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Treatment of tanning effluent using nanofiltration followed by reverse osmosis
Separation and Purification Technology 50 (2006) 291–299
Treatment of tanning effluent using nanofiltration followed by reverse osmosis Chandan Das, Piyush Patel, Sirshendu De, Sunando DasGupta ∗ Department of Chemical Engineering, Indian Institute of Technology, Kharagpur-721302, India Received 28 May 2005; received in revised form 21 November 2005; accepted 28 November 2005
Abstract An investigation on the recovery of chromium from the effluent of a chrome-tanning bath has been performed using nanofiltration (NF) followed by reverse osmosis (RO). The experiments are conducted using a rectangular cross flow cell under laminar and turbulent regimes. Significant flux enhancement is achieved using thin wires as turbulent promoters. The performance criteria are evaluated in terms of the concentration of chromium, COD, BOD, TDS, TS, pH, and conductivity of the permeate. The effects of different operating parameters on permeate flux and observed retention of chromium are evaluated experimentally. The retention of chromium is found to be 91–98% for NF and 98.8–99.7% for RO for the experimental conditions of this study. Concentrations of chromium and COD of the final permeate are well within the permissible limits. © 2005 Elsevier B.V. All rights reserved. Keywords: Chromium recovery; Nanofiltration; Reverse osmosis; Retention; Turbulent promoter
1. Introduction Almost all leather made from lighter-weight cattle hides and from the skin of sheep, lambs, goats, and pigs is chrome tanned. Chrome tanning is performed using a one-bath process that is based on the reaction between the hide and a trivalent chromium salt, usually a basic chromium sulfate. In the typical one bath process, the hides are in a pickled state at a pH of 3 or lower, the chrome tanning materials are introduced, and the pH is raised. Following tanning, the chrome-tanned leather is piled down, wrung, and graded for the thickness and quality, split into flesh and grain layers, and shaved to the desired thickness. The grain leathers from the shaving machine are then separated for retanning, dyeing, and fat liquoring [1]. Extensive studies have been made to recycle the spent tanned liquor [2,3]. But recycling of chromium solution for tanning leads to accumulation of neutral salts, which reduces the uptake of chromium during tanning [4]. Electrodialysis may be useful for selective separation of neutral salts from spent tanned liquor. However, the economic viability of the technique is yet to be established [5]. The exhausted bath from chromium tannage contains about 30–40% of initial salt and it is nor-
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mally sent to a cleaning-up plant. Here chromium salts end into the sludge creating serious problems for their disposal [6]. The traditional method for chromium recovery is based on the precipitation of chromium salt with NaOH followed by the dissolution of Cr(OH)3 in sulfuric acid [7,8]. However, the quality of the recovered solutions is not always optimal due to the presence of metals, lipidic substances, and other impurities. In order to improve the quality of the recycled chromium, an alternative method using membrane processes was studied [9–11]. Membrane-based separation processes may be an attractive alternative and they are gradually emerging as a technically significant and commercially viable ‘cleaner technology’ for the treatment of wastewater from textile industries, leather industries, paint industries, paper and pulp industries, petrochemical industries, etc. [12–16]. In recent years, membrane technologies have been developing rapidly and their cost is continuing to reduce while the application possibilities are ever extending [17,18]. The main advantage of a membrane based process is that concentration and separation is achieved without a change of state and without use of chemicals or thermal energy, thus making the process energy-efficient and ideally suited for recovery applications [19]. The possibility of applying membrane processes in the treatment of the exhausted bath of a single step offers interesting perspectives for the survival of this industry and for recovering
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and recycling of primary resources [6]. Application of RO for removal of unreacted chromium from spent tanning effluent was studied in pilot scale [20]. This was followed by a study of nanofiltration for the recovery of Cr(III) from tannery effluent by Ortega et al. [21]. Cassano et al. have discussed in detail a general overview on the potential of membrane processes involving microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) in the treatment of aqueous effluents from leather industry. The experimental results demonstrated that it is feasible to recuperate Cr (III), depending on the membrane type, as well as reusing the water in the process. Laboratory and industrial pilot scale experiments have demonstrated the economic advantages of recovering chromium using membrane processes [1,21,22]. However, application of dualmembrane systems including MF/NF, UF/RO, and NF/RO for treatment of tannery waste and recovery of chrome and/or other chemicals is challenged by the presence of considerable BOD load and proteins, which may cause fouling and subsequent system failure temporarily or permanently [23]. In the present work, nanofiltration followed by reverse osmosis technique has been used to treat the chrome tanning effluent in a cross flow cell. Retentate stream of NF may be recycled to the tanning chamber after make up of the required chromium concentration. The permeate stream of NF (which contains most of the natural salts) is passed through a reverse osmosis unit to get clean water and concentrated salt solution for reuse. The effects of different operating conditions, e.g., transmembrane pressure drop and cross flow on permeate flux and observed retention are studied. The experiments are conducted in both laminar and turbulent regimes as well as using thin wires as turbulent promoters. 2. Experimental 2.1. Membranes The component of interest in the effluent is basic chromic sulphate (2Cr(OH)(SO4 ) + Na2 SO4 ) with a molecular weight of 472. Therefore, an organic thin film composite membrane of molecular weight cut off 400, consisting of a polyamide skin over a polysulphone support is used for nanofiltration. A polyamide membrane is used for reverse osmosis. The membranes are supplied by Genesis Membrane Sepratech, Mumbai, India. Fig. 1. Membrane module assembly.
2.2. Experimental set up A rectangular cross flow cell, made of stainless steel, is designed and fabricated. The cell consists of two matching flanges as shown in Fig. 1a. The inner surface of the top flange is mirror polished. The bottom flange is grooved, forming the channels for the permeate flow. The channels in the bottom flange with the internal grid structure are shown in Fig. 1b. A porous stainless steel plate is placed on the lower plate that provides mechanical support to the membrane. Two neoprene rubber gaskets are placed over the membrane; the top view of which is shown in Fig. 1c.
Sixteen equispaced thin wires of diameter 0.19 mm are placed laterally (along the width of the channel) in between the two gaskets (shown in Fig. 1d), as turbulent promoters. The spacing between the turbulent promoters is 14.0 mm. The two flanges are tightened to create a leak proof channel. The effective length and width of the membrane available for flltration are 26 and 4.9 cm, respectively. The height of the flow channel is determined by the thickness of the gaskets after tightening the two flanges and is found to be 0.72 mm. The obstruction in the flow path due to the wires promotes localized turbulence.
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Fig. 2. Experimental setup for cross flow membrane module.
