Coagulation–flocculation of anaerobically treated sugarcane stillage

Abstract

Anaerobic digestion applied to stillage usually results in treatment performances. However, effluents from anaerobic reactors still present a residual polluting load due to the presence of organic recalcitrant compounds. Additional treatment methods, such as coagulation–flocculation, may be utilized to improve the final effluent quality. In this study, we assessed the processes of coagulation and flocculation for sugarcane stillage samples previously submitted to anaerobic digestion, aiming to obtain optimal conditions for the physicochemical treatment. Natural corn starch and ferric chloride were tested as coagulants. While starch was considered as not suitable for the treatment for the tested conditions, ferric chloride assays presented satisfactory results. The investigated parameters included coagulant solution dose, rapid mixing gradient and time, flocculation gradient and time, and sedimentation time. Their adjusted values at which better performances obtained were, respectively, 1.6 g L⁻¹, 325 rpm, 10 s, 65 rpm, 20 and 20 min. The best color, turbidity, and chemical oxygen demand removal efficiencies were 95, 97, and 65%, respectively. Stillage pH variation to alkaline conditions did not result in improved removal efficiencies. Although relatively high removal efficiencies of constituents were obtained, the final effluent characteristics did not fit regulations of water reuse in the agriculture through fertigation. However, stillage can definitely become more easily managed if better final effluent quality control parameters are achieved, enabling for example effluents discharge in water bodies.

Keywords:

• Stillage
• Anaerobic digestion
• Coagulation–flocculation
• Reuse
• Fertigation

Acknowledgments

The authors are grateful to the São Paulo Research Funding Agency (FAPESP), processes 2010/04101-8, 2011/02183-0 and 2011/05956-0, and to the National Council for Scientific and Technological Development (CNPq), process 155024/2011-6, for supporting the development of this project. We are also grateful to the Ph.D. student Antônio Djalma Nunes Ferraz Júnior from LPB, for providing the anaerobically digested stillage samples, to Nheel Química Ltda., for providing the ferric chloride used in the coagulation-flocculation tests, to the technicians from CEA, for their assistance during analyses, and to the reviewers of Desalination and Water Treatment, whose suggestions were important to improve the quality of this work.