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Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes
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Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes A. Gallego-Molina a , J.A. Mendoza-Roca a,∗ , D. Aguado b , M.V. Galiana-Aleixandre a a
Universitat Politècnica de València, Instituto de Seguridad Ambiental, Radiofísica y Medio Ambiente, Camino de Vera s/n, 46022 Valencia, Spain b Universitat Politècnica de València, Instituto de Ingeniería del Agua y Medio Ambiente, Camino de Vera s/n, 46022 Valencia, Spain
a b s t r a c t This paper presents experimental results from the implementation of two measures aimed at reducing the nitrogen concentration in a tannery wastewater. Specifically, this research has focused on the wastewater from the deliming/bating operations. The proposed measures are the replacement of ammonium salts by carbon dioxide in the deliming process and the reuse of wastewater and chemicals after membrane filtration of the deliming/bating liquor. The experimental study covered different wastewater pretreatment alternatives and experiments with two membranes (with different separation properties): one in the range of microfiltration (MF) and one in the range of the ultrafiltration (UF). Results of the pretreatment study indicated that neither settling nor protein precipitation were feasible. Only a security filtration prior to membrane filtration was recommended. The tested MF membrane was selected due to the higher flux (around 25 L/(m2 h)) in comparison with the UF membrane. The MF permeate was successfully reused in the deliming/bating process. The delimed leather quality was excellent according to both visual and organoleptic inspection from process technicians and phenolphthalein test, confirming the technical feasibility of the proposal. Globally, the implementation of the above mentioned two measures resulted in 53% total nitrogen reduction. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Bating; Deliming; Membranes; Microfiltration; Tannery wastewater; Wastewater reuse
1.
Introduction
Tanneries generate high quantities of wastewater that are characterized by high concentrations of organic matter, nitrogen, trivalent chromium, sulfate and other ions. These pollutants come from both the raw hides (mainly conservation salt and non useful parts) and the auxiliary chemicals added in the wet operations involved. The first operations in a tannery process prepare the hides for the tanning, which is the operation where the hide (a degradable stuff) is converted into leather (a non degradable stuff). These previous operations consist of soaking, for the removal of the conservation salt and impurities, liming/unhairing for epidermis and hair removal, flesh removal, for eliminating this part of the hide, deliming, for the removal of the lime added in the lime/unhairing and for pH lowering and bating, for the preparation of the collagen fibres before the
tanning and cleaning the hides removing impurities from the previous operations by means of enzymes. Deliming and bating operations are carried out in the same drum. The enzymes needed for the bating are added after deliming without a previous discharge of the deliming liquor. Thereby, the discharge from the drum can be considered as wastewater coming from both operations (deliming and bating). A typical tannery treating bovine hides discharges between 1 and 4 m3 /t of wastewater in this operation (European Commission, 2003; Gutterres et al., 2008). As previously indicated, the aim of the deliming is to remove the lime that was previously added in the liming/unhairing process to raise the pH up to 12–13 in order to open up the fibrillar structure by swelling to remove the epidermis from the hide. Since the pH should be slowly lowered, ammonium salts are commonly used for this purpose in the deliming operation. Consequently, wastewater from the
∗
Corresponding author. Tel.: +34 96 3877630; fax: +34 96 3877639. E-mail address: [email protected] (J.A. Mendoza-Roca). Received 26 March 2012; Received in revised form 19 May 2012; Accepted 2 August 2012 0263-8762/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cherd.2012.08.003 Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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Raw maer (hide from beamhouse operaons)
Water Ammonium salts / CO2 Surfactant
DELIMING AND BATING
2
ARTICLE IN PRESS Product (hide to pickling and tanning)
Wastewater
Volaleorganic acids Enzymes for bang
Reject stream to management
MF
Fig. 1 – Scheme of the process illustrating the two measures proposed: replacing of ammonium salts by CO2 and wastewater recycling after membrane filtration.
deliming operation is characterized by high nitrogen content, coming from both the hide structure and from the ammonium sulfate used as auxiliary chemical. Nitrogen removal from domestic wastewater is easily achieved with the conventional biological nitrification/denitrification processes in activated sludge wastewater treatment plants (WWTP). However, nitrogen removal from tannery wastewater is more complex, since ammonium oxidizing bacteria are difficult to maintain in the reactor biomass due to the high conductivity and to the presence of inhibiting substances (Murat et al., 2002). Therefore, from an environmental point of view, the reduction in nitrogen concentration in the tannery wastewater is of paramount importance. In order to reduce the nitrogen concentration in a tannery wastewater two concomitant measures are studied in this work: the replacement of ammonium salts by carbon dioxide (CO2 ) in the deliming process and the reuse of wastewater and chemicals after membrane filtration of the deliming/bating liquor (Fig. 1). The strategy used is in agreement with the approach of Sutter (Sutter, 1987). This author described a process considering that an improvement in the effluent can be achieved by changing the raw matter or the auxiliary matters, by changing the process itself or its efficiency or by reusing direct or after a separation process water and chemicals from the effluent. The replacement of ammonium salts in the deliming has been studied by several authors (Klaasse, 1990; C¸olak and Kilic¸, 2008; Kolomaznik et al., 1996; Yuansen et al., 2009). It is no doubt that ammonium salts (ammonium sulfate and ammonium chloride) are ideally suited to meet the technical requirements of the deliming operation due to the quick reaction with the lime, their low cost and their buffer properties which avoid a diminution in the hide quality even if these auxiliary chemicals are added in excess (Klaasse, 1990). C¸olak and Kilic¸ (2008) studied the effect of deliming sheep skin exclusively with weak acids. Particularly, these authors tested boric acid, acetic acid and citric acid achieving total nitrogen reductions in wastewater ranging from 77 to 95%. However, the cost associated to the high organic acid consumption and the increase in the effluent COD (because of the acetic acid and citric acid) was not considered. Kolomaznik et al. (1996) tested magnesium lactate to replace the ammonium salts, and although the deliming was carried out successfully the cost of the chemical made it unfeasible. Thus,
these authors suggested the recovery of this chemical from dairy wastewaters. Yuansen et al. (2009) proposed an alternative liming using sodium silicate and enzyme that implied the absence of the conventional deliming and subsequently the diminution of the total nitrogen in the wastewater. This reduction in the total nitrogen in the wastewater was around 50%. Carbon dioxide (CO2 ) seems to be the most appropriate auxiliary chemical for replacing ammonium salts. Using CO2 , the hide pH is reduced more gradually and its main advantages are the easy handling by operators and the COD reduction in the effluent when it is compared with the organic acids (Klaasse, 1990). On the other hand, the reuse of different single exhausted baths of the various wet-phase operations in tanneries either directly or after filtration by membranes has been suggested by several authors for operations previous to tanning. Cassano et al. (1997, 2001) performed experiments on the reuse of unhairing and degreasing wastewaters after ultrafiltration (UF) and proposed for further research the reuse of deliming–bating and pickling wastewaters after UF and reverse osmosis (RO), respectively. More recently, new advances in the reuse of the unhairing/liming and degreasing wastewaters after UF (De Pinho, 2009; Brites Alves and Silva, 2006; Mendoza-Roca et al., 2010; Wang et al., 2011) and in the recycling of unhairing wastewater in the soaking operation (Galiana-Aleixandre et al., 2011) have been reported. Focusing on the deliming wastewater, it has to be mentioned that little research efforts have been devoted to the treatment or reuse of the wastewater from this wet operation. A few authors propose the separated treatment of this effluent. In this way, Zengin et al. (2002) proposed the nitrogen removal by means of precipitation as magnesium ammonium phosphate and recently other authors proposed the reuse of the deliming/bating effluent to diminish the ammonium concentration in wastewater and to save water (Hu et al., 2011; Aquim et al., 2010; Gutterres et al., 2010). However, Zhang et al. (2009) warned about possible damages in the hides (rough grain) if the liquors were directly reused. Thus, membrane filtration of the liquor may be an appropriate technique in order to avoid this hide quality deterioration. The aim of this work is to provide experimental results from two concomitant measures to treat the wastewater coming from the deliming and bating operations in a tannery.
Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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These measures are the replacement of ammonium salts by carbon dioxide in the deliming process and the reuse of wastewater and chemicals after membrane filtration of the deliming/bating liquor. Technical viability of the proposal is assessed analyzing the final leather quality in terms of both visual inspection and phenolphthalein test.
2.
Materials and methods
2.1.
Methodology
In this work, experiments can be divided into the four following groups: - Characterization of the two different wastewaters resulting from deliming baths with and without ammonium salts addition. - Pre-treatment study to avoid membrane damage and minimize membrane fouling. - Membrane tests varying the transmembrane pressure. Two membranes are compared, one with separation properties in the range of microfiltration (MF) and the other in the range of the ultrafiltration (UF). - Reuse tests using membrane permeate as deliming bath.
2.2.
Wastewater samples
Wastewater samples were provided by a tannery located in Comunidad Valenciana (Spain). They were taken from the deliming/bating residual bath once the process drum had been emptied and the wastewater had been collected in a stirred tank. Samples from two different deliming processes were studied: deliming with ammonium salts (DAS) and deliming using carbon dioxide instead of the ammonium salts (DCD). The DAS process was carried out with ammonium sulfate. Carbon dioxide for the DCD process was supplied to the deliming drum from a pressurized tank connected to the drum, where was mixed with the deliming solution. In the DCD process, organic volatile acids (lactic and acetic acids) have to be added to the bath in order to decrease the operation time.
2.3.
Analyses
Chemical oxygen demand (COD), total nitrogen (TN) and Calcium were measured by means of cell tests from MERCK. COD and TN were analyzed both in raw and filtered (0.45 m) samples. For the COD measurements samples were diluted 1:1 since the range of the cell tests was 500–10,000 mg/L (ref. 14555). For the calcium analysis the dilution used was 1:5 (measurement with cell tests for the range 10–250 mg/L, ref. 00854). Finally, TN was measured after diluting 1:20 the wastewater samples (measurement with cell tests for the range 10–150 mg/L, ref. 14763). pH and conductivity were measured with a CRISON pH Meter GLP 21+ and a CRISON EC-Meter GLP 31+, respectively. Suspended solids were measured in triplicate for each sample. Cellulose acetate membrane filters (0.45 m, 47 mm of diameter) from LABSCIENCE were used. Particle size distribution (PSD) was determined by means of Mastersizer 2000. This equipment measures the particle size by laser diffraction; thereby the measurements are based on the particle volume. Thus, if there are a few large particles, results would indicate a great quantity of large particles in percentage. However, results can be converted in particle number
3
by a mathematical procedure in the equipment software. In this way, the results will be presented and discussed in terms of PSD particle number.
2.4.
Pre-treatment experiments
Four different types of experiments were tested to determine their suitability for being used in this particular application as membrane pre-treatment: Sedimentation, Selective protein precipitation, Jar-Tests and Filtration with 5 and 8 m pore size filters. Sedimentation was tested in glass-clear Imhoff 1-L sedimentation cones, allowing the wastewater samples to settle for 4 h. Protein precipitation at its isoelectric point was carried out reducing gradually the pH of 500 mL wastewater samples with hydrochloric acid 37%. Jar-tests were carried out in a multiple stirrer apparatus from SELECTA. The procedure consisted in introducing 900 mL of the samples in the jars; then, the coagulant was added and rapidly mixed (120 rpm) for 3 min. After that, the paddles velocity was decreased down to 30 rpm and the flocculant was added. The jars were stirred for 15 min and then the samples were settled for 30 min. Ferric chloride (30,000 mg/L solution prepared from 30% aqueous solution supplied by PANREAC) was used as coagulant and an anionic polyelectrolyte (FLOCUDEX-AS/15 supplied by LAMIRSA) was added as flocculant. Coagulant concentration used in the tests ranged between 100 and 400 mg/L and the polyelectrolyte concentration was maintained constant (0.5 mg/L). Turbidity in the supernatants was measured with a turbidimeter from DINKO. Security filtration was performed with cellulose acetate membrane filters with 0.5 m and 0.8 m pore size from LABSCIENCE.
2.5.
Membrane experiments
Two types of membranes were tested and compared: Microfiltration membrane and Ultrafiltration membrane. The MF membrane used was a poliethersulfone (PES) membrane from ORELIS with a pore diameter 0.1 m. The tested UF membrane was a membrane typically used for Membrane Biological Reactor (MBR) applications supplied by Microdyn-Nadir. It is also a PES membrane and its molecular weight cut-off is 150 kDa (equivalent to 0.04 m). This UF membrane has to be operated at very low transmembrane pressure (PT up to 0.4 bar). PT is set up in a MBR by vacuum conditions at the membrane permeate side. However, in this research, the TMP was set up by increasing the pressure at the membrane feed side. The used membrane module was RAYFLOW X100 supplied by TECHSEP. This is a module for testing flat-sheet membranes. The membrane active surface was 100 cm2 . Fig. 2 shows a scheme of the laboratory plant used for the experiments. Temperature was maintained at 20 ◦ C by the thermostatic bath and transmembrane pressure was adjusted by the valve located at the retentate side of the module. Both retentate and permeate streams were recycled back to the feed tank in order to maintain a constant feed concentration. Fluxes were calculated as the quotient between the measured permeate flow rate and the active surface (L m−2 h−1 ). Membrane permeability (L m−2 h−1 bar) was measured before and after UF/MF with deliming/bating wastewaters using deionized water. The fouling process was quantitatively assessed using the resistance in series model; thereby the
Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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ultrafiltrated for half an hour in order to measure Jw . After that, the membrane was fed with the deliming wastewater and consequently Jww was measured. At last, the cake was withdrawn from the membrane surface by rinsing the membrane with was measured. deionized water and Jw
7 8
5 6
2.6.
4
Reuse tests were performed with the membrane permeate. Samples of 3 L were put in a battery of 5 little drums (STENI tannery plants and automation) of 5 L of maximum capacity each one. The rotating speed was 22 rpm. In these drums, hide samples of 20 cm × 20 cm were delimed. The quality of the process was checked by optical comparison with original deliming liquor and by adding phenolphthalein to the hide after cutting a hide sample. In this test, no color observation means that the deliming will have fulfilled its aim since the pH reduction would imply that lime has been removed. On the other hand, when the hide is not correctly delimed, the phenolphthalein added would be coloured pink.
9
3
Reuse tests
1 2
1. Feed tank. 2. Thermostac bath 3. Feed pump. 4. Flowmeter. 5. Manometer. 6. Membrane module 7. Retentatevalve. 8. Reject stream. 9. Permeatestream Fig. 2 – Scheme of the membrane laboratory-scale wastewater treatment plant. total resistance (Rt ) was calculated as the sum of the intrinsic membrane resistance (Rm ), the resistance caused by particles deposited (or weakly attached) onto the membrane surface (i.e., reversible fouling which can be removed by backwashing) (Rc ) and the resistance caused by pore obstruction and strong attachment of compounds into membrane pores or onto the surface (i.e., irreversible fouling which cannot be removed by physical cleaning) (Rf ). Eqs. (1)–(3) were used to calculate each term. Rm =
PT · Jw
(1)
Rf =
PT − Rm · J w
(2)
RC =
PT − Rm − Rf · Jww
(3)
where J is the permeate flux (m3 m−2 s−1 ): Jw the permeate flux the permeate flux after removfiltrating deionized water, Jw ing the superficial fouling layer with deionized water, Jww the permeate flux filtrating deliming/bating wastewater at steadystate conditions; PT the transmembrane pressure (Pa) and is the dynamic viscosity of the permeate (Pa s). The different fluxes were determined according to the procedure by Bae and Tak (2005). Thus, deionized water was firstly
3.
Results
3.1.
