Pre-acclimation of a wastewater inoculum to cellulose in an aqueous–cathode MEC improves power generation in air–cathode MFCs Shaoan Cheng a,b , Patrick Kiely b , Bruce E. Logan b, * a State Key Laboratory of Clean Energy Utilization, Department of Energy Engineering Zhejiang University, Hangzhou 310027, PR China b Department of Civil and Environmental Engineering, 212 Sackett Building, Penn State University, University Park, PA 16802, USA article info Article history: Received 10 March 2010 Received in revised form 21 May 2010 Accepted 26 May 2010 Available online 1 July 2010 Keywords: Single-chamber MFC Cellulose Electricity generation Hydrolysis Microaerophilic abstract Cellulose has been used in two-chamber microbial fuel cells (MFCs), but power densities were low. Higher power densities can be achieved in air–cathode MFCs using an inoculum from a two-chamber, aqueous–cathode microbial electrolysis cell (MEC). Air–cathode MFCs with this inoculum produced max- imum power densities of 1070 mW m 2 (cathode surface area) in single-chamber and 880 mW m 2 in two-chamber MFCs. Coulombic efficiencies ranged from 25% to 50%, and COD removals were 50–70% based on total cellulose removals of 60–80%. Decreasing the reactor volume from 26 to 14 mL (while maintaining constant electrode spacing) decreased power output by 66% (from 526 to 180 mW m 2 ) due to a reduction in total mass of cellulose added. These results demonstrate that air–cathode MFCs can produce high power densities with cellulose following proper acclimation of the inoculum, and that organic loading rates are important for maximizing power densities from particulate substrates. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The development of improved types of microbial fuel cells (MFCs) has proceeded at a rapid pace in recent years due to their potential to produce renewable energy from various types of waste biomass (Logan, 2009). Most studies have been conducted with specific types of chemicals, such as glucose and various carbohy- drates, volatile fatty acids, amino acids and proteins (Pant et al., 2009). More complex sources of organic matter sources such as domestic wastewater have also been used in MFCs (Ahn and Logan, 2009), but power densities are generally lower than those obtained with single substrates (Pant et al., 2009). Many wastewaters and waste biomass consist of a high percentage of particulate biomass, and this form of substrate requires longer reaction times in MFCs due to the need to hydrolyze the substrate into smaller molecules that can be directly taken into the cell. Cellulose is the most abundant biopolymer in the world and is considered as to be an ideal source of organic matter for renewable energy production. However, power densities produced in MFCs directly from cellulose have been very low, and successful power generation has relied on using specialized cultures in two-chamber MFCs or in sediment MFCs. Power was produced by Enterobacter cloacae from cellulose, but the power density was very low (4.9 mW m 2 )(Rezaei et al., 2009). A co-culture of Clostridium cel- lulolyticum that can ferment cellulose to hydrogen and volatile fatty acids and solvents, and the exoelectrogen Geobacter sulfurre- ducens, were used in co-culture to produce low power densities (<60 mW m 2 ) from cellulose (type MN301) in a two-chamber MFC (Ren et al., 2007). Similarly low power densities (55 mW m 2 ) were achieved using mixed cultures with a rumen inoculum (Rismani-Yazdi et al., 2007). Low power densities were initially achieved by Rezaei et al. (2008) using a wastewater inoculum in a two-chamber MFC with a cation exchange membrane, but they found that reproducible cycles of power could not be achieved over multiple cycles without the addition of cellulase enzymes. A power density of 29–62 mW m 2 was produced with a sediment MFC using anodes wrapped around cellulose particles that were placed in a sediment (Rezaei et al., 2007). The primary reason for the low power densities reported in pre- vious studies is the high internal resistance of two-chamber MFCs. There have been no reports of direct power generation in air–cath- ode MFCs with cellulose as a sole substrate, although cellulose-con- taining wastewaters were shown to produce 627 mW m 2 power when phosphate buffer was added to the wastewater (Huang and Lo- gan, 2008). In preliminary studies in our laboratory, we found that we were unable to generate appreciable power from cellulose using a wastewater inoculum in an air–cathode MFC. However, we had shown current generation in a microbial electrolysis cell (MEC) with cellulose (Cheng and Logan, 2007). We wondered if the relatively high rate of oxygen diffusion into an air–cathode MFC could have limited development of the anode biofilm for power generation, as oxygen can diffuse into the reactor. In an MEC, the reactor is sealed off from air. To avoid oxygen during biofilm development 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.05.083 * Corresponding author. Tel.: +1 814 863 7908; fax: +1 814 863 7304. E-mail address: blogan@psu.edu (B.E. Logan). Bioresource Technology 102 (2011) 367–371 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech