Filtration of water supplies was documented early in recorded history. Today a wide range of filtration technologies are available for water treatment. Limitations of conventional water treatment, for the removal of protozoan pathogens from municipal water supplies, have led to recommendations that susceptible populations utilize point of use (POU) treatment technologies. Specialized filters for POU water treatment can address these advanced treatment needs.
Historical record of water filtration
Filtration technology today achieves pure water by removing the smallest of microbes from drinking water sources, but recognition of the health benefits derived from filtration dates back to a time when microbes weren’t even identified as causative agents of disease. Prior to 2000 BC, Sanskrit writings advised boiling and filtering water. In 400 BC, Hippocrates wrote of the need to boil and/or strain water to prevent ill health. There are many reports from the early 18th century describing the use of filtering stones and other water treatment methods. As early as 1732, sedimentation and sand filtration was popular among individual households and, by 1832, widely used on municipal water supplies. These practices were all taking place long before scientists identified that pathogenic microbes caused infectious disease and thus were primarily driven by improving the taste, odor and other aesthetic properties of water.
From 1880-90 methods of chemical coagulation and rapid sand filtration were being developed and patented, adapting filtration to more turbid water sources. By 1907, coagulant use was widespread in water treatment plants and usually involved the use of sulfate, iron or lime. The introduction of sand filtration in the U.S. resulted in a dramatic decrease of typhoid fever. For example, the use of sand filtration in Lawrence, Massachusetts (1907) corresponded to a 79 percent drop in deaths caused by typhoid fever and a 10 percent reduction in the overall death rate (death by all causes) in that area.
Today, filtration technologies are utilized for the removal of a wide variety of microbial contaminants, including protozoa, bacteria and viruses (previously defined at “filterable agents”). Filtration options range from the crude to the highly specialized and are available in both municipal water treatment and POU applications.
Filtration and municipal water treatment
Drinking water supplies in the U.S. are protected by a multi-barrier approach aimed at safeguarding the raw water from contamination, appropriate treatment of the source water and secure distribution to the individual consumer’s tap. The quality of the source water generally dictates the level of treatment required to produce potable water.
Conventional water treatment generally involves disinfection, coagulation, flocculation, sedimentation, filtration and final disinfection. Hydrolyzing metal salts (i.e., alum, ferric sulfate) are added as coagulants and the water is gently stirred to promote aggregation of particles (flocculation) that are then allowed to settle via gravitational forces (sedimentation). These pre-treatments allow for increased efficiency of further treatment processes of filtration and disinfection (typically via chlorination). Filtration methods commonly involve the passage of water through a medium of sand, gravel, coal or activated charcoal to further clarify particulates.
The multi-barrier approach is necessary for contaminated source waters due to variable characteristics of microbial pathogens. Viruses and bacteria are not always blocked by filtration, but are readily killed by chlorination; protozoa are not effectively inactivated by conventional chlorination, but are removable by filtration. Recent evidence, however, suggests that the potential commonly exists for protozoan pathogens to escape the conventional filtration barrier.
Overtaxing the treatment train
Protozoan parasites, particularly Cryptosporidium and Giardia, are a primary concern for water municipalities. Although ozone and ultraviolet light can be effective for inactivating protozoa, chlorine disinfectants, used in conventional water treatment, are not. Crypto-sporidium are approximately 6-8 µm and Giardia are 8-12 µm and filtration is the primary barrier for their removal in conventional water treatment but it is not 100 percent effective. Recently, researchers found 27 percent of 82 samples of drinking water positive for Cryptosporidium, despite the use of filtration and no record of suboptimal treatment plant operation.1 History has shown that even the best of systems can be overtaxed by a high density of pathogens entering source waters over a short time period.
Overall, protozoa are responsible for 19 percent (148/782) of the documented U.S. drinking water outbreaks from 1971-2002.2 In 1993, Cryptosporidium caused a massive waterborne disease outbreak in Milwaukee, Wis., where approximately 400,000 (about 40 percent of the community) were infected, 5,000 hospitalized and 104 died.
