There are two different modes of filtration in hollow fiber membrane depending on the direction of permeate flow, e.g. outside-in and inside-out as illustrated in Fig. 1.
The biggest advantage of outside-in filtration is the capability of handling very high suspended solids. All the commercially available hollow fiber modules for MBR run at outside-in modes. These modules are directly immersed in mixed liquor without pressure vessel, where feed water (mixed liquor) contacts the shell side of the membrane and permeate is collected in the lumen by vacuum. Air scouring is performed either continuously or intermittently in the shell side to prevent solids accumulation on membrane surface. ZW500 (GE Water), MemPulseTM (Semens), MUNC-620A (Asahi Kasei), etc. fall into this category.
Some outside-in modules used to treat the water with low suspended solids (<50 mg/L) run under dead-end mode, where all the feed water passes through membranes leaving solids on membrane surface. A large amount of solids can be stored in the spaces among the fibers before the module fails to produce design flow rate at the maximum pressure allowed. Backflushing is performed periodically to remove the accumulated solids whenever pre-determined cycle time is reached or trans-membrane pressure (TMP) exceeds the threshold. Permeate is obtained by vacuum for ZW1000 module, where hollow fibers are horizontally mounted without pressure vessel. This module is immersed in feed water and air scouring is performed only when filtration is stopped. Most other modules for low suspended solids have the hollow fibers enclosed in a pressure vessel, which run under positive pressure at dead-end mode. ZW1500 (GE Water), UNA-620A (Asahi), etc. fall into this category.
Inside-out type hollow fibers are good to maintain uniform hydrodynamics in the lumen, but it is practically not possible to generate the turbulence that is crucial to mitigate cake layer formation due to the small inner diameter and relatively low water velocity. (Note – Reynolds number is proportional to the fiber diameter and the water velocity) In addition, only the water with low suspended solids such as surface water can be filtered by this type membrane since fiber lumen can be plugged up by the debris contained in feed water.
Plugged fibers can be partially or fully recovered by backwashing. If one fiber is plugged in the middle, the concentrate from other unplugged fibers in the same module can be fed to the plugged fibers from the downstream, which can at least partially compensate the loss of feed flow to the downstream of the plugged area. Interestingly, X-Flow XIGA™ modules have dedicated feed water bypass channels through which some feed water bypasses and feed plugged fibers from the downstream. The bypassed water also dilutes the concentrate before it goes to the next module.
|Fig. 1. Outside-in and inside-out filtration modes in hollow fiber membrane|
© Seong Hoon Yoon