.38 42.75
18.25 46.17 35.58
47.6 49.56 2.84
8.63 53.25 38.12
12.15 67.21 20.64
21.7 62.08 16.22
45.32 52.54 2.14
3.2. Effects of membrane pore size Fig. 3a shows the raw wastewater permeate flux with 0.5, 0.2, 0.05 and 0.02 μm ceramic membranes at TMP of 0.1 MPa and CFV of 3.0 m/s, and Fig. 3b shows the pre-treated wastewater permeate flux under the same condition. From Fig. 3a, it can be observed that the initial flux decreases with the size of membrane pore, and generally, permeate flux decreased at the beginning of the experiment. This behavior is commonly observed in the UF process, indicating that a higher amount of foulants is driven towards the membrane surface leading to the flux decline. Consequently, the total filtration resistance increased with the filtration process [43]. It was observed that the permeate flux decreasing with time was more apparent for the 0.5, 0.2 and 0.05 μm membranes. According to Table 1, the Rm/RT × 100% of raw wastewater membrane fouling are 7.84%, 12.87% and 18.25%, respectively. With smaller pore size, the membrane resistance becomes higher. Rr/RT × 100% of raw wastewater membrane fouling are 37.79%, 44.38% and 46.17%, respectively, indicating that larger membrane pore size results in a lower hydraulically reversible resistance, which can be cleaned by physical method. Accordingly, the hydraulically irreversible resistance Ri/RT × 100% of raw wastewater membrane treatment are 54.37%, 42.75% and 35.58%, respectively. Therefore, a higher amount of solutes and foulants is convectively driven towards the larger pore size membrane surface. In this way, the larger membrane pore size enables more solutes to be adsorbed and causes more irreversible resistance. Accordingly, membranes with a larger pore size are prone to fouling [44]. Furthermore, for 0.5, 0.2 and 0.05 μm ceramic membranes used in raw wastewater treatment, the observed reduction was sharp and abrupt at the beginning of the experiment (for the first 30 min), followed by a steady permeate flux. As discussed, the adsorption of foulant on the membrane surface and the rapid pore blockage lead to the sharp decline in the permeate flux during the first 30 min of the process [45]. With increasing time, the permeate flux becomes more stable.
Fig. 4. Effect of ceramic membrane with different pore sizes on turbidity and rejection rate of pretreated water (TMP = 0.10 MPa, CFV = 3.0 m/s).
Fig. 5. Flux of pretreated water changed with time at different TMPs (CFV = 3.0 m/s).
The steady fluxes of 0.5, 0.2, 0.05 and 0.02 μm ceramic membranes were 148.4, 124.3, 160.5 and 91.8 L·m− 2·h−1, respectively, which accounted for the proportion of pure water flux of 11%, 17%, 27% and 64%, respectively. This means that the 0.5, 0.2, and 0.05 μm membranes were subjected to serious fouling. Moreover, based on the steady flux results, the ceramic membrane of 0.05 μm pore size was more suitable to treat the wastewater. However, the Rm/RT, Rr/RT, and Ri/RT of 0.02 μm raw wastewater ceramic membrane were 47.6%, 49.56% and 2.84%, respectively and the reduction was slightly attributed to the different fouling mechanisms as compared to the other membranes. The membrane intrinsic resistance and reversible resistance are the main resistances for 0.02 μm ceramic membrane. However, with the pre-treatment of coagulation–flocculation, the permeate flux shown in Fig. 3b was more smooth than the flux shown in Fig. 3a, and a steady permeate flux was observed in the first 30 min. The decrease in flux was caused by the settlement of flocs on the membrane surface which formed the cake layer [46]. The combined process using coagulation–flocculation with PAFC and UF showed higher permeate fluxes compared with UF of raw wastewater permeate flux under the same TMP and CFV. Thus the membrane coupled with coagulation–flocculation had a significant performance in the permeate flux. The pretreatment can be efficiently used for the removal of natural organic matter and reduction of membrane fouling. From Table 1, the Rr/RT × 100% of the ceramic membrane of the pre-treated wastewater was higher than that without the pretreatment. This is due to the increase of particles in wastewater and their accumulation on the
Fig. 6. Flux of pretreated water changed with time at different CFVs (TMP = 0.10 MPa).
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Table 2 The results of wastewater quality after each process.
Raw Pretreatment UF NF
Table 3 EDX of the fouled UF membrane surface.
Turbidity (NTU)
COD (mg/L)
TSS (mg/L)
TOC (mg/L)
Weight%
C
O
Na
Cl
Ca
K
Fe
Al
11,549 6461 5.70 0.03
9850 6637 2179 40.6
8945 2464 0.55 0
5057 2174 1056 14.8
Case 1 Case 2
43.97 51.26
52.66 37.33
1.11 1.46
1.06 4.18
0.69 0.38
0.52 0.79
1.59
3.01
membrane surface, thus justifying the use of PAFC as a coagulant prior to the UF step for wastewater treatment. The effect of ceramic membrane with different pore sizes on turbidity and rejection rate of treated water is presented in Fig. 4. UF was found to be very efficient in removing turbidity. As expected, the reduction in turbidity of the raw wastewater was nearly 100%. However, with the increase of membrane pore size, the removal of turbidity was decreased; the treated water turbidities of 0.2 and 0.5 μm were greater than 1 NTU which cannot meet the requirement of the feed water for NF. We also considered the influence of TOC and COD removal by UF membrane, but it was not as obvious as the impact on the permeate flux. Comparing to the 0.02 and 0.05 μm ceramic membranes, although the treated water turbidity of 0.02 μm is better than 0.05 μm, the steady permeate flux of 0.02 μm is only one-third of 0.05 μm steady permeate flux. Therefore, considering both the membrane flux and water turbidity, 0.05 μm ceramic membrane is suitable for the membrane integrated process.
