Effect of dissolved oxygen concentration on the bioflocculation process in high loaded MBRs L. Faust a,b,* , H. Temmink a,b , A. Zwijnenburg a , A.J.B. Kemperman c , H.H.M. Rijnaarts b a Wetsus-Centre of Excellence for Sustainable Water Technology, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands b Sub-department of Environmental Technology, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands c Membrane Science & Technology, MESAþ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands article info Article history: Received 3 May 2014 Received in revised form 14 July 2014 Accepted 18 August 2014 Available online 27 August 2014 Keywords: Bioflocculation Membrane bioreactor Dissolved oxygen Extracellular polymeric substances abstract High-loaded membrane bioreactors (HL-MBRs), i.e. MBRs which are operated at extremely short sludge and hydraulic retention times, can be applied to flocculate and concentrate sewage organic matter. The concentrated organics can be used for energy recovery, or for the production of more valuable organic chemicals. Little is known about the effect of the dissolved oxygen concentration (DO) on this bioflocculation process. To examine this ef- fect, two HL-MBRs were operated, respectively at a low (1 mg L 1 ) and a higher (4 mg L 1 ) DO. The higher DO resulted in a better flocculation efficiency, i.e. 92% of the colloidal COD in the sewage flocculated compared to 69% at the lower DO. The difference was attributed to a higher microbial production of extracellular polymeric substances at a DO of 4 mg L 1 and to more multivalent cations (calcium, iron and aluminium) being distributed to the floc matrix. In addition, the HL-MBR that was operated at a DO of 4 mg L 1 gave a bigger mean floc size, a lower supernatant turbidity, better settleability and better membrane filter- ability than the HL-MBR that was operated at a DO of 1 mg L 1 . © 2014 Elsevier Ltd. All rights reserved. 1. Introduction The organic compounds in municipal wastewater typically represent a chemical energy content of 1.9 kWh per m 3 (McCarty et al., 2011). Generally, this municipal wastewater is treated by activated sludge processes. These processes not only consume a considerable amount of energy for aeration (0.3e0.7 kWh per m 3 of wastewater according to Metcalf and Eddy (2003)), but also mineralise the organic compounds and thus destroy their chemical energy. Recent interest in improving the sustainability of municipal wastewater treat- ment systems has provided the impetus for new process de- signs, addressing issues such as maximizing water and energy recovery and producing inorganic and organic fertilizers (Verstraete and Vlaeminck, 2011). In this context sewage organic matter should not be aerobically mineralised, but * Corresponding author. Wetsus-Centre of Excellence for Sustainable Water Technology, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands. Tel.: þ31 642256392. E-mail address: lena.faust1@gmail.com (L. Faust). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres water research 66 (2014) 199 e207 http://dx.doi.org/10.1016/j.watres.2014.08.022 0043-1354/© 2014 Elsevier Ltd. All rights reserved.