Water Research 38 (2004) 1248–1258 Micro-profiles of activated sludge floc determined using microelectrodes Baikun Li, Paul L. Bishop* Department of Civil and Environmental Engineering, University of Cincinnati, 765G Baldwin Hall, Cincinnati, OH 45221-0071, USA Received 12 July 2002; received in revised form 21 October 2003; accepted 19 November 2003 Abstract The microbial activity within activated sludge floc is a key factor in the performance of the activated sludge process. In this study, the microenvironment of activated sludge flocs from two wastewater treatment plants (Mill Creek Wastewater Treatment Plant and Muddy Creek Wastewater Treatment Plant, with aeration tank influent CODs of 60–120 and 15–35 mg/L, respectively) were studied by using microelectrodes. Due to microbial oxygen utilization, the aerobic region in the activated sludge floc was limited to the surface layer (0.1–0.2 mm) of the sludge aggregate at the Mill Creek plant. The presence of an anoxic zone inside the sludge floc under aerobic conditions was confirmed in this study. When the dissolved oxygen (DO) in the bulk liquid was higher than 4.0mg/L, the anoxic zone inside the activated sludge floc disappeared, which is helpful for biodegradation. At the Muddy Creek plant, with its lower wastewater pollutant concentrations, the redox potential and DO inside the sludge aggregates were higher than those at the Mill Creek plant. The contaminant concentration in the bulk wastewater correlates with the oxygen utilization rate, which directly influences the oxygen penetration inside the activated sludge floc, and results in redox potential changes within the floc. The measured microprofiles revealed the continuous decrease of nitrate concentration inside the activated sludge floc, even though significant nitrification was observed in the bulk wastewater. The oxygen consumption and nitrification rate analyses reveal that the increase of ammonia flux under aerobic conditions correlates with nitrification. Due to the metabolic mechanisms of the microorganisms in activated sludge floc, which varies from one treatment plant to another, the oxygen flux inside the sludge floc changes accordingly. r 2003 Elsevier Ltd. All rights reserved. Keywords: Microelectrode; Microenvironment; Activated sludge floc; Redox potential; Nitrification; Dissolved Oxygen 1. Introduction Biological nutrient removal (BNR) using activated sludge processes is accomplished by the uptake of pollutants by microorganisms retained in the activated sludge floc or free swimming in the bulk wastewater. In the interior microenvironment of the activated sludge floc, electron acceptors (oxygen) and donors (soluble substrates) are subject to mass transfer limitations. Gradients around and inside the activated sludge flocs are maintained by the balance between bacterial uptake (utilization rate) and diffusion of the compounds (renewal rate) from the bulk solution. As the basic unit of activated sludge treatment systems, activated sludge floc are critical to the pollutant removal efficiency of the activated sludge process. However, our current knowl- edge of microbial community structure (e.g. who are they?) and function (e.g. what do they do?), and consequently a microbiological understanding of the activated sludge process itself, is still very limited. The microbial ecology in environmental samples (such as biofilm and activated sludge) has usually been investi- gated by classical microbiological techniques, such as isolation and cultivation. However, these methods have ARTICLE IN PRESS *Corresponding author. Tel.: +1-513-556-3675; fax: +1- 513-556-6741. E-mail address: Paul.Bishop@UC.edu (P.L. Bishop). 0043-1354/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2003.11.019