Efficient azo dye removal in bioelectrochemical system and post-aerobic bioreactor: Optimization and characterization Dan Cui a , Yu-Qi Guo b , Hyung-Sool Lee c , Hao-Yi Cheng a , Bin Liang a , Fan-Ying Kong a , You-Zhao Wang a , Li-Ping Huang d , Mei-Ying Xu e , Ai-Jie Wang a,⇑ a State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 202 Haihe Road, Harbin 150090, PR China b The Architecture Design and Research Institute of Harbin Institute of Technology, No.202 Haihe Road, Harbin, 150090, PR China c Department of Civil and Environmental Engineering, University of Waterloo, 200 University Avenue West Waterloo, Ontario N2L 3G1, Canada d Key Laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, PR China e State Key Laboratory of Applied Microbiology, Guangdong Institute of Microbiology, 100 Central Xianlie Road, Guangzhou 510070, PR China highlights  A new refractory wastewater treatment process (UBER–ABOR) was developed.  Alizarin Yellow R as the mode of azo dyes was efficiently removed.  The effect of cathode size on the performance of UBER was investigated.  The HRTs of UBER and ABOR were optimized.  The degradation mechanism of azo dye was discussed. article info Article history: Received 18 July 2013 Received in revised form 24 October 2013 Accepted 27 October 2013 Available online 4 November 2013 Keywords: Up-flow bio-electrocatalyzed electrolysis reactor (UBER) Cathode size Hydraulic retention time (HRT) AYR by-products Aerobic bio-contact oxidation reactor (ABOR) abstract A new process of an up-flow bio-electrocatalyzed electrolysis reactor (UBER) connected with an aerobic bio-contact oxidation reactor (ABOR) was developed for treating azo dye wastewater. Alizarin Yellow R (AYR), used as a model dye, was efficiently decolorized in UBERs (97.5 ± 1.0%) and further mineralized in the subsequent aerobic bio-contact oxidation reactor (ABOR). Decolorization efficiency was improved with increasing cathode size in UBERs, but AYR removal rate and current density were not increased in proportion to cathode size, mainly due to the limitation of anodic reactions. AYR decolorization rate was optimized at a cathode size of 90 cm 3 in an UBER where the charge transfer resistance R ct (39.5 X) was minimal. We assessed the effect of hydraulic retention time (HRT: 6.5 h, 4.5 h, 3.5 h, 2.0 h) on the removal of residual by-products (p-phenylenediamine (PPD) and 5-aminosalicylic acid (5-ASA) in the ABOR. The concentrations of PPD and 5-ASA decreased down to 0.28 ± 0.01 and 0.27 ± 0.03 mg L 1 , respectively, in an optimum HRT 3.5 h. Decolorization efficiency and COD removal efficiency was 93.8 ± 0.7% and 93.0 ± 0.5% in the combined process of UBER and ABOR in overall HRT 6 h (HRT 2.5 h in UBER + HRT 3.5 h in ABOR). The Chroma in ABOR effluent was 80 times. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Azo dyes are the largest chemical class of dyes and are fre- quently used for textile dying and paper printing industries due to cheap costs, firmness, and a variety of colors compared to natu- ral dyes [1]. However, intensive color of dye-containing wastewa- ter leads to severe aesthetic problems and obstructs light penetration and oxygen transfer into water bodies, adversely affecting aquatic life [2]. For these reasons, the color removal from dye-containing wastewater is one of major concerns in China where textile industry has grown exponentially in recent years [3]. Azo dyes are characterized by the number of azo groups (AN@NA), which was typically recalcitrant to microbial aerobic oxidation but could be reduced easily [4]. The azo group is substi- tuted with benzene or naphthalene groups, which can contain many different substituents, such as chloro (ACl), methyl (ACH 3 ), nitro (ANO 2 ), amino (ANH 2 ), hydroxyl (AOH), and carboxyl (ACOOH). Azo dyes typically remain in tail water after biological wastewater treatment processes because of their recalcitrance; by-products of azo dyes are even toxic and mutagenic [5]. Thus, the complete mineralization of azo dyes into carbon dioxide is desirable to protect ecosystem and human health. A number of re- moval technologies have been developed, which include dyestuff 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.10.082 ⇑ Corresponding author. Tel./fax: +86 451 86282195. E-mail address: waj0578@hit.edu.cn (A.-J. Wang). Chemical Engineering Journal 243 (2014) 355–363 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej