Membrane performance can be affected by the orientation of hollow fiber membranes. Many different orientations such as vertical, horizontal, slanted, radially slanted like umbrella, etc. have been tested, but vertical and horizontal configurations are considered most practical due to the easiness of the module fabrication and the high packing densities. Slanted modules were also studied, but no particular advantages were found in terms of membrane performance (Sridang, 2004).
Unfortunately there is no published data comparing the vertical and horizontal configurations in field conditions, but some lab scale studies have been published. When hollow fibers were mounted either parallel or transversal to the flow in a channel of plate and frame module and yeast suspensions (5g/L) were filtered under a constant low TMP, the vertical fibers mounted parallel to the flow showed higher steady state flux (Chang, 2000). Similar observations were also made in other experiments (Fane, 2002; Chang, 2002b). It was postulated that the rising bubbles near the membrane fiber generated strong falling film effects and wake effect that scoured the membrane surface efficiently. On the other hand, the horizontal fibers supposedly suffer from particle deposition due to the eddy flows in the half of the fibers that is not facing the flow while there is a potential advantage of physical fiber contacts if packing densities are high enough.
In spite of above observations, it is not clear whether the observations in the controlled lab experiment can be directly applicable to the poorly controlled large scale operations. For instance, the lab scale experiment used hollow fibers mounted in a plate and frame type module, where all the bubbles must pass near the fibers. But, this is not the case in larger scale, where direct membrane scouring by bubbles is not likely the major fouling mitigation mechanism as discussed here.
One potential disadvantage of horizontally mounted membrane is the internal pressure loss caused by stagnant bubbles can hinder the flow in the lumen. In immersed membrane processes, dissolved gas in permeate partially escape inside the membrane fiber/module due to the lower gas solubility under the vacuum pressure. The bubbles typically move together with permeate and are discharged, but some can stick on hydrophobic domains inside the water channel as shown in Fig. 1. The presence of an air bubble induced by a ‘dry point’ or unwetted point within the hollow fiber can cause apparent increases in local flow resistance (Chang, 2008). The fibers can suffer more from the dry points produce less permeate while the other fibers must produce more permeate to compensate the loss. Though the bubble attachment in membrane lumen can occur in any membrane configuration, it can be more significant in horizontally mounted membranes since the buoyancy of bubble does not help discharge the bubbles in this configuration.
Fig. 1. Images of stagnant and mobile bubbles detected in hollow fiber lumen by X-ray microimaging (Chang, 2008).
Other disadvantage of horizontally mounted hollow fiber membrane when it is used for MBR is the vulnerability to fiber breakages due to the tensile stress given to the both ends of the fibers. While the rising two phase flow pushes the horizontally mounted fibers upward, eddies behind the fiber pull the fiber to the same direction. As all the drag forces transfer to the both ends of the fiber, fiber breakage in both ends becomes a concern. The maximum possible lengths for a given fiber with a certain degree of tensile strength is limited by the magnitude of the stress transferred to the fiber ends in two phase flow. Since the maximum allowable membrane lengths are limited at relatively low range, the specific membrane area per header become relatively low, too, which acts as a disadvantage in terms of module costs.
Horizontally mounted hollow fiber membranes were commercialized in 1990’s as Sterapore SUR™ by Mitsubishi Rayon Co. of Japan targeting MBR market. The polyethylene (PE) membrane fibers were known to be surface treated to hydrophilic and ID and OD were 0.27 and 0.41 micron, respectively. Although two-side suctions were performed, the small fiber diameter and relatively long fiber length (~900 mm) in the original version allowed relatively low average fluxes, e.g. <12 LMH, in spite of the high scouring air flow rate at ~100 m3/m2/hr based on footprint. The membrane length was reduced to 400-600 mm in newer versions later. The similar horizontal membrane concept was adapted by Zenon Environmental Inc. of Canada in its ZW1000® module in 2003 targeting the water with low suspended solids (not MBR). The packing density of this module is much higher than those for MBR as a consequence of the low suspended solids in feed water. Air scouring is performed only during the backwash cycles, which reduces the chance of fiber damages significantly.
© Seong Hoon Yoon