Case Study The role of EPS concentration in MBR foaming: Analysis of a submerged pilot plant Gaetano Di Bella ⇑ , Michele Torregrossa 1 , Gaspare Viviani 2 Dipartimento di Ingegneria Idraulica ed Applicazioni Ambientali, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy article info Article history: Received 19 May 2010 Received in revised form 7 September 2010 Accepted 8 September 2010 Available online 17 September 2010 Keywords: EPS Foaming Foam tests MBR Viscosity abstract Foaming in Membrane BioReactor (MBR) is a frequently discussed topic. Some authors reported that the phenomenon is due to filamentous organisms, like at Conventional Activated Sludge (CAS) plants. How- ever, in recent years, other authors reported that the Extra-cellular Polymer Substances (EPSs) concentra- tion is an important factor for controlling foam as well. Nevertheless, even if a number of MBR plants are affected by foaming, presently there are no suitable methods to evaluate the phenomenon. To facilitate the study of this controversial phenomenon in an MBR system, certain foam tests proposed in the past for CASPs were investigated. The results of the tests were able to adequately measure quantity, stability and quality of the foam. In particular, the Scum Index increased proportionally with the EPS concentration and mixed liquor viscosity; Foam Power was mainly correlated with the protein concentration of in the EPS; Foam Rating was also correlated with the EPS concentration. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In many wastewater treatment plants, foaming occurs and causes serious operational drawbacks (Soddell, 1999). Foaming is a common operational problem in Conventional Activated Sludge (CAS) processes, and it often adversely affects the operation of the plant and the quality of the treated effluent (Jenkins et al., 2004; Tandoi et al., 2006). Generally, a viscous layer composed of different substances appears on the surface of the aeration tank and secondary clarifier, leading to undesirable and ‘‘un-aesthetic” problems. The foaming can be due to chemical or biological agents. In particular, the presence of abiotic compounds, mainly an excess of surfactants and other surface-active compounds, has been re- lated to the development of chemical foaming (Mara and Horan, 2003). However, biological foaming has also been correlated with some hydrophobic compounds that are synthesised by certain mi- cro-organisms (Greenfield et al., 1984; Madoni et al., 2000). Unlike the detergent-type white foam commonly observed in treatment plants, almost all recent foaming problems are associated with a biological foam that is more viscous and stable than chemical foam. For this reason, biological foaming has been of more interest in scientific research, especially with regard to CAS systems. The problems with biological foam are numerous, mainly, the high retention of suspended solids inside the foam that compli- cates sludge control in the aeration tank, produces a bad smell and reduce effluent quality (Madoni and Davoli, 1997). Biological foaming in CAS has been commonly associated with bacterial population dynamics. An abundance of hydrophobic mi- cro-organisms, such as Nocardioforms or Microthrix parvicella, is known to be related to foaming in wastewater treatment plants (Pipes, 1978; Stratton et al., 1993, 1998; Iwahori et al., 1997; Rittmann and McCarty, 2001). The growth of these micro-organ- isms can be encouraged by high Sludge and Hydraulic Retention Times (SRTs–HRTs), warm temperature, a low food-micro-organ- ism ratio (F/M), high Mixed Liquor Suspended Solid (MLSS) concen- trations, the presence of foam trapping units in the plant and the recycling of trapped foam (Jenkins et al., 2004; Tandoi et al., 2006). In recent years, biological foaming has also been analysed in new advanced activated sludge systems. A large amount of foam is sometimes also produced in Membrane BioReactor (MBR) plants that allow the total retention of all the solids larger than 0.01– 01 lm(Stephenson et al., 2000). In the MBR process, the mem- brane replaces the sedimentation tank of the traditional activated sludge process and all the bacteria are retained in the reactor. Therefore, the operational conditions of an MBR plant promote foam development as a result of high MLSS concentration in the aeration tank, high SRT and low F/M ratio (Di Bella et al., 2008; Smith, 2006). Furthermore, the tank where the membrane is placed represents a real foam trapping unit. Generally, the microbial community of a MBR system is quite different from that of a CAS system (Judd, 2006). Unfortunately, lit- tle information is available about the typical microbial community of this process, and almost nothing is known about potential foam-forming organisms. Some authors have reported that 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.09.028 ⇑ Corresponding author. Tel.: +39 09123896520; fax: +39 0916657749. E-mail addresses: dibella@idra.unipa.it (G. Di Bella), mtorre@idra.unipa.it (M. Torregrossa), gviv@idra.unipa.it (G. Viviani). 1 Tel.: +39 09123896542; fax: +39 0916657749. 2 Tel.: +39 09123896534; fax: +39 0916657749. Bioresource Technology 102 (2011) 1628–1635 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech