By Kelly A. Reynolds, MSPH, Ph.D.
Microbes vary widely in size, which can be an important consideration in the design, engineering and application of water treatment technology. Many water purification methods (i.e., ozone, ultraviolet (UV) light disinfection, chlorination) are designed to eliminate microbes via oxidation of surface proteins or destruction of nucleic acids, rendering them harmless. Other methods, utilized by municipal water treatment plants and point of use (POU) treatment systems, rely on filtration to create a barrier, based on size exclusion technology, that microbes cannot pass through. Understanding the physical characteristics of a microbe and the limitations of size exclusion treatment technology has proven important in determining the success of this approach for water purification.
Long before scientists identified that microbial agents were the cause of infectious disease, filtration practices were being applied for water treatment. The desire to improve the aesthetic qualities (i.e., taste, odor, color) of water was the primary driving force for such applications; however, an understanding that water contaminants could cause human disease was increasing by the mid-1800s. According to Sanskrit writings, dating as far back as 2000 BC, water decontamination efforts, including filtration and boiling, were commonly recommended. In 400 BC, Hippocrates wrote of waters contributing to adverse health outcomes and the need to boil or strain water. Beginning in the early 18thh century, there were many reports of the use of filtering stones and other additives, aimed at purifying drinking water. Sedimentation and sand filtration of water were popular practices among individual households beginning around 1732 (POU treatment technology has come a long way since then!) and municipal water supplies began widespread use of conventional sand filtration in 1832.
The use of filtration undoubtedly had a positive impact on public health, achieving a level of removal of some important pathogenic microbes; however, our understanding of the limitations of this treatment method was far from complete. Today we know that filtration is an important step for the removal of pathogens, like Cryptosporidium, that are resistant to conventional chemical disinfectants. For other pathogens, in particular viruses, filtration is not a practical method for large-scale treatment compared to disinfectants.
In the US, there are three general catagories of microbes that we are concerned about with regard to waterborne disease: viruses, bacteria and protozoan parasites. Human enteric viruses primarily infect the gastrointestinal tract and range in size from 0.01 to 0.1 micron (µm). They are obligate intracellular parasites, meaning that they cannot replicate outside of host cells and thus do not multiply in the environment. They are, however, capable of long-term survival in the environment (weeks to months). Although over 100 human enteric viruses are transmissible in water, only a few have actually been documented in waterborne disease outbreaks. Viruses (and their potential health effects) of greatest concern in water include enteroviruses (diarrhea, meningitis, myocarditis, fever, respiratory disease, nervous system disorders and birth defects), hepatitis A (hepatitis, liver damage), noroviruses (diarrhea), astrovirus (diarrhea), adenovirus (diarrhea, respiratory disease, eye infections, heart disease) and reovirus (respiratory infections). Conventional drinking water treatment is effective for eliminating most viruses from source water; however, contamination in the distribution system (post-municipal treatment) and lack of treatment of both public and private groundwater sources are a concern.
Bacteria range in size from 0.1 to 10 µm. Waterborne (i.e., enteric) bacteria can colonize the human intestinal and gastrointestinal tract; however, most infections require a high (100 to 10 million) dose of organisms. Sources of bacterial pathogens can include human and agricultural (animal) wastes. Some pathogenic bacteria can enter a dormant state, eluding conventional methods of detection and survive weeks to months in the environment. Others are known to proliferate in the environment. Pathogenic bacteria of primary concern in water include, Salmonella (typhoid, diarrhea), Shigella (diarrhea), Campylobacter (diarrhea, nervous system disorders), Vibrio cholerae (diarrhea) and Escherichia coli (certain strains: diarrhea, hemorrhagic colitis). Legionella is a pathogenic bacterium, commonly found in tap water, spread by the aerosol route and causing pneumonia and respiratory infections.
In recent years, Helicobacter pylori was recognized as the primary cause of duodenal (90 percent) and gastric (80 percent) ulcers. It is considered a class A carcinogen, meaning that infections can lead to gastric cancer, the second most common cause of cancer worldwide. H. pylori has been found in biofilms of water distribution systems.
