Environ. Sci. Technol. 43, 9130–9135
Biologically treated effluents from the leather industry pose severe problems for the environment due in part to both the inorganic charge and the high nitrogen content associated with the organic charge. Pressure-driven membrane processes, namely ultrafiltration/nanofiltration (UF/NF) technology, were investigated for their selective retention of the organics and permeation of the inorganic fraction. Permeation experiments were carried out with two model solutions representative of a treated tannery effluent. UF and NF of these model solutions were assessed in terms of both their inorganic/organic fractionation capability and their permeation productivity. The UF membranes with MWCOs ranging from 10 000 to 1000 Da yield retentate streams enriched in organic compounds and permeate streams enriched in salts. Despite their high capacity for pure water permeation, they displayed low permeation fluxes, as the result of concentration polarization and fouling phenomena. NF 200 and NF 270 membranes associated fractionation capability with high permeation rates. Furthermore, these membranes demonstrated the highest permeate fluxes -30 kg/h/m2 and 16 kg/h/m2 for different model solutions, at the transmembrane pressure of 8 bar. Although these membranes had lower hydraulic permeabilities relative to the other membranes tested, they exhibited the best characteristics in terms of minimization of colloidal fouling.
worldwide this figure approaches 330 million m3 for tannery effluent (3, 4). Despite the intense research and development carried out on water recovery at the level of the different process operations such as tanning (5-10), soaking and liming (5, 11, 12), pickling (13), and dyeing (14), the end of pipe wastewater and the effluent coming from the secondary biological treatment are still an environmental concern. Currently, secondary treatment faces serious technoeconomical problems to meet regulatory standards for discharge. To achieve the standards of process water is a more challenging task. In order to meet the standards for process water and allow water recycling for pulp and paper wastewaters, Geraldes and Pinho (15) proposed a hybrid process of nanofiltration (NF)/electrodialysis (ED). In fact, NF removed 100% of the organic and organo-chlorinated compounds and the ED of the NF-permeate decreased the salt content to 300 ppm. Previous studies (2, 16) approached this complexity through a hybrid process of photoelectro-chemical oxidation (PEO) and electrodialysis (ED) to comply with process water reuse. By this treatment, 90 to 100% of the organic compounds were removed by PEO and ED of the product water removed around 95% of the concentrated salts. Some salts were removed by the PEO process as well as by ammonium nitrogen, which was partially converted to nitrate, and removed from the product water by ED. The organic fraction of the leather processing effluents covers a wide range of molecular weights. Membrane pressure-driven processes such as ultrafiltration/nanofiltration (UF/NF) display the necessary versatility in terms of membranes and operating characteristics for the selective retention of the organics and permeation of the inorganic fraction. The UF/NF of an effluent with a tanning fraction presents potential membrane fouling characteristics (17, 18). Therefore, in this work, special attention was given to the selection of the membrane molecular weight cutoff (MWCO) and to the operating conditions of transmembrane pressures and feed circulation velocities. The main objectives to be addressed were as follows: (1) Assessment of the UF/NF performance for the organic/ inorganic fractionation of the leather processing effluents. (2) Optimization of the NF/UF operating conditions to minimize colloidal fouling. This investigation was conducted with model solutions that simulated the average parameters of an effluent normally discharged from the secondary treatment of the leather processing wastewaters.
Introduction
Experimental Section
Ultrafiltration/Nanofiltration for the Tertiary Treatment of Leather Industry Effluents K A T I A F . S T R E I T , †,‡ JANE ZOPPAS FERREIRA,‡ ´ A M. BERNARDES,‡ AND ANDRE M A R I A N O R B E R T A D E P I N H O * ,† Universidade Te´cnica de Lisboa, Instituto Superior Te´cnico (IST), Depto. De Engenharia Quı´mica e Biolo´gica, Av. Rovisco Paes, 1 - 1049-001, Lisboa, Portugal and, Universidade Federal do Rio Grande do Sul (UFRGS), Programa de Po´s-Graduac¸a˜o em Engenharia de Minas, Metalu ´ rgica e de Materiais (PPGEM), Av. Bento Gonc¸alves, 9500, Porto Alegre/RS, Brasil
Received July 28, 2009. Revised manuscript received October 29, 2009. Accepted November 3, 2009.