Characterization of the deliming wastewaters
Table 1 shows the mean values and the standard deviation of the measured parameters in the wastewater samples characterization of both deliming baths (DAS and DCD). Comparing the mean values obtained, the pH of the DCD samples was significantly lower than the pH of DAS samples, which is an advantage since pH has to be lowered to very low values in the following process in the tannery. Thus, the lower the pH in the deliming bath, the lower the chemical dosage in subsequent stages of the process. Conductivity of the DAS samples was always higher than that measured for the DCD samples, which is explained by the presence of ammonium salts and by the high specific conductivity of the OH− ions, which are in the DAS wastewater due to the high pH (near 9). Concerning calcium ions, the remaining concentrations are very similar, which implies that both deliming baths have a similar efficiency in the process of calcium removal from the hide. COD values were significantly higher for the DCD wastewater due to the addition of high concentrations of volatile organic acids in the process to gradually reduce the pH. This COD can be considered as rapidly biodegradable; thereby it can be easily eliminated by biological treatment in a conventional WWTP. From an environmental point of view, the reduction in nitrogen concentration in the wastewater from a tannery is of paramount importance. This reduction is achieved in this
Table 1 – Characterization of the wastewater samples from DAS and DCD deliming baths (n = 15). DAS Mean value pH Conductivity (mS/cm) Calcium (mg Ca2+ /L) Total COD (mg O2 /L) Soluble COD (mg O2 /L) TN (mg/L) Soluble TN (mg/L) Suspended solids (mg/L)
8.7 25.2 665 15,347 5310 1679 1025 2350
DCD Standard deviation 0.5 4.1 201 4379 2567 28 22 198
Mean value 6.9 14.74 642 21,948 9835 1257 635 2822
Standard deviation 0.3 3.26 199 2465 2255 32 15 220
Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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Table 2 – Suspended solids concentrations removal with filters of 8 and 5 m. After 8 m filtration
Raw sample Suspended solids (mg/L) Removed suspended solids (%)
2844 –
2364 16.9
work by replacing the ammonium salts by carbon dioxide in the deliming process (soluble TN was reduced from 1025 mg/L to 635 mg/L). In spite of this diminution, a total nitrogen concentration near 1200 mg/L was measured in the DCD samples, which was due to both solubilized proteins from the hide and to the enzymes coming from the bating. The enzymes added in the bating operation and the particulate impurities removed from the hide explain the high suspended solid concentrations in the wastewaters of both types of deliming (recall that deliming and bating are carried out in the same drum and consequently the wastewater comes from both operations). In the following sections, results of the regeneration and reuse of the deliming wastewaters are exclusively referred to DCD deliming exhausted baths, since the reduction in the total nitrogen achieved by replacing the ammonium salts by CO2 was considered significant and a clear advantage that will favour the use of this type of deliming.
3.2.
Pretreatment tests
To avoid membrane damage and minimize membrane fouling, different pretreatment strategies of the DCD samples before their membrane filtration were assessed: sedimentation, jartests, selective protein precipitation and filtration with 5 and 8 m pore size filters. No suspended solids removal was obtained with direct sedimentation of the samples; thereby jar tests using ferric chloride as coagulant and an anionic polyelectrolyte as flocculant were performed. Results of the jar-tests in terms of turbidity removal are shown in Fig. 3. As can be observed in Fig. 3, the optimal ferric chloride concentration was 300 mg/L since an increase in its concentration in 100 mg/L led to an almost negligible decrease in turbidity (5.5%). Anyway, results were not good enough since only a 29% of turbidity and 21% of suspended solids removal efficiencies were achieved at the selected FeCl3 concentration. Total COD and total nitrogen removal efficiencies were 18.9% and 22.2%, respectively. These low removal efficiencies imply that it is not worth the addition of chemicals that increase the dissolved solids in wastewater and the economical cost of the process.
2328 18.1
Another strategy tested in the pretreatment was the precipitation of proteins at the isoelectric point. This is based on the variation of the protein solubility with the pH. The minimum solubility is reached with proteins not charged, and this occurs at a particular pH (isoelectric point). The pH was gradually lowered with HCl as commented in materials and method section. The best results were obtained at pH 4.1 (it was adopted as isoelectric point). However, total COD and total nitrogen removal efficiencies were very similar to those obtained with the jar-tests and turbidity of the supernatants was even higher than the wastewater turbidity (480.3 NTU, i.e. 20% higher than the turbidity of the wastewater) since wastewater changed its colour to white. This wastewater colour change was probably due to a degradation of the surfactant used as auxiliary chemical in the deliming and to calcium precipitation, which was checked by measuring calcium in the supernatant (38% of calcium was eliminated). Finally, filtration studies using filters with different pore sizes were carried out. This study was performed with a DCD sample with a suspended solids concentration of 2844 mg/L. Table 2 illustrates the results obtained. As can be observed in Table 2, only 18.1% of suspended solids removal was achieved with a 5-m filter. This result indicates that the size of most of the suspended solids was close to 0.45 m. In order to check it, particle size distribution measurements were carried out for the raw wastewater sample and for the filtrates from the 5 and 8 m filters. The results of PSD in particle number are shown in Fig. 4. As can be observed in Fig. 4, according to the results of the PSD analysis, the size of the 90% of the particles in raw wastewater is below 1.195 m. These results explain the difficulty in removing by physical or physic-chemical methods the suspended solids of the samples. In fact, the results of the PSD in terms of percentiles 10, 50 and 90 are very similar for the filtrates. According to these results, the only recommended pretreatment was a security filtration in case the wastewater sample could eventually contain large particles that could damage the membrane. This filtration was done with a 8-m filter. RAW WW
8 microns
5 microns
20
450
18
400
16
350
14
300
Number (%)
Turbidity (NTU)
After 5 m filtration
250 200 150
12 10 8 6
100
4
50
2
0 0
50
100
150
200
250
300
350
400
FeCl3 concentraon (mg/L)
Fig. 3 – Turbidity in the jar-tests for different ferric chloride concentrations (flocculant concentration = 0.5 mg/L).
0 0.01
0.1
1
10
Particle size (µm)
Fig. 4 – Results of the particle size distribution analysis (m) using Mastersizer 2000.
Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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(a)
(b)
30
20
20 15
TMP =0.5 bar TMP =1 bar
10
TMP =1.5 bar
Flux (L·m -2 ·h -1 )
Flux (L·m -2 ·h -1 )
25 15
10
5
5 0 0
20
40
60
80
0
100
0
20
40
Time (min)
60
80
100
Time (min)
Fig. 5 – Flux evolution in the membrane filtration tests: (a) MF membrane; (b) UF membrane.
3.3. Membrane experiments varying the transmembrane pressure Fig. 5 shows the variation of the flux along time in the membrane filtration tests. For the MF membrane, three transmembrane pressures (TMP) were tested. It can be observed that higher fluxes were obtained with higher TMP as expected. After the initial flux decay, permeate fluxes remained practically constant for the three studied TMPs. Due to the characteristics of the UF membrane used in this research, the only tested TMP was 0.3 bar. The flux behaviour in the experiment with the UF membrane was very similar to that observed for the MF membrane at 0.5 bar. The application of this UF membrane, specifically designed for Membrane Bioreactors (MBR), to conventional UF could be promising for treating small wastewater volumes, i.e. for applications in that operating at low TMP makes it economically unfeasible. Table 3 shows the distilled water permeability of the MF membrane before and after the filtration of the deliming/bating wastewater (after water rinsing), and the corresponding permeability decrease (in percentage) which indicates the irreversible fouling that had to be eliminated via chemical cleaning. As can be observed, the highest irreversible fouling was produced at the highest TMP as expected, although values were relatively similar (ranging from 20 to 28%). For the UF membrane the irreversible fouling clearly dominated the permeability reduction reaching a 83% of permeability decrease (from 583.53 L m−2 h−1 bar−1 of initial permeability to 95.23 L m−2 h−1 bar−1 ). This fact can be due to the operation at 0.3 bar of TMP which is close to the maximal recommended for the type of UF membrane used in this research. Therefore, the results are not comparable with those obtained for the MF membrane. For this reason, the MF
Fig. 6 – Contribution of the membrane resistance (Rm ), cake resistance (Rc ) and fouling resistance (Rf ) to the total membrane resistance in the MF membrane test (TMP = 1.5 bar). membrane was selected for continuing the tests, and from now on all the reported results are referred to this membrane. It has to be explained that UF membrane permeability was very high because it is a membrane designed for MBR applications as mentioned in Materials and Methods section; thereby the working TMP is very limited, what implies in the practice lower permeate fluxes than those obtained with the MF membrane as expected. The results of the resistance in series model (Fig. 6) showed that the resistance caused by particles deposited (or weakly attached) onto the membrane surface (Rc ) was higher than the others, thus reversible fouling (which can be removed by the backwashing shear forces) predominated over the irreversible fouling (which requires chemical cleaning). The total resistance (Rt ) at the end of the microfiltration at 1.5 bar TMP was 1.88323 × 1013 m−1 .
Table 3 – Distilled water permeability of the membrane before and after the MF tests. Initial permeability (L m−2 h−1 bar−1 ) TMP = 0.5 bar TMP = 1 bar TMP = 1.5 bar
Permeability after water rinsing (L m−2 h−1 bar−1 )
223.7 216.86 211.57
178.96 162.65 152.33
% permeability decrease 20 25 28
Table 4 – Characterization of the permeates of the MF tests. Permeate COD (mg/L) TMP = 0.5 bar TMP = 1 bar TMP = 1.5 bar
13,100 11,700 11,360
COD rejection (%) 36.0 42.9 44.5
Permeate total nitrogen (mg/L) 900 850 830
Total nitrogen rejection (%) 23.1 27.3 29.0
Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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Fig. 7 – (a) Hide samples in the laboratory drums in the deliming process with the membrane permeate. (b) Results of the phenolphthalein tests obtained after reusing the permeate (left side of the figure) compared to the results of a incorrectly delimed sample (right side of the figure). Table 4 shows the characterization of the permeate and the rejection of the MF membrane in terms of COD and total nitrogen. It should be highlighted that COD and total nitrogen concentrations in the permeate are still very high. For the COD, this is explained by the presence of peptidic chains of low molecular weight coming from the hidrolized proteins of the hide and by the use of high concentrations of volatile organic acids in the process. Regarding the total nitrogen, the main contribution is given by the amino acids from the peptidic chains coming from the hydrolyzed proteins of the hide which can cross the membrane. Thus, these elevated COD and TN concentrations would not affect negatively the possible reuse of the permeate. Anyway, this hypothesis will be experimentally verified in the following section. Regarding the rejection values shown in Table 4, it has to be commented that the rejection percentages increased with the TMP increase due to a higher water passage through the membrane. Considering the effect of both measures implemented in this work to reduce the nitrogen concentration in a tannery wastewater, the results have shown that TN was reduced from 1679 mg/L to 1257 mg/L due to the replacement of ammonium salts by carbon dioxide (CO2 ) in the deliming process and from 1257 mg/L to 900 mg/L after membrane filtration of the deliming/bating liquor. Both measures have resulted in a 53% TN global reduction.
3.4.
Permeate reuse
Membrane permeate was reused in the deliming process with the aim of saving water and auxiliary chemicals. To experimentally demonstrate the technical viability of reusing the permeate from the deliming/bating liquor, hide samples were set in the laboratory drums together with the membrane permeate (Fig. 7a). Once the process had finished, the hide samples were taken out of the drums and supervised, examining their appearance and the pH lowering after lime removal. According to the technicians from the tannery, the visual and other organoleptic properties of the hide after reusing the permeate of the first exhausted bath of the deliming/bating operations were excellent and identical to those obtained when fresh water was used. Moreover, phenolphthalein test resulted satisfactory (left side of Fig. 7b): since the deliming was correct, the hide did not change its colour. Recall that when the hide is not correctly delimed, the phenolphthalein added through the
hide after cutting it is coloured pink, as it is shown as example in the right side of Fig. 7b.
4.
Conclusions
In this work two concomitant measures to reduce the nitrogen concentration in a tannery wastewater have been implemented and experimentally assessed. These measures were the replacement of ammonium salts by carbon dioxide in the deliming process and the reuse of wastewater and chemicals after membrane filtration of the deliming/bating liquor. The main conclusions that can be drawn from this study are: • The proposal is technically feasible, since the final leather quality is excellent according to both visual and organoleptic inspection from process technicians and phenolphthalein test. • The implementation of both measures have resulted in 53% total nitrogen reduction when compared with deliming using ammonium salts and not reusing. • The COD concentration in the wastewater from the deliming using CO2 is higher than that when ammonium salts are used, due to the addition of volatile organic acids to gradually reduce the pH. However, since this COD is biodegradable, it can be easily removed in the biological treatment of a conventional wastewater treatment plant. • The characterization of the deliming/bating liquor samples, confirms the significant pollution from these wet operations of the tanning industry. • Both deliming baths (using ammonium salts and using CO2 ) have shown a similar efficiency in the process of calcium removal from the hide. • Deliming/bating liquor pretreatment tests (jar tests and selective protein precipitation) have shown to be unnecessary. Thus, only a security filtration (8 m filter) prior to membrane filtration was recommended. • The small size of particles in deliming/bating wastewater (90% or particles ≤1.195 m) joined with their low density, made solids settling an ineffective treatment option. • The MF membrane used was suitable for the treatment of the deliming/bating wastewaters both from the point of view of the flux and of the rejection. The UF membrane tested, specifically designed for MBR applications, showed severe fouling as it was operated near the maximal recommended TMP.
Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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• The cake resistance (i.e., the reversible fouling which can be removed by backwashing) was the predominant resistance in the deliming/bating microfiltration. Summarizing, the reuse of the wastewater from the deliming/bating operations provides environmental and economic benefits due to the water consumption reduction and the reduction in nitrogen and salts discharge, which are essential for the sustainable development of the leather industry in the future. However, further research is needed to determine how many times the first exhausted bath can be reused without reduction in the final leather quality, and the economical viability of the proposal.
Acknowledgement Authors thank Industrias del Curido S.A. (INCUSA) for its support in this work.
References Aquim, P.M., Gutterres, M., Trierweiler, J.O., 2010. Assessment of water management in tanneries. J. Soc. Leather Technol. Chem. 94, 253–258. Bae, T.H., Tak, T.M., 2005. Interpretation of fouling characteristics of ultrafiltration membranes during the filtration of membrane bioreactor mixed liquor. J. Membr. Sci. 264, 151–160. Brites Alves, A.M., Silva, N., 2006. Recovery of water and sulfides from liming wastewaters. Desalination 200, 748–749. Cassano, A., Drioli, E., Molinari, R., 1997. Recovery and reuse of chemicals in unhairing degreasing and chromium tanning processes by membranes. Desalination 113, 251–261. Cassano, A., Molinari, R., Romano, M., Drioli, E., 2001. Treatment of aqueous effluents of the leather industry by membrane processes. A review. J. Membr. Sci. 181, 111–126. C¸olak, S.M., Kilic¸, E., 2008. Deliming with weak acids: effects on leather quality and effluent. J. Soc. Leather Technol. Chem. 92, 120–123. De Pinho, M.N., 2009. Membrane-based treatment for tanning wastewaters. Can. J. Civ. Eng. 36, 356–362. 2003. European Commission Reference Document on Best Available Techniques for the Tanning of Hides and Skins.
