UMN researchers study how bacteria can contribute to safer drinking water.
By Shayna Korol and Charlie Kidder
Clean water doesn’t happen by accident. Before it is ready to drink, water must be purified of microbes and other pollutants that are harmful to human health. Most drinking water in the United States is treated through biofiltration, a process that uses filter media, such as sand, anthracite coal, or granular activated carbon (GAC), with attached communities of bacteria to remove harmful microorganisms as well as dissolved contaminants. It seems counterintuitive to cultivate bacteria in a drinking water treatment facility, but it is important to realize that not all bacteria are harmful. In fact, many bacteria are beneficial and can be utilized to improve water quality.
Biofilters use a granular filter medium such as sand, which is covered with a thin bacterial layer known as a biofilm. As untreated water passes through the biofilter, the grains of filter medium collect the microorganisms and other particles from the incoming water while the biofilm “catches” dissolved pollutants, including nutrients and organic contaminants like pesticides. While water treatment never completely eliminates microbes, biofiltration aims to keep the concentration of harmful pathogens low enough to prevent people from getting sick.
Not all biofilters are created equal. Some biofilters that use sand as a filter medium are known as slow sand filters. As the name suggests, these filters work relatively slowly, making it difficult for an urban water treatment facility (like the city of Saint Paul’s) to process 50 million gallons a day. Tim LaPara and Ray Hozalski, University of Minnesota professors in the department of Civil, Environmental, and Geo-Engineering and the BioTechnology Institute, study differences between biofilters that can impact water quality. “The slow sand filters don’t work well [for large metropolitan areas like the Twin Cities],” says LaPara. Water treatment plants for large cities typically employ “rapid filters”, which process water at a rate that is more than 10 times greater than slow sand filters. Hence, the number of filters needed is much more manageable, and the plant size is reasonable.
In the early 2000s, Saint Paul had a water quality issue because of a substance called geosmin. Although it is not harmful to human health, geosmin has an unpleasant taste and odor. In order to mitigate geosmin’s effects on the water supply – and cut down on hundreds of yearly complaints from Saint Paul residents – the city installed granular activated carbon (GAC) filters. “The idea was the geosmin would stick to those filters, and water would taste better,” says LaPara.
In Saint Paul, water quality complaints plummeted by about 90 percent after the GAC filters were put in place. “They went from 250 to 300 complaints down to 15 to 30 complaints per year,” says LaPara, a tremendously low number for a city as big as Saint Paul.
Like Brita water filters in kitchens across the world, the thought was that the GAC filters would eventually have to be replaced. Implementing the filters cost $5 million, and removing and replacing them would cost the city another $5 million each time. Hozalski estimated that the filters would work for at least five years. Surprisingly, the filters kept working well even after five years. Instead of simply accumulating on the GAC filters as the researchers assumed would eventually happen, the geosmin also was being consumed by the bacterial communities on the biofilm as it passed through the filter. The GAC “takes up all the geosmin during the summer and then slowly dissolves it out the rest of the year, and it dissolves it so slow[ly] that the microbes eat it and nobody ever taste[s it],” explains LaPara. Ten years passed, and the researchers found that the filters worked as well as they did the day they were installed. Thus, the GAC filter media was being bioregenerated, allowing for a sustainable treatment process.
“If the bacteria can biodegrade the compounds [that need to be removed], then you don’t have to replace the media because they can basically take care of it naturally,” says Hozaski. The work by the bacteria on the filters provided millions of dollars in savings.
As an outgrowth of the GAC filter research that began in the early 2000s, the scientists attempted to understand how the microbial communities in these biofilters evolve and function. In a 2018 journal article, Ph.D. student Ben Ma, together with Hozalski and Prof. Bill Arnold, demonstrated that biofilters can remove a wide variety of trace organic contaminants, including pesticides and pharmaceuticals. Hozalski also lead a study by a research team, which included Ma, LaPara, and Ashley N. Evans from the consulting firm Arcadis, in which samples of filter media were collected from biofilters throughout North America. The researchers found that the microbiome of drinking water biofilters is affected by both environmental factors and filter design.
They published the basic science study in FEMS Microbiology Ecology. “We were trying to understand how different these biofilters are from location to location,” LaPara says. While Minneapolis filters are very similar to Saint Paul filters, they are different from filters in California.
“Geographic location seems to have some bearing on the [microbial] communities that evolve, and so the closer that water plants are together, the more similar the communities are; the further they are apart, the more different,” Hozalski explains.
In 2020, the researchers published a paper in Environmental Science & Technology investigating the effects of biofilter design on both the microbiome of the filter media and filtered water itself. This study, also an extension of the earlier GAC filter research, used GAC-sand and anthracite-sand biofilters. Hozalski also served as lead researcher.
While the biofilters reduced bacterial abundance in the water by about 70 percent, they did not significantly affect the microbial composition that remained in the filtered water. These results suggest the biofilms mostly affect water quality by removing pollutants and nutrients rather than changing the microbial composition of the filtered water. “The biofilter does a lot, but it doesn’t add different microbes to our water,” LaPara says.
By shedding further light on how biofilters function, the researchers are setting the groundwork for better filters – and cleaner water. Hozalski said, “I have been working on biofilters since my Ph.D. studies in the early 1990s, and I still have a lot to learn! They are both simple in design yet decidedly complex when you dig into them.”
