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When the
weather warms up each spring, the calls start rolling into St. Paul's
water plant, complaints about the water having an earthy or fishy
odor and taste.
Usually the problem lasts a week or two at a time. During one particularly
long hot spell in June 2001, the plant logged 150 calls.
"For us that's a huge number,” said Jim Bode, water quality manager
for St. Paul Regional Water Services. “For every person who calls,
there are 10 who don't. When you get 150 calls, no one's happy."
The city water plant serves 415,000 water users in St. Paul, Maplewood,
West St. Paul, Mendota Heights and Falcon Heights.
City water managers have tried for years to find a way of controlling
geosmin, a chemical produced by algae that contributes to the musty
taste and odor of water.
With the help of civil engineering graduate student Robert “Bo”
Johnston and faculty advisers Raymond Hozalski and Michael Semmens,
they've finally found a solution.
Water utility workers recently began installing granular activated
carbon (GAC)—essentially creating giant Brita water filters—to
remove the geosmin.
“The goal is to have 12 of our 24 filters operational by May 2006,”
Bode said.
The source of the taste and odor problem goes back to the city's
water supply.
The water is drawn from the Mississippi and filtered through a
chain of lakes, including Lake Vadnais, a reservoir outside the
city, before it is piped into the water plant.
Because the lakes act as giant sediment basins, settling out most
impurities from the river, the quality of the raw water coming into
the plant is good, said Hozalski, who supervised Johnston in a recent
project to test the GAC filters for the city.
“Rivers are prone to a very high level of turbidity. With rains,
lots of sediment is churned up,” Hozalski said. “Minneapolis has
to deal with that. St. Paul, because they have these lakes, it gives
them great water quality coming into the plant,” he said.
However, when the weather warms up, the lakes are prone to algal
blooms.
City water managers have tried several techniques to reduce the
amount of algae and geosmin in the water. They've tried to reduce
the amount of phosphorous, a key nutrient, seeping into the lakes.
They've diluted the lakes with well water to reduce nutrient content.
And they've tried removing geosmin at the water plant by putting
carbon powder directly in the water, Bode said.
But none of these methods completely removed the geosmin or ended
the complaints. Geosmin, it turns out, is a compound that people
can detect in very small concentrations in water, in the order of
parts per trillion.
“Although it's not a public health issue, it is an important aesthetic
issue for the city,” Hozalski said.
City water managers have been working with the department's environmental
engineering graduate students and faculty on a variety of research
projects since the early 1990s.
When a feasibility study recommended that the city install GAC
filters to remove the geosmin, Bode talked the project over with
Hozalski and Semmens, who suggested a pilot study to Johnston as
his master's thesis.
Johnston set up a pilot-scale treatment system to compare the city's
current coal and sand filters with two other techniques: using the
GAC filters alone and using the GAC filters with ozone to further
remove organic compounds from the water.
“We know ozone will work,” Bode said. “The big question Bo was
trying to answer was how effective the GAC filters were and how
often you have to replace them.”
Johnston used water quality tests to measure the amount of geosmin
and other impurities in the water. But he also turned to another
source, a “taste and odor” panel made up of trained water utility
employees.
The panel met once a week, from December to early February to sample
the water.
“We wanted some basic qualitative data for what we were seeing
quantitatively in the removal of the geosmin,” Johnston said. “The
goal was to get as close to odor-free as we could.”
Water quality tests showed that the GAC filters were removing most
of the geosmin and the filters with the ozone were removing even
more. The taste and odor panel confirmed the findings.
To keep down costs, water plant managers chose to install the GAC
filters without the ozone first. Workers will finish installing
all 24 filters this fall. Then managers will wait to see how many
calls come in during the next warm spell.
"Ultimately that's the proof in the pudding. We can use all the
analytical measurements we want, but if the water doesn't smell
bad, then we've done our job." Hozalski said.
Reprinted with permission from the winter 2006 edition of Civil
Engineer, a publication of Department of Civil Engineering.
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