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Flocculating device Aeration/Air lift Air scouring Concentrate/sludge Fig. 1. Schematic of pilot plant configuration. 3. Results and discussion Variations in wastewater quality are significant for membrane fouling and studies have identified dominating fouling mechanisms in MBRs to be related to soluble sugars and proteins (extracellular substances, EPS) and suspended solids concentrations, in particular the submicron particle size fraction [3–7]. Results were therefore analyzed on the hypothesis that colloids are mainly responsible for fouling. There was no clear correlation found between values measured for SS and COD/FCOD and membrane performance, however, the PSD measurements showed a relatively good relationship. In cases where the differential number percentage of submicron particles was high an increase in membrane fouling rates was observed. This is in agreement with results observed in similar studies with the same BF-MBR process configuration [8]. Comparison between the two membrane reactors showed that the unit with an integrated flocculation device generally had less submicron particles in the concentrate and a better performance with regard to membrane fouling rates. The sludge generated in the reactor with the flocculating device generally displayed better settling characteristics, less submicron particles, and the average SS experienced by the membrane was lower during operation. This may account for some the enhanced performance Changing concentrate characteristics by introducing a flocculation device in the membrane reactor was observed to enhance the membrane performance. The performance could not be directly correlated to SS and COD/FCOD measurements but with trends observed on number and size distribution of submicron particles in the reactor. This paper will show how design and operation of the membrane reactors in a hybrid BF-MBR can impact membrane performance and reduce fouling rates.