The schematic of the experimental setup is shown in Fig. 2. The leather effluent is placed in a stainless steel feed tank of 10 l capacity. A high pressure reciprocating pump is used to feed the leather effluent into the cross flow membrane cell. The retentate stream is recycled to the feed tank routed through a rotameter. The permeate stream is also recycled to maintain a constant concentration in the feed tank. A bypass line from the pump delivery to the feed tank is provided. The two valves in the bypass and the retentate lines are used to vary the pressure and the flow rate through the cell, independently. 2.3. Operating conditions Nanofiltration runs are conducted at three pressures (828, 966, and 1104 kPa) with cross flow velocities of 0.47 m/s (flow rate of 1.0 lpm), 0.71 m/s (flow rate of 1.5 lpm), and 0.94 m/s (flow rate of 2.0 lpm) in laminar regime both with and without promoter. Cross flow velocities of 3.29 m/s (flow rate of 7.0 lpm), 3.76 m/s (flow rate of 8.0 lpm), and 4.23 m/s (flow rate of 9.0 lpm) are used in turbulent regime. Reverse osmosis experiments are conducted at four different pressures of 1380, 1518, 1725, and 1932 kPa with cross flow velocities of 3.29 (flow rate of 7.0 lpm), 3.76 (flow rate of 8.0 lpm), and 4.23 m/s (flow rate of 9.0 lpm) in turbulent regime. In laminar regime, with and without promoter, the RO experiments are conducted at 1725 kPa pressure and at 0.47 and 0.71 m/s of cross flow velocities. 2.4. Experimental procedure The steps used in the experiments are as follows: 1. The tannery effluent is cloth-filtered first to remove suspended impurities. 2. Compaction of membranes: A fresh membrane is compacted at a pressure of 690 kPa for 3 h using distilled water. 3. Determination of membrane permeability: Membrane permeability is determined using distilled water. Flux values at various operating pressures are measured and the slope of flux versus pressure plot gives the permeability. 4. Conduction of the experiments: By regulating the valve in the bypass line, the pressure is set in the cross flow cell. Controlling the valve before the rotameter, the flow rate is controlled independently. Cumulative volumes of permeate are collected during the experiment. Values of permeate flux
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are determined from the slopes of cumulative volume versus time plot. Permeate samples are collected at different times for analysis. The duration of the cross flow experiment is 1 h. 5. Determination of new permeability: Once an experimental run is over, the membrane is thoroughly washed, in situ, with distilled water for 15 min applying a maximum pressure of 200 kPa. The cross flow channel is dismantled next and the membrane is dipped in acid solution for 3 h. Next it is washed carefully with distilled water to remove traces of acid. The cross flow cell is reassembled and the membrane permeability is again measured. It is observed that the membrane permeability remains almost constant between successive runs. 2.5. Sample analysis 2.5.1. Measurement of chromium concentration Chromium present in the effluent and supernatant of subsequent NaOH treatment is estimated by measuring the pink violate complex formed by diphenyl carbazide at 541 nm according to APHA [24]. 2.5.2. Measurements of chemical oxygen demand COD is the measure of oxygen consumed during the oxidation of the organic matter in water by a strong oxidizing agent. COD value of each stream is measured by gravimetric analysis [25]. 2.5.3. Measurement of conductivities and total dissolved solid The conductivities and TDS of all the streams are measured by an auto ranging conductivity meter (Chemito 130, manufactured by Toshniwal Instruments, India). 2.5.4. Measurement of pH After each experiment, pH of the sample is measured by a pH meter, supplied by Toshniwal Instruments, India. 2.5.5. Measurement of biological oxygen demand BOD is the measure of the degradable organic material present in a sample, and can be defined as the amount of oxygen required by the microorganisms in stabilizing the biologically degradable organic matter under aerobatic condition. The principle of the method involves, measuring the difference of the oxygen concentration between the sample and after incubating it for 5 days [25]. 2.5.6. Measurement of total solids Total solids (TS) of all the samples are measured by weighing a known volume of sample in a petri dish and keeping it in an oven maintained at 105 ± 2 ◦ C, till complete drying of the sample. 3. Results and discussions This section is divided into two parts. In the first part, nanofiltration of the effluent is conducted to measure the variations of permeate flux, permeate quality at different pressures and cross
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Table 1 Characterization of effluent
Chromium concentration (ppm) COD (ppm) BOD (ppm) pH Conductivity (mho/cm) TDS (ppm) TS (ppm)
Feed
After NF
After RO
2300 1150 442 4.1 75 49500 126100
57.33 402 154 4.44 53.9 35.6 64.2
0.2 60.5 23.3 5.6 4.0 1.4 1.4
flow velocities are discussed with and without turbulent promoter in laminar flow and in purely turbulent flow conditions. In the second part, the performance of reverse osmosis in terms of flux and permeate quality is discussed. 3.1. Characterization of effluent Forty litres of effluent of the basic chrome sulphate process of tanning is colleted from Alison Tannery, Kolkata, India. The characterization of the pretreated effluent has been performed and the results are presented in Table 1. 3.2. Nanofiltration 3.2.1. Transient flux decline Figs. 3 and 4 represent the flux decline behavior of the effluent at 828 and 1104 kPa pressures, respectively. It can be clearly seen from these two figures that the time required to reach steady state decreases with increase in cross flow velocity. For example, it can be observed from Fig. 3 that the steady state is attained in about 1520 s, at 7.0 lpm and 828 kPa pressure, whereas at the same pressure but at 9.0 lpm, the steady state is attained within 1300 s. In addition, the flux decline is about 36% of the initial value for 7.0 lpm, about 33% with increase in velocity to 8.0 lpm, and 24% at 9.0 lpm. As the cross flow velocity increases, the growth of the concentration boundary layer over the membrane surface is arrested faster. This leads to the onset of the steady state at an earlier time. For the above reason, the resistance to the solvent flux also decreases with the cross flow velocity, resulting in higher permeate flux. Therefore, the flux decline is lower at higher cross flow velocities as observed. It can also be seen from Figs. 3 and 4 that at a fixed cross flow velocity, the steady
Fig. 3. Transient flux data at 828 kPa pressure in NF.
Fig. 4. Transient flux data at 1104 kPa pressure in NF.
state is attained faster with an increase in operating pressure. For example in Fig. 4, steady state is attained in about 1214 s for an operating condition of 7.0 lpm and at 1104 kPa pressure. Whereas at the same cross flow velocity, the time required to attain steady state is about 1520 s for an operating pressure 828 kPa. It is also observed that the steady state is achieved faster using turbulence promoter compared to laminar flow. For example, in Fig. 3, at 1.0 lpm and 828 kPa, the steady state is attained in about 2675 s without promoter and about 2046 s with promoter at the same operating condition. The flux decline is about 26% without promoter at 1.5 lpm and 828 kPa pressure; but only 18% using promoter at the same operating condition. Similar trends are observed in Fig. 4 as well. Use of the turbulence promoters creates local turbulence, thus reducing the concentration polarization at the membrane surface and the growth of the concentration boundary layer is checked quickly, establishing steady state earlier than the case of without promoter. Since the concentration polarization is reduced due to the presence of the promoters, the flux decline is also less than the no promoter case. 3.2.2. Steady state The variations of steady state permeate flux with pressure at different cross flow rate under laminar flow and with turbulent promoters are shown in Fig. 5. The variations of permeate flux under turbulent regimes at different operating conditions are shown in Fig. 6. The figures show the usual trend that the permeate flux increases with the operating pressure and cross flow rate. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with cross flow rate is because of decreasing concentration polarization as discussed earlier. The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions are presented in Fig. 7. All the increases are calculated taking the laminar flow results under same operating conditions as the base. The concentration boundary layer over the membrane surface is significantly disturbed in presence of the turbulent promoters.
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Fig. 5. Variation of permeate flux with pressure drop in laminar regime in NF.
This causes reduction in the membrane surface concentration and thereby increase in the effective driving force (P–π, π is the osmotic pressure) and hence an increase in permeate flux. It may be observed from Fig. 7 that the flux increment is in the range of 31–57% for laminar flow with promoter. The results for turbulent flow regime cannot be directly compared with laminar
Fig. 6. Variation of permeate flux with pressure drop in turbulent regime in NF.
