Available online at www.sciencedirect.com Chemical Engineering and Processing 47 (2008) 184–191 Fluidized ASBR treating synthetic wastewater: Effect of recirculation velocity Denise Marques Pinheiro a , Suzana Maria Ratusznei a , Jos´ e Alberto Domingues Rodrigues a,∗ , Marcelo Zaiat b , Eugˆ enio Foresti b a Escola de Engenharia Mau´ a, Instituto Mau ´ a de Tecnologia (IMT), Pra¸ ca Mau ´ a 1, CEP 09.580-900, S ˜ ao Caetano do Sul, SP, Brazil b Departamento de Hidr ´ aulica e Saneamento, Escola de Engenharia de S ˜ ao Carlos, Universidade de S ˜ ao Paulo (USP), Av. Trabalhador S˜ ao-Carlense 400, CEP 13.566-590, S ˜ ao Carlos, SP, Brazil Received 5 July 2006; received in revised form 22 December 2006; accepted 8 March 2007 Available online 16 March 2007 Abstract An investigation has been performed on the effect of liquid phase recirculation velocity and increasing influent concentration on the stability and efficiency of an anaerobic sequencing batch reactor (ASBR) containing granular biomass. The reactor treated 1.3 L synthetic wastewater at 30 ◦ C in 6 h cycles. Initially the effect of recirculation velocity was investigated employing velocities of 5, 7 and 10 m/h and influent concentration of 500 mg COD/L. At these velocities, filtered sample organic matter removal efficiencies were 83, 85 and 84%, respectively. A first order kinetic model could also be fitted to the experimental organic matter concentration profiles. The kinetic parameter values of this model were 1.35, 2.36 and 1.00 h -1 at the recirculation velocities of 5, 7 and 10 m/h, respectively. The recirculation velocity of 7 m/h was found to be the best operating strategy and this value was maintained while the influent concentration was altered in order to verify system efficiency and stability at increasing organic load. Influent concentration of 1000 mg COD/L resulted in filtered sample organic matter removal efficiency of 80%, and a first order kinetic parameter value of 1.14 h -1 , whereas the concentration of 1500 mg COD/L resulted in an efficiency of 82% and a kinetic parameter value of 1.31 h -1 . © 2007 Elsevier B.V. All rights reserved. Keywords: ASBR; Volumetric organic load; Liquid circulation; Superficial velocity 1. Introduction Application of anaerobic sequencing batch reactors (ASBRs) still depends on insights into some fundamental and technolog- ical aspects. Among the fundamental aspects should be pointed out mass transfer phenomena, kinetic behavior, as well as hydro- dynamic behavior. The technical aspects include operational assessments of these systems and applicability in treating several types of wastewater with systems employing auto-immobilized biomass. Within the fundamental scope it becomes necessary to assess superficial velocity in ASBR with liquid-phase recir- culation, as its increase may improve mass transfer and hence process efficiency. However, special attention should be given to biomass retention. ∗ Corresponding author. E-mail address: rodrigues@maua.br (J.A.D. Rodrigues). According to Dague et al. [1] the basic characteristic of inter- mittent flow in ASBRs is wastewater feeding at the beginning and discharge at the end of the treatment, repeating this opera- tion with each new batch. The reactor content is mixed, allowing good contact between wastewater and biomass. Treatment takes place in a single tank in an operation sequence comprising basically the following steps: (i) filling with wastewater; (ii) treatment by means of biotransformations of the wastewater constituents by microbial activity; (iii) settling of the biologi- cal sludge after reaction termination; (iv) discharge of reactor by withdrawing the treated, clarified liquid. Zaiat et al. [2] state that the main factors affecting the over- all performance of the anaerobic sequencing batch reactors are agitation, initial ratio between substrate and biomass concen- trations (F/M), geometric configuration of reactor and feeding strategy. Agitation in the ASBR should be intense enough to promote homogeneity of conditions in the reactor, such as temperature, 0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.03.004