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Historically, pathogens of concern in municipal drinking water applications (e.g., Giardia, Cryptosporidium, viruses, etc.) have been effectively removed using a series of processes like coagulation, flocculation, sedimentation and media filtration. Known as conventional treatment, these processes work in concert to achieve particulate removal by first adding a coagulant to destabilize the particles and then applying gentle mixing energy to facilitate agglomeration. The largest, densest particle agglomerates settle out while the remainder is filtered, reducing pathogen and turbidity levels.
The U.S. Environmental Protection Agency (EPA) credits a well-designed and conventional treatment process that achieves a filtrate turbidity of 0.3 NTU or below with 2.5-log Giardia, 3-log Cryptosporidium and 2-log virus reduction—allowances that incorporate safety.
Despite its name, conventional treatment is becoming a misnomer as the variety of processes used to treat potable water steadily diversifies. Perhaps the most common modern alternative is membrane filtration—microfiltration (MF) and ultrafiltration (UF)—which has become an increasingly popular means of removing pathogens and other particulate matter.
A brief history
Prior to the 1990s, MF and UF were primarily used as sterilizing filters in laboratory and industrial applications such as pharmaceutical production. In the late 1980s and early 1990s, small commercial scale membrane filtration systems were first marketed to municipalities for drinking water treatment as an alternative to conventional treatment. The first municipal membrane filtration system of substantial capacity (more than 1 mgd), an alternative to conventional treatment for meeting state and federal pathogen reduction standards, was installed in Saratoga, Calif., by the San Jose Water Co. This installation was a watershed milestone for membrane filtration technology, as the 3.6-mgd size represented a roughly 100-fold increase over the capacity of other municipal systems at the time. The success of this facility set an important precedent that initiated a period of substantial growth in both the number and total installed capacity of municipal membrane filtration systems throughout the U.S.
While both media and membrane filtration are effective for removing pathogens and other particulate matter, a comparison of the different mechanisms of filtration demonstrates the reason for the enhanced efficiency of MF and UF.
Media filters are generally composed of two layers (dual media) of porous materials such as sand and anthracite that are graded by density so that the coarser material rests atop the finer. The media is contained in an open basin that represents a break in the hydraulic head of a water treatment plant so that the filters operate by flow of gravity. (Note that variants of this filter configuration and operating scheme may be utilized.)
The ability to filter particulate matter and pathogens relies on the intersection of these particles with the media as water flows through the interstices. Consequently, the probability of intersection (and thus removal) is increased by augmenting the size of the particles through coagulation and flocculation pretreatment. Sedimentation reduces the load of particulate matter on the media filters in a conventional treatment scheme, thereby making the filters more efficient by reducing fouling and backwashing frequency.
Membranes utilize a layer of polymeric material with discrete submicron-sized pores to reject particulate matter on the basis of size exclusion, acting as a barrier. Although every membrane has a distribution of pore sizes, this characteristic is typically listed as either nominal (average or standard) or absolute (maximum) in terms of microns. MF membranes are generally considered to have a pore size range of 0.1 to 0.2 microns, allowing for the rejection of critical pathogens such as Giardia (5 to 15 µm) and Cryptosporidium (3 to 7 µm). UF pore sizes generally range from 0.01 to 0.05 microns or less, also allowing for the rejection of some viruses, which generally are not retained by clean MF membranes. MF and UF membranes used for municipal water treatment applications are manufactured as small hollow fibers and bundled into modules that are operated under either positive (for membranes in pressure vessels) or negative (vacuum) pressure (for modules submerged in an open basin).
Both membrane and media filters benefit from the build-up of filtered materials in the matrix during a cycle that either blocks the pores or occludes the interstices, respectively; this allows for the retention of both more and potentially smaller particulate matter (viruses) than otherwise would be possible.
Because membrane filtration relies on the principle of size exclusion for particulate removal, an integral membrane is essentially a full barrier to pathogens that are larger than the absolute pore. This performance has been demonstrated in numerous studies showing 5-, 6- and 7-log removal of Giardia and Cryptosporidium for both MF and UF, and similar virus removals for UF; in many cases the concentrations of these pathogens were reduced to levels below the detection limit of the technique used to measure them. In addition, filtered water turbidity levels are consistently 0.05 NTU or less.
Notably, membrane filtration does not require pretreatment to achieve more efficient turbidity and pathogen reduction than media filters, and the high quality of the membrane filtrate is generally independent of the feedwater quality. As with conventional treatment, however, the use of coagulation/flocculation can enable the reduction of organic compounds—the precursor material for disinfection byproducts—which is otherwise generally unaffected by membrane filtration. Similarly, pre-sedimentation can improve overall performance by increasing the length of time between required backwashing and chemical cleaning.
Growth of membrane filtration
Today there are more than 200 membrane filtration systems treating municipal drinking water in North America. The numbers are steadily increasing, having doubled since 2000. MF and UF systems used in the municipal market are also becoming larger, with 100-mgd facilities currently in design and construction.
As membrane filtration has become a more prominent pathogen removal option used throughout the country, state and federal regulators have more closely examined the manner in which membrane filtration systems fit into the existing regulatory framework established under the authority of the federal Safe Drinking Water Act.