Combining submerged membrane technology with anaerobic and ...

Introductionof 2% NaOH and 0.5% citric acid. This kind of cleaning is normally carried out every 1-2months. Maintenance chemical cleaning. Maintenance cleaning is conducted in situ and isused to maintain membrane permeability and helps reduce the frequency of intensivecleaning. Maintenance cleaning is normally carried out moderate reagent concentrations of200-500 mg·L -1 NaOCl and a frequency of 1-2 weeks or days for MBRs or TMFapplications, respectively. Alternatively, a low concentration of chemical cleaning agentcan be added to the backwash water to produce a “chemically enhanced backwashing”(CEB) on a daily basis. In the case of TMF applications, CEB is commonly used with afrequency of 1-2 hours and moderate reagent concentrations of 20-200 mg·L -1 NaOCl(Zheng et al., 2011).Apart from the measures of physical or chemical cleaning, a second strategy can beapplied to limit the consequences of membrane fouling, which is to reduce its causes. Thisstrategy is based on modifying the physical and chemical nature of the membrane, thecomposition of the wastewater and the characteristics of mixed liquorThe characteristics of the mixed liquor are the factor on which a major research efforthas been made regarding membrane fouling.Membrane fouling in MBRs has been largely attributed to extracellular polymericsubstances (EPS) (Chang and Lee, 1998; Cho and Fane, 2002; Nagaoka et al., 1996;Nagaoka et al., 1998; Rosenberger and Kraume, 2002) and soluble microbial products(SMP) (Chang et al., 2002; Liu et al., 2005; Evenblij and van der Graaf, 2004; Ji and Zhou,2006) althought SMP have been reported to have a greater impact on membrane foulingas will be mentioned later.EPS consist of insoluble materials (sheaths, capsular polymers, condensed gel,loosely bound polymers and attached organic material) secreted by the cell, shed from thecell surface or generated by cell lysis (Jang et al., 2005). On the other hand SMP aredefined as the pool of organic compounds that are released into solution from substratemetabolism (usually with biomass growth) and biomass decay (Barker and Stuckey, 1999).The EPS or SMP are constituted by proteins, polysaccharides, nucleic acids, lipids, andhumic acids.A number of different studies have indicated a direct relationship between thecarbohydrate level in SMP fraction and MBR membrane fouling (Lesjean et al., 2005; Le-Clech et al., 2005; Rosenberger et al., 2005). The hydrophilic nature of carbohydrate mayexplain the apparently higher fouling propensity of this fraction over that of proteins, given43