Pour a glass of water from the tap and drink it down. You probably don’t stop to wonder where that water came from, whether it’s clean, or what it’s been hanging around with in the twists and turns of pipes underground.

But when Francis DiGiano, professor of environmental sciences and engineering, pulls out a piece of castiron pipe crusted with what looks like crumbly burnt fudge, you give it a second thought. These deposits, called “tubercles,” are made of mostly iron and manganese. By themselves, they’re harmless. But, when they build up in pipes, they form nodules and crevices—places where bacteria can cling.

DiGiano and Don Francisco, clinical professor of environmental biology, have been studying how tubercles and other conditions encourage bacterial growth in the pipes that eventually feed our faucets. It’s not certain that these particular growths can be harmful to human health. But, Francisco says, “It certainly raises a flag and says, `this is something we need to watch.’”

People don’t want to take chances with their drinking water. Talk to the researchers who study it, and it seems that what they worry about most is not the obvious contaminants—they’re taken care of by modern water-treatment practices. It’s the bacteria or chemicals in our water that we don’t know about, or that have unknown effects, or the chance that our disinfectants might fail, that concern these scientists.

First, it helps to know how drinking water is cleaned. Before adding disinfectant, we eliminate discoloration and cloudiness (called “turbidity”). In North Carolina, especially, water gets a yellow-brown color from natural organic matter (NOM) that finds its way into reservoirs from surrounding forests and vegetation. And one cause of turbidity is our abundance of clay soil.

To remove some NOM and most of the turbidity, a coagulant such as alum or ferric chloride is added. Then the water goes through “flocculation”—it’s slowly mixed with paddles to help the coagulant clump with the NOM and other particles. From there it flows into a sedimentation tank, where the particles formed during flocculation are allowed to settle. Then the water is filtered. Finally, disinfectant is added to kill bacteria, viruses, and such.

Certain conditions can cause disinfectant to dissipate, making pipes ripe for bacterial regrowth. DiGiano and Francisco’s study shows that bacteria can regrow even when small amounts of disinfectant are present. Most of the growth is attached to pipe walls, forming “biofilm.” But some bacteria escape into the water, reaching our faucets.

All water utilities are required to take weekly samples to determine the amount of disinfectant and bacteria in the water at different places in the distribution system. If monitoring does show problems, DiGiano and Francisco’s research offers ways that systems can help solve them.

In a study of systems in Raleigh and Durham, the team found that dead ends—places in the network of pipes where water just sits—make it more likely that disinfectant will dissipate. The best pipe systems keep water circulating, DiGiano says. Pipe material also makes a difference. When there’s equal amounts of disinfectant in the water, bacteria are more likely to grow on pipes made of ductile and cast iron, which have rough surfaces where bacteria can easily cling. But once disinfectant is gone, bacteria can grow even on smooth PVC or glass pipes.

The very disinfectants that discourage bacterial growth can cause another problem when they react with any NOM left in the water. This reaction creates chemicals called disinfection byproducts (DBPs). Hundreds of DBPs have been identified, but scientists aren’t sure what effect they may have on human health. One class of DBPs, the trihalomethanes (THMs), has been found to cause cancer in laboratory animals. But it’s not clear how these THMs, such as chloroform, will affect humans, or how much we’d have to drink to simulate the doses used in animal studies. A 1987 study by the National Cancer Institute did suggest that people drinking chlorinated water had a significantly higher risk of bladder cancer compared to those drinking nonchlorinated water.

The Environmental Protection Agency (EPA) has regulated THMs since 1979, when the agency established a maximum contaminant level of 100 micrograms of chemical per liter of water. Philip C. Singer, professor of environmental sciences and engineering, says that this was a compromise position. “No one knew whether it was economically or technologically feasible to reduce THM formation to that level, or what effect it would have on public health,” Singer says. Researchers knew that lowering chlorine doses in the water would reduce DBPs, but they were afraid that it might also increase the risk of infectious organisms in the water. Since then, water plants have been able to reduce THM levels to 100 micrograms per liter without increasing risk of infection or spending too much money. So the EPA has lowered the limit again, to 80 micrograms per liter, Singer says, and will regulate other DBPs suspected of causing health problems.

Working in the lab and the field, Singer and his students devise techniques for lowering DBP levels. They collect water samples that contain different amounts of NOM and test them with various disinfectants and coagulants under different conditions. They have also helped water plants reduce DBPs in such North Carolina cities as Raleigh, Durham, Chapel Hill, and Greenville, as well as in South Carolina and Virginia.

One solution is to add chlorine or other disinfectants later in the treatment process, after much of the NOM has been removed, Singer says. Or water plants can use another disinfectant instead of chlorine—ozone, for instance. The same stuff that forms the protective layer around the earth, ozone is produced when oxygen is exposed to ultraviolet light. It’s more expensive than chlorine, but it produces virtually no potentially harmful byproducts, Singer says. Wilmington, N.C. is now using ozone to treat its water, and Raleigh is about to begin doing so.

Other new technologies can reduce DBPs because they require less disinfectant, but they’re not yet widely used. DiGiano studies ways to improve membrane technology, which uses very thin layers of synthetic material rolled tightly into long tubes. Water is then pushed with great pressure through large bundles of these tubes. The membranes filter out nearly all of the NOM and bacteria, so much less disinfectant is needed.

But membrane technology is expensive because the high pressure requires a lot of electricity. The membranes also get dirty easily and must be cleaned or replaced. Working with Joe DeSimone, professor of chemistry, DiGiano is testing new membrane materials that are less susceptible to fouling and can treat larger amounts of water. Despite the expense of membrane technology, people are beginning to try it. “If you buy your water out of a machine at Wellspring grocery or Weaver Street market, when you hear that machine go on, that’s a pump pushing water through a membrane,” DiGiano says. And some small water plants like one in Kill Devil Hills, N.C., use membranes.

DiGiano also studies uses for filters that contain granular activated carbon, a black, grainy substance made from crushed coal or wood. Activated carbon, common in home water-filtration systems, soaks up contaminants such as detergents, insecticides, and NOM. Carbon filters also improve the odor, taste, and color of water. For now, they’re more economically feasible than membranes, DiGiano says. “I would be very happy if we used activated carbon to treat our water here.”

Treatment technology is improving. But Daniel A. Okun, professor emeritus of environmental engineering, feels that we should not depend entirely on that technology. The water we drink, he says, should start out clean.

Drinking water should come from the purest sources available. We have the technology to treat polluted water, but technology can and does fail,” Okun says. He gives the example of a 1993 incident in Milwaukee when 400,000 people became sick and 100 people died from infection with the parasite Cryptosporidium. “We used to think that filtration and chlorination would protect us from pathogens,” Okun says. “But Cryptosporidium, for example, is not completely inactivated by chlorine.”

Okun has advocated that we protect our drinking-water reservoirs. For example, 10 years ago, Asheville, N.C. proposed a $40 million bond issue to pump drinking water from the French Broad River, which is highly polluted with wastewater. The bond issue was supported by the state health department, but, with Okun’s help, the “Citizens for Safe Water” used debates and newspaper articles to defeat it.

Trace chemicals, including common drugs such as antibiotics and birth control pills, show up in wastewater, Okun says, and conventional treatment may not entirely remove them. “We don’t know yet what the risks are.” Okun says. DiGiano agrees that there are unknown risks. “It’s impossible to live in an urban environment and not expect to have water that contains some man-made chemicals,” he says. Cities downstream of other water systems have treated wastewater in their drinking-water supply. Raleigh, for instance, gets its water from Falls of the Neuse lake, which is fed by the Neuse River, which takes treated wastewater from Durham and also agricultural runoff.

There are limits set for contaminants such as pesticides, and cities do monitor for them, though probably only about once a year, DiGiano says. Okun believes that, rather than discharging all our treated wastewater into rivers and streams, we can reclaim it for such uses as landscape irrigation, air conditioning, and toilet flushing, preserving valuable high-quality water for drinking. Treating wastewater for such reuse also costs less than treating it to put back into streams and lakes. The practice is becoming more common, especially in places with water shortages, such as the Southwest and Florida.

While water utilities and researchers work to ensure we drink clean water, consumers, too, need to stay aware. Often we take for granted that water and other products are safe, Singer says. He picks up an empty Coke bottle. “Do you know what’s in there? Or how about this apple? Do you think it might still have some pesticides on it? We trust that some regulatory agency is looking out for us, making sure the things we consume are safe. And usually they are.” But if something with your water seems amiss, call your water company.

So, does Singer drink tap water? “Sure,” he says, “three or four glasses a day.” Then he grins. “Want to see me?”

Philip Singer and Dan Okun are members of the National Academy of Engineering.