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New system could remove two water pollutants from ag fields

Two types of industrial waste products tested in phosphorus filters that may be paired with nitrate traps.

Algae blooms in the Gulf of Mexico use up the majority of the oxygen in the water, leading to “dead zones” that cannot support fish or other wildlife. The culprit is nitrate, which runs off agricultural fields through tile drainage systems, but nitrate is only part of the problem, according to the University of Illinois. Algae in freshwater lakes and ponds flourish when exposed to a different pollutant -- phosphorus -- and the tiniest amount is enough to trigger a bloom.

The majority of the water in Illinois and 11 other states drains into the Mississippi River, so they have set aggressive goals to reduce nitrate and phosphorus pollution in the Gulf of Mexico, the university said. To achieve those goals, large point sources of phosphorus, such as wastewater treatment plants, will need to invest in new infrastructure. However, new research suggests that farmers could play a role as well.

Laura Christianson, assistant professor of water quality in the University of Illinois department of crop sciences, is an expert in woodchip bioreactors and has done extensive work to demonstrate the potential of woodchip-filled trenches in removing nitrate from tile drainage water in Illinois croplands.

“The woodchips and the nitrate are necessary for the bacteria to complete their life cycles. As they consume the nitrate, it is removed from the water. It’s a biological process,” Christianson explained.

In a recent study, Christianson and several colleagues looked at whether they could also remove phosphorus by adding a special “P-filter” designed to trap the fertilizer-derived pollutant. The team tested two types of industrial waste products in the P-filters: acid mine drainage treatment residual (MDR) and steel slag. Phosphorous binds to elements such as iron, calcium and aluminum contained in these products, removing it from the water.

Rather than mixing MDR or steel slag with woodchips in one big nitrate- and phosphorus-removing machine, the team placed a separate P-filter upstream or downstream of a laboratory-scale bioreactor. They ran wastewater from an aquaculture tank through the system and measured the amount of nitrate and phosphorus at various points along the way.

Nitrate removal was consistent, regardless of the P-filter type and whether the P-filter was upstream or downstream of the bioreactor. However, MDR was far superior as a phosphorus filter. “It removed 80-90% of the phosphorus at our medium flow rate,” Christianson said. “That was really, really good -- amazing.”

Steel slag, on the other hand, removed only about 25% of the phosphorus, “but steel slag is a lot easier to find in the Midwest. According to the Illinois Nutrient Loss Reduction Strategy, we’re only trying to remove 45% of the phosphorus we send downstream. Since agriculture is only responsible for half of that, 25% would be pretty good,” Christianson noted.

The system clearly shows potential, but several unknowns remain. Paired bioreactors and P-filters have yet to be tested in real-world conditions, although a handful have been installed in the U.S. Perhaps what's more important is that researchers don’t have a good handle on how much phosphorus is running off agricultural fields in tile drainage.

“We suspect our tile drainage in Illinois doesn’t have much phosphorus in it, but we know there is some,” Christianson said. “We’re getting a better handle right now on just how much phosphorus we have. We know that phosphorus moves more readily in surface runoff. When you have soil eroding and the water is murky and brown, there’s generally phosphorus attached to the soil. The easy way to sum it up is if you have tile drainage, you should be more concerned about losing nitrate in that water, but if you have hillier land, you should be more concerned about soil erosion and losing phosphorus.”

The article, “Denitrifying Woodchip Bioreactor & Phosphorus Filter Pairing to Minimize Pollution Swapping,” was published in Water Research.

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