INTRODUCTION

Surfactants are in widespread use throughout the world. The use of surfactant is gradually increased day by day; the world surfactant production was 1.7, 1.8 and 9.3 million tons in the years 1984, 1987 and 1995, respectively. A wide range of these products are actually synthesized and used in several domains such as textiles, fibbers, food, paints, polymers, cosmetics, pharmaceuticals, microelectronic, mining, oil recovery, pulp-paper industries, etc. Anionic surfactants are the major class of surfactants used in detergent formulations and represent about 59 % of surfactants used in 1989 (Mathis, 1992). Surfactants are among the most widely disseminated xenobiotics that may enter waste streams and the aquatic environment (Sigoillot and Nguyen, 1992; Margesin and Schinner, 1998; Eichhorn, et al., 2001, 2002). They are harmful to human beings, fishes and vegetation; and are responsible to cause foams in rivers and effluent treatment plants and to reduce the quality of water. Surfactants cause short term as well as long-term changes in ecosystem. Due to all these reasons, many environmental and public health regulatory authorities have fixed stringent limits for anionic detergent as standard 0.5 mg/Lfor drinking water and relaxable up to 1.0 mg/Lfor other purposes (Rao, 1995). Surfactants removal operations involve processes such as chemical and electrochemical oxidation (Lissens, et al., 2003; Mozia, et al., 2005), membrane technology (Sirieix-Plénet, et al., 2003; Kowalska, et al., 2004; Fernández , et al., 2005), chemical precipitation (Shiau, et al., 1994; Talens-Alesson, et al., 2002), photocatalytic degradation (Rao and Dube, 1996; Ohtaki, et al., 2000; Zhang, et al., 2003), adsorption (Ogita, et al., 2000; Lin, et al., 2002; Adak, et al., 2005) and various biological methods (Matthew and Malcolm, 2000; Dhouib, et al., 2003; Chen, et al., 2005). Each has its merits and limitation in application. Surfactants containing wastewaters treatment by biological processes such as activated sludge is problematic due to the low kinetics of degradation and to foam production (Dhouib, et al., 2003). Among the currently employed chemical unit processes in wastewater treatment, coagulation-flocculation has received considerable attention for yielding high pollutant removal efficiency. This process can be directly applied to wastewaters without being affected by the toxicity in the wastewater and could constitute a simple, selective and economically acceptable alternative. The objectives of this study are the examination of coagulation precipitation process efficiency for the treatment of industrial wastewaters with high surfactants content, especially in terms of organic matter and surfactants removals. More specifically, the aim is the determination of the most appropriate iron chloride dose, the examination of pH effects on removal capacity and the identification of optimum experimental conditions for the efficient application of this process.

MATERIALS AND METHODS

Sampling procedures

Samples of effluent are collected from outfall of a microelectronic factory situated in an industrial park of the Casablanca city, Morocco. The generated effluent is discharged into the sea without any treatment. Sampling of the wastewater is carried out according to standard methods for the examination of wastewater.

Chemical coagulation and analysis

Laboratory scale evaluation of chemical coagulation and flocculation is performed using a four-place jar test apparatus. The experimental process consisted of three subsequent stages: initial rapid mixing stage at 160 rpm took place for 5 min, followed by a slow mixing stage for 20 min at 30 rpm, and the final settling step lasted for another 1 h. Four equal volume polyethylene beakers are used to examine the four different dosages of coagulant or initial pH values in each run. Sample bottles are thoroughly shaken, for resuspension of possibly settling solids and the appropriate volume of sample is transferred to the corresponding jar test beakers. First, the optimum pH for the function of iron is determined. A known volume of prepared ferric chloride solution is added to a jar containing 1 L of wastewaters at different pH values adjusted with H₂(SO)₄ and NaOH. To investigate the optimum coagulant dose, the pH valueofthe wastewater is maintained at an optimum pH as determined above; and varying doses of iron are then added. After 60 min settling, the supernatant is withdrawn for analyses. To asses the efficacy of iron on wastewater treatment, the following are determined: turbidity, chemical oxygen demand (COD) and surfactants. Turbidity: the turbidity is determined by turbidity meter (HI 93703 Microprocessor turbidity meter).

Chemical oxygen demand: COD and other physicochemical parameters (BOD, NTK, Pt….) for wastewater characterisation measurement are performed according to Standard Methods (AFNOR, 1999). Surfactants: detergent compounds are determined using solvent extraction-spectrophotometric with Ethyl Violet method (Motomizu, et al., 1982).

DISCUSSION AND CONCLUSION

Figs. 2 and 3 indicate that a minimum dosage of FeCl₃ of 900 mg/Lis required on the reduction of COD and surfactants load from the wastewater by coagulation to an acceptable level. At this optimum dosage, the COD removal reach 88 % and it is reduced to approximately 400 mg/L, and then the treated wastewaters respect the Moroccan acceptable environmental standard in terms of COD (500 mg/L).The process is also effective in the reduction of surfactants which reached 99 %. However, residual surfactants in treated wastewaters (Table 2) are slightly above than this recommended by Moroccan legislation (3 mg/L). Then, a further treatment is necessary to allow the surfactants guide level. The effect of iron on pollutants reduction shows that there is a comparable evolution between COD and surfactants removals. Fig. 4 shows positive correlation between COD and surfactants removals, R² = 0.99. It indicates that there exists similar mechanism for the removal of surfactants and organic matters. Thus the conditions for precipitation coagulation of the surfactant correspond with the conditions for precipitation coagulation of the other substances. Talens-Alesson, et al. (2002) report that Fe³⁺ can be used as an alternative to Al³⁺ in adsorptive micellar flocculation (AMF). They concluded that the pH range of Fe³⁺ solutions resulting from flocculation of anionic surfactant sodium dodecyl sulphate solutions with Fe³⁺ is 1.8-2.5. On the basis of the results obtained on surfactants and COD removals and on the pH of coagulated wastewaters at pH = 2.4 (Table 2), it seem that, in addition to precipitation coagulation process, AMF mechanism contributes to the removal of surfactants and organic matters from this rejection. The cations Fe³⁺ binds itself to a micelle, causing two simultaneous effects: it suppresses repulsion between micelles flocculating them and effectively removing micellar surfactant from solution in the form of an aggregate, and it binds organic compounds to the flocs (Porras et Talens, 1999, 2000; Talens-Alesson, 2001). It can be concluded that, in addition to precipitation coagulation process, adsorptive micellar flocculation mechanism seems contribute to the removal of surfactants and organic matters from this rejection. Many authors use the ratio BOD/COD as biodegradability index (Chamarro, et al., 2001; Scott et Ollis, 1995). Table 2 shows that BOD₅/COD ratio for initial and treated wastewaters changed from 0.16 to 0.41. It can be seen that the process leads to noticeable improvement in the biodegradability index for the microelectronic plant wastewaters. This improvement in biodegradabilityis due to reduction in theproportion of COD not amenable to biological mineralization. Wastewater can be considered readily biodegradable if it has a BOD₅/COD ratio between 0.4 and 0.8 (Metcalf and Eddy, 1985; Al-Momani, et al., 2002). This limit is reached for the microelectronic plant wastewaters, which indicates that a biological treatment can be applied as a further treatment to enhance the pollutants removal and to more reducing of surfactants contents to allow the guide level.

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