3.3. Effects of TMP and CFV on permeate flux Fig. 5 shows the TMP of 0.05 μm ceramic membrane ranging from 0.05 to 0.20 MPa. We found that the initial permeate flux increased with the increasing TMP. The reason is that the TMP played an important role in UF. The flux declined during the initial stages of filtration for 0.15 and 0.20 MPa. However, the permeate flux under 0.05 and 0.10 MPa had small fluctuations and the steady permeate flux of 0.10 MPa was higher than the other permeate flux. This could be attributed to the critical flux. Studies have suggested that the flux increased first linearly with TMP independently of velocity and then leveled off to a plateau. Once the critical flux is reached, lots of foulants build up at the membrane surface. Furthermore, particles would direct into membrane pore with the increased pressure, and flux declined sharply at 0.15 and 0.20 MPa during the 30 min before becoming stable. Also, the membrane fouling will become more serious and lead to hydraulically irreversible resistance, which make membrane harder to be cleaned [47]. The CFV is a factor that significantly influences the separation properties and permeates quality, where larger hydrodynamic shear was
produced at higher CFV, causing disturbance to the foulants on the membrane surface. As presented in Fig. 6, it can be seen that the steady flux increases with CFV. Generally the increase of CFV will lead to increase in the shear stress flow, which sweeps more deposited foulants on the membrane away, reducing the filtration fouling resistance effectively [48]. It also reduces the impact of concentration polarization. When the CFV was raised up from low velocity to 3.0 m/s, the permeate flux was also increased. However, the steady permeate flux grew little at 4.0 m/s, which means residual deposited foulants cannot be swept off by increasing shear stress. In addition, the high velocity also consumes more energy [49]. According to the above results, 0.10 MPa and 3.0 m/s were selected for the membrane filtration experiment.
3.4. NF process for treating the following wastewater Table 2 presents the wastewater characteristics of the coagulation– flocculation process to reduce COD, TSS, turbidity and TOC to about 50%. In the following UF stage, the permeate characterization and the efficiency of the UF membrane process to reduce COD and turbidity, and the results obtained with the UF membrane process show a reduction in the range of 51–99%. From those results, the most relevant conclusion that can be drawn is the fact that, although treatment of the instant tea wastewater by coagulation–flocculation and UF provides relatively high reduction of turbidity and TSS, the rejection of COD and TOC is clearly insufficient to comply with the legislation requirement for effluent discharge. Under these circumstances, it was decided to use smaller pore size membrane such as NF membranes to reject the remaining low molecular weight organic matters contributed to the rest COD dissolved in wastewater. In the NF process, the flux of NF membrane has a small fluctuation, indicating that salt ions have a contribution to the fouling formation on the membrane surface. However, this foulants can be easily cleaned by deionized water. Table 2 also shows the results of wastewater characterization after the NF process. From the table, the removal ratios of turbidity, COD and the TOC were all over 99.9%, and the total suspended solids could not be detected in the water. The characterization of permeate after NF complies to China standard [50]. The concentrate fluid with high COD and turbidity (large amount of tea polyphenol, tea polysaccharide and impurities, etc.) could be treated by advanced oxidation combined with biological treatment processes [51].
Fig. 7. FE-SEMs of ceramic UF membrane, surface. (a) New membrane. (b) Fouled membrane.
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bond. The peak at 1695 cm− 1 and 1447 cm−1can be ascribed to the C_O and C_C bonds that originated from benzene ring, respectively. Resonance at 1024 cm−1 can be assigned to the C–O bond. All of the peaks represent the main functional group of instant tea such as tea polyphenols, saccharide, and caffeine, which indicate the presence of organic material deposit on the membrane surface [54].
Fig. 8. IR spectra of the foulants scraped from the membrane.
