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Improving biomass properties is a practical approach towards minimizing fouling in membrane bioreactors (MBRs). A possible and promising direction for reaching such purposes is the use of a hybrid growth MBR (HG-MBR) which combines activated sludge and biofilm processes. Four types of MBR of different design principles were examined in the present study: (i) an airlift MBR (A-MBR) equipped with a draft tube but without carriers (as a control); (ii) an HG-MBR containing carries but without a draft tube; (iii) an airlift HG-MBR (AHG-MBR) equipped with a draft tube and carriers, and; (iv) a fixed-bed HG-MBR (FHG-MBR) equipped with a draft tube and carriers, including two meshes around the bottom and top of the draft tube. For each of these configurations, a mixed wastewater consisting of domestic wastewater and chicken manure was treated, using a hollow fiber UF membrane module of ZW-10 under ambient desert conditions. The results demonstrate that generating adequate balance between MLSS and biofilm fractions is important in order to achieve high removal efficiency and low fouling tendency in an HG-MBR. The use of AHG-MBR and FHG-MBR has some limitations. Operation of the HG-MBR under appropriate aeration conditions which allows maintaining carriers’ recirculation and is advantageous. (C) 2011 Elsevier B.V. All rights reserved.