Cryptosporidium is now regulated by the federal government as a primary drinking water contaminant and over the last decade, several treatment rules have been promulgated by the United States Environmental Protection Agency (U.S. EPA) aimed at reducing the risks of Cryptosporidium in drinking water.
- The Surface Water Treatment Rule requires that public water systems utilizing a surface water source achieve at least a 3- and 4-log removal or inactivation of Giardia and viruses, respectively.
- The Interim Enhanced Surface Water Treatment Rule requires a minimum 2-log (99 percent) removal efficiency of Cryptosporidium with a maximum contaminant level goal of zero for water utilities using surface water or groundwater under the direct influence of surface water and serving >10,000 people.
- The Long Term 1 and 2 Enhanced Surface Water Treatment Rule, collectively strengthen microbial controls for large and small systems (serving <10,000 people).
In addition to federal safety regulations, in 1999 the USEPA and Centers for Disease Control and Prevention (CDC) issued a guidance for persons with severely weakened immune systems to purify their tap water by boiling for one minute, as a safeguard against waterborne exposures to Cryptosporidium. As an alternative, the agencies recommend POU devices with reverse osmosis (RO) treatment, labeled as absolute one-micrometer filters, or that have been certified by NSF International under Standard 53 for cyst removal. A recent publication from the CDC further recommends that homeowners with individual groundwater wells purchase appropriately designed POU devices (CDC, 2004).
Current treatment technologies are capable of addressing most drinking water contaminants; however, the site of application is critical. An appropriate barrier at the point of use can minimize health risks resultant from treatment failures, untreated source waters and distribution system contamination and could be a life-saving choice for susceptible populations. Many POU treatment devices are designed to improve water aesthetics, such as taste, color and odor, while others specialize in removal of harmful microbes, such as Cryptosporidium or viruses. Comprehensive systems are available that accomplish both levels of treatment, but the consumer must evaluate that the units have been independently tested and certified to meet specific treatment needs. Stringent NSF International and USEPA standards have been developed for testing POU devices for claims of a microbiological purifier (NSF P231), health contamination reduction—including VOCs and cyst removal (NSF Standard 53) and reduction of potential bioterrorism microbes—including viruses and bacteria (USEPA/NSF International Environmental Technology Verification Program).
A major limitation of POU devices is the need for regular maintenance. POU device components typically have a finite lifespan that varies with use and source water quality. Regular testing, backwashing, system regeneration, filter replacement or professional maintenance may be required. Neglecting to properly maintain a POU device may result in poorer quality water than the original inlet water source.
Filtration technology plays a vital role in water treatment and aids in the production of high quality drinking water, suitable for consumption by some of the most susceptible populations. Understanding the limitations of various filtration methods, at a specific point in the conventional treatment train, allows the consumer to evaluate individual needs for advanced POU treatment applications.
- LeChevallier, M.W., W.D. Norton, et al., 1991. Giardia and Cryptosporidium spp. In filtered drinking water supplies. Applied Environmental Microbiology 57:2617-21; and R. Aboytes, G.D. DiGiovanni, F.A. Abrams, C. Rheinecker, W. McElroy, N. Shaw, M.W. LeChevallier. 2004. Detection of infectious Cryptosporidium in filtered drinking water. Journal AWWA 96:88-98.
- Centers for Disease Control and Prevention. Surveillance for Waterborne-Disease Outbreaks Associated with Drinking Water- United States, 2001-2002. B.G. Blackburn, G.F. Craun, J.S. Yoder, V. Hill, R.L. Calderon, N. Chen, S.H. Lee, D.A. Levy, M.J. Beach. MMWR Surveillance Summaries. October, 22, 2004. 53 (SS08); 28-45.
About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of rapid methods for detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P technical review committee since 1997. She can be reached via email, [email protected]