3.5. Membrane cleaning 3.5.1. Fouling morphology and structure Fig. 7 presents the SEM images for the 0.05 μm ceramic membrane surface under the condition of 0.1 MPa TMP and 3.0 m/s CFV. Comparing to the relatively smooth new membrane surface (Fig. 7a), the fouled membrane (Fig. 7b) surface displayed an uneven surface which was similar to the surface of earth covering with ravine crossbar. This phenomenon is caused by the crack of the organic foulants on the membrane after filtration induced by drying at 80 °C for 8 h. Another reason is that the pressure filtration process shows a very high tendency to foulant cracking [52]. From Fig. 7b, we can see a thick foulant cake layer which means that the membrane was heavily polluted. Table 3 shows the EDX results of the fouled membrane surface, case 1 is the membrane foulants without pretreatment, and case 2 is the wastewater fouling with pretreatment (coagulation–flocculation). In both cases, carbon and oxygen are the highest weight element originated from the main constitution of organic wastewater while other low amount ones (eg. Na, Ca and K) come from the wastewater due to salt deposition, a common process in wastewater [53]. In case 1, Fe and Al were not detected, and Cl element in case 1 was less than that in case 2. Therefore Fe, Al, and partly Cl were mainly derived from the flocculant PAFC. Fig. 8 illustrates the FTIR spectra of the foulants. A strong peak at 3380 cm− 1 corresponds to O–H bond, peaks at 2912 and 2834 cm− 1 can be assigned to the symmetry flex vibration of C–H
3.5.2. Cleaning method Three different cleaning methods were applied and the cleaning results of different methods are given in Fig. 9. From the results, the method of hydraulic flushing in 0.02 μm could achieve high FRR of 94.4%. However, the FRR of 0.05 and 0.2 μm ceramic membranes is around 40%, while 0.5 μm ceramic membrane showed much lower FRR. This phenomenon is ascribed to the different fouling mechanisms, according to resistance-in-series model, the main resistance of 0.02 μm ceramic membrane is the hydraulically reversible resistance, which can be cleaned by the water flushing method. However, the irreversible resistance was a predominant issue in the other three membranes. Chemical method must be adopted for membrane washing. From Fig. 9, the most effective method was to use 0.5 wt.% NaClO as the cleaning agent, achieving 99.0% FRR for all the fouled membranes. In contrast, the fouling of NF membrane was insignificant compared to that of the UF membrane. The NF membrane foulants, on the other hand, can be cleaned up by deionized water flushing which is not in the scope of the study. 4. Conclusion In this study, the combinations of coagulation–membrane treatments were applied to the instant tea powder factory wastewater. The results highlighted by this work have shown that coagulation was an excellent pre-treatment which can increase the permeate flux. The optimum results were obtained in terms of COD removal (77.3%) and turbidity reduction (99.8%) for coupling coagulation–flocculation and UF. However, a few low molecular weight organic matters that contributed to the remaining COD were still dissolved in the wastewater which could be removed by a subsequent NF process. The removal of turbidity, COD and the TOC was all over 99.9%, and the total suspended solids were not detected in the water. The characterization of sewerage after NF meets China standard (integrated wastewater discharge standard GB 8978-1996). The optimum parameter of coagulation is at the dosage of 800 mg/L, pH 5 and 20 °C, and under those conditions, the permeate flux of wastewater of ceramic membrane showed great improvement. 0.5 wt.% NaClO was found to be the most effective cleaning agent. Furthermore, this work represents the first successful application of coagulation–flocculation, UF and NF in the treatment of instant tea powder factory wastewater. Acknowledgments The financial support comes from the National Natural Science Foundation of China (no. 21276124), the Key Projects in the National Science & Technology Pillar Program (no. 2011BAE07B09-3), and the Jiangsu Province Industrial Supporting Project (no. BE2012399). References
Fig. 9. Effects of different cleaning methods.
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Integrated membrane process for wastewater treatment from production of instant tea powders Bin Chen a, Xiaohui Xiong a, Zhong Yao a, Na Yin b, Ze-Xian Low c, Zhaoxiang Zhong a,⁎ a b c
College of food science and light industry, Nanjing University of Technology, National Engineering Research Center for Special Separation Membrane, Nanjing 210009, PR China Applied Chemistry & Environmental Engineering faculty, Bengbu College, Bengbu 233030, PR China Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia
H I G H L I G H T S • Design an integrated process to treat wastewater from the production of instant tea powders • Optimize operating parameters and analyze separation mechanism to improve the water quality • Propose an effective cleaning method with a flux recovery ratio higher than 99%
a r t i c l e
i n f o
Article history: Received 16 June 2014 Received in revised form 6 October 2014 Accepted 21 October 2014 Available online xxxx Keywords: Integrated membrane process Instant tea wastewater Fouling mechanism
a b s t r a c t This study investigated an integrated process to treat the wastewater from an instant tea powder factory. The wastewater was pretreated by combination of coagulation–flocculation. Polyaluminum ferric chloride (PAFC) was used as the coagulating agent. The optimum operating conditions with respect to contaminant removal were determined to be pH 5, 20 °C and 800 mg/L of PAFC, with polyacrylamide (PAM) as coagulant aid at a dosage of 20 mg/L. The discharged water from the coagulation–flocculation was treated with ceramic UF membranes. It was found that interactions between the molecules of PAFC and organic foulants reduced the membrane fouling and improved the removal efficiencies effectively. The permeate flux of UF membrane was almost three times higher than typical flux obtained without pretreatment. In the subsequent NF process, the removal efficiencies of turbidity, COD and TOC were all higher than 99.9%, and the TSS wasn't detected in the permeate. The effects of cleaning methods on permeate flux were also investigated. It was found out that 0.5 wt.% NaClO flushing produced the highest flux recovery ratio of 99% and showed good reproducibility. This study showed the feasibility of applying coagulation–flocculation, UF and NF in the treatment of instant tea powder factory wastewater. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Tea is the second most commonly consumed drink in the world. It is a popular, widely accepted, pleasant, economical and safe drink that is enjoyed every day by hundreds of millions of people across all continents [1]. As one type of tea products, instant tea is consumed regularly around the world and therefore constitutes an important pillar in human consumption. Its popularity is due mostly to its availability of varieties of tastes. Moreover, instant tea is also consumed for health reasons because it contains large quantity of tea polyphenols, saccharide, protein, caffeine, vitamins, inorganic elements and trace elements such as physiological active element [2,3]. Nowadays instant tea powder is manufactured from processed leaves, tea wastes or fermented leaves via digestion, followed by disc separation and vacuum concentration ⁎ Corresponding author. E-mail address: [email protected] (Z. Zhong).
http://dx.doi.org/10.1016/j.desal.2014.10.029 0011-9164/© 2014 Elsevier B.V. All rights reserved.
before drying to form powder. All the equipments from the production process of instant tea powder need to be washed by water, so the wastewater produced may contain substances such as salt, alkali, acids, organic additives, dissolved solids and suspended solids. Therefore, more instant tea factories are under increasing pressure to reduce the environmental impact of their wastewater streams. Wastewater must be properly treated to comply with the national standards for discharge into surface water or reuse for factory application [4]. Few researches focus on the treatment of instant tea wastewater. The mechanism of such wastewater is assumed to be similar to that of organic wastewater, because the main components are also organics. Several common techniques are used in the treatment of organic wastewater, such as coagulation, flocculation, conventional biological treatment and advanced oxidation processes [5–7]. Low efficiency, high operating cost, prone to corrosion and recontamination are the general disadvantages of these methods. For example, conventional biological treatment requires a long reaction time and has a large footprint [8]. Additionally, a high
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content of non-reacted chemicals is consumed in the wastewater treatment [9]. In the last decade, membrane processes have attracted lots of attention over conventional separation processes, due to their high efficiency, high quality product, ease of operation, low footprint and low energy requirement comparing with biological and chemical treatment methodologies. Membrane technology (especially UF and NF) has often been considered as a promising method for water reuse and wastewater clean-up. The UF membrane is useful for the removal of suspended solids, organic components and microorganisms larger than its pore size. Also, UF acts as a disinfection barrier by removing bacteria as well as viruses; however, UF does not remove low molecular weight and soluble compounds. Organic compounds with low molecular weight can be removed by using NF membranes. Membrane technology has been applied in a wide variety of fields, such as pharmaceutical industry [10], dairy and food technology [11], pulp and paper industry [12], textile industry [13] and a wide diversity of other new applications [14]. However, membrane fouling, which will remarkably deteriorate membrane permeability and consequently result in high energy consumption, has become a major obstacle to further the application of UF technology in wastewater treatment [15]. The dissolved organics on the membrane surface are considered to be the most important foulants in the processes of microfiltration (MF) and UF [16]. Moreover, the fouling results in deterioration of membrane performance, which increases the use of chemical cleaning agents and energy consumptions, ultimately increasing the maintenance and operation costs [17]. Membrane fouling is caused by the interactions of membrane and foulant, and also by foulant–foulant interactions which results in flux decline [18]. The internal pore blocking by the retained molecules may lead to a sharp and rapid flux decline. The further flux decline may be attributed to the formation and subsequent growth of the gel and cake layer, which come from the deposition and accumulation of rejected particles [19]. The common main foulants include dissolved organic or inorganic components, colloidal matter, soluble microbial products, bacteria and suspended solids [20]. In the case of instant tea wastewater treatment, organic fouling is a significant issue. To a large extent, the size of organic matter, which falls in the lower region of the organic matter size distribution, is smaller than the pore size of UF membrane. Thus, the dissolved and colloidal organic matter may permeate through the membrane and cause solute adsorption onto the pore walls. Consequently, pre-treatment has been used to alleviate the membrane fouling, such as pre-filtration [21], oxidation [22], adsorption [23], and coagulation/flocculation [24]. Because of the low capital cost and excellent performance, coagulation/flocculation has been regarded as one of the most successful pre-treatment [25]. High dissolved organic
components can be pre-removed by proper selection of coagulant/flocculant aids, initial pH, dosage of coagulant and temperature. After pre-treatment via coagulation/flocculation, an integrated membrane process was used in the present study to treat the wastewater from an instant tea factory, and the complex ceramic membrane fouling was also examined. In the treatment of coagulation/flocculation, the optimizations of the parameters such as initial pH, dosage of coagulant, and temperature were studied. The effects of transmembrane pressure (TMP) and cross-flow velocity (CFV) were also studied. Moreover, experimental and instrumental methods were coupled with modeling to analyze the resistances and fouling in the UF process. Based on these results, an effective cleaning method was developed to clean the fouled membranes. 2. Materials and methods 2.1. Experimental setup Fig. 1 is a schematic representation of the experimental set-up, including both UF and NF. In the UF process, the feed wastewater was filtered through the membrane by a centrifugal pump at different pressures. The TMP was monitored with a manometer. In the experimental process, the valves A, C and E were closed and the retentate was recirculated to the feed tank, while the penetrants were collected from valve D. The setup was operated in a continuous or recycling mode, the permeated solution was removed or recycled continuously and the membranes were fouled by the retentate which was recycled to the feed tank in each mode. Four different 19-channel zirconia ceramic UF membranes with average pore size of 0.5, 0.2, 0.05 and 0.02 μm (corresponding pure water flux was 1312.5, 780.4, 604.3 and 201.1 L·m−2·h−1, respectively; 0.1 MPa, 20 °C) were used in this UF process study (Jiuwu High-Tech Co. Ltd., Jiangsu, China). The membrane area for mass transfer is 0.11 m2 with channel diameter of 4 mm. Also, the NF set-up had a similar working mode. The NF polyamide composite spiral-wound membrane (NF90-2540; pure water flux was 56.3 L·m−2·h−1; 1.0 MPa, 20 °C) was purchased from Dow Chemical Company (USA). 2.2. Wastewater characteristics and measuring method Turbidity was measured by HACH turbidity meter (2100N), and COD was determined by the fast digestion spectrophotometric method followed by the Chinese National Method [26]. Total organic carbon (TOC) was measured by Shimadzu (ASI-V). After a continuous mode, the fouled ceramic membrane was broken into small samples and
10 P
4 5 P T
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T
I
11
J
D H
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p
4 8
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G P-38
(a)
(b)
Fig. 1. Setup of UF (a) and NF (b) processes: (1) feed tank, (2) centrifugal pump,(3) rotameter, (4) manometer, (5) thermometer, (6) UF membrane, (7) feed tank, (8) centrifugal pump, (9) HP pump, (10) manometer, (11) thermometer, (12) NF membrane, and (13) rotameter.
