Biodiesel Wastewater Treatment by Coagulation-Flocculation: Evaluation and Optimization of Operational Parameters

Abstract

In this study the coagulation-flocculation process was evaluated as an alternative for treatment of biodiesel wastewater. The role of two coagulants, Al 3+ and Fe 3+ , as well as its dosage, pH, treatment time, stirring and aeration were evaluated. It was observed that in the treatment using Al 3+ the pH of the effluent (9.7) does not need to be adjusted, while for Fe 3+ a previous adjustment to pH 5.0 was necessary. On the other hand, a high concentration of Al 3+ (243 mg L -1) and a relatively long treatment time (70 min) were required to reach more than 96% of turbidity removal, when compared to the processing using Fe 3+ (56 mg L -1 and 53 min), attributed to the formation of different mole fractions of hydrolyzed cationic species. Under the optimized conditions, more than 96% of turbidity, apparent color and suspended solids, and 82% of oil and fats were removed. Keywords: aluminum sulfate, biodiesel effluent, ferric chloride, physical-chemical treatment, turbidity IntroductionThe quality of the biodiesel is directly dependent on its refinement process, which produces large volumes of wastewater inappropriate for disposal in water bodies. [1][2][3][4] In general, biodiesel wastewater is characterized by expressive values of chemical oxygen demand (COD) and biochemical oxygen demand (BOD 5 ), respectively between 9,500 and 230,000 mg L -1 and 1,650 and 3,200 mg L -1 , due the presence of glycerin, alcohols (methanol and/ or ethanol), oil, fats and other contaminants; 5 turbidity between 189 and 2,550 NTU; oils and fats between 124 and 7,200 mg L -1 ; color between 361 and 1,500 mg Pt-Co L -1 ; total solids between 3.7 and 25 g L -1 ; residual methanol between 8 and 29% (m/m) and residual glycerin between 0.6 and 1.6 (m/m). 3,4 In terms of oils and fats, the Brazilian legislation establishes as 50 mg L -1 the maximum disposal limit for wastewaters derived from the production of biodiesel.6 So, it is extremely important the study and proposition alternative processes for the efficient treatment of this kind of wastewater. This is a research field relatively new, once the first studies on biodiesel wastewater treatment started on 2005. 4 Three reviews on biodiesel wastewater treatment have been recently published. 1,4,5 One of them summarizes a significant number of possible processes and discusses their advantages and disadvantages as well as the possibilities of application for the improvement of the treatment of biodiesel wastewaters. The improvement of the operational parameters of the coagulation-flocculation process, based on the coupling of acidification and coagulation, has been pointed as promisor for this purpose. 4 Since biodiesel wastewaters contain soaps (presence of polar anionic and nonionic groups), oils and fats (presence of carboxylic groups), the cycle of emulsificationdemulsification tends to be strongly pH dependent. Low pH values favor the demulsification through the reduction of the electrical forces or by the commitment of the electrical double-layer, 7 with the opposite effect at...

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