Combining submerged membrane technology with anaerobic and ...

Chapter 4days 57 (period I) and 316 (end of period III), probably due to the low MLVSSconcentration in the membrane chamber that did not prevent membrane pore blocking(Drews, 2010). Maintenance chemical cleanings were performed fortnightly, except onperiod II. Chemical reagents were not necessary during period II.Permeabilities between 100 and 200 L·m -2·h -1·bar -1 were normally observed duringthe two operational periods (figure 4.4a). These values were slightly better than thoseobserved during the operation of similar membrane modules, (Judd, 2002; Bouhabila etal., 2001), and also were higher than permeabilities observed in AnMBR (Spagni et al.,2010; Zhang et al., 2007). The reason of such behaviour is still unclear, but it is consideredthat aerobic biomass, both in suspension and in biofilm retained or degraded somefoulants generated in the methanogenic reactor. This fact made feasible to achieve theobserved permeabilities. The highest permeability values and the lowest fouling rates wereobserved in period II (table 4.1). Rapid permeability drops were observed during period I,whereas fouling rate was almost negligible during period II (figure 4.4a). In fact, asmentioned before, several maintenance cleanings and one recovery cleaning on day 57were performed during period I. Although the possible causes responsible for membranefouling will be analyzed in deep in Chapter 5, the worse membrane performance observedduring period I, coincided with a higher concentration of SMP carbohydrate fraction(SMPc) and a lower MLVSS concentration in membrane filtration chamber (table 4.1).During periods III and IV, SMPc was not so high than in period I, probably due to thehigher F/M ratio, but the lower MLVSS concentration led to a strong membrane fouling andlower operational fluxes, especially in period IV.The membrane filtration chamber in the MBR of the proposed system worked withMLVSS concentrations much lower than those recommended in the literature (Judd,2011). In figure 4.5 are represented to different TMP profiles corresponding to days onwhich MLVSS concentration was very low (around 0.5 gMLVSS·L -1 ) and moderate (around3.0 gMLVSS·L -1 ). The other operational parameters such as SRT, SAD m SMP cconcentration or flux were similar. As can be observed, at higher biomass concentrationsthe TMP profile is typical of cake layer formation fouling (Drews, 2010), which is easilyremoved by mechanical cleaning. On the other hand, at lower biomass concentrationsTMP profile changed significantly, indicating that the lack of protection by the cake layerled to an irreversible membrane fouling provoked by pore clogging of soluble and colloidalbiopolymers. In fact this kind of fouling is more harmful since it can be only removed bychemical cleaning. The fouling rate increased more than a 60 % from one scenario toanother, being MLVSS the only different parameter.124