Combining submerged membrane technology with anaerobic and ...

Denitrification with dissolved methane in an MBR after a methanogenic pre-treatment at ambient temperaturebiomass. Anaerobic methanogenic bacteria (ANME), which are capable to carry outreversed methanogenesis and convert methane into acetic acid/acetate, (Knittel andBoetius, 2009; Valentine and Reeburgh, 2000), were also found. It could be anotherpossible explanation for methane oxidation observed in the reactor even though theoxygen molar ratio was always lower than the one given by stoichiometry of the aerobicmethane oxidation pathway (eq. 6.1; table 6.3).Moreover, FISH analyses of the biofilms indicated the abundance of large clusters ofanammox bacteria. Therefore, as previously reported by Waki et al. (2009), nitrogenremoval in the presence of CH 4 and O 2 seemed to be a complex mixture ofmethanotrophic, denitrifying, ammonia-oxidizing and anammox processes.6.4.5. Membrane performanceMembrane critical flux did not varied significantly during the six experimental periods,with an average value of 20.8±2.0 L·m -2·h -1 . The flux applied (14.5 ± 1.0 L·m -2·h -1 ) wasbelow the critical flux, thus it was expected that reversible fouling was predominant. In fact,it was observed during all the operation time that permeability was almost fully recoveredwhen a physical cleaning with tap water was carried out. Only two maintenance chemicalcleanings were performed during the operation, at the beginning of periods III and V.As can be observed in figure 6.6, permeabilities normally ranged between 150 and230 L·m -2·h -1·bar -1 . Although the fluxes obtained were lower than those typically reported inaerobic MBRs operating with similar membrane modules, being between 20 and 25 L·m -2·h -1 , observed permeability values were similar (Judd, 2002; Wen et al., 2004).As recommended in Chapter 5, colloidal BPC concentration was measured in orderto establish a possible relationship between the operational conditions of the system andthe membrane fouling. A slight increase on cBPC concentration was observed at thebeginning of period II, when the environment in first chamber of the MBR was changedfrom aerobic to anoxic. As a consequence of the high MLVSS concentrations maintainedin the MBR, between 4 and 8 g·L -1 , it was not observed a significant impact of cBPCconcentration on membrane performance.Nevertheless, the remarkable increase observed during period IV, when methanewas desorbed from the UASB effluent (figure 6.6), was accomplished by a significant dropon permeability, being necessary a maintenance chemical cleaning. cBPC concentrationdid not decrease instantly when dissolved methane desorption was stopped but aprogressive diminution was observed during periods V and VI.167