The role of fouling mechanisms in a membrane bioreactor G. Di Bella*, F. Durante*, M. Torregrossa*, G. Viviani*, P. Mercurio** and A. Cicala** *Dipartimento di Ingegneria Idraulica ed Applicazioni Ambientali dell’Universita ` di Palermo, Viale delle Scienze 90146, Palermo, Italy. (E-mail: dibella@idra.unipa.it; durante@idra.unipa.it; mtorre@idra.unipa.it; gviv@idra.unipa.it) **AMAP S.p.A., Via Volturno 2, Palermo, Italy Abstract The present study has aimed to quantify the role of pore blocking and cake layer in a laboratory scale hollow fibre membrane module in submerged configuration. The membrane reactor (MBR) was fed with raw wastewater, only screened with a 2-mm sieve, collected from the Palermo WWTP. The MBR was characterised by an operating volume of 190 L and equipped with an aeration system located on the bottom of the reactor. The MBR operated for 65 days. The permeate was extracted by imposing a constant flux through the membrane (21 Lh 21 m 22 ). The results confirm the importance of pore blocking control during start-up. In particular, it provides a rapid irreversible fouling that takes place at the beginning of the filtration process, before the deposition mechanism. Therefore, low suspended solids concentration in the initial phase causes a fast irreversible fouling. This circumstance creates the need for more frequent chemical cleaning after start-up without inoculum. Finally, the results underline that the cake has a mainly reversible feature. Keywords Fouling; MBR; membrane bioreactor; resistance; start-up; transmembrane pressure Introduction In the last years much attention has been addressed towards new technologies to treat wastewater. Among these, the membrane bioreactor (MBR) represents a good innovative process where the traditional gravity settling is replaced by a membrane separation module. More specifically, such a module reproduces microfiltration or ultrafiltration processes that enable, on one hand, the separation of the solid–liquid and serve, on the other hand, as an advanced treatment unit for specific pollution agents (for example, coliform bacteria and suspended solids), that cannot be completely removed by conventional processes. Furthermore, complete retention of biomass particles by membrane process makes it possible to minimise sludge wastage by maintaining a low food to microorganism ratio (F/M) and producing an effluent of good quality. However, some problems, related basically to the membrane fouling, need to be solved for competitive use of the MBR processes for domestic wastewater treatment. Fouling in MBR may be physical and biological, inorganic and organic. There are different fouling mechanisms, such as pore blocking or cake deposition. In particular, fouling phenomena are affected by biomass concentration and composition, feed water, process configuration, membrane material and process operation, including hydrodynamic phenomena. The nature and extent of fouling are strongly affected by physical and chemical inter- actions between membrane and mixed liquor; in particular, the mixed liquor composition defines fouling characteristics. However, there are several problems relating to defining the specific composition of mixed liquor; in fact, the separation of each component is technically impossible. Water Science & Technology Vol 55 No 8–9 pp 455–464 Q IWA Publishing 2007 455 doi: 10.2166/wst.2007.290 Downloaded from https://iwaponline.com/wst/article-pdf/55/8-9/455/439833/455.pdf by guest on 11 June 2020