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then dried at 80 °C for 8 h. The foulants on the ceramic membrane were scraped and collected, and then dried in the same way. The morphologies of collected foulants were characterized and analyzed by field emission scanning electron microscope (FE-SEM) and energydispersive X-ray spectroscopy (EDX; S-4800, Hitachi, Japan). The foulants on ceramic membranes were also determined by FTIR Avatar 360 (Thermo Nicolet, USA). The instant tea wastewater was obtained from Nanjing Rongdian Foodstuff Co. Ltd. The wastewater was mainly comprised of dissolved organic components and colloidal matter with the following characteristics — COD (5000–40,000 mg/L), turbidity (1000–10,000 NTU) and TOC (3000–20,000 mg/L). The detailed values of wastewater characteristics are shown as follows: turbidity (11,549 NTU), COD (9850 mg/L), pH 7.5 and TOC (5057 mg/L), and the concentration of tea polyphenol (9.39 g/kg) and caffeine (6.52 g/kg). In the pre-treatment step, variable doses of coagulant agent such as polyaluminum ferric chloride (PAFC; 200–1200 mg/L) and flocculant anionic polyacrylamide (PAM; 20 mg/L) were added to wastewater at different pH values which ranged between 4 and 8. The coagulant solutions were added to the wastewater and the mixture was stirred at a rapid speed of 200 rpm for 3 min. Then PAM was added at a low stir speed of 30 rpm for 15 min. After that, the flocs were formed and allowed to settle for 30 min. The optimal conditions of coagulant/flocculant were determined in terms of coagulant and flocculant dosing and pH, considering the COD and turbidity removal. The COD and turbidity removal rate, R, are defined as (C0 − C1) / C0, where C0 stands for the COD and turbidity of raw wastewater and C1 stands for the COD and turbidity of pre-treated wastewater. After each filtration run, different cleaning methods evaluated by the flux recovery ratio, were investigated, including cleaning by pure water, NaOH and HNO3 solution, and NaClO solution. The fouled membranes were cleaned in a recycling mode, which was shown in Fig. 1a. Pump provides the energy for heating. Unless otherwise indicated, the detailed cleaning conditions were at temperature of 55 °C for 45 min at low TMP (0.01 MPa) and high CFV (5 m/s). The fouling degree m = (J0 − J1) / J0 × 100% and the flux recovery ratio FRR = (J2 / J0) × 100% were used to determine the anti-fouling performance of each cleaning method [27], where J0 and J1 refer to the water fluxes of membranes before and after fouling, respectively, and J2 stands for the water flux of cleaned membrane. During the filtration of the wastewater, according to the resistancein-series model [28], the main resistances including both hydraulically reversible resistance and irreversible resistance are determined. The flux of permeate (Jp) follows Eq. (1) JP ¼
ΔP ΔP ΔP ¼ ¼ μ w RT μ w ðRm þ R f Þ μ w ðRm þ Ri þ Rr Þ
ð1Þ
where μw is the permeate viscosity (10−3 Pa·s at 25 °C), ΔP is the transmembrane operating pressure (Pa). RT is the total resistance to solvent permeation, defined as the sum of the membrane resistance (Rm) and the total fouling resistance (Rf). Herein, Rf is provided by the sum of hydraulically irreversible resistance (Ri) and the hydraulically reversible resistance (Rr). Rm was determined from the intrinsic membrane permeability. RT was obtained from the steady-state permeate flux, and the sum of Rm and Ri was obtained from the pure water flux measurements performed after rinsing. Rr was finally determined by RT − Rm − Ri [29]. 3. Result and discussion 3.1. Effects of PAFC dosage, pH and temperature in coagulation/flocculation process To reduce the fouling on the membrane and further improve the efficiency of membrane filtration, the coagulation–flocculation process was used as the pre-treatment. Due to the reduction in the surface
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ionic charge of the organic suspended solids and colloid, the coagulation–flocculation process causes the agglomeration of organic suspended solids and colloidal matter, which form large aggregates (known as flocs). The flocs then contact with each other, further increasing in size. When these flocs become larger, they can be removed through sedimentation. Also the larger size flocs cannot flow easily through the membrane [30]. Fe-salt and Al-salt are the most commonly used coagulants in water treatment. PAFC is developed by partially neutralizing AlCl3 and FeCl3, which has been widely used [31–33]. Thus, PAFC has the advantages of both polyaluminum and polyferric coagulants, which includes low price, large particle size, wide pH range and excellent adsorption ability. Fig. 2 shows the variations of wastewater turbidity and COD at different PAFC concentrations, pH values and temperatures. As plotted in Fig. 2a, the percentage COD reduction of wastewater increases with the PAFC dosage, and the COD reduction efficiency nearly reached a plateau when the dosage was higher than 800 mg/L. This can be explained by the role of aluminum and iron salts as coagulant which can generally be divided into two distinct mechanisms: charge neutralization of negatively charged colloids by cationic hydrolysis products and incorporation of impurities in an amorphous hydroxide precipitate (‘sweep flocculation’) [34]. On the one hand, when the dosage of Fe3+ in PAFC increases, the zeta potential of the wastewater after coagulation would increase [35], illustrating that the positively charged ferric was growing with increasing PAFC dosage, and thus charge neutralization between ferric and negatively charged phenolic hydroxyl group of organic material was enhanced with the increase of PAFC dosage, leading to the increase of COD removal efficiency. On the other hand, the polyaluminum would change to hydroxide