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flow results for flux enhancement calculations, as the operating conditions are different. However, it can be clearly seen from Figs. 5 and 6 that the permeate flux in turbulent flow at a specific pressure is considerably higher than either the laminar or the turbulent promoter enhanced cases. A resistance in series type of model is used to analyze the experimental results to observe the effects of the hydrodynamic conditions. Uses of resistance in series approach involving similar resistances are quite common in the membrane literature. Aimar et al. [26] have postulated that the decrease in flux during ultrafiltration is a result of two resistances, namely (i) liquid macromolecular concentration polarization (osmotic pressure resistance) and (ii) the deposition on the membrane (gel-type resistance). This deposited layer cannot go on increasing like a cake in dead-end filtration. In cross flow mode, scouring will limit the thickness of the deposited layer. A general approach to resistance in series model for the flux decline in membrane filtration is available (Song [27]; Ho and Zydney [28]). The three major resistances to permeate flow are the hydraulic (membrane) resistance, osmotic pressure resistance and a geltype layer resistance. The hydraulic resistance of the membrane is constant between the experimental runs as has been found by the nearly constant values of the membrane permeability. An estimate of the magnitude of the osmotic pressure resistances is made and it has been found to be small compared to the overall driving force for the experiments considered herein. For example, in laminar flow NF experiments, the maximum osmotic pressure for the worst polarizing conditions (maximum pressure, minimum cross flow velocity) is only about 3% of the transmembrane pressure. For turbulent flow, the osmotic pressure contribution is about 2%. In RO, this value is even less, about 1%. Thus, in the present case, the gel-type layer resistance and the hydraulic resistances are considered in the resistance in series model and the gel-type resistances are estimated from the experimental flux values. The effect of cross flow velocity on this resistance is investigated next. The steady state permeate flux (vw ) can be written using classical cake filtration theory as, vw =
P µ(Rm + Rsp )
(1)
where Rm is the hydraulic resistance and RsP is the gel-type resistance at steady state. The membrane permeability Lp is experimentally measured using pure distilled water and the hydraulic resistance of the membrane is calculated from the following relation. Rm =
Fig. 7. Flux enhancement with cross flow velocity and pressure in laminar regime with promoter in NF.
1 µLP
(2)
The value of Rm is found to be 38.46 × 1012 m−1 for the NF membrane having MWCO 400 and for the RO membrane this value is 12.82 × 1013 m−1 . RsP are calculated from Eq. (1) using the experimental steady state values of the permeate fluxes at different operating conditions. It has been found that RsP accounts for about 95% of the total resistance in the case of laminar flow
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Fig. 9. Variation of observed retention of chromium with pressure drop in laminar and with promoter in NF.
Fig. 8. Variation of the ratio of gel-type and hydraulic resistances at steady state with the cross flow velocity during NF.
with lowest velocity, whereas even at the highest cross flow velocity in turbulent flow RsP accounts for 90% of the total resistance. The variation of gel-type layer resistances with cross flow velocity for some typical experiments is presented in Fig. 12. As can be observed from Fig. 8, the gel-type layer resistance RsP decreases with increase in cross flow velocity in all the cases. For example, for a transmembrane pressure drop of 828 kPa in laminar flow, the ratio of the gel-type and hydraulic resistance reduces from 20 to 17.6 with an increase in cross flow velocity from 0.47 to 0.71 m/s. Significant reductions in geltype layer resistance, compared to laminar flow, are achieved using turbulent promoters at the same cross flow velocity. At the same operating velocity (0.47 m/s) and transmembrane pressure (828 kPa), the presence of turbulent promoters reduces the resistance to 12.38 compared to 20 in laminar flow. For the case with the promoters, the gel-type layer resistance decreases significantly due to the enhanced forced convection near the membrane surface induced by the promoters. This reduction in RsP is more than 61% in some of the experiments leading to a significant enhancement of the permeate flux. The figure also shows further reductions for the case of purely turbulent flows for reasons already discussed. Similar behavior has also been observed for the RO experiments. 3.2.3. Permeate quality The permeate quality is expressed in terms of retention of chromium. Variations in permeate quality with transmembrane pressure at three different cross flow velocities in laminar and turbulent regimes are shown in Figs. 9 and 10, respectively. It is observed that with increase in pressure drop and cross flow velocity, the permeate quality improves. With increase in pressure, the water flux increases in a linear fashion, while solute flux is nearly independent of pressure for less open membranes (RO and in some cases for NF membranes) [29]. The result is that, with increasing pressure, more water passes through the membrane along with a fixed amount of the solute; the water is thus purer and hence the permeate quality (expressed as observed
rejection) improves. The effects of turbulence promoter are also investigated in the laminar flow regime. The results are summarized in Fig. 9. It can be seen from Fig. 9 that at 828 kPa pressure and 2.0 lpm flow rate, chromium retention characteristic improves slightly (by about 3%) in presence of promoter compared to the base case (laminar at same operating conditions). Percentage improvement in quality is found to be in about the same range as in the turbulent regime as shown in Fig. 10. Fig. 10 illustrates that the chromium retention increases with cross flow velocity and pressure for reasons already described earlier. For example, at 1104 kPa, an increase in cross flow velocity from 7.0 to 9.0 lpm results in an increase in chromium retention from 95.65 to 97.8%. At 7.0 lpm cross flow rate, as the transmembrane pressure increases from 828 to 1104 kPa, the observed retention of chromium increases by 5.1%. The permeate quality after NF, for various operating conditions, is presented in Table 2. It may be observed from the table that for the laminar regime, the retention of chromium varies from about 92 to 97.3%. The COD of the permeate remains quite high, higher than the permissible limit (250 mg/l) in India. The conductivity of the permeate is same as the feed which signifies that almost all the salt present in the feed solution has
Fig. 10. Variation of observed retention of chromium with pressure drop in turbulent regime NF.
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Table 2 Permeate Analysis after nanofiltration S.No.
Pressure (kPa)
Flow rate (lpm)
TDS (ppm)
TS (ppm)
pH
Conductivity (mS)
COD (ppm)
BOD (ppm)
R0 of chromium
Turbulent regime 1 828 2 3 4 966 5 6 7 1104 8 9
7.0 8.0 9.0 7.0 8.0 9.0 7.0 8.0 9.0
42800 42500 42300 42300 42300 43000 43000 43000 43500
73.0 71.4 71.0 72.0 72.1 69.0 68.3 67.1 67.6
4.4 4.4 4.4 4.4 4.4 4.4 4.4 4.4 4.4
64.8 64.4 51.8 48.6 46.2 43.1 50 45.8 43.6
859.1 822.4 797.9 675.5 614.4 569.5 532.8 496.1 381.9
330.4 316.3 306.9 259.8 236.3 219.0 204.9 190.8 146.9
91.0 91.5 91.9 93.6 94.5 95.1 95.7 96.2 97.8
Laminar regime 10 828 11 12 13 966 14 15 16 1104 17 18
1.0 1.5 2.0 1.0 1.5 2.0 1.0 1.5 2.0
42900 42000 42000 43000 42000 42000 40900 40900 37300
71.4 71.8 72.2 69.4 70.8 68.2 67.2 68.5 68.2
4.3 4.3 4.4 4.4 4.4 4.4 4.4 4.4 4.4
65 58.6 56.6 51.6 49.4 46.3 61.8 59.4 56.6
797.9 740.8 695.9 712.3 622.5 512.4 577.7 512.4 418.6
306.9 284.9 267.7 273.9 239.4 197.1 222.2 197.1 161
91.9 92.7 93.3 93.1 94.4 95.9 95.0 95.9 97.3
With turbulent promoter 19 828 20 21 22 966 23 24 25 1104 26 27
1.0 1.5 2.0 1.0 1.5 2.0 1.0 1.5 2.0
37200 35600 42300 42600 36900 38900 39200 37300 35600
68.1 67.7 67.5 67.1 65.3 65.1 65.1 64.5 64.2
4.4 4.4 4.3 4.3 4.4 4.3 4.3 4.4 4.4
56.4 51.1 50.9 64.5 61.2 58.9 59.4 56.6 53.9
691.9 565.4 512.4 659.2 504.2 426.7 540.9 475.7 402.3
266.1 217.5 197.1 253.5 193.9 164.1 208.1 183 154.7
93.4 95.2 95.9 93.9 96.1 97.2 95.5 96.5 97.5
permeated through the NF membrane. From Table 2, it can be observed that the presence of turbulent promoters or high cross flow velocity (turbulent regime) does not affect the chromium retention characteristics of the membrane to a great extent, rather it contributes to the considerable increase in the permeate flux. 3.3. Reverse osmosis The permeate from the NF is collected and treated using RO in the same cross flow cell in purely laminar, laminar with turbulent promoters and in turbulent conditions at different operating conditions. 3.3.1. Transient flux decline Fig. 11 presents the flux decline behavior with transmembrane pressure and cross flow velocity in RO. The results clearly show that as in the case of NF, the time required to reach steady state decreases with increase in cross flow velocity and applied pressure or in presence of turbulence promoters. Extent of flux decline also follows similar trends for reasons already discussed in the section describing NF operations. It can be observed that the flux decline is about 26% of the initial value at a cross flow velocity of 1.5 lpm and 1725 kPa pressure and is 20% at the same operating conditions but with promoters. The system reaches steady state in about 800 s with promoter compared to
about 1700 s without promoter in laminar regime. It may also be observed from Fig. 11 that at lower operating pressure in turbulent regime, flux decline is marginal; whereas at higher operating pressure, it is significant due to concentration polarization effects. For example, flux decline is about 26% at 1932 kPa and 9.0 lpm flow rate. 3.3.2. Steady state flux The values of flux obtained in the turbulent regime are significantly higher than that of laminar and laminar with turbulent promoters due to the higher operating pressure (driving force) and turbulence present near the membrane surface. The effects
Fig. 11. Transient flux data in RO.
