Activated sludge quality is one of the major factors influencing flux decline in membrane bioreactors (MBRs). Sludge filterability is a recognized parameter to characterize the physical properties of activated sludge. Decrease in filterability is linked to a higher number of submicron particles. In our present research we studied whether particle counting techniques can be used to indicate deflocculation of the sludge suspended fraction to submicron particles, causing the aforementioned filterability decrease. A total number of 105 activated sludge samples were collected in four full scale municipal MBRs. Samples were tested for filterability and particle counting in the range 2–100 μm. In 88% of the membrane tank samples the filterability varied between good and poor, characterized by the ΔR20, being 0 < ΔR20 < 1. Filterability varied following the season of the year, stability of the MBR operation and recirculation ratio. The membrane tank filterability can be improved by applying low recirculation ratio between MBR tanks. The applied particle counting methodology generated reproducible and reliable results in the range 10–100 μm. Results show that differences in filterability cannot be explained by variations in particle size distribution in the range 10–100 μm. However, measurable deflocculation might be masked by the large numbers of particles present. Therefore, we cannot exclude the suspended particles as a possible source of submicron particles that are subsequently responsible for MBR sludge filterability deterioration.

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