Tracing membrane foulants in membrane bioreactors by filtration characterisation and fractionation S.P. Geilvoet*, M. Remy**, H. Evenblij*, H. Temmink** and J.H.J.M. van der Graaf* *Department of Water Management, Section for Sanitary Engineering, Delft University of Technology, PO Box 5048, 2600 GA Delft, The Netherlands (E-mail: S.P.Geilvoet@CiTG.TUDelft.nl ) **Wetsus, Centre for Sustainable Water Technology, PO Box 1113, 8900 CC Leeuwarden, The Netherlands Abstract 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 mm, 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. Keywords Extracellular polymeric substances; filtration characterisation; fouling; fractionation; membrane bioreactor Introduction Membrane bioreactor (MBR) technology is considered a promising treatment technology for municipal wastewater. Combining the activated sludge process and membrane separ- ation offers several advantages over the conventional activated sludge process with sludge sedimentation in secondary clarifiers. Most mentioned benefits are the smaller footprint, the superior effluent quality and the possibilities for a flexible and phased extension of existing wastewater treatment plants (Stephenson et al., 2000). Notwith- standing these advantages the widespread application of MBR technology is constrained by the relatively high costs due to the expensive membranes, the relatively high energy consumption and the membrane fouling that takes place during filtration. Membrane foul- ing is considered to be the most serious problem affecting system performance (Chang et al., 2001; Kim et al., 2001). Periodical cleaning measures are required, leading to an increase of maintenance and operating costs. In recent years much research has been devoted to investigating, modelling and controlling fouling processes. However, because the behaviour of foulants is very complex and depends on various factors, fouling is still a poorly understood process. Many MBR studies have identified extracellular polymeric substances (EPS) as the most significant biological factor responsible for membrane fouling. EPS is a generic term for a wide range of organic compounds produced by micro-organisms. Among the many functions of EPS are the formation of flocs and a protective layer around the micro-organisms (Wingender et al., 1999). EPS in activated sludge occur in two forms: Water Science and Technology: Water Supply Vol 6 No 1 pp 165–172 Q IWA Publishing 2006 165 doi: 10.2166/ws.2006.020 Downloaded from https://iwaponline.com/ws/article-pdf/6/1/165/418035/165.pdf by guest on 23 May 2020