An estimated 1,000 new synthetic compounds are introduced every year — 

 some of them inevitably seep into drinking-water sources. 



Drought in Australia. Water shortages in northern 

 China. The desertification of western Africa. 

 Almost daily, such headlines roll off the presses 

 and issue from the airwaves. 

 Undoubtedly, diminished access to freshwater 

 is a dire threat to people around the world. But 

 consider the condition of the water when it finally trickles 

 down people's throats. Infectious pathogens and harmful 

 chemicals — from parasites to poisons — contaminate the 

 world's freshwater and contribute to the deaths of mil- 

 lions of people worldwide every year. Understanding the 

 effects of those contaminants holds the key to protecting 

 our drinking water. And figuring out how we are exposed 

 to harmful agents is the first order of business in choosing 

 proper water-treatment techniques. 



The burden of those agents weighs heavily on commu- 

 nities around the world. Nearly 2 million people — most 

 of them children under five — die every year from diar- 

 rheal diseases. That statistic is not surprising when you 

 realize just how much dirty water flows, or in many 

 cases lies stagnant, across the continents. Nearly 20 

 percent of the 6.6 billion people in the world lack ac- 

 cess to a supply of clean water, and 40 percent lack safe 

 sanitation facilities. No new headlines there: as far back 

 as 1981 the United Nations recognized the need for 

 improved water supplies and sponsored a water-themed 

 decade through 1990, in hopes of rallying international 

 aid. Yet the percentage of people who have sufficient access 

 to clean water supplies has remained fairly static. 



Arguably, the battle is uphill. As quickly as innovative 

 filters and water-transport systems enter the market, new 

 contaminants and diseases arise, populations grow, and 

 competing demands for water increase. Certain micro- 

 organisms can be elusive, causing severe illness at doses as 

 low as one infectious organism per drink of water. And 

 those disease-causing organisms don't stand still while we 

 figure out how to combat them: dirty water can lead to 

 increased virulence, as in the case of antibiotic-resistant 

 bacteria. Battling, let alone eliminating, those ever- 

 changing organisms, along with the plethora of synthetic 

 contaminants, seems only to be getting more difficult. 



One thing will never change: people need water for 

 survival. Circulating inside, outside, and across our cells, 

 water constitutes as much as 70 percent of our body weight. 

 Although we may survive four weeks without food, our 

 bodies last, at best, only a few days without water. Fur- 

 thermore, we use water for the most basic daily activities: 

 drinking, cooking, bathing, washing, and sanitation. 



For at least the past six thousand years, civilizations 

 have understood the need to engineer water treatment 

 techniques. Greek and Sanskrit texts discuss approaches to 

 water sanitation that include boiling, straining, exposing 

 to sunlight, and charcoal filtering. The ancient Egyptians 

 employed coagulants — chemicals that are frequently used 

 even today to remove suspended particles in drinking 

 water — and other methods of purification. The earliest 

 large-scale water treatment plants, such as the one built in 

 1804 to serve the city of Paisley, Scotland, used slow-sand 

 filtration. By the 1850s London was sending all of its city 

 water through sand filters and saw a dramatic reduction 

 in cholera cases. 



The discovery of chlorine as a microbicide in the 

 early 1900s was a turning point in drinking-water 

 engineering. That, in turn, led to a major advance 

 in public health. Chlorination was initiated in the United 

 States around 1910, and during the next several decades 

 change was evident: the previously high mortality rate 

 from typhoid fever — twenty-five deaths per 100,000 — 

 plummeted to almost zero. Although chlorine readily 

 inactivates viruses and bacteria, its killing power flags 

 when faced with hardy protozoan oocysts (developing 

 cells), such as those of Cryptosporidium parvum — an agent 

 of diarrheal disease. Another, and perhaps even nastier, 

 drawback is that chlorine and organic matter may create 

 carcinogenic by-products when they mix in the treatment 

 plant. Nevertheless, chlorine is still one of the cheapest 

 and most effective disinfectants in use today. 



No panacea for water disinfection exists, however. To 

 ensure that the water supply is clean enough to drink, 

 most modern drinking-water plants amass an arsenal of 

 treatment options. A multibarrier approach might include 

 physical processes such as coagulation and flocculation 

 (creating clumps of particles), sedimentation, and filtra- 

 tion, in conjunction with disinfectants such as chlorine, 

 chlorine dioxide, chloramines, or ozone. 



Such systems for cleansing community water are public 

 investments that pay dividends. Clean w ater improves 

 general health and reduces health-care costs, thereby en- 

 abling greater productivity among community members 

 and redirection of public funds to other pressing needs. 

 Unfortunately, rural and low-income localities cannot 

 afford the infrastructure required for large, centralized 

 drinking-water facilities. 



On a global scale, of course, an ideal filter is natural 

 vegetation. Protecting entire watersheds could vastly im- 



November 2007 natural his i urn 



4 7 