Galiana-Aleixandre, M.V., Mendoza-Roca, J.A., Bes-Piá, A., 2011. Reducing sulfates concentration in the tannery effluent by applying pollution prevention techniques and nanofiltration. J. Clean Prod. 19, 91–98. Gutterres, M., Passos, J.B., Aquim, P.M., Severo, L.S., Trierweiler, J.O., 2008. Reduction on water demand and treatment cost in tanneries through reuse techniques. J. Am. Leather Chem. Assoc. 103, 138–143. Gutterres, M., Aquim, P.M., Passos, J.B., Trierweiler, J.O., 2010. Water reuse in tannery beamhouse process. J. Clean Prod. 18, 1545–1552. Hu, J., Xiao, Z., Zhou, R., Deng, W., Wang, M., Ma, S., 2011. Ecological utilization of leather tannery waste with circular economy model. J. Clean Prod. 19, 221–228. Klaasse, M.J., 1990. CO2 deliming. J. Am. Leather Chem. Assoc. 85, 431–441. Kolomaznik, K., Blaha, A., Dedrle, T., Bailey, D.G., Taylor, M.M., 1996. Non-ammonia deliming of cattle hides with magnesium lactate. J. Am. Leather Chem. Assoc. 91, 18–20. Mendoza-Roca, J.A., Galiana-Aleixandre, M.V., Lora-García, J., Bes-Piá, A., 2010. Purification of tannery effluents by ultrafiltration in view of permeate reuse. Sep. Purif. Technol. 70, 296–301. Murat, S., Genceli, E.A., Tasli, R., Artan, N., Orhon, D., 2002. Sequencing batch reactor treatment of tannery wastewater for carbon and nitrogen removal. Water Sci. Technol. 46 (9), 219–227. Sutter, H., 1987. Vermeidung and Verwertung von Sonderabfällen, 3rd ed. Erich Schmidt Verlag, Berlin. Wang, H., Wang, Y., Zhou, L., 2011. Purification and recycling of tannery degreasing wastewater by ultrafiltration with polyimide membrane. In: International Conference on Remote Sensing, Environment and Transportation Engineering (RSETE), http://dx.doi.org/10.1109/RSETE.2011.5964341. Yuansen, L., Chen, Y., Haoujun, F., Zhou, H., Zhaoyang, L., Shi, B., 2009. Environment friendly lime-free liming. J. Soc. Leather Technol. Chem. 93, 56–60. Zengin, G., Ölmez, T., Dogruel, S., Kabdaslı, I., Tünay, O., 2002. Assessment of source-based nitrogen removal alternatives in leather tanning industry wastewater. Water Sci. Technol. 12, 205–215. Zhang, Z.D., Yang, Y.L., Xu, F., Feng, C.Q., Liu, T., 2009. Recycling of wastewater from raw hide to wet blues in leather manufacture. In: XXX Congress of the International Union of Leather Technologists & Chemists Societies, Proceedings, pp. 119–128.
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Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes A. Gallego-Molina a , J.A. Mendoza-Roca a,∗ , D. Aguado b , M.V. Galiana-Aleixandre a a
Universitat Politècnica de València, Instituto de Seguridad Ambiental, Radiofísica y Medio Ambiente, Camino de Vera s/n, 46022 Valencia, Spain b Universitat Politècnica de València, Instituto de Ingeniería del Agua y Medio Ambiente, Camino de Vera s/n, 46022 Valencia, Spain
a b s t r a c t This paper presents experimental results from the implementation of two measures aimed at reducing the nitrogen concentration in a tannery wastewater. Specifically, this research has focused on the wastewater from the deliming/bating operations. The proposed measures are the replacement of ammonium salts by carbon dioxide in the deliming process and the reuse of wastewater and chemicals after membrane filtration of the deliming/bating liquor. The experimental study covered different wastewater pretreatment alternatives and experiments with two membranes (with different separation properties): one in the range of microfiltration (MF) and one in the range of the ultrafiltration (UF). Results of the pretreatment study indicated that neither settling nor protein precipitation were feasible. Only a security filtration prior to membrane filtration was recommended. The tested MF membrane was selected due to the higher flux (around 25 L/(m2 h)) in comparison with the UF membrane. The MF permeate was successfully reused in the deliming/bating process. The delimed leather quality was excellent according to both visual and organoleptic inspection from process technicians and phenolphthalein test, confirming the technical feasibility of the proposal. Globally, the implementation of the above mentioned two measures resulted in 53% total nitrogen reduction. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Bating; Deliming; Membranes; Microfiltration; Tannery wastewater; Wastewater reuse
1.
Introduction
Tanneries generate high quantities of wastewater that are characterized by high concentrations of organic matter, nitrogen, trivalent chromium, sulfate and other ions. These pollutants come from both the raw hides (mainly conservation salt and non useful parts) and the auxiliary chemicals added in the wet operations involved. The first operations in a tannery process prepare the hides for the tanning, which is the operation where the hide (a degradable stuff) is converted into leather (a non degradable stuff). These previous operations consist of soaking, for the removal of the conservation salt and impurities, liming/unhairing for epidermis and hair removal, flesh removal, for eliminating this part of the hide, deliming, for the removal of the lime added in the lime/unhairing and for pH lowering and bating, for the preparation of the collagen fibres before the
tanning and cleaning the hides removing impurities from the previous operations by means of enzymes. Deliming and bating operations are carried out in the same drum. The enzymes needed for the bating are added after deliming without a previous discharge of the deliming liquor. Thereby, the discharge from the drum can be considered as wastewater coming from both operations (deliming and bating). A typical tannery treating bovine hides discharges between 1 and 4 m3 /t of wastewater in this operation (European Commission, 2003; Gutterres et al., 2008). As previously indicated, the aim of the deliming is to remove the lime that was previously added in the liming/unhairing process to raise the pH up to 12–13 in order to open up the fibrillar structure by swelling to remove the epidermis from the hide. Since the pH should be slowly lowered, ammonium salts are commonly used for this purpose in the deliming operation. Consequently, wastewater from the
∗
Corresponding author. Tel.: +34 96 3877630; fax: +34 96 3877639. E-mail address: [email protected] (J.A. Mendoza-Roca). Received 26 March 2012; Received in revised form 19 May 2012; Accepted 2 August 2012 0263-8762/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cherd.2012.08.003 Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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Raw maer (hide from beamhouse operaons)
Water Ammonium salts / CO2 Surfactant
DELIMING AND BATING
2
ARTICLE IN PRESS Product (hide to pickling and tanning)
Wastewater
Volaleorganic acids Enzymes for bang
Reject stream to management
MF
Fig. 1 – Scheme of the process illustrating the two measures proposed: replacing of ammonium salts by CO2 and wastewater recycling after membrane filtration.
deliming operation is characterized by high nitrogen content, coming from both the hide structure and from the ammonium sulfate used as auxiliary chemical. Nitrogen removal from domestic wastewater is easily achieved with the conventional biological nitrification/denitrification processes in activated sludge wastewater treatment plants (WWTP). However, nitrogen removal from tannery wastewater is more complex, since ammonium oxidizing bacteria are difficult to maintain in the reactor biomass due to the high conductivity and to the presence of inhibiting substances (Murat et al., 2002). Therefore, from an environmental point of view, the reduction in nitrogen concentration in the tannery wastewater is of paramount importance. In order to reduce the nitrogen concentration in a tannery wastewater two concomitant measures are studied in this work: the replacement of ammonium salts by carbon dioxide (CO2 ) in the deliming process and the reuse of wastewater and chemicals after membrane filtration of the deliming/bating liquor (Fig. 1). The strategy used is in agreement with the approach of Sutter (Sutter, 1987). This author described a process considering that an improvement in the effluent can be achieved by changing the raw matter or the auxiliary matters, by changing the process itself or its efficiency or by reusing direct or after a separation process water and chemicals from the effluent. The replacement of ammonium salts in the deliming has been studied by several authors (Klaasse, 1990; C¸olak and Kilic¸, 2008; Kolomaznik et al., 1996; Yuansen et al., 2009). It is no doubt that ammonium salts (ammonium sulfate and ammonium chloride) are ideally suited to meet the technical requirements of the deliming operation due to the quick reaction with the lime, their low cost and their buffer properties which avoid a diminution in the hide quality even if these auxiliary chemicals are added in excess (Klaasse, 1990). C¸olak and Kilic¸ (2008) studied the effect of deliming sheep skin exclusively with weak acids. Particularly, these authors tested boric acid, acetic acid and citric acid achieving total nitrogen reductions in wastewater ranging from 77 to 95%. However, the cost associated to the high organic acid consumption and the increase in the effluent COD (because of the acetic acid and citric acid) was not considered. Kolomaznik et al. (1996) tested magnesium lactate to replace the ammonium salts, and although the deliming was carried out successfully the cost of the chemical made it unfeasible. Thus,
these authors suggested the recovery of this chemical from dairy wastewaters. Yuansen et al. (2009) proposed an alternative liming using sodium silicate and enzyme that implied the absence of the conventional deliming and subsequently the diminution of the total nitrogen in the wastewater. This reduction in the total nitrogen in the wastewater was around 50%. Carbon dioxide (CO2 ) seems to be the most appropriate auxiliary chemical for replacing ammonium salts. Using CO2 , the hide pH is reduced more gradually and its main advantages are the easy handling by operators and the COD reduction in the effluent when it is compared with the organic acids (Klaasse, 1990). On the other hand, the reuse of different single exhausted baths of the various wet-phase operations in tanneries either directly or after filtration by membranes has been suggested by several authors for operations previous to tanning. Cassano et al. (1997, 2001) performed experiments on the reuse of unhairing and degreasing wastewaters after ultrafiltration (UF) and proposed for further research the reuse of deliming–bating and pickling wastewaters after UF and reverse osmosis (RO), respectively. More recently, new advances in the reuse of the unhairing/liming and degreasing wastewaters after UF (De Pinho, 2009; Brites Alves and Silva, 2006; Mendoza-Roca et al., 2010; Wang et al., 2011) and in the recycling of unhairing wastewater in the soaking operation (Galiana-Aleixandre et al., 2011) have been reported. Focusing on the deliming wastewater, it has to be mentioned that little research efforts have been devoted to the treatment or reuse of the wastewater from this wet operation. A few authors propose the separated treatment of this effluent. In this way, Zengin et al. (2002) proposed the nitrogen removal by means of precipitation as magnesium ammonium phosphate and recently other authors proposed the reuse of the deliming/bating effluent to diminish the ammonium concentration in wastewater and to save water (Hu et al., 2011; Aquim et al., 2010; Gutterres et al., 2010). However, Zhang et al. (2009) warned about possible damages in the hides (rough grain) if the liquors were directly reused. Thus, membrane filtration of the liquor may be an appropriate technique in order to avoid this hide quality deterioration. The aim of this work is to provide experimental results from two concomitant measures to treat the wastewater coming from the deliming and bating operations in a tannery.