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Table 3 Permeate analysis after reverse osmosis S.No.
Pressure (kPa)
Turbulent regime 1 1380 2 3 4 1518 5 6 7 1725 8 9 10 1932 11 12
Flow rate (lpm)
TDS (ppm)
TS (ppm)
pH
Conductivity (mS)
COD (ppm)
BOD (ppm)
R0 of chromium
Cr Concentration (ppm)
7.0 8.0 9.0 7.0 8.0 9.0 7.0 8.0 9.0 7.0 8.0 9.0
5350 4800 5020 5800 5740 5610 4360 5360 6360 7360 8360 9360
5563 4991 5220 6031 5969 5834 4534 5574 6614 7653 8693 9733
6.2 6.1 5.9 5.7 5.9 6.1 6.1 6.4 5.8 5.7 5.7 5.6
10.1 9.3 8.0 10.9 10.5 10.2 13.9 10.9 9.0 14.9 11.8 9.6
160.3 148.7 127.0 94.1 94.1 78.4 94.1 78.4 62.7 77.1 71.1 63.8
61.7 57.2 48.8 36.2 36.2 30.2 36.2 30.2 24.1 29.7 27.4 24.6
98.9 99.0 99.1 99.4 99.6 99.6 99.5 99.6 99.7 99.6 99.7 99.7
1 0.907 0.733 0.493 0.36 0.32 0.467 0.347 0.293 0.333 0.307 0.227
of transmembrane pressure and cross flow velocity on steady state flux in RO are shown in Fig. 12. The figure shows that the permeate flux increases with operating pressure and cross flow velocity as in NF. At 1932 kPa, an increase in cross flow velocity from 7.0 to 9.0 lpm results in about 64% increase in permeate flux. 3.3.3. Permeate quality The effects of transmembrane pressure and cross flow velocity on permeate quality in terms of chromium retention for turbulent regime, in RO, are shown in Fig. 13. The figure illustrates that the improvement of permeate quality with cross flow velocity and pressure is marginal. For example, at 1932 kPa, an increase in cross flow velocity from 7.0 to 9.0 lpm results in an increase in retention of chromium from 99.62 to 99.74%. The permeate qualities in terms of other properties for various operating conditions are presented in Table 3. It may be observed from Table 3 that the concentration of chromium in the permeate varies from about 1.0 to 0.2 ppm in the pressure range of 1380–1932 kPa which is within the permissible limit (1.0 ppm
Fig. 13. Variation of observed retention of chromium with pressure drop in turbulent regime RO.
[1]). Furthermore, the COD of permeate is substantially lower than the permissible limit (250 mg/l). From Table 3, it may also be observed that the conductivity of the permeate is very small signifying that almost all the salt present in the feed has been retained by the RO membrane. This salt rich retentate stream can be recycled to the tanning process. The information are essential for choosing the operating conditions and thereby improving the economics of the process without loss of product quality. 4. Conclusion
Fig. 12. Variation of permeate flux with pressure drop in RO.
Effluent from a chrome tanning unit has been successfully treated by a combination of NF followed by RO process. The retentate of the NF, rich in chromium, can be recycled. However, most of the salt is not retained by the process. The time required to reach steady state decreases with increase in cross flow velocity and applied pressure. The use of turbulence promoters in laminar regime results in substantial increase in flux compared to the laminar case. Effluent quality also increases with increase in pressure and cross flow velocity. Nevertheless, the permeate
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contains higher than permissible amounts of chromium and higher COD. The treatment of the permeate of the NF process by RO successfully addresses these problems including retaining most of the dissolved salts. The permeate RO has less than 1 ppm of chromium with low values of COD and BOD. The gel-type resistance is evaluated using a resistance in series model from the experimental data. It is found that the resistance decreases with increase in cross flow velocity and significantly decreases by the presence of turbulent promoters. References [1] A. Cassano, R. Molinari, M. Romano, E. Drioli, Treatment of aqueous effluents of the leather industry by membrane process. A review, J. Membr. Sci. 181 (2001) 111–126. [2] M.H. Davis, J.G. Scroggie, Theory and practice of direct chrome liquor recycling, Das Leder 31 (1980) 1–8. [3] N.P. Slabbert, Recycling in the tanning industry, J. Soc. Leather Traders Chem. 64 (1980) 89–92. [4] J.R. Rao, Recovery and reuse techniques for chromium salts in tanneries, Leather Sci. 34 (1987) 175–182. [5] R.R. Rao, B.G.S. Prasad, V. Narasimhan, T. Ramasami V., P.R. Shah, A.A. Khan, Electro dialysis in the recovery and reuse of chromium from industrial effluents, J. Membr. Sci. 46 (1989) 215–224. [6] J. Gauglhofer, Environmental aspects of tanning with chromium, J. Soc. Leather Technol. Chem. 70 (1) (1986) 11–13. [7] Technical Brochure, Impianto centralizzato per il recupero del solfato basico di cromo da reflui di conceria, Consorzio Recupero Cromo, S. Croce S/Arno, Italy, 1992. [8] A. Simoncini, M. Tomaselli, Recupero del cromo dai bagni di concia esausti, Cuoio, Pelli. Mat. Concianti 53–3 (1977) 251. [9] A. Cassano, E. Drioli, R. Molinari, Recovery and reuse of chemicals in unhairing, degreasing and chromium tanning processes by membranes, Desalination 113 (1997) 251–261. [10] A. Cassano, E. Drioli, R. Molinari, C. Bertolutti, Quality improvement of recycled chromium in the tanning operation by membrane processes, Desalination 108 (1996) 193–203. [11] A. Cassano, R. Molinari, E. Drioli, Saving of water and chemicals in tanning industry by membrane processes, Water Sci. Technol. 40 (4–5) (1999) 443–450. [12] M. Marcucci, G. Ciardelli, A. Matteucci, L. Ranieri, M. Russo, Experimental campaigns on textile wastewater for reuse by means of different membrane processes, Desalination 149 (1–3) (2002) 137–143.
