Tracing membrane foulants in membrane bioreactors by filtration characterisation and fractionation

A pilot MBR was fed with two different influent loading conditions, namely dry weather flow (DWF) and DWF with an additional discharge of wastewater from a local cheese factory. For both situations sludge was sampled from different tanks of the pilot (predenitrification, denitrification, nitrification and membrane tank). All sludge samples and the supernatant of sludge from the membrane tank were filtered under exactly the same conditions with a filtration characterisation unit. Besides this all samples were subdivided in different fractions (supernatant, 0.45 and 0.03 m, 500, 100, 10 and 1 kDa) and analysed for their EPS concentrations (proteins and polysaccharides). The results show that the filterability of the sludge changes with the sampling point and differs between the two influent loading conditions. Contrary to DWF conditions, for cheesy influent conditions sludge from the membrane tank has better filterability than its supernatant. This suggests that the sludge forms a cake layer that protects the membrane against fouling by particles in the free water. No clear relation between filterability and EPS concentrations of the different fractions was found.