Membrane Filtration
Membrane filtration does not require the use of chemicals or additives, making them safe to use and cost-effective in specialized applications such as blood purification for dialysis or filtering milk for cheese production. Additionally, membrane filters require minimal energy and are easily integrated into pre-existing flow pipes. As with all filter media, membranes may become clogged with excessive use and must be cleaned or replaced regularly to ensure optimal functionality. Backwashing, process flow disruption and diafiltration all allow for automatic in-place cleaning. Alternatively, filters can be removed, washed and sterilized in an autoclave. Filtering systems with successive membranes significantly reduce the maintenance needs and improve productivity. In this arrangement, a series of films are put in place with progressively smaller pore size to first remove large particulates, allowing secondary filters to trap micro particles and microorganisms. The trapped slurry can sometimes be recycled or returned to the manufacturing process for reuse, further reducing operating costs while improving productivity. Successive membrane filtration allows for very specific slurry separation and retrieval.
Nanofiltration, ultrafiltration, microfiltration and reverse osmosis techniques all employ membrane filters. Manufacturers discriminate between the groupings based upon the porosity of the membrane films. The basic function of each is that an air or fluid process stream is drawn through a thin material. The clarified fluid or gas is small enough to pass through the pores while excess particulates are not permitted. The composition of the membrane must be carefully calculated to exclude undesirable particles while remaining semi-permeable. The molecular composition and size of both desirable materials and contaminants must be taken into account during membrane selection and installation. The continuous lattice or matrix of pores can be classified as either polymeric or ceramic. Although ceramics can be used in air filtration systems, they are more commonly used in liquid filtration applications. Polymeric and other advanced synthetics are more predominant in air or gaseous applications. These materials vary in thickness, a significant factor in material selection for a specific filtration application. Nitrocellulose, cellulose acetate, coated PTFE, nylon, polycarbonate and mixed cellulose ester materials are common membrane materials in both liquid and air systems. In addition to porosity and thickness, there are several considerations integral to the success of membrane filtration. Effectiveness, absorption, flow and binding should be considered as well as the frame or holder design. Membrane filters are becoming more popular as they remove the possibility or filter contamination due to shedding particles or out-gassing, which occurs in other fibrous filters.
