Fouling is one of the most vexing problems in water-treatment systems that use membranes to remove suspended and dissolved particles. The deposition and accumulation of these substances on membrane surfaces inevitably lead to loss of performance for the membrane . While there are several strategies for reducing fouling, the right solution varies with the process and type of fouling.
Increasingly specified for process water and wastewater treatment, membranes provide better removal efficiencies than conventional filters because they have smaller pore sizes. Also, membranes have fewer consequences for the environment because they eliminate or significantly reduce the need for coagulants and flocculants, as well as the handling and auxiliary equipment required for traditional chemical-treatment methods.
Fouling can dramatically reduce the efficiency and economic benefits of a membrane process. The type of fouling and how strongly it appears depends on several parameters:
• nature of solutes and solvents
• membrane process
• pore-size distribution
• membrane surface characteristics and material of construction
• hydrodynamics of the membrane module
• process conditions
The most common fouling types are shown in Table 1.
Fouling can dramatically reduce the economic benefits of a membrane process, especially during the filtration of wastewater, when the filtered water is in economic competition with fresh water sources. Certain economic aspects have to be taken into account and a systematic analysis of the various options has to be considered. As illustrated in Figure 1, there are several basic tools, or parameters, that can be influenced to reduce fouling:
• bulk properties
• membrane properties
• concentration polarization
• permeate flux
Influence of the bulk solution
Parameters that can be used to influence the fouling behavior of a bulk solution are primarily pH, temperature and particle size. An example is the scaling of calcium sulfate. Operation at low pH and the addition of scaling inhibitors, which prevent scaling by changing the solubility of the salts, can significantly reduce the precipitation of calcium sulfate on the membrane.
Proteins are a special group of macromolecules. Variations in temperature, ionic strength, concentration and pH can influence the tendency of membranes to foul. If protein denaturation occurs, fouling becomes much stronger. While these examples show the influence of the bulk properties on membrane fouling, whether it is possible to influence any of these parameters depends on actual process conditions and the economic considerations involved.
Concentration polarization is the accumulation of rejected particles, especially during microfiltration and ultrafiltration, to an extent that transport to the membrane surface becomes limited. High flux through the membrane can cause the rejected particles to accumulate on the surface of the membrane (Cm), as shown in Figure 1. Concentration polarization reduces the permeability of the solvent and can lead to a limiting flux, in which an increase in pressure does not correspond to a rise in flux. Reducing concentration polarization leads to higher limiting flux and lower fouling tendency.
Depending on the solute and process conditions, three effects can reduce flux:
• rising resistance, which leads to lower flux
• increasing osmotic pressure due to retained macromolecules, which reduces transmembrane pressure
• changing physico-chemical properties, such as viscosity within the membrane boundary layer
Controlling concentration polarization is essential for the process to be economically beneficial. It can be controlled by increasing crossflow velocity or by increasing turbulence on the membrane surface, which increases the back transport of the particle away from the membrane.
To varying degrees, the type of membrane module used in the water-treatment process affects the influence of concentration polarization. Additionally, it is difficult to balance high fluxes and low fouling with low investment and operating costs. For example, tubular modules can accommodate high cross flow and large particles, however, their capital costs and ratio of relative price to membrane area are considerably higher than those for spiral-wound modules. The most commonly used module type, spiral-wound modules, enjoy the advantages of lower installed costs and easier changeout. Channel height can be varied by the use of distance keepers, also known as spacers.
Capillary membrane modules, constructed of hollow fibers arranged in parallel, can be backwashed inline during filtration to remove particles from the membrane or to add chemicals from the permeate side. Like tubular modules, they have high investment costs, but their ability to backwash at regular intervals reduces the potential for fouling.
Other types of modules, such as rotational or vibrating systems can be used. Because they have high energy demands, it is important to calculate the energy demand for the required amount of permeate. The filtrate determines whether a more-expensive module type or enhanced pretreatment will provide a stable process with the best economic benefits. A precise evaluation with pilot trials is necessary for each individual case.
As an essential part of the membrane process, the membrane itself has a strong influence on fouling. Typically, hydrophilic membranes are specified because they exhibit an affinity for water, which is one of the main tools used to reduce the adsorption of foulants onto the membrane surface. A hydrophilic membrane is surrounded by water molecules, which work as a protective layer. The hydrophilicity and hydrophobicity of some polymeric membrane materials are shown in Table 2.
Because hydrophilic membranes have lower chemical resistance than hydrophobic ones, their chemical stability and cleanability have to be evaluated as part of the selection process. It should be noted that most membranes are polymer blends. Most polyethersulfone (PES) membranes contain some polyvinylpyrolidone (PVP) to increase hydrophilicity.
The problem is that PVP is not stable against oxidizing agents, which may lead to changes in membrane porosity if not closely monitored.
Porosity is one parameter that can reduce fouling during microfiltration and ultrafiltration. The strongest fouling is caused by the blocking of membrane pores; therefore, their pore size should be smaller than the average particle size. Typically, Ergo, a membrane with a narrow pore-size distribution, is preferred to avoid the blocking of bigger pores.
Consider, for example, a process water stream that is pretreated with a 100-µm pre-filter. After pretreatment, the stream is treated directly with a capillary module constructed of a very hydrophilic cellulose-triacetate ultrafiltration membrane. Operating at moderate pressure, fouling is reduced significantly compared to that achieved with standard PES membranes. Chemical cleaning with citric acid to further reduce scaling is only required every three to six months, versus a weekly cleaning with chlorine for PES. Although more expensive than PES, cellulose triacetate provides a more stable process and requires less chemical treatment.
Influence of the permeate flux
Critical flux is another factor that can be influenced to reduce fouling in microfiltration, ultrafiltration and nanofiltration processes. It is defined as the flux below which a decline in flux with time does not occur, while above this flux, fouling starts . In this modus, the amount of particles transported to the membrane is similar to the amount of particles that diffuse away from the membrane.
There are two hypotheses for this concept. In the strong form, the flux of the solution is equivalent to the clean-water flux at a certain transmembrane pressure. In the weak form, a constant flux is rapidly established but it is below the pure-water flux. This may appear due to fast adsorption on the membrane, but afterward the flux remains constant. Concepts of the weak and strong forms are illustrated in Figure 2.
It should be noted that the theory of critical flux has no correlation to limiting flux. Limiting flux is a phenomenon that appears due to concentration polarization, while critical flux is related to fouling. In operation below critical flux, the fluxes are reversible, which means that as long as flux is below critical flux, membrane permeability is not changed by fouling.
The advantages of critical flux are that constant fluxes and membrane properties can be sustained for longer periods. On the other hand, the conditions of critical flux require lower pressure, hence a higher membrane area is required. Hence, the tradeoff between higher investment costs and lower fouling tendencies have to be assessed for each process.
The challenge ahead
Fouling is and will continue to be the Achilles heel of membrane filtration. The control and reduction of these phenomena are essential to establishing viable membrane processes in new applications. Especially during the treatment of problematic solutions, the economic benefits are drastically reduced if the fouling is not rigidly understood and controlled. Therefore, it is important to reduce fouling with effective but simple methods.
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