Fouling characteristics specific to submerged outside-in

As stated earlier, the pilot experiment that this work is based on used hollow fiber membrane modules. The design for hollow fiber modules is complex and fairly unique, thus the fouling occurring in these modules can be expected to have different characteristics from other module types. One of the key parameters defining fouling in hollow fiber modules is packing density. In membrane literature, two different definitions for packing density are commonly used. First one is membrane area divided by the volume of the module (Shimizu et al. 1996). The other common definition is the ratio between the cross sectional area of the membrane divided by the total cross sectional area, where the unit of packing density is percent. While high packing density is important for obtaining high effective surface area per process volume demand, it has been shown that increasing packing density too much can have a negative effect on the filtration process efficiency mainly due to increased fouling. (Günther et al. 2010).

There has been little research about how different areas of these type of modules or individual fibers perform during hollow fiber filtration. In study by Yeo and Fane (2005), it was shown that even in bundles of just 9 fibers, the permeate flux of the middle fiber can be 50 % smaller than

the average flux in the bundle. Yeo and Fane (2005) presented two possible explanations for this: uneven hydrodynamic conditions and merging cake layers. Uneven hydrodynamic conditions refers to stagnant areas inside the bundle, where the cross flow is especially low.

This decreases shear stress on the membrane surface and increases fouling of all sorts. The other explanation refers to a phenomenon, where cake layers around individual membranes merge creating areas between the membranes that are inactive during the filtration (Shimizu et al. 1996). The stagnant zones and merged cake layers are very likely made more frequent by high packing density. Thus, lowering packing density can improve the filtration performance of middle part of the module. Other methods for achieving this include aeration, increasing cross flow in the module and frequent cleaning. (Yeo and Fane, 2005)

As stated above, the performance of an individual fiber in a bundle is depended on its location, for example fibers in the middle of the module can have lower permeate flux than fibers in the outer areas under certain conditions. In addition to this, the permeate flux at each point of the same fiber is rarely uniform. As seen from equations (2) and (7), the permeate flux is affected by the transmembrane pressure. The pressure inside the fiber has an axial gradient due to the the suction pressure located at the open-end. If the pressure outside the fiber is constant, this means that the permeate flux is the highest near the open-end, where the pressure inside the fiber is the lowest. This effect has been demonstrated by Carroll & Booker (2000) in experiments with hollow fiber membranes of different lengths. Increasing the fiber length yielded diminishing results on the total flux until increasing the fiber length had no noticeable results in total permeate flux. When the filtration was kept going for longer time, the fluxes of membranes of different lengths started to decrease until reaching a constant state.

The study done by Carroll & Booker (2000) considered cases with constant pressure outside the membrane. However, in most real-life modules this is not the case. Günther et al. (2010) applied computational fluid dynamics (CFD) to model submerged hollow fiber filtration. In the case, where the pressure at the bottom of the module is higher than at the top, there will most likely be some amount of vertical flow outside the fiber. This is most relevant for open-bottom modules, where the pressure at the bottom of the module is close to the surrounding tank pressure but can apply to other module types as well. The bottom to top flux around the membranes will cause pressure drop, which decreases transmembrane pressure towards the open-end of the fibers. High packing density reduces the space the vertical flux has and

increases the pressure drop outside the fiber. The model that was developed by Günther et al.

(2010) predicted that with high enough packing density (at least > 0.6) the permeate flux would be higher near the bottom of the membrane. It should be noted that the model makes some big assumptions like uniform filtration conditions and doesn’t account for critical effects like cake layer formation, which most likely would have large effect with high packing density.

Because the permeate flux is different in different areas of the fiber, these areas also foul at different rates. The basic idea behind this it that areas with high local permeate flux will have foul faster than areas with lower local permeate flux. Thus, at the start of the filtration fouling will occur mainly in areas with high permeate flux, but as the fouling evolves in these areas, the local permeate flux will drop, and fouling will start to increase in other areas of the module.

(Carroll & Booker, 2000). If the filtration is kept going long enough, the module will reach a state of uniform permeate flux distribution, where ∆p/Rt is constant at each location, as stated in equation (7).

Lee & Kim (2012) have studied to local fouling in pilot plant using a PVDF hollow fiber membrane (nominal pore size of 0.1 μm) for filtrating river water. The pilot process used coagulation (not in-line) as pretreatment for separating particulate matter and operated for 3 months. After the pilot process had ended, Lee & Kim (2012) tested the degree of fouling of different fiber samples by conducting individual filtration experiments, where the fouling resistance terms (Rr and Rir) were calculated for each sample. The results did not show correlation between reversible fouling and the distance of the fiber sample from the open-end, but irreversible fouling did correlate with this distance, when the SS loading was low (average 12.6 mg/L). Possible explanation for this, according to Lee & Kim (2012), was that areas with high permeate flux initially developed high amounts of irreversible fouling, which in turn decrease the reversible fouling developed after chemical cleanings, resulting in close to uniform permeate flux distribution along the fiber. This explanation appears logical, and for example Zhuang et al. (2015) have also presented the idea that by manipulating the membrane resistance, the cake layer distribution and permeate flux could reach a uniform state along the fiber.