Short Communication Anaerobic digestion of municipal wastewater sludges using anaerobic fluidized bed bioreactor Nizar Mustafa a , Elsayed Elbeshbishy b , George Nakhla a,c,⇑ , Jesse Zhu c a Department of Civil and Environmental Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada b Department of Civil and Environmental Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada c Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada highlights  Performance of AnFBR for treating primary and waste activated sludges was evaluated.  Different organic loading rates (OLRs) ranged from 4.2 to 93 kg/m 3 -d were evaluated.  70% solids destruction of primary sludge was achieved at OLR of 19 kg/m 3 -d.  56% solids destruction of waste activated sludge was achieved at OLR of 8 kg/m 3 -d.  First-order coefficient for primary sludge was higher than waste activated sludge. article info Article history: Received 7 July 2014 Received in revised form 16 September 2014 Accepted 17 September 2014 Available online xxxx Keywords: Anaerobic digestion Fluidized bed bioreactor Primary sludge TWAS abstract The anaerobic digestion of primary sludge (PS) and thickened waste activated sludge (TWAS) using an anaerobic fluidized bed bioreactor (AnFBR) employing zeolite particles as the carrier media was investi- gated at different organic loading rates (OLRs). PS was tested at OLRs from 4.2 to 39 kg COD/m 3 -d corre- sponding to hydraulic retention times (HRTs) from 1.0 to 8.9 days. The highest COD removal and VSS destruction efficiencies for primary sludge of 85% and 88%, respectively, were achieved at an HRT of 8.9 days and OLR of 4.2 kg COD/m 3 -d. For TWAS, VSS destruction efficiencies varied from 42% at an HRT of 2.6 days and OLR of 13.1 kg COD/m 3 -d to 69% at an HRT of 8.8 days and an OLR of 4.2 kg COD/ m 3 -d. The first-order COD biodegradation rates in the AnFBR for PS and TWAS were 0.4 d 1 and 0.1 d 1 , respectively, almost double the rates in conventional high-rate digesters. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The primary challenge associated with the anaerobic treatment of biosolids in conventional systems is the slow rate of biodegrada- tion not only requiring very long retention times and large reactor volume but also achieving low solids destruction efficiencies (Bunrith, 2008). The common low rate digestion system is contin- uously stirred tank reactor (CSTR), and high rate systems are upflow anaerobic sludge bed (UASB), expanded granular sludge bed (EGSB), fluidized bed bioreactor (FBR), anaerobic membrane bioreactor (AnMBR) (De Mes et al., 2003). Although high rate anaerobic digesters such as UASB, AnMBR, EGSB, and FBR are not suitable for high solids or thickened wastes, high rate systems can be used as a part of multi-stage system for treating high solids wastes (Angelidaki et al., 2003). Typically conventional completely mixed anaerobic digesters (CAD) treating municipal wastewater sludge have solids retention times (SRT) of 15 days or more and are characterized by relatively low organic solids destruction effi- ciencies of less than 50%. Grady et al. (1999) observed that a lower SRT limit of 10 days at a temperature of 35 °C is sufficient to ensure an adequate safety factor against washout of the methanogenic population, and incremental changes in volatile solids destruction are relatively small for SRT values above 15 days. Fluidized bed reactors have been used as a substitute for the conventional bio- processes in various biotechnological applications e.g. fermenta- tion, production of enzymes, and bioconversions (Grady et al., 1999). The structure and physical properties of natural zeolite such as channel and pore cavities, high specific surface area, low bulk density, high exchange (CEC) and adsorption capacities make it ideal for use in biological purification wastewater processes (Christidis, 1998; Wong and Yeung, 2007; Carretero and Pozo, 2009; Marty et al., 2010; Park et al., 2010). Consequently, the use http://dx.doi.org/10.1016/j.biortech.2014.09.081 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Civil and Environmental Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada. E-mail address: gnakhla@eng.uwo.ca (G. Nakhla). Bioresource Technology xxx (2014) xxx–xxx Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech Please cite this article in press as: Mustafa, N., et al. Anaerobic digestion of municipal wastewater sludges using anaerobic fluidized bed bioreactor. Bior- esour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.081