Harmful contaminants often taint drinking water drawn
directly from a river, but a low-cost natural filter may
lie just beyond the banks. Johns Hopkins researchers have
found that the soil alongside a river can remove dangerous
microbes and organic material as water flows through it.
The cleaner water is then pumped to the surface through
wells drilled a short distance from the river.
This technique, called riverbank filtration, has been
used in Europe for more than 50 years to improve the taste
and smell of drinking water and to remove some hazardous
pollutants such as industrial solvents. But after studying
these natural filtration processes for six years at three
rivers in the midwestern United States, Johns Hopkins
researchers have determined that passing river water
through nearby sediment can produce other health benefits
and may cut water treatment costs.
Josh Weiss, a doctoral student in the Department of Geography and Environmental
Engineering, presented the most recent research results
Aug. 25 at the 228th national meeting of the American
Chemical Society, held in Philadelpha. He reported that
riverbank filtration appears to significantly decrease the
presence of bacteria and viruses. Water analyses also
showed encouraging, though not definitive, signs that this
technique can curtail Giardia and Cryptosporidium, two
waterborne microorganisms that cause serious digestive
ailments.
The latest results confirm the value of riverbank
filtration, Weiss said. "It sounds counterintuitive to
drill wells nearby when water can be taken directly from a
river," he said. "But our research indicates that riverbank
filtration can naturally remove pathogens and organic
material that can cause health problems, including some
microbes that are able to survive conventional disinfection
systems. If you think about how much it costs to build a
full-scale treatment plant to make river water safe to
drink, you can see how this could be very beneficial."
The research has been supported by Environmental
Protection Agency grants awarded to a team led by Weiss'
doctoral adviser,
Edward J. Bouwer, a professor in DOGEE.
The team has been studying water drawn from commercial
wells located beside the Wabash, Ohio and Missouri rivers
near Terre Haute, Ind.; Louisville, Ky.; and Kansas City,
Mo.
In several recent papers published in peer-reviewed
journals, Weiss, Bouwer and their colleagues have reported
that riverbank filtration helps remove organic material
left behind by decaying plants. In its natural state, this
material poses no health hazards, but exposure to common
water treatment chemicals such as chlorine can transform
the material into cancer-causing compounds called
disinfection byproducts.
"For this reason, it's a good idea to remove as much
of this organic matter as we can from the water before it's
treated with chemicals," Bouwer said. "Our research
indicates that with riverbank filtration, we wind up with
fewer of these dangerous disinfection byproducts in the
drinking water."
Riverbank filtration, Bouwer added, doesn't completely
eliminate the need for water treatment. "But it should
lower the treatment costs and reduce the risks of mixing
chlorine with the organic material that can become
carcinogenic," he said.
The researchers studied wells that had been
constructed at varying distances, from 90 to 580 feet, from
the three rivers. Over a period of days or weeks, river
water moves outward toward these wells. As it travels
through the sediment, the water is exposed to physical,
chemical and biological processes that help remove
impurities, the researchers say. Large particles may be
pulled out by a straining process. Some of the chemical
contaminants and microbes react with components in the
sediment and remain behind, too. As a result, the water
that reaches the wells is significantly cleaner than it was
when it left the river.
In a campus laboratory, the Johns Hopkins researchers
are trying to learn more about this natural filtration
process by sending samples of river water through glass
columns filled with sediment. They believe that soil
characteristics and environmental factors such as the
amount of river flow may also affect the natural filtration
process.
Weiss, who is preparing his doctoral thesis on
riverbank filtration, said the technique may not be
appropriate in some areas, such as regions of the western
United States where rivers typically dry up in the summer.
But in communities that depend on rivers for a year-round
supply of drinking water, Weiss expects riverbank
filtration to become more common in the coming years. "We
definitely think riverbank filtration is worthwhile," he
said. "We're letting nature maintain the system, minimizing
the need for external maintenance and the associated
costs."
Weiss, who is from Aiken, S.C., earned a bachelor's
degree in civil engineering at Georgia Tech before
enrolling in the graduate program at Johns Hopkins.
Collaborating with Weiss and Bouwer on the ACS
presentation were Charles R. O'Melia of the Johns Hopkins
Department of Geography and Environmental Engineering;
Ramon Aboytes of Belleville Laboratory, American Water; and
Binh T. Le and Kellogg J. Schwab, both of the Department of
Environmental Health Sciences, Johns Hopkins Bloomberg
School of Public Health.
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