NAMS 2002 Workshop - ICOM 2008

Membrane Modeling III - Process Simulations – 4
Wednesday July 16, 11:15 AM-11:45 AM, O’ahu/Waialua
Hydrodynamic CFD Simulation of Mixing in Full-Scale Membrane
Bioreactors with Field Experimental Validation
Y. Wang (Speaker), The University of New South Wales, Sydney, Australia
M. Brannock, The University of New South Wales, Sydney, Australia
G. Leslie, The University of New South Wales, Sydney, Australia - yuanw@student.unsw.edu.au
Membrane bioreactors (MBR) represent the ‘state of the art’ for the treatment of
municipal wastewater. The optimisation of MBR units requires knowledge of
biological treatment, membranes and hydrodynamics/mixing. Good mixing can
ensure the effective use of the entire reactor volume and can affect nutrient
removal efficiency. The degree of mixing and membrane configuration (e.g. flat
sheets and hollow fibres) affects the output response describing the system’s
flow regimes and expressed by the residence time distribution (RTD) profiles.
The authors’ research group has investigated the mixing efficiency of pilot scale
MBRs [1] and full-scale MBRs [2] with different membrane configurations via
RTD analysis. Recently, we have developed a CFD model that has been
validated with field experiments to show how membrane configurations can affect
mixing conditions in the reactor.
CFD simulations were conducted using the commercial software package
Fluent® on a 2.2 MLD hollow fibre membrane MBR in Sydney and a 2.5 MLD
double deck flat sheet membrane MBR in South Australia. A 3-dimensional flow
field consisting of the interacting phases of water and air were computed using
the Eulerian-Eulerian multiphase model. The simulation results showed good
agreement with the measured field RTD data. The hollow fibre MBR has a Peclet
number of 0.24 and number of completely mixed tanks in series of 1.08, while the
flat sheet MBR has a Peclet number of 0.37 and 1.13 of completely mixed tanks
in series, which showed that the two MBRs were both close to completely mixed
conditions. However, the mixing energy contributed by the mixer, bioreactor and
membrane aeration, and recirculation pumps was 55.8 kW in total of the flat
sheet MBR while 42.9 kW of the hollow fibre MBR, which indicated that the use
of flat sheet membranes was 20% higher in mixing energy to create the same
degree of mixing.
In conclusion, the development of MBR CFD model can provide the access to
evaluate the effects of membrane configurations on energy consumption with the
view of achieving the optimum mixing conditions at the lowest possible energy
inputs for the design of large installations.