Microfiltration membrane module configurations

Microfiltration and UF membranes are available in tubular, spiral wound, and hollow fiber membrane module configurations. Tubular and spiral MF and UF modules are similar to RO tubular and spiral wound membrane modules described in Chapters 4.3.2 and 4.3.3. However, while the thickest feed spacer in a spiral RO module is 34-mil, UF and MF modules nominally have up to a 45-mil spacer due to the relatively high concentration of suspended solids these membranes are called upon to treat (TriSep Corporation offers a special 65-mil spacer for dairy applications). 

Table 16.3 Advantages and limitation of various module configurations for microfiltration and ultrafiltration membranes.

In this chapter, we will introduce fundamental concepts of the membrane and membrane-separation processes, such as membrane definition, membrane classification, membrane formation, module configuration, transport mechanism, system design, and cost evaluation. Four widely used membrane separation processes in water and wastewater treatment, namely, microfiltration (MF), ultrafiltration (UF), nanofiltrafion (NF), and reverse osmosis (RO), will be discussed in detail. The issue of membrane foufing together with its solutions will be addressed. Several examples will be given to illustrate the processes. 

When porous ultra-or microfiltration membranes are employed, the capillaries mosdy have a gradient in pore size across the membrane. In this case the location of the smallest pores (inside or outside) determines which of the two configurations is used. A packing density of about 600 - 1200 rn /m is obtained with modules containing capillaries, in between those existing in tubular and hollow Hber modules. 

Microfiltration (MF) is a membrane filtration in which the filter medium is a porous membrane with pore sizes in the range of 0.02-10 pm. It can be utilized to separate materials such as clay, bacteria, and colloid particles. The membrane structures have been produced from the cellulose ester, cellulose nitrate materials, and a variety of polymers. A pressure of about 1-5 atm is applied to the inlet side of suspension flow during the operation. The separation is based on a sieve mechanism. The driving force for filtration is the difference between applied pressure and back pressure (including osmotic pressure, if any). Typical configurations of the cross-flow microfiltration process are illustrated in Fig. 2. The cross-flow membrane modules are tubular (multichannel), plate-and-frame, spiral-wound, and hollow-fiber as shown in Fig. 3. The design data for commercial membrane modules are listed in Table 1. 

Air Bacfiflush A configuration unique to microfiltration feeds the process stream on the shell side of a capillary module with the permeate exiting the tube side. The device is run as an intermittent deadend filter. Every few minutes, the permeate side is pressurized with air. First displacing the liquid permeate, a blast of air pushed backward through the membrane pushes off the layer of accumulated solids. The membrane skin contacts the process stream, and while being backwashed, the air simultaneously expands the capillary and membrane pores slightly. This momentary expansion facilitates the removal of imbedded particles. 

Microfiltration units can be configured as plate and frame flat sheet equipment, hollow fiber bundles, or spiral wound modules. The membranes are typically made of synthetic polymers such as Polyethersulfone (PES), Polyamide, Polypropylene, or cellulosic mats. Alternate materials include ceramics, stainless steel, and carbon. Each of these come with its own set of advantages and disadvantages. For instance, ceramic membranes are often recommended for the filtration of larger particles such as cells because of the wider lumen of the channels. However, it has been shown that spiral wound units can also be used for this purpose, provided appropriate spacers are used. 

Electrofiltration is related to the application of an electric field to improve the efficiency of pressure-driven membrane filtration [110], Figure 10.47 shows the basic configuration of electrofiltration, where an electric field is applied across microfiltration or UF membranes in flat sheet modules, tubular modules, and SWMs. The electrode is installed on either side of the membrane with the cathode in the permeate side and the anode in the feed side. Usually, the membrane support is made of stainless steel or the membrane itself is made of conductive materials, to form the cathode. Titanium coated with a thin layer of a noble metal such as platinum could, according to Bowen [111], be one of the best anode materials. Wakeman and Tarleton [112] analyzed the particle trajectory in a combined fluid flow and electric field and suggested that a tubular configuration should be more effective in the use of electric power than the flat and multitubular module. 

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