The nitrogen and phosphorus that nurture crops become pollutants when they drain into lakes and streams. Woodchip bioreactors have proven useful in removing nitrates from tile drainage water, but researchers are still searching for low-cost methods of removing phosphates.
Assistant professor of civil and environmental engineering Guanghui Hua of South Dakota State University is using steel byproducts to trap phosphates in simulated tile drainage water. He collaborates with associate professor of agricultural and biosystems engineering and SDSU Extension water management engineer Chris Hay, who has been testing woodchip bioreactors since 2011. Hay envisions installing a steel-containing cartridge as an add-on to nitrate-capturing bioreactors.
Their research is supported by a U.S. Geological Survey grant through the South Dakota Water Resources Institute with matching funds from the East Dakota Water Development District, SDSU Water and Environmental Engineering Research Center and the Department of Civil and Environmental Engineering. The project, which began in March 2014, has received $140,653 in funding.
Protecting lakes, rivers
"Phosphates are the leading cause for algae growth in natural water bodies," Hua pointed out. Because dissolved phosphorus is readily available, algae can use it easily, Hay added. In addition to consuming oxygen, some algae species release toxic substances into the water, Hua explained.
In the last decade, more South Dakota farmers have installed tile drainage to increase crop production, particularly in the last five years, Hay explained. Tile drainage generally reduces total phosphorus losses from farmland, which helps reduce phosphorus loading to lakes and streams. However, the drains can still be a source of dissolved phosphorus, which is what this research addresses.
Experimenting with steel byproducts
Hua and graduate student Morgan Salo are testing four types of steel byproducts from machine shops to determine their phosphate removal capacity. When the steel shavings rust, the iron oxides that form on their surfaces react chemically, binding with phosphate ions and thus removing phosphates from the drainage water.
The researchers performed batch adsorption tests to determine the maximum capacity on a per mass basis for each type of steel material.
Carbon steel works better than stainless steel, Hua pointed out, noting that the iron oxide on carbon steel surfaces is highly reactive with phosphates. The researchers selected a steel mixture containing small and large pieces for subsequent reactor experiments.
They are now optimizing the procedure by pumping simulated drainage water first through a column filled with wood chips and then one filled with steel byproduct in an upflow reactor.
The civil engineering researchers are evaluating a 24-hour flow time through the wood chips and eight hours through the steel byproduct. During three months of continuous operation, the column reactors consistently exhibited 100 percent removal of nitrates and phosphates.
In addition, they are exploring factors such as varying flow-through time and influent nutrient concentrations using the column reactors. The researchers will also conduct batch experiments to determine the impact of pH, temperature and reaction time on adsorption.
Moving to field trials
In the bioreactor, water will flow through a bed of woodchips first and then a steel-loaded barrel before entering the last control structure, according to Hay. An underground bioreactor in the neighborhood of 15 to 20 feet wide and 100 to 120 feet long can handle the runoff from a 30- to 40-acre field.
The researchers plan to add a phosphorus unit to a woodchip bioreactor at Baltic this fall through funding from the South Dakota Soybean Research and Promotion Council. The goal is to remove 80 to 90 percent of the phosphates and nitrates from the tile drainage water.
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