Southwest Environment

Stories written by University of Arizona students

Department of Soil, Water and Environmental Science
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UA researchers find treated biosolids safe as fertilizers

By Ruth Hook
sample of biosolidPhoto by Ruth Hook
A biosolid sample at the Water and Environmental Technology Center in Tucson is ready for use as fertilizer.

Piles up to 15 feet high of dark, rich organic matter ready to be used as fertilizer is a common sight for those who have ever been around a farm. These piles in western Arizona are unique in that they are not your common fertilizer source. The rich organic matter they contain comes not from cows, but from humans.

They result from the process of wastewater treatment designed to remove the germs that could be harmful to human health. What remains is a material that can provide nutrients to plants – as long as people are willing to accept the concept. University of Arizona researchers are working to change people’s perceptions about potential hazards from land application of this organic matter. (See related story.) 

For centuries, cities across the nation had been dumping raw sewage into rivers, lakes and ocean bodies – and leaving some of the solids anywhere on the landscape, including parking lots, noted Chuck Gerba, a professor in the UA Department of Soil, Water and Environmental Science. It wasn’t until 1972, when the U.S. Congress passed the Clean Water Act, that further regulations were put in place to regulate sewage effluent and biosolids.

Biosolids are a result of the treatment of the millions of gallons of raw sewage that pass through wastewater treatment plants across the nation. Despite the name, they can range from a liquid to a solid, depending on the drying process. As the biosolids decompose, they release essential nutrients such as nitrogen, phosphorus and potassium into the soils for plants to absorb.

Gerba and others conduct some of this research at the Water and Environmental Technology Center, located near Tucson International Airport. Here the labs fill multiple buildings affectionately known as the Water Village. Though the center focuses on multiple research areas, Gerba and others focus on microorganisms in water and in biosolids.

The application of biosolids to farms, timberlands and pastures could provide not only nutrients but water, which can be important in the arid lands of the Southwest, Gerba pointed out. Farmers also appreciate biosolids, in part due to the rising cost of inorganic fertilizers.

“Cities in the West ship out their biosolids and give it away to the farmers because there is nowhere else for them to put it in the cities,” remarked Gerba, who conducts research on bacteria in biosolids.

Another advantage to applying biosolids to the land relates to the carbon cycle. Typically biosolids were incinerated as waste. Now more than 60 percent of the material is applied to land.

“The burning of biosolids produces carbon dioxide that is then released into the atmosphere,” explained Ian Pepper, a UA professor in Soil, Water and Environmental Science and director of the National Science Foundation Water and Environmental Technology Center in Tucson. Carbon dioxide is a greenhouse gas with heat-trapping properties that contributes to climate change.

mine tailingsIn contrast, applying biosolids to land helps pull carbon dioxide from the atmosphere into plants and soils, Pepper added. As a fertilizer, it promotes the growth of plants, which build their tissues out of carbon dioxide and water. Soils can also benefit from the application of biosolids. In fact, Gerba is using biosolids to help convert mine tailings to fertile sites.

Photo by Ruth Hook
Mine tailings of discarded rocks and soil serve as a backdrop in this Green Valley, Arizona, neighborhood.
Mine tailings are a waste product of mining operations.  These barren sites can be seen across Arizona and any other mining states. They become wastelands due to high concentrations of heavy metals and other pollutants. The major concern about leaving mine tailings untreated is that the pollutants in the soil can become airborne in the wind and pose a health risk to surrounding communities.

“In Arizona, applying biosolids to mine tailings is a going to a major part” of restoration efforts, said Gerba. “One hundred and fifty to 200 tons of biosolids can be applied to one acre of mine tailings.”

Applying biosolids to mine tailings can provide a nutrient base for plants to grow on. This will also lessen the potential for wind erosion, thus preventing heavy metal contaminants from becoming airborne. Using biosolids on mine tailings restores organic matter to the soil, Gerba explained .

“At first, after application, invasive species take over because of all the organic matter,” he said. “Eventually it will return to native desert vegetation and you don’t have this bare land staring at you.”

Ruth Hook is an environmental science major in the Department of Soil, Water and Environmental


Related story by same author: Changing perception on biosolids

The Water and Environmental Technology (WET) Center

UA SWES Water Quality Center Laboratory

Environmental Protection Agency guide to biosolids rule

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