Mixing Criteria Development for Aerobic Granular Sludge Reactor Operations : The Influence of Mixing Conditions to the Settling Behavior of Aerobic Granular Sludge A.Nor-Anuar 1 , Z.Ujang 1, M.C.M. van Loosdrecht 2 , M.K. de Kreuk 2 and G.Olsson 3 1 Institute of Environmental and Water Resources Management (IPASA), Universiti Teknologi Malaysia, 81310, Johor, Malaysia.(E-mail: aznah@utm.my ; zaini@utm.my ) 2 Delft University of Technology, Department of Biotechnology, Julianalaan 67, NL-2628 BC Delft, The Netherlands. (E-mail: M.C.M.vanLoosdrecht@tudelft.nl ; M.K.deKreuk@tudelft.nl ) 3 Lund University, Department of Industrial Electrical Engineering and Automation, S-221 00 Lund, Sweden (E-mail: gustaf.olsson@iea.lth.se ) Annotation Aerobic granular sludge technology has recently received growing attention by researchers and technology developers, worldwide. It has good settling properties and high capacity for biomass retention. Furthermore, it is suggested that aerobic granules can be established and profitably used in activated sludge plants, provided adequate design specifications and process conditions are applied. In the operation of aerobic granule reactors, either mechanical or aerated mixing has been commonly used to introduce shear effects in the system. Sufficient shear stress is needed for the formation of stable and dense aerobic granules. However, excessive shear stress (due to excessive mixing) might negatively affect the aerobic granules settling characteristics. In pilot or full-scale implementations, as the size of the reactor increases, the effect of the mixing mechanism strategy becomes more significant. Therefore, this study was conducted to explore the influence of mixing conditions (both aerated and mechanical mixing) on the settling characteristics of the aerobic granules. Special attention was given to three parameters that have a strong influence on the aerobic granules settling velocity (ν), i.e. (a) the stirrer speed (N), (b) the shear rates (γ) and (c) the ratio of blade and vessel diameters (D/T v ) in mechanical mixing operation. For aerated mixing, two parameters have a major influence on the aerobic granules settling velocity (ν); (a) the airflow rate (Q air ) and (b) the bubble size (r 2 ). Parameters that relate to aerobic granules settling characteristics, such as settling velocities of aerobic granules are compared with activated sludge flocs from sequencing batch reactor system, extended aeration and conventional activated sludge plants. Optimum mixing conditions required for maintaining aerobic granules, defined by high speed settling characteristics (ν > 10 mh -1 ) are highlighted in this study. Keywords : Aerobic granules, settling characteristics, settling velocity, mixing operation INTRODUCTION The ability of an activated sludge culture to form flocculent and dense flocs, which can settle quickly and be easily separated in the secondary clarifier determines in general the efficiency of a wastewater treatment system. A well-compacted sludge also reduces the costs related with waste sludge dewatering and disposal. Unfortunately, the biomass does not always have these characteristics and is not kept in the system at desirable concentrations. Regularly the process fails due to poor solid-liquid separation. After the activated sludge process was invented, various modified processes have been developed and applied to municipal and industrial wastewater treatment. Since the performance of the activated sludge processes is limited by the capacity of a clarifier for the separation of the activated sludge from the treated effluent, the self immobilization of sludge is an attractive option because of its prompting solid-liquid separation in the clarifier. Therefore, AGS technology has recently received growing attention by researchers and technology developers, worldwide. It has good settleability and high capacity for biomass retention [1-5]. The results suggest that aerobic granular sludge can be established and profitably used in activated sludge plants, provided sufficient design specifications and process conditions are applied. This study was aimed to explore the influence of mixing conditions (both aerated and mechanical mixing) on the settling characteristics of the aerobic granules. For mechanical-mixing, the outcomes of the study on aerobic granules settling profile and its settling characteristics have been published [6]. Further conclusions about the granules settling behavior are elaborated in this paper with special emphasized on effects of aerated-mixing conditions.