precipitating gradually until saturation [36]. From Table 2, the negatively charged suspended solids reacted with PAFC, which settle with other impurities under the action of gravity, which can be reflected from the removal rate of TSS (72.5%). However, when PAFC dosage increased beyond 800 mg/L, there was a little excess of PAFC in wastewater, which could lead to restabilization of colloids, thus decreasing turbidity removal efficiency. The removal of turbidity and COD at 800 mg/L PAFC was also studied at various pH values, as plotted in Fig. 2b. The reduction of COD and turbidity was found to be the most effective at pH 5 as efficiency was reduced beyond pH 5. Studies have suggested that the dominating mechanism for organic matter coagulation–flocculation is dictated by the nature of the hydrolyzed Al and Fe [37]. In alkaline conditions, the hydrolysis of PAFC was enhanced, which resulted in lower positive iron salt [38]. The solubility products of Fe3 + and Al3 + are shown in Eq. (2) h i h i 3þ − 3 ; KSP1 ¼ Fe OH
h i h i 3þ − 3 KSP2 ¼ Al OH
ð2Þ
where the Ksp1 and Ksp2 stand for the solubility constant of the ferric hydroxide and aluminum hydroxide, respectively. Increase in Fe3 + and Al3+ or OH− ion concentration beyond Ksp1 and Ksp2 value will lead to the formation of the hydroxide [39]. Meanwhile, the functional groups (carboxylic or phenolic acids) of natural organic material in alkaline region were deprotonated, which led to the increase of negative charge of the organic material [40]. It was also observed that COD and turbidity removal efficiencies were lower at pH 4 than that at pH 5. This was because the organic wastewater was neutral, while Al3+ in PAFC was positively charged at low pH. When PAFC reacted with organic material in wastewater, small portion of the complexes undergone a charge reversal mechanism (from negative to positive charge) [41], which caused the repulsion between organic material and PAFC. As a result, the removal of the complexes was weakened at pH b5. Moreover, Fig. 2c shows the effect of temperature on the removal of turbidity and COD. Temperature is an important factor which affects the coagulation efficiency. Based on the turbidity and COD reduction, flocs formed at temperature beyond 20 °C were much poorer. It was suggested that the high temperature may result in poor coagulation performance as
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Fig. 2. The effect of PAFC dosage (pH = 5, 20 °C) (a), pH (PAFC dosage = 800 mg/L, 20 °C) (b) and temperature (PAFC dosage = 800 mg/L, pH = 5) (c) of coagulation–flocculation process.
particles move quickly at higher temperature [42]. Also, high temperature influences the hydrolysis of coagulant and solubility of Al and Fe hydroxides which affect the coagulation performance. Moreover, a more important reason is that the coagulation of PAFC becomes
unstable at higher temperature due to faster motion of organic molecules which result in deflocculation. The pretreatment process can be efficiently used for the removal of natural organic matter and to relieve the burden to the following UF treatment.
Fig. 3. Effect of membrane pore size on permeate flux. (a) Raw wastewater permeate flux. (b) Pre-treated wastewater permeate flux (TMP = 0.1 MPa, CFV = 3.0 m/s).
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Table 1 Resistances of four different pore size membranes. Wastewater
Raw wastewater
Membrane pore size/μm
0.5
0.2
0.05
0.02
0.5
Pre-treated wastewater 0.2
0.05
0.02
Rm/RT × 100% Rr/RT × 100% Ri/RT × 100%
7.84 37.79 54.37
12.87 44.38 42.75
18.25 46.17 35.58
47.6 49.56 2.84
8.63 53.25 38.12
12.15 67.21 20.64
21.7 62.08 16.22
45.32 52.54 2.14
3.2. Effects of membrane pore size Fig. 3a shows the raw wastewater permeate flux with 0.5, 0.2, 0.05 and 0.02 μm ceramic membranes at TMP of 0.1 MPa and CFV of 3.0 m/s, and Fig. 3b shows the pre-treated wastewater permeate flux under the same condition. From Fig. 3a, it can be observed that the initial flux decreases with the size of membrane pore, and generally, permeate flux decreased at the beginning of the experiment. This behavior is commonly observed in the UF process, indicating that a higher amount of foulants is driven towards the membrane surface leading to the flux decline. Consequently, the total filtration resistance increased with the filtration process [43]. It was observed that the permeate flux decreasing with time was more apparent for the 0.5, 0.2 and 0.05 μm membranes. According to Table 1, the Rm/RT × 100% of raw wastewater membrane fouling are 7.84%, 12.87% and 18.25%, respectively. With smaller pore size, the membrane resistance becomes higher. Rr/RT × 100% of raw wastewater membrane fouling are 37.79%, 44.38% and 46.17%, respectively, indicating that larger membrane pore size results in a lower hydraulically reversible resistance, which can be cleaned by physical method. Accordingly, the hydraulically irreversible resistance Ri/RT × 100% of raw wastewater membrane treatment are 54.37%, 42.75% and 35.58%, respectively. Therefore, a higher amount of solutes and foulants is convectively driven towards the larger pore size membrane surface. In this way, the larger membrane pore size enables more solutes to be adsorbed and causes more irreversible resistance. Accordingly, membranes with a larger pore size are prone to fouling [44]. Furthermore, for 0.5, 0.2 and 0.05 μm ceramic membranes used in raw wastewater treatment, the observed reduction was sharp and abrupt at the beginning of the experiment (for the first 30 min), followed by a steady permeate flux. As discussed, the adsorption of foulant on the membrane surface and the rapid pore blockage lead to the sharp decline in the permeate flux during the first 30 min of the process [45]. With increasing time, the permeate flux becomes more stable.