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These measures are the replacement of ammonium salts by carbon dioxide in the deliming process and the reuse of wastewater and chemicals after membrane filtration of the deliming/bating liquor. Technical viability of the proposal is assessed analyzing the final leather quality in terms of both visual inspection and phenolphthalein test.
2.
Materials and methods
2.1.
Methodology
In this work, experiments can be divided into the four following groups: - Characterization of the two different wastewaters resulting from deliming baths with and without ammonium salts addition. - Pre-treatment study to avoid membrane damage and minimize membrane fouling. - Membrane tests varying the transmembrane pressure. Two membranes are compared, one with separation properties in the range of microfiltration (MF) and the other in the range of the ultrafiltration (UF). - Reuse tests using membrane permeate as deliming bath.
2.2.
Wastewater samples
Wastewater samples were provided by a tannery located in Comunidad Valenciana (Spain). They were taken from the deliming/bating residual bath once the process drum had been emptied and the wastewater had been collected in a stirred tank. Samples from two different deliming processes were studied: deliming with ammonium salts (DAS) and deliming using carbon dioxide instead of the ammonium salts (DCD). The DAS process was carried out with ammonium sulfate. Carbon dioxide for the DCD process was supplied to the deliming drum from a pressurized tank connected to the drum, where was mixed with the deliming solution. In the DCD process, organic volatile acids (lactic and acetic acids) have to be added to the bath in order to decrease the operation time.
2.3.
Analyses
Chemical oxygen demand (COD), total nitrogen (TN) and Calcium were measured by means of cell tests from MERCK. COD and TN were analyzed both in raw and filtered (0.45 m) samples. For the COD measurements samples were diluted 1:1 since the range of the cell tests was 500–10,000 mg/L (ref. 14555). For the calcium analysis the dilution used was 1:5 (measurement with cell tests for the range 10–250 mg/L, ref. 00854). Finally, TN was measured after diluting 1:20 the wastewater samples (measurement with cell tests for the range 10–150 mg/L, ref. 14763). pH and conductivity were measured with a CRISON pH Meter GLP 21+ and a CRISON EC-Meter GLP 31+, respectively. Suspended solids were measured in triplicate for each sample. Cellulose acetate membrane filters (0.45 m, 47 mm of diameter) from LABSCIENCE were used. Particle size distribution (PSD) was determined by means of Mastersizer 2000. This equipment measures the particle size by laser diffraction; thereby the measurements are based on the particle volume. Thus, if there are a few large particles, results would indicate a great quantity of large particles in percentage. However, results can be converted in particle number
3
by a mathematical procedure in the equipment software. In this way, the results will be presented and discussed in terms of PSD particle number.
2.4.
Pre-treatment experiments
Four different types of experiments were tested to determine their suitability for being used in this particular application as membrane pre-treatment: Sedimentation, Selective protein precipitation, Jar-Tests and Filtration with 5 and 8 m pore size filters. Sedimentation was tested in glass-clear Imhoff 1-L sedimentation cones, allowing the wastewater samples to settle for 4 h. Protein precipitation at its isoelectric point was carried out reducing gradually the pH of 500 mL wastewater samples with hydrochloric acid 37%. Jar-tests were carried out in a multiple stirrer apparatus from SELECTA. The procedure consisted in introducing 900 mL of the samples in the jars; then, the coagulant was added and rapidly mixed (120 rpm) for 3 min. After that, the paddles velocity was decreased down to 30 rpm and the flocculant was added. The jars were stirred for 15 min and then the samples were settled for 30 min. Ferric chloride (30,000 mg/L solution prepared from 30% aqueous solution supplied by PANREAC) was used as coagulant and an anionic polyelectrolyte (FLOCUDEX-AS/15 supplied by LAMIRSA) was added as flocculant. Coagulant concentration used in the tests ranged between 100 and 400 mg/L and the polyelectrolyte concentration was maintained constant (0.5 mg/L). Turbidity in the supernatants was measured with a turbidimeter from DINKO. Security filtration was performed with cellulose acetate membrane filters with 0.5 m and 0.8 m pore size from LABSCIENCE.
2.5.
Membrane experiments
Two types of membranes were tested and compared: Microfiltration membrane and Ultrafiltration membrane. The MF membrane used was a poliethersulfone (PES) membrane from ORELIS with a pore diameter 0.1 m. The tested UF membrane was a membrane typically used for Membrane Biological Reactor (MBR) applications supplied by Microdyn-Nadir. It is also a PES membrane and its molecular weight cut-off is 150 kDa (equivalent to 0.04 m). This UF membrane has to be operated at very low transmembrane pressure (PT up to 0.4 bar). PT is set up in a MBR by vacuum conditions at the membrane permeate side. However, in this research, the TMP was set up by increasing the pressure at the membrane feed side. The used membrane module was RAYFLOW X100 supplied by TECHSEP. This is a module for testing flat-sheet membranes. The membrane active surface was 100 cm2 . Fig. 2 shows a scheme of the laboratory plant used for the experiments. Temperature was maintained at 20 ◦ C by the thermostatic bath and transmembrane pressure was adjusted by the valve located at the retentate side of the module. Both retentate and permeate streams were recycled back to the feed tank in order to maintain a constant feed concentration. Fluxes were calculated as the quotient between the measured permeate flow rate and the active surface (L m−2 h−1 ). Membrane permeability (L m−2 h−1 bar) was measured before and after UF/MF with deliming/bating wastewaters using deionized water. The fouling process was quantitatively assessed using the resistance in series model; thereby the
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ultrafiltrated for half an hour in order to measure Jw . After that, the membrane was fed with the deliming wastewater and consequently Jww was measured. At last, the cake was withdrawn from the membrane surface by rinsing the membrane with was measured. deionized water and Jw
7 8
5 6
2.6.
4
Reuse tests were performed with the membrane permeate. Samples of 3 L were put in a battery of 5 little drums (STENI tannery plants and automation) of 5 L of maximum capacity each one. The rotating speed was 22 rpm. In these drums, hide samples of 20 cm × 20 cm were delimed. The quality of the process was checked by optical comparison with original deliming liquor and by adding phenolphthalein to the hide after cutting a hide sample. In this test, no color observation means that the deliming will have fulfilled its aim since the pH reduction would imply that lime has been removed. On the other hand, when the hide is not correctly delimed, the phenolphthalein added would be coloured pink.