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[13] M.D. Afonso, M.N.D. Pinho, Membrane separation processes in the pulp and paper industry, Desalination 85 (1) (1991) 53–58. [14] S. Chakraborty, M. Purkait, S. DasGupta, S. De, J.K. Basu, Nanofiltration of textile plant for color removal and reduction in COD, Sep. Purif. Technol. 31 (2003) 141–151. [15] S. Chakraborty, B.C. Bag, S. DasGupta, S. De, J.K. Basu, Separation and fractionation of dye solution by nanofiltration, Sep. Purif. Technol. 38 (2003) 219–235. [16] A.S. J¨onsson, G. Tr¨ag˚ardh, Ultrafiltration applications, Desalination 77 (1990) 135–179. [17] R.W. Baker, Membrane Separation Systems—Recent Development, Future Direction, Noyes Data Corporation, 1991, p. 329. [18] P. Ball, Scale-up and scale down of membrane based separation processes, Membr. Technol. 117 (1999) 10–13. [19] W. Scholz, W. Bowden, Application of membrane technology in the tanning industry, Leather 201 (4694) (1999) 17–18. [20] A.I. Hafez, M.S. El-Manharawy, M.A. Khedr, RO membrane removal of unreacted chromium from spent tanning effluent. A pilot-scale study, Part 2, Desalination 144 (2002) 237–242. [21] L.M. Ortega, R. Lebrun, I.M. No¨el, R. Hausler, Application of nanofiltration in the recovery of chromium (III) from tannery effluents, Sep. Purif. Technol. 44 (2005) 45–52. [22] W.S. Winston Ho, T.K. Poddar, New membrane technology for removal and recovery of chromium from waste waters, Environ. Prog. 20 (2001) 44–52. [23] H.F. Shaalan, M.H. Sorour, S.R. Tewfik, Simulation and optimization of a membrane system for chromium recovery from tanning wastes, Desalination 141 (2001) 315–324. [24] American Public Health Association, Standard Methods for Examination of Waters and Wastewaters, 16th ed., APHA, Washington, DC, 1985. [25] R.K. Trivedy, P.K. Goel, Chemical and Biological Methods for Water Pollution Studies, Environmental Publications, Karad, 1986. [26] P. Aimar, C. Taddei, J.P. Lafaille, V. Sanchez, Mass transfer limitations during ultrafiltration of cheese whey with inorganic membrane, J. Membr. Sci. 38 (1988) 203–221. [27] L. Song, Flux decline in crossflow microfiltration and ultrafiltration: mechanisms and modeling of membrane fouling, J. Membr. Sci. 139 (1998) 183–200. [28] C. Ho, A.L. Zydney, A combined pore blockage and cake filtration model for protein fouling during microfiltration, J. Colloid Interface Sci. 232 (2000) 389–399. [29] P.M. Bungay, H.K. Lonsdale, M.N. de Pinho, Synthetic membranes: science, in: Engineering and Application, D. Reidel Publishing Company, 1983, p. 312.
Treatment of tanning effluent using nanofiltration followed by reverse osmosis Chandan Das, Piyush Patel, Sirshendu De, Sunando DasGupta ∗ Department of Chemical Engineering, Indian Institute of Technology, Kharagpur-721302, India Received 28 May 2005; received in revised form 21 November 2005; accepted 28 November 2005
Abstract An investigation on the recovery of chromium from the effluent of a chrome-tanning bath has been performed using nanofiltration (NF) followed by reverse osmosis (RO). The experiments are conducted using a rectangular cross flow cell under laminar and turbulent regimes. Significant flux enhancement is achieved using thin wires as turbulent promoters. The performance criteria are evaluated in terms of the concentration of chromium, COD, BOD, TDS, TS, pH, and conductivity of the permeate. The effects of different operating parameters on permeate flux and observed retention of chromium are evaluated experimentally. The retention of chromium is found to be 91–98% for NF and 98.8–99.7% for RO for the experimental conditions of this study. Concentrations of chromium and COD of the final permeate are well within the permissible limits. © 2005 Elsevier B.V. All rights reserved. Keywords: Chromium recovery; Nanofiltration; Reverse osmosis; Retention; Turbulent promoter
1. Introduction Almost all leather made from lighter-weight cattle hides and from the skin of sheep, lambs, goats, and pigs is chrome tanned. Chrome tanning is performed using a one-bath process that is based on the reaction between the hide and a trivalent chromium salt, usually a basic chromium sulfate. In the typical one bath process, the hides are in a pickled state at a pH of 3 or lower, the chrome tanning materials are introduced, and the pH is raised. Following tanning, the chrome-tanned leather is piled down, wrung, and graded for the thickness and quality, split into flesh and grain layers, and shaved to the desired thickness. The grain leathers from the shaving machine are then separated for retanning, dyeing, and fat liquoring [1]. Extensive studies have been made to recycle the spent tanned liquor [2,3]. But recycling of chromium solution for tanning leads to accumulation of neutral salts, which reduces the uptake of chromium during tanning [4]. Electrodialysis may be useful for selective separation of neutral salts from spent tanned liquor. However, the economic viability of the technique is yet to be established [5]. The exhausted bath from chromium tannage contains about 30–40% of initial salt and it is nor-
∗
Corresponding author. Tel.: +91 3222 283922; fax: +91 3222 275303. E-mail address: [email protected] (S. DasGupta).
1383-5866/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2005.11.034
mally sent to a cleaning-up plant. Here chromium salts end into the sludge creating serious problems for their disposal [6]. The traditional method for chromium recovery is based on the precipitation of chromium salt with NaOH followed by the dissolution of Cr(OH)3 in sulfuric acid [7,8]. However, the quality of the recovered solutions is not always optimal due to the presence of metals, lipidic substances, and other impurities. In order to improve the quality of the recycled chromium, an alternative method using membrane processes was studied [9–11]. Membrane-based separation processes may be an attractive alternative and they are gradually emerging as a technically significant and commercially viable ‘cleaner technology’ for the treatment of wastewater from textile industries, leather industries, paint industries, paper and pulp industries, petrochemical industries, etc. [12–16]. In recent years, membrane technologies have been developing rapidly and their cost is continuing to reduce while the application possibilities are ever extending [17,18]. The main advantage of a membrane based process is that concentration and separation is achieved without a change of state and without use of chemicals or thermal energy, thus making the process energy-efficient and ideally suited for recovery applications [19]. The possibility of applying membrane processes in the treatment of the exhausted bath of a single step offers interesting perspectives for the survival of this industry and for recovering
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and recycling of primary resources [6]. Application of RO for removal of unreacted chromium from spent tanning effluent was studied in pilot scale [20]. This was followed by a study of nanofiltration for the recovery of Cr(III) from tannery effluent by Ortega et al. [21]. Cassano et al. have discussed in detail a general overview on the potential of membrane processes involving microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) in the treatment of aqueous effluents from leather industry. The experimental results demonstrated that it is feasible to recuperate Cr (III), depending on the membrane type, as well as reusing the water in the process. Laboratory and industrial pilot scale experiments have demonstrated the economic advantages of recovering chromium using membrane processes [1,21,22]. However, application of dualmembrane systems including MF/NF, UF/RO, and NF/RO for treatment of tannery waste and recovery of chrome and/or other chemicals is challenged by the presence of considerable BOD load and proteins, which may cause fouling and subsequent system failure temporarily or permanently [23]. In the present work, nanofiltration followed by reverse osmosis technique has been used to treat the chrome tanning effluent in a cross flow cell. Retentate stream of NF may be recycled to the tanning chamber after make up of the required chromium concentration. The permeate stream of NF (which contains most of the natural salts) is passed through a reverse osmosis unit to get clean water and concentrated salt solution for reuse. The effects of different operating conditions, e.g., transmembrane pressure drop and cross flow on permeate flux and observed retention are studied. The experiments are conducted in both laminar and turbulent regimes as well as using thin wires as turbulent promoters. 2. Experimental 2.1. Membranes The component of interest in the effluent is basic chromic sulphate (2Cr(OH)(SO4 ) + Na2 SO4 ) with a molecular weight of 472. Therefore, an organic thin film composite membrane of molecular weight cut off 400, consisting of a polyamide skin over a polysulphone support is used for nanofiltration. A polyamide membrane is used for reverse osmosis. The membranes are supplied by Genesis Membrane Sepratech, Mumbai, India. Fig. 1. Membrane module assembly.