Fig. 4. Effect of ceramic membrane with different pore sizes on turbidity and rejection rate of pretreated water (TMP = 0.10 MPa, CFV = 3.0 m/s).
Fig. 5. Flux of pretreated water changed with time at different TMPs (CFV = 3.0 m/s).
The steady fluxes of 0.5, 0.2, 0.05 and 0.02 μm ceramic membranes were 148.4, 124.3, 160.5 and 91.8 L·m− 2·h−1, respectively, which accounted for the proportion of pure water flux of 11%, 17%, 27% and 64%, respectively. This means that the 0.5, 0.2, and 0.05 μm membranes were subjected to serious fouling. Moreover, based on the steady flux results, the ceramic membrane of 0.05 μm pore size was more suitable to treat the wastewater. However, the Rm/RT, Rr/RT, and Ri/RT of 0.02 μm raw wastewater ceramic membrane were 47.6%, 49.56% and 2.84%, respectively and the reduction was slightly attributed to the different fouling mechanisms as compared to the other membranes. The membrane intrinsic resistance and reversible resistance are the main resistances for 0.02 μm ceramic membrane. However, with the pre-treatment of coagulation–flocculation, the permeate flux shown in Fig. 3b was more smooth than the flux shown in Fig. 3a, and a steady permeate flux was observed in the first 30 min. The decrease in flux was caused by the settlement of flocs on the membrane surface which formed the cake layer [46]. The combined process using coagulation–flocculation with PAFC and UF showed higher permeate fluxes compared with UF of raw wastewater permeate flux under the same TMP and CFV. Thus the membrane coupled with coagulation–flocculation had a significant performance in the permeate flux. The pretreatment can be efficiently used for the removal of natural organic matter and reduction of membrane fouling. From Table 1, the Rr/RT × 100% of the ceramic membrane of the pre-treated wastewater was higher than that without the pretreatment. This is due to the increase of particles in wastewater and their accumulation on the
Fig. 6. Flux of pretreated water changed with time at different CFVs (TMP = 0.10 MPa).
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Table 2 The results of wastewater quality after each process.
Raw Pretreatment UF NF
Table 3 EDX of the fouled UF membrane surface.
Turbidity (NTU)
COD (mg/L)
TSS (mg/L)
TOC (mg/L)
Weight%
C
O
Na
Cl
Ca
K
Fe
Al
11,549 6461 5.70 0.03
9850 6637 2179 40.6
8945 2464 0.55 0
5057 2174 1056 14.8
Case 1 Case 2
43.97 51.26
52.66 37.33
1.11 1.46
1.06 4.18
0.69 0.38
0.52 0.79
1.59
3.01
membrane surface, thus justifying the use of PAFC as a coagulant prior to the UF step for wastewater treatment. The effect of ceramic membrane with different pore sizes on turbidity and rejection rate of treated water is presented in Fig. 4. UF was found to be very efficient in removing turbidity. As expected, the reduction in turbidity of the raw wastewater was nearly 100%. However, with the increase of membrane pore size, the removal of turbidity was decreased; the treated water turbidities of 0.2 and 0.5 μm were greater than 1 NTU which cannot meet the requirement of the feed water for NF. We also considered the influence of TOC and COD removal by UF membrane, but it was not as obvious as the impact on the permeate flux. Comparing to the 0.02 and 0.05 μm ceramic membranes, although the treated water turbidity of 0.02 μm is better than 0.05 μm, the steady permeate flux of 0.02 μm is only one-third of 0.05 μm steady permeate flux. Therefore, considering both the membrane flux and water turbidity, 0.05 μm ceramic membrane is suitable for the membrane integrated process.
3.3. Effects of TMP and CFV on permeate flux Fig. 5 shows the TMP of 0.05 μm ceramic membrane ranging from 0.05 to 0.20 MPa. We found that the initial permeate flux increased with the increasing TMP. The reason is that the TMP played an important role in UF. The flux declined during the initial stages of filtration for 0.15 and 0.20 MPa. However, the permeate flux under 0.05 and 0.10 MPa had small fluctuations and the steady permeate flux of 0.10 MPa was higher than the other permeate flux. This could be attributed to the critical flux. Studies have suggested that the flux increased first linearly with TMP independently of velocity and then leveled off to a plateau. Once the critical flux is reached, lots of foulants build up at the membrane surface. Furthermore, particles would direct into membrane pore with the increased pressure, and flux declined sharply at 0.15 and 0.20 MPa during the 30 min before becoming stable. Also, the membrane fouling will become more serious and lead to hydraulically irreversible resistance, which make membrane harder to be cleaned [47]. The CFV is a factor that significantly influences the separation properties and permeates quality, where larger hydrodynamic shear was
produced at higher CFV, causing disturbance to the foulants on the membrane surface. As presented in Fig. 6, it can be seen that the steady flux increases with CFV. Generally the increase of CFV will lead to increase in the shear stress flow, which sweeps more deposited foulants on the membrane away, reducing the filtration fouling resistance effectively [48]. It also reduces the impact of concentration polarization. When the CFV was raised up from low velocity to 3.0 m/s, the permeate flux was also increased. However, the steady permeate flux grew little at 4.0 m/s, which means residual deposited foulants cannot be swept off by increasing shear stress. In addition, the high velocity also consumes more energy [49]. According to the above results, 0.10 MPa and 3.0 m/s were selected for the membrane filtration experiment.