Operating processes associated with the leather industry produce large volumes of wastewater with a high organic/ inorganic charge that potentially may pose severe pollution problems for the environment. For example, processing of salted bovine hides generates around 1 m3 effluent/hide equivalent to ∼30-35 L.kg-1 (1, 2). In Brazil, leather production leads to 45 million m3 of wastewater annually and * Corresponding author e-mail:
[email protected]. † Universidade Te´cnica de Lisboa, Instituto Superior Te´cnico (IST), Depto. De Engenharia Quı´mica e Biolo´gica. ‡ Universidade Federal do Rio Grande do Sul (UFRGS), Programa de Po´s-Graduac¸a˜o em Engenharia de Minas, Metalu ´ rgica e de Materiais (PPGEM). 9130
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 24, 2009
Leather Processing Effluent. Effluents from the secondary biological treatment of 20 Brazilian tanning industries were collected and analyzed by an ISO 17025 accredited laboratory that is specialized on tannery wastewater analysis. The parameters analyzed were COD, conductivity, total nitrogen, and ammonium nitrogen. Conventional wastewater treatment is not very efficient for some parameters, especially for nitrogen, and in this case the tannery industries are facing difficulties to reach the required limits established on the Brazilian wastewaters regulations published on 2005 (19). The conductivity and COD parameters were analyzed in order to characterize the inorganic and organic fractions of the treated effluent. The range of values obtained is shown in Table 1. 10.1021/es902105q
2009 American Chemical Society
Published on Web 11/18/2009
TABLE 1. Average Characteristics of Leather Treated Effluent parameter
concentration
COD total nitrogen ammonium nitrogen conductivity
100-400 mg O2 L-1 20-300 mg L-1 20-250 mg L-1 1-12 mS cm-1
TABLE 2. Model Solution Composition component
model solution 1 concentration (g L-1)
model solution 2 concentration (g L-1)
0.76 3.00 4.00 0.21
0.76 3.00 4.00
NH4Cl Na2SO4 MgCl2 CO(NH2)2 soy peptone tannin
0.30
FIGURE 2. Pure water membrane permeability, Lp (kg/h/m2/bar) for the tested membranes.
0.50 0.30
TABLE 3. Characteristics of Model Solutions parameter
model solution 1 concentration
model solution 2 concentration
COD TOC total nitrogen ammonium nitrogen conductivity
492 mg O2 L-1 130 mg L-1 240 mg L-1 230 mg L-1 10.0 mS cm-1
800 mg O2 L-1 329 mg L-1 190 mg L-1 180 mg L-1 8.65 mS cm-1
Model Solutions. Treated tannery effluent was simulated by preparing two model solutions according to the composition presented in Table 2. The nitrogen source in Model Solution 1 is the low molecular weight compound, urea, and in Model Solution 2, it is the high molecular weight compound, soy peptone. Their carbon contributions are, as well, different. All reagents used were p.a. grade and supplied by Merck. These model solutions were characterized in terms of COD, TOC, total nitrogen, ammonium nitrogen, and conductivity. These data are showed in Table 3. Membranes. The commercial membranes of ultrafiltration (UF) were supplied by Alfa Laval (Denmark). The ETNA01PP and ETNA10PP are Composite fluoro polymer membranes with molecular weight cutoff (MWCO) of 10 000
FIGURE 3. Permeate flux variation (Vp) versus transmembrane pressure (∆P) for different feed flow rates. Model Solution 1 (MS1) and 2 (MS2). and 1000, respectively. The GR81PP is a Polyethersulphone membrane with a MWCO of 10 000. The support material of these membranes is polypropylene. The commercial membranes of nanofiltration (NF), supplied by FILMTEC, are Polyamide Thin-Film Composite membranes. Ultrafiltration/Nanofiltration Permeation Experiments. The UF/NF experiments were performed in laboratory flatcell units, previously described by Afonso et al. (20). A schematic is presented in Figure 1. The membrane surface area of the permeation cells was 13.2 × 10-4 m2. All UF/NF experiments were carried out at laboratory temperature (20-25 °C). Membrane conditioning was carried out through the circulation of pure water (conductivity