2.2. Experimental set up A rectangular cross flow cell, made of stainless steel, is designed and fabricated. The cell consists of two matching flanges as shown in Fig. 1a. The inner surface of the top flange is mirror polished. The bottom flange is grooved, forming the channels for the permeate flow. The channels in the bottom flange with the internal grid structure are shown in Fig. 1b. A porous stainless steel plate is placed on the lower plate that provides mechanical support to the membrane. Two neoprene rubber gaskets are placed over the membrane; the top view of which is shown in Fig. 1c.
Sixteen equispaced thin wires of diameter 0.19 mm are placed laterally (along the width of the channel) in between the two gaskets (shown in Fig. 1d), as turbulent promoters. The spacing between the turbulent promoters is 14.0 mm. The two flanges are tightened to create a leak proof channel. The effective length and width of the membrane available for flltration are 26 and 4.9 cm, respectively. The height of the flow channel is determined by the thickness of the gaskets after tightening the two flanges and is found to be 0.72 mm. The obstruction in the flow path due to the wires promotes localized turbulence.
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Fig. 2. Experimental setup for cross flow membrane module.
The schematic of the experimental setup is shown in Fig. 2. The leather effluent is placed in a stainless steel feed tank of 10 l capacity. A high pressure reciprocating pump is used to feed the leather effluent into the cross flow membrane cell. The retentate stream is recycled to the feed tank routed through a rotameter. The permeate stream is also recycled to maintain a constant concentration in the feed tank. A bypass line from the pump delivery to the feed tank is provided. The two valves in the bypass and the retentate lines are used to vary the pressure and the flow rate through the cell, independently. 2.3. Operating conditions Nanofiltration runs are conducted at three pressures (828, 966, and 1104 kPa) with cross flow velocities of 0.47 m/s (flow rate of 1.0 lpm), 0.71 m/s (flow rate of 1.5 lpm), and 0.94 m/s (flow rate of 2.0 lpm) in laminar regime both with and without promoter. Cross flow velocities of 3.29 m/s (flow rate of 7.0 lpm), 3.76 m/s (flow rate of 8.0 lpm), and 4.23 m/s (flow rate of 9.0 lpm) are used in turbulent regime. Reverse osmosis experiments are conducted at four different pressures of 1380, 1518, 1725, and 1932 kPa with cross flow velocities of 3.29 (flow rate of 7.0 lpm), 3.76 (flow rate of 8.0 lpm), and 4.23 m/s (flow rate of 9.0 lpm) in turbulent regime. In laminar regime, with and without promoter, the RO experiments are conducted at 1725 kPa pressure and at 0.47 and 0.71 m/s of cross flow velocities. 2.4. Experimental procedure The steps used in the experiments are as follows: 1. The tannery effluent is cloth-filtered first to remove suspended impurities. 2. Compaction of membranes: A fresh membrane is compacted at a pressure of 690 kPa for 3 h using distilled water. 3. Determination of membrane permeability: Membrane permeability is determined using distilled water. Flux values at various operating pressures are measured and the slope of flux versus pressure plot gives the permeability. 4. Conduction of the experiments: By regulating the valve in the bypass line, the pressure is set in the cross flow cell. Controlling the valve before the rotameter, the flow rate is controlled independently. Cumulative volumes of permeate are collected during the experiment. Values of permeate flux
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are determined from the slopes of cumulative volume versus time plot. Permeate samples are collected at different times for analysis. The duration of the cross flow experiment is 1 h. 5. Determination of new permeability: Once an experimental run is over, the membrane is thoroughly washed, in situ, with distilled water for 15 min applying a maximum pressure of 200 kPa. The cross flow channel is dismantled next and the membrane is dipped in acid solution for 3 h. Next it is washed carefully with distilled water to remove traces of acid. The cross flow cell is reassembled and the membrane permeability is again measured. It is observed that the membrane permeability remains almost constant between successive runs. 2.5. Sample analysis 2.5.1. Measurement of chromium concentration Chromium present in the effluent and supernatant of subsequent NaOH treatment is estimated by measuring the pink violate complex formed by diphenyl carbazide at 541 nm according to APHA [24]. 2.5.2. Measurements of chemical oxygen demand COD is the measure of oxygen consumed during the oxidation of the organic matter in water by a strong oxidizing agent. COD value of each stream is measured by gravimetric analysis [25]. 2.5.3. Measurement of conductivities and total dissolved solid The conductivities and TDS of all the streams are measured by an auto ranging conductivity meter (Chemito 130, manufactured by Toshniwal Instruments, India). 2.5.4. Measurement of pH After each experiment, pH of the sample is measured by a pH meter, supplied by Toshniwal Instruments, India. 2.5.5. Measurement of biological oxygen demand BOD is the measure of the degradable organic material present in a sample, and can be defined as the amount of oxygen required by the microorganisms in stabilizing the biologically degradable organic matter under aerobatic condition. The principle of the method involves, measuring the difference of the oxygen concentration between the sample and after incubating it for 5 days [25]. 2.5.6. Measurement of total solids Total solids (TS) of all the samples are measured by weighing a known volume of sample in a petri dish and keeping it in an oven maintained at 105 ± 2 ◦ C, till complete drying of the sample. 3. Results and discussions This section is divided into two parts. In the first part, nanofiltration of the effluent is conducted to measure the variations of permeate flux, permeate quality at different pressures and cross
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Table 1 Characterization of effluent
Chromium concentration (ppm) COD (ppm) BOD (ppm) pH Conductivity (mho/cm) TDS (ppm) TS (ppm)
Feed
After NF
After RO
2300 1150 442 4.1 75 49500 126100
57.33 402 154 4.44 53.9 35.6 64.2
0.2 60.5 23.3 5.6 4.0 1.4 1.4
flow velocities are discussed with and without turbulent promoter in laminar flow and in purely turbulent flow conditions. In the second part, the performance of reverse osmosis in terms of flux and permeate quality is discussed. 3.1. Characterization of effluent Forty litres of effluent of the basic chrome sulphate process of tanning is colleted from Alison Tannery, Kolkata, India. The characterization of the pretreated effluent has been performed and the results are presented in Table 1. 3.2. Nanofiltration 3.2.1. Transient flux decline Figs. 3 and 4 represent the flux decline behavior of the effluent at 828 and 1104 kPa pressures, respectively. It can be clearly seen from these two figures that the time required to reach steady state decreases with increase in cross flow velocity. For example, it can be observed from Fig. 3 that the steady state is attained in about 1520 s, at 7.0 lpm and 828 kPa pressure, whereas at the same pressure but at 9.0 lpm, the steady state is attained within 1300 s. In addition, the flux decline is about 36% of the initial value for 7.0 lpm, about 33% with increase in velocity to 8.0 lpm, and 24% at 9.0 lpm. As the cross flow velocity increases, the growth of the concentration boundary layer over the membrane surface is arrested faster. This leads to the onset of the steady state at an earlier time. For the above reason, the resistance to the solvent flux also decreases with the cross flow velocity, resulting in higher permeate flux. Therefore, the flux decline is lower at higher cross flow velocities as observed. It can also be seen from Figs. 3 and 4 that at a fixed cross flow velocity, the steady
Fig. 3. Transient flux data at 828 kPa pressure in NF.
