Membrane Water Treatment, Vol. 5, No. 4 (2014) 281-293 DOI: http://dx.doi.org/10.12989/mwt.2014.5.4.281 281 Copyright © 2014 Techno-Press, Ltd. http://www.techno-press.org/?journal=mwt&subpage=7 ISSN: 2005-8624 (Print), 2092-7037 (Online) Decolorization kinetics and characteristics of the azo dye acid red 18 in MSBR system at various HRTs and SRTs M. Hasani Zonoozi ª, M.R. Alavi Moghaddam  and R. Maknoon b Civil and Environmental Engineering Department, Amirkabir University of Technology, Hafez Ave., Tehran15875-4413, Iran (Received May 25, 2014, Revised October 21, 2014, Accepted October 22, 2014) Abstract. The present work aimed to study the decolorization kinetics and characteristics of a selected azo dye under the influence of two key operational parameters including hydraulic retention time (HRT) and solid retention time (SRT). The decolorization efficiency and the two important criteria of k and normalized k (k/MLSS) were evaluated in lab-scale membrane sequencing batch reactors (MSBRs) at various HRTs of 48, 24 and 16 h (with constant SRT) and in addition, at various SRTs of infinity, 40 and 10 d (with constant HRT). According to the obtained results, both zero and first-order kinetics were properly fitted the decolorization profiles of the selected azo dye in all of the applied HRTs and SRTs. Increase of both HRT and SRT positively affected the decolorization efficiency. More MLSS concentrations corresponded to the lower HRTs and the higher SRTs resulted in higher decolorization rate constants (k). However, the effect of reducing the HRT was not compensated by increase of the MLSS concentration in order to reach higher decolorization efficiency. In addition, increase of the decolorization efficiency, as a consequence of the higher MLSS concentrations at longer SRTs, was restrained by decrease of the time-limited decolorization capability of biomass (represented by normalized k). Evaluation of both k and normalized k is suggested in order to have a more precise study on the decolorization kinetics and characteristics. Keywords: decolorization kinetics and characteristics; hydraulic retention time; solid retention time; decolorization rate constant (k); normalized k 1. Introduction Azo dyes are known as one of the most important groups of synthetic dye compounds applied in many industrial sectors. They account for more than 50% of all dyes used in textile, food, cosmetics and paper printing manufactories (Stolz 2001, van der Zee and Villaverde 2005, Saratale et al. 2011). Not only for the aesthetics problems and affecting the water transparency, but also due to the toxicity, carcinogenicity and mutagenicity of many azo dyes and their degradation products, removal of these pollutants from the effluents before release into the receiving bodies is a major environmental concern (van der Zee and Villaverde 2005, Atacag Erkurt 2010, Firmino et al. 2010, Saratale et al. 2011). Corresponding author, Associated Professor, E-mail: alavim@yahoo.com, alavi@aut.ac.ir a Ph.D. Student, E-mail: maryamzonooz@yahoo.com, zonoozi_maryam@aut.ac.ir b Assistant Professor, E-mail: rmaknoon@yahoo.com, maknoon@aut.ac.ir