3.4. NF process for treating the following wastewater Table 2 presents the wastewater characteristics of the coagulation– flocculation process to reduce COD, TSS, turbidity and TOC to about 50%. In the following UF stage, the permeate characterization and the efficiency of the UF membrane process to reduce COD and turbidity, and the results obtained with the UF membrane process show a reduction in the range of 51–99%. From those results, the most relevant conclusion that can be drawn is the fact that, although treatment of the instant tea wastewater by coagulation–flocculation and UF provides relatively high reduction of turbidity and TSS, the rejection of COD and TOC is clearly insufficient to comply with the legislation requirement for effluent discharge. Under these circumstances, it was decided to use smaller pore size membrane such as NF membranes to reject the remaining low molecular weight organic matters contributed to the rest COD dissolved in wastewater. In the NF process, the flux of NF membrane has a small fluctuation, indicating that salt ions have a contribution to the fouling formation on the membrane surface. However, this foulants can be easily cleaned by deionized water. Table 2 also shows the results of wastewater characterization after the NF process. From the table, the removal ratios of turbidity, COD and the TOC were all over 99.9%, and the total suspended solids could not be detected in the water. The characterization of permeate after NF complies to China standard [50]. The concentrate fluid with high COD and turbidity (large amount of tea polyphenol, tea polysaccharide and impurities, etc.) could be treated by advanced oxidation combined with biological treatment processes [51].
Fig. 7. FE-SEMs of ceramic UF membrane, surface. (a) New membrane. (b) Fouled membrane.
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bond. The peak at 1695 cm− 1 and 1447 cm−1can be ascribed to the C_O and C_C bonds that originated from benzene ring, respectively. Resonance at 1024 cm−1 can be assigned to the C–O bond. All of the peaks represent the main functional group of instant tea such as tea polyphenols, saccharide, and caffeine, which indicate the presence of organic material deposit on the membrane surface [54].
Fig. 8. IR spectra of the foulants scraped from the membrane.
3.5. Membrane cleaning 3.5.1. Fouling morphology and structure Fig. 7 presents the SEM images for the 0.05 μm ceramic membrane surface under the condition of 0.1 MPa TMP and 3.0 m/s CFV. Comparing to the relatively smooth new membrane surface (Fig. 7a), the fouled membrane (Fig. 7b) surface displayed an uneven surface which was similar to the surface of earth covering with ravine crossbar. This phenomenon is caused by the crack of the organic foulants on the membrane after filtration induced by drying at 80 °C for 8 h. Another reason is that the pressure filtration process shows a very high tendency to foulant cracking [52]. From Fig. 7b, we can see a thick foulant cake layer which means that the membrane was heavily polluted. Table 3 shows the EDX results of the fouled membrane surface, case 1 is the membrane foulants without pretreatment, and case 2 is the wastewater fouling with pretreatment (coagulation–flocculation). In both cases, carbon and oxygen are the highest weight element originated from the main constitution of organic wastewater while other low amount ones (eg. Na, Ca and K) come from the wastewater due to salt deposition, a common process in wastewater [53]. In case 1, Fe and Al were not detected, and Cl element in case 1 was less than that in case 2. Therefore Fe, Al, and partly Cl were mainly derived from the flocculant PAFC. Fig. 8 illustrates the FTIR spectra of the foulants. A strong peak at 3380 cm− 1 corresponds to O–H bond, peaks at 2912 and 2834 cm− 1 can be assigned to the symmetry flex vibration of C–H
3.5.2. Cleaning method Three different cleaning methods were applied and the cleaning results of different methods are given in Fig. 9. From the results, the method of hydraulic flushing in 0.02 μm could achieve high FRR of 94.4%. However, the FRR of 0.05 and 0.2 μm ceramic membranes is around 40%, while 0.5 μm ceramic membrane showed much lower FRR. This phenomenon is ascribed to the different fouling mechanisms, according to resistance-in-series model, the main resistance of 0.02 μm ceramic membrane is the hydraulically reversible resistance, which can be cleaned by the water flushing method. However, the irreversible resistance was a predominant issue in the other three membranes. Chemical method must be adopted for membrane washing. From Fig. 9, the most effective method was to use 0.5 wt.% NaClO as the cleaning agent, achieving 99.0% FRR for all the fouled membranes. In contrast, the fouling of NF membrane was insignificant compared to that of the UF membrane. The NF membrane foulants, on the other hand, can be cleaned up by deionized water flushing which is not in the scope of the study. 4. Conclusion In this study, the combinations of coagulation–membrane treatments were applied to the instant tea powder factory wastewater. The results highlighted by this work have shown that coagulation was an excellent pre-treatment which can increase the permeate flux. The optimum results were obtained in terms of COD removal (77.3%) and turbidity reduction (99.8%) for coupling coagulation–flocculation and UF. However, a few low molecular weight organic matters that contributed to the remaining COD were still dissolved in the wastewater which could be removed by a subsequent NF process. The removal of turbidity, COD and the TOC was all over 99.9%, and the total suspended solids were not detected in the water. The characterization of sewerage after NF meets China standard (integrated wastewater discharge standard GB 8978-1996). The optimum parameter of coagulation is at the dosage of 800 mg/L, pH 5 and 20 °C, and under those conditions, the permeate flux of wastewater of ceramic membrane showed great improvement. 0.5 wt.% NaClO was found to be the most effective cleaning agent. Furthermore, this work represents the first successful application of coagulation–flocculation, UF and NF in the treatment of instant tea powder factory wastewater. Acknowledgments The financial support comes from the National Natural Science Foundation of China (no. 21276124), the Key Projects in the National Science & Technology Pillar Program (no. 2011BAE07B09-3), and the Jiangsu Province Industrial Supporting Project (no. BE2012399). References
Fig. 9. Effects of different cleaning methods.
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