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Microfiltration (MF) is the term applied to the use of a microporous membrane to remove contaminants from a fluid. The common classification of microfiltration encompasses pore sizes between 0.1 and 10 microns. This places microfiltration on a spectrum (see Chart 1) between fine particulate filtration and ultrafiltration.
Chart 1 shows four classifications of membrane filter, ranging from microfiltration to reverse osmosis. Each of these membrane technologies extends the range of particulate filtration to smaller dimensions.
Above microfiltration lies the full array of filtration and separation media applied to particle filtration. Such media include other polymer and paper sheets, nonwoven fiber sheets and cylinders, molded or granular carbon, molded ceramics and graded granular media of many materials.
Ultrafiltration (UF) and microfiltration share many characteristics and differ primarily in the range of pore dimensions. Both are commonly used in pressurized conventional flow. Their pressure drop is low enough to flow adequately at the pressure within most water supply systems. All feed water passes through the membrane in pressurized conventional flow, so there is no water waste.
Less common are applications of MF and UF in a cross flow configuration, where a portion of the feed water passes through the membrane and the remainder is sent to the drain as concentrate.
Nanofiltration (NF) and Reverse Osmosis (RO) are most commonly operated in a cross flow configuration. The pore size is typically too small to provide sufficient flux unless pressurized to levels uncommon in most home or commercial settings. Cross flow operation imposes some geometry and equipment parameters that impact application opportunities.
For instance, a larger membrane surface area is required and an accumulator tank is commonly used to enhance water delivery at the time of use. These do not allow NF or RO to be used as faucet filters, for example.
The filter membrane material can be flat sheets, either spiral wound or pleated. They can be hollow fibers which are typically potted into a casing to support and protect the fibers (Figure 1). A spiral wound sheet is usually employed with cross flow, while pleated sheets and hollow fiber media are commonly used in a straight through flow path. Filters are made from various polymers. Common polymers are polysulfone, polyethersulfone, polypropylene, polyvinylidene fluoride and nylon.
The MF pore size range overlaps with cells, bacteria, cysts and spores, plus some macromolecules and emulsions. Early applications were in microbiology, and other uses in microelectronics and drinking water followed.
Since the major pathogens of drinking water are retained by MF, microfiltration was adopted by pharmaceutical and biotechnology industries for solution sterilization.
Membrane filters can be scaled readily, so applications range from small residential point-of-use (POU) filters to larger commercial point-of-entry (POE) devices to immense arrays of filters for municipal water utilities. The principles of operation are the same at any scale.
Microfiltration is commonly applied in food and beverage processing systems. Fermentation systems such as beer, wine or cheese production use MF to clarify fluids and segregate yeasts, bacteria and other biological entities.
Industrial and commercial applications requiring fluids of low particulate count include electronics, paints, inks, coatings and chemical processing. This article will focus on microfiltration applied to drinking water purification.
Microfiltration is generally a low-cost, low energy and safe treatment process. The membrane flux is adequate at relatively low water pressures. Gravity operated systems are commonly applied.
As a barrier technology, no chemical treatment is necessary to disinfect water. This simplifies the devices involved and opens microfiltration to many applications. The pore structure rejects components predominantly by an exclusion mechanism, although some associative forces contribute to membrane filtration. Thus, turbidity resulting from suspended solids is reduced along with biological pathogens.
The high flux means small MF devices are suitable for space-limited applications. Conversely, a manageable array of large MF modules can produce sufficient water for a municipal water supply.
MF systems protect against bacterial and cyst hazards without eliminating dissolved minerals and salts, an advantage since these contribute to the health and taste of water. RO systems can significantly reduce dissolved mineral and salt components from water, and this is often viewed as an unfortunate side-effect that must be compensated.
Microfiltration systems will not remove dissolved contaminants, such as nitrates, fluoride, dissolved metals, sodium and VOCs. Colors, tastes and odors are also untreated.
Membrane fouling is a serious phenomenon affecting performance and service of membrane filters. Fouling can arise from several mechanisms: Surface associative materials, such as colloids and other partially charged macromolecules; biofilm growth resulting from microbiological activity of the accumulated pathogens; suspended solids; and precipitation of insoluble salts experiencing a concentration gradient across the membrane device.
Each of these fouling mechanisms can be addressed to some degree by pretreatment of the influent water. Removal of suspended solids and organic colloids can be approached with various filtration media.
Fouling is a phenomenon of all polymer membranes, to varying degrees, so using one membrane to pretreat another is not a meaningful option.
Market research organizations have predicted significant worldwide growth for microfiltration products ever since their popular growth in the 1990s. Predictions for the future show continued growth of about 9 percent per year for MF in all drinking water sectors: Municipal, POU and POE. This growth is far larger than general GDP growth. Growth of UF and RO membrane categories are predicted to be even faster than MF.
Some of the MF growth will actually be misidentified as UF products. There is an unfortunate popular misuse of the term UF that classifies all membrane products with any microbiological protection as UF. Admittedly, there is room for some overlap of the particle size ranges addressed by the two classifications. Yet, most cases of misuse originate as a superlative as in the following statement: “The standard filter is good and the ultrafilter is even better.”
Countless products labeled as ultrafilters have specifications indicating a minimum pore size of 0.1 µm. It is my preference to classify microfiltration and ultrafiltration for the drinking water industry as “rejects all bacteria” and “rejects all viruses.”
Related to drinking water purification, this definition is appropriate in that few viruses are rejected at 0.1 µm while all viruses should be rejected by a membrane with an absolute cutoff at 0.01 µm. This classification also corresponds with the performance requirements for a Purifier in the EPA Guide Standard and Protocol for Testing Microbiological Purifiers.
Bacteria rejection does not provide ultimate microbiological protection. Still, MF has legitimate applications with dramatic market opportunities. MF fiber product costs much less than true UF fibers and its flux is much higher at a given pressure drop. Thus, MF may be the best compromise among cost and flow in applications with low water pressure or where the economic situation cannot support a higher purchase cost.