Drinking-Water Disinfection: Fighting Microbes with Microbes
by Kristine Bradof
This article originally appeared in the July 1996 issue of the Wellspring newsletter, published by the MTU Regional Groundwater Education in Michigan (GEM) Center, now the Center for Science and Environmental Outreach at Michigan Technological University.
In April 1993, an outbreak of waterborne disease in Milwaukee sickened more than 400,000 people and led to the deaths of about 100 whose immune systems were weakened by AIDS or cancer (Sandler and Manning 1996). The culprit was a microscopic protozoan called Cryptosporidium. Although hardly a household name at the time, "Crypto" had already caused at least two major disease outbreaks in Texas and Georgia during the 1980s, which affected thousands of people (Huntoon 1993, 1995).
In fact, waterborne disease is far more common in the U.S. than many Americans realize. Most cases go unreported because the symptoms of diarrhea and vomiting in otherwise healthy people are usually dismissed as "stomach flu" or "something I ate." The Centers for Disease Control and Prevention (CDC) estimate that microbiologically contaminated drinking water sickens 940,000 and kills as many as 900 people each year (Olson 1993, Huntoon 1995).
The Natural Resources Defense Council (1995) leads a coalition of more than 200 groups in the Campaign for Safe and Affordable Drinking Water. Their report, The Dirty Little Secret About Our Drinking Water, is based on CDC data. The report documents 116 disease outbreaks—and at least 127 deaths—in nine years, all traced to microbes like E. coli, Shigella, Salmonella, Giardia, and Cryptosporidium in contaminated drinking water.
Cryptosporidium and chlorine
Cryptosporidium is a particularly troublesome disease-causing organism, or pathogen, because it is not killed by the most common means of disinfection, chlorination. Oocysts, the dormant form of Crypto, are able to survive in bleach (50,000 ppm free chlorine) even after 24 hours (Huntoon 1993). Crypto is relatively common in surface water contaminated by animal or human waste. It has also been found in well water (Schleicher 1995).
A study published in the September 1991 issue of Applied and Environmental Microbiology found that 97 percent of raw water samples taken at 66 water utilities in fourteen states and one Canadian province contained either Cryptosporidium or Giardia, another chlorine-resistant protozoan. The study noted, however, that not all of the organisms may have been viable, that is, able to cause disease (Olson 1993).
Milwaukee has been testing its water supply for Crypto since the 1993 disease outbreak. About 100 other large water systems also test for Crypto. They will be joined in 1997 by the rest of the 300 largest water systems, as ordered by the U.S. Environmental Protection Agency (EPA) in May 1996 (Sandler and Manning 1996).
In a June 11, 1996, news release, EPA Administrator Carol Browner said, "Although two million fewer people now are at risk from being exposed to Cryptosporidium in their drinking water than two years ago, 10 million people are still at risk." The news release announced a new EPA report, "Environmental Indicators of Water Quality in the United States."
Conventional watershed protection, coagulation, and filtration can remove more than 97 percent of Cryptosporidium oocysts (Pontius 1995). The operational turbidity of the filter plant must be less than 0.1 nephelometric turbidity unit (ntu) to remove oocysts reliably. To reduce the possibility of repeating the Milwaukee disease outbreak, ozone treatment will be added to the city’s water filtration plants at a cost of $45 million. Ozone has been shown to inactivate Crypto cysts.
Northwestern University Professor of Environmental Engineering Bruce Rittmann advocates another potential treatment method using biofilms. The treatment borrows from both Rittmann’s research on hazardous contaminants in groundwater and methods used for years in European drinking water systems (Russell 1995). During the 1970s, Rittmann studied how biofilms, thin layers of bacteria attached to surfaces such as soil and sand particles, removed toxins from groundwater. By 1980 he realized that European water supplies drawn from polluted rivers like the Seine were commonly passed through either the natural river bank or constructed sand or gravel beds that supported biofilms. Little or no chlorination was required at the end of the treatment process because the bacteria in the biofilms grew by degrading and feeding on organic matter in the water, including toxic chemicals and pathogens.
Biologically stable water ¾the new wave?
In 1984, Rittmann promoted the concept of "biological stability" for drinking water, achieved through use of biofilms (Russell 1995). The concept, which essentially pits microbes against microbes and toxic contaminants, has since gained acceptance by the EPA and the American Water Works Association. Sand filters already in use for filtering particles out of water can be modified to encourage growth of biofilms that also remove biodegradable molecules. The major obstacle to more widespread use of biofilm technology in the U.S. is the reluctance to trust "good" microorganisms to help remove "bad" microorganisms from the water we drink, although the concept has been well accepted in wastewater treatment for years.
Chlorination of drinking water has been popular in the U.S. for most of this century. It is an inexpensive, broadly effective disinfection method that has the additional advantage of leaving a chemical residual to suppress the growth of microorganisms in water distribution pipes.
Since the 1970s, however, chlorination itself has been linked to health problems resulting from reactions with organic matter, either in solution or as particles, in the distribution system. More recently, chlorine has failed to safeguard the water supply against pathogens like Cryptosporidium. Most people also object to the taste and odor left by chlorine in their drinking water.
Yet, without chlorination, microorganisms in the water distribution system, although not usually health-threatening, produce undesirable tastes, odors, and turbidity. As Professor Rittmann observes, "If we don’t want the tastes, odors and chemical risks of chlorine, we need to get the biodegradable molecules that are causing the growth of microorganisms out of the water" (Russell 1995). One promising way of producing such biologically stable—and better-tasting—water is to incorporate biofilm processes into water treatment, enlisting the help of beneficial microorganisms to eliminate the ones that cause disease.
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Olson, E. D. Think Before You Drink. The Failure of the Nation’s Drinking Water System to Protect Public Health. New York: Natural Resources Defense Council, September 1993.
Pontius, F. Part II—What you should know about Cryptosporidium. In Huntoon, E. (ed.) Improve Ground Water Protection by Sharing Information. 1995 Continuing Education Program for Well Drillers & Pump Installers. Technical Guide. Lodi, WI:Wisconsin Water Well Association. 1995, pp. 50-51.
Russell, S. On tap. Environmental engineer Bruce Rittmann explains how biological processes can reduce chlorine in drinking water. Northwestern Perspective [Northwestern University], Fall 1995, pp. 24-27.
Sandler, L. and J. Manning. City sets pace for Crypto testing. Milwaukee Journal Sentinel, May 3, 1996.
Schleicher, C. L. How to battle Cryptosporidium and win. Ozone, information are your best weapons. In Huntoon, E. (ed.) Improve Ground Water Protection by Sharing Information. 1995 Continuing Education Program for Well Drillers & Pump Installers. Technical Guide. Lodi, WI:Wisconsin Water Well Association. 1995, pp. 48-49.
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