Effects Of Membrane Fouling And Factors That Influence

Published Date: 02 Nov 2017

Chapter 2

2.1 Introduction

In this chapter, effect of membrane fouling and factors influencing membrane fouling will be discussed. As a result from membrane fouling, flux through the membrane will greatly decrease. The other effects of membrane fouling are permeating quality deterioration and energy consumption increase. These happened due to the membrane resistance increase with time and the foulants tend to accumulate on the membrane surface. Resistances can be differentiating into four categories which are pore blocking, adsorption, formation of gel layer, and concentration polarization. Besides, decreasing of membrane flux may also cause by the decreasing in driving force. The factors influencing membrane fouling was studied based on feed properties, membrane properties, and also process properties.

2.2 Effect of membrane fouling on flux

From simplified model-Hagen Poiseuille equation (Eq 2.1), the effect of membrane fouling can be examined.

Where,

: flux

: posity of membrane

: viscosity of fluid

: effective thickness of the membrane

: mean pore diameter of membrane

: transmembrane pressure

Basically, this equation provided is for the laminar flow regime within membrane pores and also for the pores that are round in shape. For non-round pores, this equation can be expanded by replacing in equation 2.1 with hydraulic diameter,

Where the equation change to

According to equation (2.1), flux of a membrane depends on porosity of the membrane, hydraulic diameter of the flow channel in pores, and also the effective thickness of the membrane for a given transmembrane pressure and also its viscosity. When a membrane is fouled, it is also mean by decreases in flux. This affects the changes in membrane porosity, hydraulic diameter, and also effective thickness of the membrane. Based on equation 2.2, porosity and the hydraulic diameter are decreases, whilst the effective thickness increases when there is the flux is decreases due to the fouling effect.

2.3 Factors Influencing Membrane Fouling

Membrane fouling is a major problem for every membrane application. Factor that influence membrane fouling is discussed further based on feed solution, membrane properties, and also process parameter.

2.3.1 Feed solution properties

Concentration, pH and ionic strength of feed solution contribute to the amount of membrane fouling. In addition, fouling also influences by the interaction of different components in multicomponent mixtures.

According to Bhattacharjee (1993) and Doulia (1992), when there is increase in feed concentration, permeate flux will decrease. This is explained by higher rate of deposition will occur when concentration of feed solution increases. However, this is more likely to be happened for surface fouling as there is formation of gel layer on the membrane surface. When concentration of feed increases, gel layer thickness will also increases. The increasing gel layer thickness leads to increase of total hydrodynamic resistance to the membrane flux and thus less permeate.

Other than concentration, pH and ionic strength of feed solution also influence the membrane fouling. Mutual repulsion occurred preventing clustering of proteins at higher or lower pH in which the proteins are charged. In addition, the solubility in water increases and the affinity for the membrane decreases if the solutes are charged. Thus, a looser cake layer will be formed where then leads to a higher flux. However, the charged of the solutes is affected by the ionic strength that causes a charge shielding. For nanofiltration performance in natural organic matter (NOM) rejection, increased ionic strength illustrated higher flux decline and NOM rejection which is shown by figure 2.1. Besides, from figure 2.2, increased solution pH also showed greater flux decline which caused by increasing salt rejection and enhancing salt concentration on membrane surface. (Jarusutthiraka et al., 2007)

C:\Users\Sin Huei\Desktop\3.pngFigure 2.1: Influence of ionic strength in the present of NOM on permeates flux. (Jarusutthiraka et al., 2007)

C:\Users\Sin Huei\Desktop\1.pngFigure 2.2: Influence of pH on permeates flux for ionic strength of CITATION Cha07 \l 1033 (Jarusutthiraka et al., 2007)

For multicomponent mixtures, filtration of one solute can affect the flux and retention behavior of another solute. When there is presence of larger components in the solution, it can affect the retention of smaller molecules. This can be explained by pore narrowing by one of the solutes which then affecting the filtration behavior of other solutes in the mixtures. Moreover, another mechanism is the formation of cake layer by larger molecular weight components that has a smaller porosity than the original membrane.

2.3.2 Membrane Properties

Membrane properties divided into two categories which are membrane material related and membrane structure related factors. These factors included hydrophobicity, charge, pore size, porosity and pore distribution of membrane, and etc.

For hydrophobic membrane, solutes tend to adsorb more through the membrane compared to the hydrophilic membrane. This phenomenon caused lower permeate flux due to the higher degree of adsorption of solutes that lead to more compact structure and also increased pore blocking.

Pore size of the membrane also one of the factor that influence the membrane fouling. When membrane with larger pore size is used, membrane fouling are tend to be occur more severe. This is due to the chance of internal fouling increase as more deposition of solutes within the large pores.

Porosity of the membrane also greatly affects the membrane fouling. When comparing ultrafiltration and microfiltration membranes, membrane with lower porosity which is ultrafiltration membrane show high fouling effects. This is because solvent flowing towards the lower porosity ultrafiltration membrane will not meet a homogeneously permeable surface, while it has to follow streamlines to the opening of isolated pores. These finally result in pore blocking and forming of gel layer along the time.

2.3.3 Process Properties

Process related factor such as temperature, transmembrane pressure, and cross flow velocity also contribute affect the membrane fouling.

Basically, flux will increase when the temperature is increase due to the increment on the diffusivity and lowering of viscosity. However, the fluxes can also decreases when temperature increase if the feed properties changed. This can be better explained with film model, and osmotic pressure model which have an inverse proportional relation to the viscosity and a proportional relation to the diffusion coefficient. For an example, fouling is more severe in filtration when clustering and aggregation of solutes at high temperatures. This happened because increasing in concentration polarization that reduces retention which then resulting in higher solute concentrations at the membrane surface.

Apart from that, transmembrane pressure will also affect the membrane fouling. Generally, increase in the membrane pressure will results in an increase in the permeate flux. But there is also an exception when the pressure applied on gel layer that is established. A temporary flux increase causes the gel layer to thicken, which later result again in decrease of the flux to its original value. Due to these phenomena, limiting flux region exists above the critical transmembrane pressure.

Aside from all those factors, cross flow velocity also influencing the membrane fouling. Increasing of cross flow velocity will reduce the boundary layer thickness which resulting in an increase of mass transfer coefficient in film model. This caused the reduction in concentration polarization and also fouling. In addition, increasing of cross flow velocity enhancing back transport mechanism by its higher shear rate. Cross flow velocity has higher effect in ultrafiltration membrane as it reduces the gel layer thickness which increases the membrane flux. This theory may not applied in every cases as in some cases, fouling will be more severe when increasing the cross flow velocity. This mainly happened due to the increased pressure drop along the module at higher cross flow velocity. Besides, cross flow velocity has no effect on the flux and retention behavior of the membrane for internal fouling mechanisms.

2.4 Summary

As discussed in this chapter, membrane fouling can be occur due to many factors which can be categorized based on three basis which are feed properties, membrane properties and also process properties. All of these properties are important for taking account in each membrane application in order to achieve high permeate flux and less fouling. However, there is no perfect achievement and condition for zero fouling of membrane which the optimum condition is required to get the best result. It should be also considerate based on the operating cost and also efficiency of the membrane.