9
3
Reuse tests
1 2
1. Feed tank. 2. Thermostac bath 3. Feed pump. 4. Flowmeter. 5. Manometer. 6. Membrane module 7. Retentatevalve. 8. Reject stream. 9. Permeatestream Fig. 2 – Scheme of the membrane laboratory-scale wastewater treatment plant. total resistance (Rt ) was calculated as the sum of the intrinsic membrane resistance (Rm ), the resistance caused by particles deposited (or weakly attached) onto the membrane surface (i.e., reversible fouling which can be removed by backwashing) (Rc ) and the resistance caused by pore obstruction and strong attachment of compounds into membrane pores or onto the surface (i.e., irreversible fouling which cannot be removed by physical cleaning) (Rf ). Eqs. (1)–(3) were used to calculate each term. Rm =
PT · Jw
(1)
Rf =
PT − Rm · J w
(2)
RC =
PT − Rm − Rf · Jww
(3)
where J is the permeate flux (m3 m−2 s−1 ): Jw the permeate flux the permeate flux after removfiltrating deionized water, Jw ing the superficial fouling layer with deionized water, Jww the permeate flux filtrating deliming/bating wastewater at steadystate conditions; PT the transmembrane pressure (Pa) and is the dynamic viscosity of the permeate (Pa s). The different fluxes were determined according to the procedure by Bae and Tak (2005). Thus, deionized water was firstly
3.
Results
3.1.
Characterization of the deliming wastewaters
Table 1 shows the mean values and the standard deviation of the measured parameters in the wastewater samples characterization of both deliming baths (DAS and DCD). Comparing the mean values obtained, the pH of the DCD samples was significantly lower than the pH of DAS samples, which is an advantage since pH has to be lowered to very low values in the following process in the tannery. Thus, the lower the pH in the deliming bath, the lower the chemical dosage in subsequent stages of the process. Conductivity of the DAS samples was always higher than that measured for the DCD samples, which is explained by the presence of ammonium salts and by the high specific conductivity of the OH− ions, which are in the DAS wastewater due to the high pH (near 9). Concerning calcium ions, the remaining concentrations are very similar, which implies that both deliming baths have a similar efficiency in the process of calcium removal from the hide. COD values were significantly higher for the DCD wastewater due to the addition of high concentrations of volatile organic acids in the process to gradually reduce the pH. This COD can be considered as rapidly biodegradable; thereby it can be easily eliminated by biological treatment in a conventional WWTP. From an environmental point of view, the reduction in nitrogen concentration in the wastewater from a tannery is of paramount importance. This reduction is achieved in this
Table 1 – Characterization of the wastewater samples from DAS and DCD deliming baths (n = 15). DAS Mean value pH Conductivity (mS/cm) Calcium (mg Ca2+ /L) Total COD (mg O2 /L) Soluble COD (mg O2 /L) TN (mg/L) Soluble TN (mg/L) Suspended solids (mg/L)
8.7 25.2 665 15,347 5310 1679 1025 2350
DCD Standard deviation 0.5 4.1 201 4379 2567 28 22 198
Mean value 6.9 14.74 642 21,948 9835 1257 635 2822
Standard deviation 0.3 3.26 199 2465 2255 32 15 220
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Table 2 – Suspended solids concentrations removal with filters of 8 and 5 m. After 8 m filtration
Raw sample Suspended solids (mg/L) Removed suspended solids (%)
2844 –
2364 16.9
work by replacing the ammonium salts by carbon dioxide in the deliming process (soluble TN was reduced from 1025 mg/L to 635 mg/L). In spite of this diminution, a total nitrogen concentration near 1200 mg/L was measured in the DCD samples, which was due to both solubilized proteins from the hide and to the enzymes coming from the bating. The enzymes added in the bating operation and the particulate impurities removed from the hide explain the high suspended solid concentrations in the wastewaters of both types of deliming (recall that deliming and bating are carried out in the same drum and consequently the wastewater comes from both operations). In the following sections, results of the regeneration and reuse of the deliming wastewaters are exclusively referred to DCD deliming exhausted baths, since the reduction in the total nitrogen achieved by replacing the ammonium salts by CO2 was considered significant and a clear advantage that will favour the use of this type of deliming.
3.2.
Pretreatment tests
To avoid membrane damage and minimize membrane fouling, different pretreatment strategies of the DCD samples before their membrane filtration were assessed: sedimentation, jartests, selective protein precipitation and filtration with 5 and 8 m pore size filters. No suspended solids removal was obtained with direct sedimentation of the samples; thereby jar tests using ferric chloride as coagulant and an anionic polyelectrolyte as flocculant were performed. Results of the jar-tests in terms of turbidity removal are shown in Fig. 3. As can be observed in Fig. 3, the optimal ferric chloride concentration was 300 mg/L since an increase in its concentration in 100 mg/L led to an almost negligible decrease in turbidity (5.5%). Anyway, results were not good enough since only a 29% of turbidity and 21% of suspended solids removal efficiencies were achieved at the selected FeCl3 concentration. Total COD and total nitrogen removal efficiencies were 18.9% and 22.2%, respectively. These low removal efficiencies imply that it is not worth the addition of chemicals that increase the dissolved solids in wastewater and the economical cost of the process.
2328 18.1
Another strategy tested in the pretreatment was the precipitation of proteins at the isoelectric point. This is based on the variation of the protein solubility with the pH. The minimum solubility is reached with proteins not charged, and this occurs at a particular pH (isoelectric point). The pH was gradually lowered with HCl as commented in materials and method section. The best results were obtained at pH 4.1 (it was adopted as isoelectric point). However, total COD and total nitrogen removal efficiencies were very similar to those obtained with the jar-tests and turbidity of the supernatants was even higher than the wastewater turbidity (480.3 NTU, i.e. 20% higher than the turbidity of the wastewater) since wastewater changed its colour to white. This wastewater colour change was probably due to a degradation of the surfactant used as auxiliary chemical in the deliming and to calcium precipitation, which was checked by measuring calcium in the supernatant (38% of calcium was eliminated). Finally, filtration studies using filters with different pore sizes were carried out. This study was performed with a DCD sample with a suspended solids concentration of 2844 mg/L. Table 2 illustrates the results obtained. As can be observed in Table 2, only 18.1% of suspended solids removal was achieved with a 5-m filter. This result indicates that the size of most of the suspended solids was close to 0.45 m. In order to check it, particle size distribution measurements were carried out for the raw wastewater sample and for the filtrates from the 5 and 8 m filters. The results of PSD in particle number are shown in Fig. 4. As can be observed in Fig. 4, according to the results of the PSD analysis, the size of the 90% of the particles in raw wastewater is below 1.195 m. These results explain the difficulty in removing by physical or physic-chemical methods the suspended solids of the samples. In fact, the results of the PSD in terms of percentiles 10, 50 and 90 are very similar for the filtrates. According to these results, the only recommended pretreatment was a security filtration in case the wastewater sample could eventually contain large particles that could damage the membrane. This filtration was done with a 8-m filter. RAW WW
8 microns
5 microns
20
450
18
400
16
350
14
300
Number (%)
Turbidity (NTU)
After 5 m filtration
250 200 150
12 10 8 6
100
4
50
2
0 0
50
100
150
200
250
300
350
400
FeCl3 concentraon (mg/L)
Fig. 3 – Turbidity in the jar-tests for different ferric chloride concentrations (flocculant concentration = 0.5 mg/L).
0 0.01
0.1
1
10
Particle size (µm)
Fig. 4 – Results of the particle size distribution analysis (m) using Mastersizer 2000.
Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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(a)
(b)
30
20
20 15
TMP =0.5 bar TMP =1 bar
10
TMP =1.5 bar
Flux (L·m -2 ·h -1 )
Flux (L·m -2 ·h -1 )
25 15
10
5
5 0 0
20
40
60
80
0
100
0
20
40
Time (min)
60
80
100
Time (min)
Fig. 5 – Flux evolution in the membrane filtration tests: (a) MF membrane; (b) UF membrane.
