计算流体力学及氧传质模型耦合对一体化A2 /O-MBR的模拟与优化

A2/O-MBR is applied in many large-scale sewage treatment  plants, because of its high removal efficiency of denitrification  and phosphorus, higher product  quality and the  smaller footprint.  However,  it  still has  disadvantages,  such as  the  long process flow and  the high energy consumption  which restrict  its further application.  In order to save the energy consumption and  optimize structure, a new kind of A integrated  A2/O-MBR,  was  studied  by  the  research  group.  It  was  found  that  the integrated A2/O-MBR  could be operated  at lower air  flow rate in the  membrane tank with lower  aeration energy consumption.  In the integrated  A2/O-MBR, the  hydraulic A2/O-MBR, the circulation  was driven  by  the  airlift  power  of membrane  tank. A  very  important question  needed  to be  answered  is  whether  the  DO concentrations  in  the  aerobic, anoxic and anaerobic unit can  meet the requirements of A2 /O process  or not under the condition of aeration in the membrane  unit. It is important not only  for inspecting and controlling  the  operation  conditions  of A2 /O-MBR,  but  also   for  establishing  the  reasonable or optimal operation parameters. To answer the questions above,  the computational fluid dynamics (CFD) coupled with  oxygen  mass   transfer  model  was   implemented  to  study  the   hydrodynamic characteristics and DO  concentration distribution within the  integrated A2/O-MBR  in this study. The results obtained are summarized as follows: 1) For the structure optimization of the integrated A2/O-MBR： a)    The  diameter   of  backflow  holes   had  no  obvious   influence  on   the velocity and wall shear stress  in up flow region. However,  it had a great influence on  down flow  region. The  velocity in down  flow region  was lower  and   the   hydrodynamic  characteristics   were  better   when  the diameter of backflow holes was set at 10mm. b)    The distance between baffles and underside  had a great influence on the velocity and  wall shear stress  within the reactor.  The velocity and  wall shear stress  became higher when  the distance was  110mm. Meanwhile,the distribution of velocity and wall shear stress was more uniform. c)    The  number  of  diffusers  had  a  great  influence  on  the  flow  field  in membrane region. When the number of diffusers  was 5, the velocity and wall  shear   stress  in   membrane  region   were  higher.   Moreover,  the distribution of  velocities and  wall shear stresses  was more  uniform, so that it was better for the scouring of the membrane. d)    The layouts of  diffusers had an  obvious effect on  the velocity and  wall shear  stress within  the  reactor. The  distribution of  velocities  and wall shear  stresses  was  more   uniform  when  the  layout  of  diffusers   was parallel. 2)    For the simulation of DO distribution by CFD-DO： a)    At  four  different aeration  intensities,  30,  60,  90,  120 L/min,  the  DO concentration  in  up  flow region  was  4  mg/L,  which  was  also called aerobic  zone. The  DO  concentration was  lower  in down  flow  region,where the upper  zone was called  anoxic zone (DO b)    Lower  aeration   rate  in   the  integrated   A2 /O-MBR  was   not  only   a requirement to  establish the necessary  reaction conditions in  anaerobic, anoxic or aerobic region, but  also an expected situation to reduce energy consumption in sewage treatment and reuse. c)    Sludge  viscosity   had  a  certain   influence  on   the  DO  concentration distribution  in the  integrated  A2 /O-MBR.  With the  increase  of sludge viscosity, the DO  concentration became lower,  but it had little  effect on the flow field. d)    Diameter of  bubbles had  an obvious  influence  on the  DO distribution and with  the increase of  the diameter of bubbles,  the DO  concentration in  the  reactor  reduced significantly,  but  the  wall  shear  stresses  were increased  at the  membrane surfaces.  However, the  variation of  bubble sizes  had no  evident  effect  on  the value  and  distribution of  velocties within the  reactor. The  results showed that  there was  a suitable bubble size   that  could   not   only  produce   satisfactory   shear  stress   at   the membrane,   but   also    establish   the   desire   DO   conditions    in   the anaerobic-anoxic-aerobic region. The  CFD-DO   model  and  the  modeling   results  in  this   study  were successfully   validated   against   the   experimental   results    from   a   pilot integrated A2 /O-MBR.  The results showed that  the coupled CFD-DO model and  the  simulation platform  established  were  feasible  and  the  simulation results showed  the effectiveness  and credibility of  the CFD-DO  model and the  simulation  platform.   It  could   be  used  as  technical   support  for  the research and development of the new integrated A 2/O-MBR.