Microfiltration (MF) membranes provide absolute removal of contaminants from a feed stream by separation based on retention of contaminants on a membrane surface. It is the "loosest" of membrane processes, having a pore size ranging from 0.05 to 5µm. As a consequence of its large pore size, it is used primarily for particle and microbial removal and can be operated under ultra-low pressure conditions. However, it should be noted that MF does not, in all cases, remove contaminants strictly based on pore size of the membrane. A cake layer, consisting of materials present in the feedwater, can form on the membrane surface and provide additional removal capabilities.
The main types of commercially available microfiltration membrane geometries include:
- Spiral Wound
- Tubular
- Hollow Fibre
Spiral Wound Membranes are flat sheet membranes wound into a spiral configuration and are categorized as crossflow separation. In crossflow separation, a fluid stream runs parallel to a membrane. There is a pressure differential across the membrane that causes some of the fluid to pass through the membrane, while the remainder continues across the surface.
Because of its configuration there are difficulties in keeping the surface of the membrane clean, which when coupled with the fact that these membranes cannot be backwashed, means they are normally employed only in specific applications.
Tubular Membranes have relatively large inner diameters, ranging from 1.0 to 2.5cm, contributing to good mechanical strength. The membranes, which can be composed of polymeric or ceramic materials, are usually placed inside stainless steel or fibreglass reinforced plastic tubes, which are sealed by means of a gasket and outer ring clamps. They may be single or multiple feed channels. The pressurised feedwater flows through the inner lumen of the tube, and the permeate is collected in the outer shell of the module. This configuration allows for mechanical cleaning of the membrane surface.
Hollow Fibre Membranes usually consist of several hundred to several thousand fibers encased in a module, the fibres bound at each end with an epoxy or urethane resin. The inner lumens or internal fibres, which are kept small to avoid collapse under pressure, range from 0.4 to 1.5mm in diameter.
The physical strength of these membrane fibres allows them to be backwashed.
There are two different flow regimes in hollow fibre MF: inside-out and outside-in. Because the water is flowing through a concentric channel or lumen, the inside-out membrane allows good control over module hydrodynamics. On the other hand, it is more difficult to control the flow of an outside-in module because it is often difficult to control flow channeling and/or dead-end zones. Moreover with this flow regime it is more difficult to flush the particles from the module when backwashing as compared to an inside-out membrane. However, an advantage of the outside-in membrane is that there is usually lower head loss through the module.