Fig. 4. Transient flux data at 1104 kPa pressure in NF.
state is attained faster with an increase in operating pressure. For example in Fig. 4, steady state is attained in about 1214 s for an operating condition of 7.0 lpm and at 1104 kPa pressure. Whereas at the same cross flow velocity, the time required to attain steady state is about 1520 s for an operating pressure 828 kPa. It is also observed that the steady state is achieved faster using turbulence promoter compared to laminar flow. For example, in Fig. 3, at 1.0 lpm and 828 kPa, the steady state is attained in about 2675 s without promoter and about 2046 s with promoter at the same operating condition. The flux decline is about 26% without promoter at 1.5 lpm and 828 kPa pressure; but only 18% using promoter at the same operating condition. Similar trends are observed in Fig. 4 as well. Use of the turbulence promoters creates local turbulence, thus reducing the concentration polarization at the membrane surface and the growth of the concentration boundary layer is checked quickly, establishing steady state earlier than the case of without promoter. Since the concentration polarization is reduced due to the presence of the promoters, the flux decline is also less than the no promoter case. 3.2.2. Steady state The variations of steady state permeate flux with pressure at different cross flow rate under laminar flow and with turbulent promoters are shown in Fig. 5. The variations of permeate flux under turbulent regimes at different operating conditions are shown in Fig. 6. The figures show the usual trend that the permeate flux increases with the operating pressure and cross flow rate. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with cross flow rate is because of decreasing concentration polarization as discussed earlier. The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions are presented in Fig. 7. All the increases are calculated taking the laminar flow results under same operating conditions as the base. The concentration boundary layer over the membrane surface is significantly disturbed in presence of the turbulent promoters.
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Fig. 5. Variation of permeate flux with pressure drop in laminar regime in NF.
This causes reduction in the membrane surface concentration and thereby increase in the effective driving force (P–π, π is the osmotic pressure) and hence an increase in permeate flux. It may be observed from Fig. 7 that the flux increment is in the range of 31–57% for laminar flow with promoter. The results for turbulent flow regime cannot be directly compared with laminar
Fig. 6. Variation of permeate flux with pressure drop in turbulent regime in NF.
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flow results for flux enhancement calculations, as the operating conditions are different. However, it can be clearly seen from Figs. 5 and 6 that the permeate flux in turbulent flow at a specific pressure is considerably higher than either the laminar or the turbulent promoter enhanced cases. A resistance in series type of model is used to analyze the experimental results to observe the effects of the hydrodynamic conditions. Uses of resistance in series approach involving similar resistances are quite common in the membrane literature. Aimar et al. [26] have postulated that the decrease in flux during ultrafiltration is a result of two resistances, namely (i) liquid macromolecular concentration polarization (osmotic pressure resistance) and (ii) the deposition on the membrane (gel-type resistance). This deposited layer cannot go on increasing like a cake in dead-end filtration. In cross flow mode, scouring will limit the thickness of the deposited layer. A general approach to resistance in series model for the flux decline in membrane filtration is available (Song [27]; Ho and Zydney [28]). The three major resistances to permeate flow are the hydraulic (membrane) resistance, osmotic pressure resistance and a geltype layer resistance. The hydraulic resistance of the membrane is constant between the experimental runs as has been found by the nearly constant values of the membrane permeability. An estimate of the magnitude of the osmotic pressure resistances is made and it has been found to be small compared to the overall driving force for the experiments considered herein. For example, in laminar flow NF experiments, the maximum osmotic pressure for the worst polarizing conditions (maximum pressure, minimum cross flow velocity) is only about 3% of the transmembrane pressure. For turbulent flow, the osmotic pressure contribution is about 2%. In RO, this value is even less, about 1%. Thus, in the present case, the gel-type layer resistance and the hydraulic resistances are considered in the resistance in series model and the gel-type resistances are estimated from the experimental flux values. The effect of cross flow velocity on this resistance is investigated next. The steady state permeate flux (vw ) can be written using classical cake filtration theory as, vw =
P µ(Rm + Rsp )
(1)
where Rm is the hydraulic resistance and RsP is the gel-type resistance at steady state. The membrane permeability Lp is experimentally measured using pure distilled water and the hydraulic resistance of the membrane is calculated from the following relation. Rm =
Fig. 7. Flux enhancement with cross flow velocity and pressure in laminar regime with promoter in NF.
1 µLP
(2)
The value of Rm is found to be 38.46 × 1012 m−1 for the NF membrane having MWCO 400 and for the RO membrane this value is 12.82 × 1013 m−1 . RsP are calculated from Eq. (1) using the experimental steady state values of the permeate fluxes at different operating conditions. It has been found that RsP accounts for about 95% of the total resistance in the case of laminar flow
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Fig. 9. Variation of observed retention of chromium with pressure drop in laminar and with promoter in NF.
Fig. 8. Variation of the ratio of gel-type and hydraulic resistances at steady state with the cross flow velocity during NF.
with lowest velocity, whereas even at the highest cross flow velocity in turbulent flow RsP accounts for 90% of the total resistance. The variation of gel-type layer resistances with cross flow velocity for some typical experiments is presented in Fig. 12. As can be observed from Fig. 8, the gel-type layer resistance RsP decreases with increase in cross flow velocity in all the cases. For example, for a transmembrane pressure drop of 828 kPa in laminar flow, the ratio of the gel-type and hydraulic resistance reduces from 20 to 17.6 with an increase in cross flow velocity from 0.47 to 0.71 m/s. Significant reductions in geltype layer resistance, compared to laminar flow, are achieved using turbulent promoters at the same cross flow velocity. At the same operating velocity (0.47 m/s) and transmembrane pressure (828 kPa), the presence of turbulent promoters reduces the resistance to 12.38 compared to 20 in laminar flow. For the case with the promoters, the gel-type layer resistance decreases significantly due to the enhanced forced convection near the membrane surface induced by the promoters. This reduction in RsP is more than 61% in some of the experiments leading to a significant enhancement of the permeate flux. The figure also shows further reductions for the case of purely turbulent flows for reasons already discussed. Similar behavior has also been observed for the RO experiments. 3.2.3. Permeate quality The permeate quality is expressed in terms of retention of chromium. Variations in permeate quality with transmembrane pressure at three different cross flow velocities in laminar and turbulent regimes are shown in Figs. 9 and 10, respectively. It is observed that with increase in pressure drop and cross flow velocity, the permeate quality improves. With increase in pressure, the water flux increases in a linear fashion, while solute flux is nearly independent of pressure for less open membranes (RO and in some cases for NF membranes) [29]. The result is that, with increasing pressure, more water passes through the membrane along with a fixed amount of the solute; the water is thus purer and hence the permeate quality (expressed as observed
rejection) improves. The effects of turbulence promoter are also investigated in the laminar flow regime. The results are summarized in Fig. 9. It can be seen from Fig. 9 that at 828 kPa pressure and 2.0 lpm flow rate, chromium retention characteristic improves slightly (by about 3%) in presence of promoter compared to the base case (laminar at same operating conditions). Percentage improvement in quality is found to be in about the same range as in the turbulent regime as shown in Fig. 10. Fig. 10 illustrates that the chromium retention increases with cross flow velocity and pressure for reasons already described earlier. For example, at 1104 kPa, an increase in cross flow velocity from 7.0 to 9.0 lpm results in an increase in chromium retention from 95.65 to 97.8%. At 7.0 lpm cross flow rate, as the transmembrane pressure increases from 828 to 1104 kPa, the observed retention of chromium increases by 5.1%. The permeate quality after NF, for various operating conditions, is presented in Table 2. It may be observed from the table that for the laminar regime, the retention of chromium varies from about 92 to 97.3%. The COD of the permeate remains quite high, higher than the permissible limit (250 mg/l) in India. The conductivity of the permeate is same as the feed which signifies that almost all the salt present in the feed solution has
Fig. 10. Variation of observed retention of chromium with pressure drop in turbulent regime NF.
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Table 2 Permeate Analysis after nanofiltration S.No.
Pressure (kPa)
Flow rate (lpm)
TDS (ppm)
TS (ppm)
pH
Conductivity (mS)
COD (ppm)
BOD (ppm)
R0 of chromium
Turbulent regime 1 828 2 3 4 966 5 6 7 1104 8 9
7.0 8.0 9.0 7.0 8.0 9.0 7.0 8.0 9.0
42800 42500 42300 42300 42300 43000 43000 43000 43500
73.0 71.4 71.0 72.0 72.1 69.0 68.3 67.1 67.6
4.4 4.4 4.4 4.4 4.4 4.4 4.4 4.4 4.4
64.8 64.4 51.8 48.6 46.2 43.1 50 45.8 43.6
859.1 822.4 797.9 675.5 614.4 569.5 532.8 496.1 381.9
330.4 316.3 306.9 259.8 236.3 219.0 204.9 190.8 146.9
91.0 91.5 91.9 93.6 94.5 95.1 95.7 96.2 97.8
Laminar regime 10 828 11 12 13 966 14 15 16 1104 17 18
1.0 1.5 2.0 1.0 1.5 2.0 1.0 1.5 2.0
42900 42000 42000 43000 42000 42000 40900 40900 37300
71.4 71.8 72.2 69.4 70.8 68.2 67.2 68.5 68.2
4.3 4.3 4.4 4.4 4.4 4.4 4.4 4.4 4.4
65 58.6 56.6 51.6 49.4 46.3 61.8 59.4 56.6
797.9 740.8 695.9 712.3 622.5 512.4 577.7 512.4 418.6
306.9 284.9 267.7 273.9 239.4 197.1 222.2 197.1 161
91.9 92.7 93.3 93.1 94.4 95.9 95.0 95.9 97.3
With turbulent promoter 19 828 20 21 22 966 23 24 25 1104 26 27
1.0 1.5 2.0 1.0 1.5 2.0 1.0 1.5 2.0
37200 35600 42300 42600 36900 38900 39200 37300 35600
68.1 67.7 67.5 67.1 65.3 65.1 65.1 64.5 64.2
4.4 4.4 4.3 4.3 4.4 4.3 4.3 4.4 4.4
56.4 51.1 50.9 64.5 61.2 58.9 59.4 56.6 53.9
691.9 565.4 512.4 659.2 504.2 426.7 540.9 475.7 402.3
266.1 217.5 197.1 253.5 193.9 164.1 208.1 183 154.7
93.4 95.2 95.9 93.9 96.1 97.2 95.5 96.5 97.5
permeated through the NF membrane. From Table 2, it can be observed that the presence of turbulent promoters or high cross flow velocity (turbulent regime) does not affect the chromium retention characteristics of the membrane to a great extent, rather it contributes to the considerable increase in the permeate flux. 3.3. Reverse osmosis The permeate from the NF is collected and treated using RO in the same cross flow cell in purely laminar, laminar with turbulent promoters and in turbulent conditions at different operating conditions. 3.3.1. Transient flux decline Fig. 11 presents the flux decline behavior with transmembrane pressure and cross flow velocity in RO. The results clearly show that as in the case of NF, the time required to reach steady state decreases with increase in cross flow velocity and applied pressure or in presence of turbulence promoters. Extent of flux decline also follows similar trends for reasons already discussed in the section describing NF operations. It can be observed that the flux decline is about 26% of the initial value at a cross flow velocity of 1.5 lpm and 1725 kPa pressure and is 20% at the same operating conditions but with promoters. The system reaches steady state in about 800 s with promoter compared to
about 1700 s without promoter in laminar regime. It may also be observed from Fig. 11 that at lower operating pressure in turbulent regime, flux decline is marginal; whereas at higher operating pressure, it is significant due to concentration polarization effects. For example, flux decline is about 26% at 1932 kPa and 9.0 lpm flow rate. 3.3.2. Steady state flux The values of flux obtained in the turbulent regime are significantly higher than that of laminar and laminar with turbulent promoters due to the higher operating pressure (driving force) and turbulence present near the membrane surface. The effects
Fig. 11. Transient flux data in RO.
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Table 3 Permeate analysis after reverse osmosis S.No.
Pressure (kPa)
Turbulent regime 1 1380 2 3 4 1518 5 6 7 1725 8 9 10 1932 11 12
Flow rate (lpm)
TDS (ppm)
TS (ppm)
pH
Conductivity (mS)
COD (ppm)
BOD (ppm)
R0 of chromium
Cr Concentration (ppm)
7.0 8.0 9.0 7.0 8.0 9.0 7.0 8.0 9.0 7.0 8.0 9.0
5350 4800 5020 5800 5740 5610 4360 5360 6360 7360 8360 9360
5563 4991 5220 6031 5969 5834 4534 5574 6614 7653 8693 9733
6.2 6.1 5.9 5.7 5.9 6.1 6.1 6.4 5.8 5.7 5.7 5.6
10.1 9.3 8.0 10.9 10.5 10.2 13.9 10.9 9.0 14.9 11.8 9.6
160.3 148.7 127.0 94.1 94.1 78.4 94.1 78.4 62.7 77.1 71.1 63.8
61.7 57.2 48.8 36.2 36.2 30.2 36.2 30.2 24.1 29.7 27.4 24.6
98.9 99.0 99.1 99.4 99.6 99.6 99.5 99.6 99.7 99.6 99.7 99.7
1 0.907 0.733 0.493 0.36 0.32 0.467 0.347 0.293 0.333 0.307 0.227
of transmembrane pressure and cross flow velocity on steady state flux in RO are shown in Fig. 12. The figure shows that the permeate flux increases with operating pressure and cross flow velocity as in NF. At 1932 kPa, an increase in cross flow velocity from 7.0 to 9.0 lpm results in about 64% increase in permeate flux. 3.3.3. Permeate quality The effects of transmembrane pressure and cross flow velocity on permeate quality in terms of chromium retention for turbulent regime, in RO, are shown in Fig. 13. The figure illustrates that the improvement of permeate quality with cross flow velocity and pressure is marginal. For example, at 1932 kPa, an increase in cross flow velocity from 7.0 to 9.0 lpm results in an increase in retention of chromium from 99.62 to 99.74%. The permeate qualities in terms of other properties for various operating conditions are presented in Table 3. It may be observed from Table 3 that the concentration of chromium in the permeate varies from about 1.0 to 0.2 ppm in the pressure range of 1380–1932 kPa which is within the permissible limit (1.0 ppm
Fig. 13. Variation of observed retention of chromium with pressure drop in turbulent regime RO.
[1]). Furthermore, the COD of permeate is substantially lower than the permissible limit (250 mg/l). From Table 3, it may also be observed that the conductivity of the permeate is very small signifying that almost all the salt present in the feed has been retained by the RO membrane. This salt rich retentate stream can be recycled to the tanning process. The information are essential for choosing the operating conditions and thereby improving the economics of the process without loss of product quality. 4. Conclusion
Fig. 12. Variation of permeate flux with pressure drop in RO.
Effluent from a chrome tanning unit has been successfully treated by a combination of NF followed by RO process. The retentate of the NF, rich in chromium, can be recycled. However, most of the salt is not retained by the process. The time required to reach steady state decreases with increase in cross flow velocity and applied pressure. The use of turbulence promoters in laminar regime results in substantial increase in flux compared to the laminar case. Effluent quality also increases with increase in pressure and cross flow velocity. Nevertheless, the permeate
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