Membrane bioreactors (MBR) technology is a well-developed wastewater treatment process; however, the integrated operation between biological reactions and physical filtration has been poorly studied. Among other operational parameters, optimal control of sludge recirculation can enhance nitrogen and phosphorous removal processes, but the effects on sludge filterability is not clear. In this paper, different recirculation flow rates were tested to evaluate consequences on sludge filterability

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... nutrient removal in a MBR-UCT pilot plant treating real municipal wastewater. Three distinct sludge recirculation flows were studied during 10 weeks [external recirculation (from the membrane tank to the anoxic reactor), anoxic recirculation (from the aerobic to the anoxic reactor) and anaerobic recirculation (from the anoxic to the anaerobic reactor)]. The obtained results have shown that anaerobic recirculation affected nutrient removal in an inversely proportional way, whereas anoxic recirculation had a directly proportional effect. Referring sludge characteristics, filterability and capillarity suction time (CST) remained independent of sludge recirculation, whereas CST is proportional to transmembrane pressure (TMP), which seems to depend on external and anoxic sludge recirculation.