3.3. Membrane experiments varying the transmembrane pressure Fig. 5 shows the variation of the flux along time in the membrane filtration tests. For the MF membrane, three transmembrane pressures (TMP) were tested. It can be observed that higher fluxes were obtained with higher TMP as expected. After the initial flux decay, permeate fluxes remained practically constant for the three studied TMPs. Due to the characteristics of the UF membrane used in this research, the only tested TMP was 0.3 bar. The flux behaviour in the experiment with the UF membrane was very similar to that observed for the MF membrane at 0.5 bar. The application of this UF membrane, specifically designed for Membrane Bioreactors (MBR), to conventional UF could be promising for treating small wastewater volumes, i.e. for applications in that operating at low TMP makes it economically unfeasible. Table 3 shows the distilled water permeability of the MF membrane before and after the filtration of the deliming/bating wastewater (after water rinsing), and the corresponding permeability decrease (in percentage) which indicates the irreversible fouling that had to be eliminated via chemical cleaning. As can be observed, the highest irreversible fouling was produced at the highest TMP as expected, although values were relatively similar (ranging from 20 to 28%). For the UF membrane the irreversible fouling clearly dominated the permeability reduction reaching a 83% of permeability decrease (from 583.53 L m−2 h−1 bar−1 of initial permeability to 95.23 L m−2 h−1 bar−1 ). This fact can be due to the operation at 0.3 bar of TMP which is close to the maximal recommended for the type of UF membrane used in this research. Therefore, the results are not comparable with those obtained for the MF membrane. For this reason, the MF
Fig. 6 – Contribution of the membrane resistance (Rm ), cake resistance (Rc ) and fouling resistance (Rf ) to the total membrane resistance in the MF membrane test (TMP = 1.5 bar). membrane was selected for continuing the tests, and from now on all the reported results are referred to this membrane. It has to be explained that UF membrane permeability was very high because it is a membrane designed for MBR applications as mentioned in Materials and Methods section; thereby the working TMP is very limited, what implies in the practice lower permeate fluxes than those obtained with the MF membrane as expected. The results of the resistance in series model (Fig. 6) showed that the resistance caused by particles deposited (or weakly attached) onto the membrane surface (Rc ) was higher than the others, thus reversible fouling (which can be removed by the backwashing shear forces) predominated over the irreversible fouling (which requires chemical cleaning). The total resistance (Rt ) at the end of the microfiltration at 1.5 bar TMP was 1.88323 × 1013 m−1 .
Table 3 – Distilled water permeability of the membrane before and after the MF tests. Initial permeability (L m−2 h−1 bar−1 ) TMP = 0.5 bar TMP = 1 bar TMP = 1.5 bar
Permeability after water rinsing (L m−2 h−1 bar−1 )
223.7 216.86 211.57
178.96 162.65 152.33
% permeability decrease 20 25 28
Table 4 – Characterization of the permeates of the MF tests. Permeate COD (mg/L) TMP = 0.5 bar TMP = 1 bar TMP = 1.5 bar
13,100 11,700 11,360
COD rejection (%) 36.0 42.9 44.5
Permeate total nitrogen (mg/L) 900 850 830
Total nitrogen rejection (%) 23.1 27.3 29.0
Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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Fig. 7 – (a) Hide samples in the laboratory drums in the deliming process with the membrane permeate. (b) Results of the phenolphthalein tests obtained after reusing the permeate (left side of the figure) compared to the results of a incorrectly delimed sample (right side of the figure). Table 4 shows the characterization of the permeate and the rejection of the MF membrane in terms of COD and total nitrogen. It should be highlighted that COD and total nitrogen concentrations in the permeate are still very high. For the COD, this is explained by the presence of peptidic chains of low molecular weight coming from the hidrolized proteins of the hide and by the use of high concentrations of volatile organic acids in the process. Regarding the total nitrogen, the main contribution is given by the amino acids from the peptidic chains coming from the hydrolyzed proteins of the hide which can cross the membrane. Thus, these elevated COD and TN concentrations would not affect negatively the possible reuse of the permeate. Anyway, this hypothesis will be experimentally verified in the following section. Regarding the rejection values shown in Table 4, it has to be commented that the rejection percentages increased with the TMP increase due to a higher water passage through the membrane. Considering the effect of both measures implemented in this work to reduce the nitrogen concentration in a tannery wastewater, the results have shown that TN was reduced from 1679 mg/L to 1257 mg/L due to the replacement of ammonium salts by carbon dioxide (CO2 ) in the deliming process and from 1257 mg/L to 900 mg/L after membrane filtration of the deliming/bating liquor. Both measures have resulted in a 53% TN global reduction.
3.4.
Permeate reuse
Membrane permeate was reused in the deliming process with the aim of saving water and auxiliary chemicals. To experimentally demonstrate the technical viability of reusing the permeate from the deliming/bating liquor, hide samples were set in the laboratory drums together with the membrane permeate (Fig. 7a). Once the process had finished, the hide samples were taken out of the drums and supervised, examining their appearance and the pH lowering after lime removal. According to the technicians from the tannery, the visual and other organoleptic properties of the hide after reusing the permeate of the first exhausted bath of the deliming/bating operations were excellent and identical to those obtained when fresh water was used. Moreover, phenolphthalein test resulted satisfactory (left side of Fig. 7b): since the deliming was correct, the hide did not change its colour. Recall that when the hide is not correctly delimed, the phenolphthalein added through the
hide after cutting it is coloured pink, as it is shown as example in the right side of Fig. 7b.
4.
Conclusions
In this work two concomitant measures to reduce the nitrogen concentration in a tannery wastewater have been implemented and experimentally assessed. These measures were the replacement of ammonium salts by carbon dioxide in the deliming process and the reuse of wastewater and chemicals after membrane filtration of the deliming/bating liquor. The main conclusions that can be drawn from this study are: • The proposal is technically feasible, since the final leather quality is excellent according to both visual and organoleptic inspection from process technicians and phenolphthalein test. • The implementation of both measures have resulted in 53% total nitrogen reduction when compared with deliming using ammonium salts and not reusing. • The COD concentration in the wastewater from the deliming using CO2 is higher than that when ammonium salts are used, due to the addition of volatile organic acids to gradually reduce the pH. However, since this COD is biodegradable, it can be easily removed in the biological treatment of a conventional wastewater treatment plant. • The characterization of the deliming/bating liquor samples, confirms the significant pollution from these wet operations of the tanning industry. • Both deliming baths (using ammonium salts and using CO2 ) have shown a similar efficiency in the process of calcium removal from the hide. • Deliming/bating liquor pretreatment tests (jar tests and selective protein precipitation) have shown to be unnecessary. Thus, only a security filtration (8 m filter) prior to membrane filtration was recommended. • The small size of particles in deliming/bating wastewater (90% or particles ≤1.195 m) joined with their low density, made solids settling an ineffective treatment option. • The MF membrane used was suitable for the treatment of the deliming/bating wastewaters both from the point of view of the flux and of the rejection. The UF membrane tested, specifically designed for MBR applications, showed severe fouling as it was operated near the maximal recommended TMP.
Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003
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• The cake resistance (i.e., the reversible fouling which can be removed by backwashing) was the predominant resistance in the deliming/bating microfiltration. Summarizing, the reuse of the wastewater from the deliming/bating operations provides environmental and economic benefits due to the water consumption reduction and the reduction in nitrogen and salts discharge, which are essential for the sustainable development of the leather industry in the future. However, further research is needed to determine how many times the first exhausted bath can be reused without reduction in the final leather quality, and the economical viability of the proposal.
Acknowledgement Authors thank Industrias del Curido S.A. (INCUSA) for its support in this work.
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Please cite this article in press as: Gallego-Molina, A., et al., Reducing pollution from the deliming–bating operation in a tannery. Wastewater reuse by microfiltration membranes. Chem. Eng. Res. Des. (2012), http://dx.doi.org/10.1016/j.cherd.2012.08.003