Protozoan parasites are single-celled animals that infect the gastrointestinal tract at very low doses (one to 10 organisms). In the US, Crytosporidium and Giardia (both diarrhea-causing microorganisms) are the primary protozoa of concern with regard to waterborne disease. As a group they range in size from one to 100 µm; however, Cryptosporidium are approximately four to six µm and Giardia are eight to 12 µm. Both produce an environmentally stable oocyst or cyst, respectively, that is highly resistant to chemical disinfectants. In addition, infectious Cryptosporidium have the ability to fold in half and pass through filter pores that are larger than their reported size of four to six µm; thus you will notice that filters approved for protozoan removal must be of an absolute one µm size as a necessary margin of safety. Although ozone and UV light can be effective for inactivating protozoa, chlorine disinfectants used in conventional water treatment are not and thus filtration is required for surface water sources (or groundwater under the influence of surface water) of drinking water. POU water treatment systems must be carefully evaluated to make sure they are capable of removing or inactivating these dangerous pathogens.
The filtration spectrum
There are many different types of filtration including microfiltration (MF), nanofiltration (NF), ultrafiltration (UF), reverse osmosis (RO) and conventional or media depth filtration. Filters remove contaminants via processes of size exclusion or adsorption. Treatments relied upon for pathogen removal, at the point of use, are generally based on size exclusion technology. Membrane filtration processes utilize various semi-permeable membranes and pressures to provide specific fractionation of proteins, salts, turbidity, organic compounds, etc. from water. Compared to conventional filtration, cross-flow filtration extends the life of the filter by avoiding build-up of matter on the filter. Systems vary by pore size, molecular weight cutoff and pressure needed. MF is loosely defined as a process that uses low pressure (five to 30 psi), a pore size membrane of 0.03 to 10 microns and a molecular weight cutoff (MWCO) of about 100,000 Daltons. Thus, microfiltration alone removes only some microbial pathogens (Cryptosporidium and some bacteria but not viruses). UF removes particles greater than 0.002 to 0.1 micron, 10,000 to 100,000 Daltons (MWCO) and operating pressures ranging from 30-100 psi, capable of removing most viruses, bacteria and protozoa. NF is yet a finer filtration process designed to remove particles greater than 0.001 micron and 1,000 to 100,000 daltons, under higher pressures (about 75-150 psi). RO is an advanced filtration process, common to POU water treatment systems, that utilizes a semi-permeable membrane and higher pressures (150-1,500 psi). While RO is effective for removing contaminants larger than water molecules (i.e., pathogens) other media are typically used concurrently with these systems to remove contaminants that are molecularly smaller than water molecules (i.e., some pesticides).
Another important factor to consider in terms of effective filtration of microbes, is that the pore sizes of filters are rated as either nominal or absolute. Absolute means the pore size given represents the largest pore and thus will exclude a microbe of greater size with 100 percent efficiency. Nominal pore size ratings provide the average pore size in the filter, meaning that the filter media retains most, but not all of those sized particles. Efficiency standards vary with application but for POU/POE water filtration it is generally defined as 85 percent. Filtration conditions can have an effect on the efficiency of the filter and thus with nominal filtration; not all of the microbes may be removed all of the time, requiring the use of combined systems of disinfection (i.e., ozone or ultraviolet light) to better protect the end user.
Protecting those most at risk
For some populations, like the immunocompromised, POU systems may be life-saving and are recommended as one treatment option by the Centers for Disease Control and Prevention (CDC) and US EPA for reducing risks of Cryptosporidium and other types of infectious agents transmitted by drinking water. Choosing the proper system is very important, however and requires an understanding of the microbe’s characteristics and filtration terminology. US EPA advises that individuals wanting extra protection boil their water for one minute as the best method for eliminating Cryptosporidium. As an alternative to boiling water, US EPA and CDC also recommend the use of POU devices with distillation or RO treatment or labeled as absolute, one micron (or smaller) filters or that have been certified by National Science Foundation International (NSF) under Standard 53 for cyst removal.
- Center for Disease Control and Prevention, http://www.cdc.gov/NCIDOD/DPD/PARASITES/cryptosporidiosis/factsht_ crypto_prevent_water.htm
About the author
Dr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a master of science degree in public health (MSPH) from the University of South Florida and a 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 at firstname.lastname@example.org