Graduate student Bo Johnston stands next to his pilot system at St. Paul Regional Water Services.
Student project could cut St. Paul water complaints
By Trisha Collopy
February 14, 2006
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.
"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.With the help of civil engineering (CE) 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 River 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 CE'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," Hozalski said. "We can use all the analytical measurements we want, but if the water doesn't smell bad, then we've done our job."