Environ. Sci. Technol. 2001, 35, 4916-4921
Flocculation/Flotation/Ultrafiltration Integrated Process for the Treatment of Cork Processing Wastewaters MIGUEL MINHALMA AND MARIA NORBERTA DE PINHO* Chemical Engineering Department, Instituto Superior Te´cnico, Avenue Rovisco Pais 1, 1049-001 Lisboa, Portugal
Membrane fouling is the major problem in the treatment by ultrafiltration (UF) of the cork processing wastewaters. This problem leads to drastic reduction on the permeate fluxes and has been associated with wastewaters phenolic/ tannic colloidal matter. The present work proposes a flocculation/flotation/ultrafiltration integrated process for the treatment of these wastewaters.A flocculation study was carried out in jar-test equipment with chitosan. The ζ-potential and the particle size were monitored at different pH values and for different chitosan concentrations. The results showed an increase of the ζ-potential when chitosan is added and a significant increase of the effluent particle size with the decrease of the pH. A dissolved-air flotation study was performed with the flocculated wastewater. The parameters varied were the operating pressure and the recycle ratio. It was observed that for higher pressures and at a recycle ratio of 0.19 the polyphenols removal was higher. The UF experiments were carried out in flat-sheet cells of 13.2 cm2 of membrane surface area at transmembrane pressures of 1-3 bar. A commercial membrane (Ropur-TS60) and four cellulose acetate membranes with molecular weight cutoff (MWCO) ranging from 4.5 to 86 kDa were used. The flocculation/flotation pretreatment led to the enhancement of the UF permeate fluxes. For the membrane with higher MWCO, the permeate flux enhancement reached 130%.
Introduction Membrane pressure-driven processes are playing a major role in wastewater treatment due to their capability of removing organic matter over a wide range of sizes, going from small solutes with the use of nanofiltration/reverse osmosis (NF/RO), macromolecules and colloids by the use of ultrafiltration (UF), to suspended matter through the use of microfiltration (MF). The wastewaters from food, pulp and paper, cork, and many other industries are very complex mixtures of vegetal extracts covering a wide range of molecular weights and very often presenting colloidal behavior. The colloidal fouling associated with great flux declines is very often more pronounced with the more permeable membranes of MF/ UF. The retention characteristics of these membranes are essentially governed by sieving mechanisms, and the UF membranes are commonly characterized by the molecular * Corresponding author telephone: 00351 218417488; fax: 00351 218499242; e-mail:
[email protected]. 4916
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 24, 2001
weight cutoff (MWCO) or the pore size distribution. However, oftentimes there is the retention of solutes that are orders of magnitude smaller than the membrane pore size (1-3). Minhalma and de Pinho (4) studied the membrane/solute interactions and concluded that, in the small polyphenols present in the cork processing wastewater the more hydrophobic ones such as the ellagic and ferulic acids are the ones responsible for the membrane fouling. Taking this into account, the present work will only focus on these two solutes. Dissolved-air flotation (DAF) and flocculation/coagulation are widely used water treatment technologies. LahoussineTurcaud et al. (5) studied the removal by coagulation of tannic acid and humic substances prior to microfiltration (MF) permeation, and their results showed that the coagulation pretreatment can effectively reduce the colloidal fouling. One other study that achieved a significant reduction of the membrane fouling was performed by Braghetta et al. (6) using DAF as a pretreatment. Combined flocculation/DAF studies were also performed in order to remove colloidal matter, like humic substances (7-9). The final results showed that these combined technologies can effectively remove organic matter with colloidal behavior. To overcome the severe membrane fouling problems mentioned above, this work proposes an integrated flocculation/flotation/UF process for the treatment of the cork processing wastewaters as the conventional flocculation and flotation process removes the colloidal fouling agents and enhances the UF performance.
Experimental Section The experimental work of this paper was divided into three parts. One regarding the flocculation study, another referring to the flotation experiments, and a final part regarding the UF permeation experiments. The setup of the optimal flocculation and flocculation/flotation conditions was carried out, and the sequence flocculation/flotation was integrated with UF to investigate the conditions of minimal membrane fouling. Characterization of Cork Wastewater. The wastewater samples were collected at a Portuguese cork processing mill, and not more than 3-4 days elapsed from the sample collection to the laboratory runs. They were kept refrigerated, at 4 °C, and brought up to room temperature for the experimental runs. The characterization in terms of pH, total organic carbon (TOC), average colloid size, ζ-potential, and total polyphenols is shown in Table 1 and in terms of the phenolic solutess gallic acid, protocatechuic acid, vanillic acid, syringic acid, ferulic acid, and ellagic acids is shown in Table 2. It is also presented as an HPLC chromatogram of the raw wastewater (Figure 1). The analysis of the HPLC chromatogram presented in Figure 1 shows that the two solutes with higher retention times are the ferulic acid and the ellagic acids, meaning that these two compounds are the more hydrophobic solutes present in the cork processing wastewater. Flocculation Experiments. The flocculation experiments were carried in jar-test equipment (supplied by P Selecta) at room temperature with chitosan (90% deacetylated) supplied by Primex, Norway. The procedure was as follows: (i) effluent pH adjustment; (ii) addition of chitosan under rapid mixing of 200 rpm for 1 min; (iii) slow mixing of 30 rpm for 10 min; (iv) effluent sedimentation during 60 min; and (v) analysis of the clarified effluent. Flotation Experiments. The flotation performance of the effluent was estimated using a laboratory-made flotation cell described in ref 10. The experimental procedure was as 10.1021/es010119n CCC: $20.00
2001 American Chemical Society Published on Web 11/10/2001
TABLE 1. Physicochemical Characterization of Cork Processing Wastewaters characteristic
value
pH TOC (mg of C/L) av colloid size (nm) ζ-potential (mV) total polyphenols (g/L galic acid)
4.9 3350 383.0 -13.2 0.958
TABLE 2. Phenolic/Tannic Solutes in Wastewaters solutes
concn (ppm)
gallic acid protocatechuic acid vanillic acid ferulic acid ellagic acid
103.7 71.1 8.0 7.7 238.7
follows (11): (i) the calibrated cylinder was partially filled with effluent (V1 volume), and the pressure chamber was filled with deionized water (
