Classification of filtration modes

 Crossflow filtration vs Dead-end filtration

Filtration modes can be divided by crossflow filtration and dead-end filtration depending the flow direction on membrane surface as shown in Fig. 1.

  • In crossflow filtration, feed moves parallel to the filter medium to generate shear stress to scour the surface (Fig. 1a). Extra energy is required to generate crossflow, but cake layer thickness can be controlled. Pseudo steady-state may exist, where scouring effect and particle deposition find a balance and cake layer hardly grows. This filtration mode is particularly effective when feed water carries high level of foulants such as suspended solids and macromolecules. All MBR processes and most of wastewater filtrations are adapting crossflow modes. Filtration systems adapted by both iMBR and sMBR discussed here are relying on crossflow filtration.
  • In dead-end filtration, no crossflow exits and feed moves toward the filter medium. All the particles that can be filtered by filter settle on the filter surface. Since the filtration is not sustainable forever without removing accumulated solids, backwashing is performed periodically and/or filter medium is replaced. This filtration mode is particularly effective when feed water carries low level of foulants. Many surface water filtrations, pretreatment for seawater RO, and tertiary filtrations are adapting dead-end modes.
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a) Crossflow mode                                                              b) dead-end mode

Fig. 1. Crossflow filtration and dead-end filtration.

 

Surface filtration vs Depth filtration

 

Filtration can be also divided by two depending on where the particle rejection occurs as shown in Fig. 2.

  • In surface filtration, particles are supposedly rejected on the filter surface and generally do not intrude into the filter medium. All the filter mediums used in membrane filtration fall into this category.
  • In depth filtration, particles can intrude filter medium and captured by the medium through collision. Since the particle removal relies on the random collision and there is no absolute barrier, some particles can pass the filter. Cartridge filter, sand filter, multi-media filter, most air filters, etc. fall into this category.
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a) surface filtration by skin layer                                                      b) depth filtration by filter medium

Fig. 2. Surface filtration and depth filtration.

 

© Seong Hoon Yoon