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Comparison of dead-end and continuous filtration conditions in a denitrification membrane bioreactor

2010; Elsevier BV; Volume: 369; Issue: 1-2 Linguagem: Inglês

10.1016/j.memsci.2010.11.061

1873-3123

E.J. McAdam, Elise Cartmell, Simon Judd,

Membrane-based Ion Separation Techniques

Abstract The impact of using dead-end filtration conditions to sustain permeability in an immersed denitrification membrane bioreactor (MBR) has been studied, and the performance compared with a more conventional MBR hydrodynamic regime using constant gas sparging. Shear imparted during constant gas sparging was sufficient to reduce median floc size ( d 50 ) from 182 μm, observed during dead-end filtration, to 55 μm. Although this reduction in floc size demonstrated erosion, a concomitant generation of soluble extracellular polymeric substance (EPS) was not observed. Whilst unexpected, this finding accords with several recent investigations studying the long-term effect of shear. However, a significant concentration of soluble EPS was recorded in the biofilm that developed when using constant gas sparging (44.0% cf . 18.8% for dead-end) and was explained by the preferential transport of soluble compounds to the membrane wall during constant shear conditions. For an imposed flux ( J 20 °C ) of 22 L m −2 h −1 , a fouling rate (d P /d t ) of 0.39 mbar h −1 was recorded during constant gas sparging compared to 0.11 μbar h −1 during dead-end filtration. The lower fouling rate recorded during dead-end conditions was sustained using only a 30 s gas sparge at the end of each dead-end filtration cycle ( ca . 10–30 min) which appeared sufficient to almost completely remove the cake layer formed. The reversibility of the cake was attributed to the simultaneous deposition of soluble, colloidal and flocculant materials which created a more heterogeneous, porous and subsequently less tenacious deposit. Cycle length, rather than gas flow rate, was identified as the critical parameter for optimum dead-end filtration; provided filtered volume was sufficiently short to limit mass deposition, low gas flow rates were sufficient to reverse deposition. This significantly impacts on energy demand; dead-end filtration provides a net energy reduction for gas sparging from 0.19 kWh m −3 for constant sparging to 0.007 kWh m −3 .