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A student conducting research by a septic tank.

Sea Grant-funded graduate student Jonathan Pundsack adds bacteria to septic tank wastewater.

Germ hunters go to prison

By Sharon Moen

December 20, 2005

Several University researchers have done time at the Northeast Regional Correction Center--a county work farm. They didn't do their time behind bars. They did it behind toilets, testing the efficacy of onsite wastewater treatment options. Using innovation and septic tank effluent, they discovered the impressive ability of peat to filter out impurities. They also compared the benefits of peat, sand, and wetlands as wastewater filters.

"The peat filters were astonishing," says Randy Hicks, a project collaborator as well as professor of biology at the University of Minnesota, Duluth (UMD). "They removed virtually all the viruses and bacteria planted in the wastewater."

"We wouldn't go to prison for just any cause," jokes Rich Axler, a senior research associate with UMD's Natural Resource Research Institute. "The correctional facility had enough wastewater and land for us to construct what amounted to an outdoor laboratory. In relatively controlled conditions, we could conduct experiments throughout the year in alternative wastewater treatment systems sized for an average home."

This is important in northern Minnesota, where septic systems ring the shorelines of sensitive lakes and are confounded by lack of space, surface bedrock, clay soils, high water tables, and frigid winters. Alternatives to conventional trench or mound systems could reduce the levels of environmental and human health hazards associated with treated sewage.

Axler and Hicks, along with lead researcher Barb McCarthy and their colleagues, followed a variety of pollutants through the systems in numerous experiments performed during the summer and winter. In one set of trials, they seeded the septic tank outflow with a bacteriophage (a virus that attacks bacteria) that has similarities to disease-causing viruses in humans. After passing the bacteriophages through the alternative wastewater systems, they recorded their dwindling concentrations over the following month.

In another trial, they spiked the wastewater with a strain of Salmonella and then tracked the bacteria's fate in a similar way. Although they achieved better removal rates in the summer, all the systems performed well in both seasons. The peat filter clearly outperformed the other systems, especially in the winter, but even the constructed wetland, which did the worst of the three, removed more than 90 percent of the study virus and a remarkable 99.6 percent or more of the "culturable" Salmonella in the summer.

Methods matter The word "culturable" is significant when considering the results of these or any reports on bacteria. Culturable bacteria reproduce into colonies in appropriate and standard Petri dish-like conditions. However, culturable cells make up only a small fraction of the total, which includes corpses of expired cells and cells too traumatized or too stubborn to blossom into colonial masses at the time of testing. Knowing this, the researchers attached an RNA probe specific to the Salmonella strain, then counted the cells that fluoresced (including all viable and dead cells). They found that, in most cases, the total and viable cell counts produced numbers that were several orders of magnitude more abundant than the conservative numbers calculated through standard cultures.

"Estimates of bacteria abundance depend on the methods you use," says former graduate student Jonathan Pundsack. "The culture-based method of estimating the abundance of our model pathogen grossly underestimated the abundance of living Salmonella. Assuming that some of the viable but unculturable cells are virulent, then it might be worth looking at this microscopic world in a different way."

These results are consistent with other studies comparing methodologies for estimating bacteria abundance. However, the fact that the counting method significantly influences the outcome complicates the answer to a fundamental question: "How germy is the water?" The standard way to evaluate the likelihood that pathogens are in the water is through cultured fecal coliform or E. coli counts.

Axler and his colleagues designed experiments using sand filters, peat filters, and constructed wetland systems at the prison from 1995 until state funding dwindled in 2003. The projects were made possible by the Legislative Commission on Minnesota Resources, Sea Grant, the Iron Range Resources and Rehabilitation Board, St. Louis County, the Natural Resources Research Institute, Minnesota Power, Minnesota Technology, Inc., and most recently, Minnesota's Lake Superior Coastal Program.

For more information about the system, see onsite wastewater treatment and University of Minnesota Extension Service.