⋅ BY JOHN LOVETT ⋅
Struvite, the same substance that makes up kidney stones and irritates sewage plant operators, could be an effective alternative to using a limited supply of mined phosphate for crop fertilizer.
According to the U.S. Department of Agriculture, 19% of the 22.02 billion tons of fertilizer used in the United States in 2015 — the most recent data available — was phosphate based.
Struvite is the common name for magnesium ammonium phosphate, a crystal-like substance that often coats the inside of sewage pipes and causes blockages. Struvite can, however, be created by chemical engineers from solid wastes or wastewater as a fertilizer with slow-release potential because most of it is not water soluble. The substance is taken up by plants as magnesium, nitrogen, and phosphorus as the roots acidify the soil around it, so excess nutrient runoff is limited.
There are two ways to precipitate struvite from a solution. The “electrochemical method” involves using an electrical current through a magnesium electrode, putting magnesium atoms into the solution that react with ammonium and phosphate in the solution. The “chemical method” usually includes adding magnesium salts to a solution, which results in a chemical reaction with the ammonium and phosphate. Using either method, struvite is produced.
Two-year field studies by the Arkansas Agricultural Experiment Station showed that struvite produced by electrochemical means performs as well as the common sources of phosphate fertilizer, and the chemically created struvite, on yields in rice, corn, and soybeans in east Arkansas soils.
“The results of our study demonstrate that electrochemically precipitated struvite has potential as an alternative fertilizer source for upland and lowland row crops in eastern Arkansas, where there is a documented nutrient deficiency,” said Kristofor Brye, University Professor of applied soil physics and pedology with the experiment station, the research arm of the University of Arkansas System Division of Agriculture.
The field study’s results are the culmination of research done with chemical engineers and agricultural economists supported by the National Science Foundation.
Recovering nutrients from wastewaters could have a positive impact on the environment and create an additional revenue stream for wastewater treatment facilities, Brye added.
A previous experiment station study indicated flood-irrigated rice grown with electrochemically precipitated struvite produced less methane, a greenhouse gas, than other phosphate fertilizers, including a chemically precipitated struvite. A confirmation study is ongoing.
“A fertilizer-phosphorus source that is environmentally friendly, as the product of recovered nutrients from wastewater, would be a substantial benefit for Arkansas row-crop producers, provided electrochemically precipitated struvite is economically cost-effective,” Brye said.
Research by the University of Arkansas chemical engineering department has been supported by the National Science Foundation to develop innovative methods of creating electrochemically precipitated struvite.
Testing the options
Prior to the two-year field studies on each crop, Brye said no field research had been conducted using electrochemically precipitated struvite in the highly agriculturally productive soils of the Mid-South and southern United States. Lauren Greenlee, former associate professor in chemical engineering for the University of Arkansas, had the idea for an electrochemical approach to making struvite from wastewaters, Brye said. She decided to prove the concept of making the material and Brye followed up with agronomic testing. Greenlee is now at Penn State University. The studies were conducted from 2018 to 2020.
Previous research leading up the field trials for each crop included lab and greenhouse studies, including a study by Niyi Omidire, now a postdoctoral fellow in crop, soil, and environmental sciences at the University of Arkansas, comparing a chemically precipitated struvite to triple super phosphate on both irrigated and non-irrigated land in a wheat-soybean double crop system in east Arkansas. The results upheld the use of chemically precipitated struvite as a potential alternative fertilizer-phosphorus and magnesium source on a silt-loam soil for crop production.
Brye also served as adviser to crop, soil, and environmental sciences graduate student Ryder Anderson’s struvite in moist-soil incubation study. Ryder’s study showed phosphorus concentrations differed among soil textures and previous management histories, indicating struvite’s “slow-release” characteristic is more dependent on soil type.
The goal of the two-year confirmation field study was to evaluate the potential effectiveness of struvite for use in rice, corn, and soybeans. Electrochemically precipitated struvite from real wastewater is now being used in a field study to test greenhouse gas emission differences in flood-irrigated rice. Researchers are also looking at rain runoff levels of each phosphorus source.
How the test was done
A uniform application rate of phosphorus was used for each fertilizer source in the field study. The chemically precipitated struvite was Crystal Green® by Ostara.
Generally, struvite contains between 11% and 26% total phosphorus depending on the initial source and production method. Because of the small amount of electrochemically precipitated struvite that could be created in the lab, the field test areas in the two-year study were equally small, 5-by-5 feet.
Brye said there were no major differences in yield performance between the electrochemically precipitated struvite and all the other mined phosphate sources used in the various studies for rice, corn, and soybeans.
While the researcher’s struvite appeared to be more effective on corn and soybeans than rice, Brye said there are contributing factors that would lead him to believe there is more to the story.
When rice is flood-irrigated, the plants can tap into a greater pool of phosphorus released by the water, but the fertilizer is less efficiently used than in corn. Also, corn has a different root structure that allows it to explore the ground better, and the slow-release characteristic of struvite better matches the physiological growth of corn and soybeans, he said. Corn also has a higher phosphorus demand than the other crops.
Economics of recycling
“By all accounts we’ve concluded this electrochemically precipitated struvite can be more than a viable alternative fertilizer phosphorus source,” Brye said. “While there is no large-scale production of electrochemically precipitated struvite, there is large-scale production of chemically precipitated struvite.”
Studies show that recovery of struvite from wastewater treatment plants is possible and would be a relief in the wastewater treatment industry, Brye said.
“Struvite works as a fertilizer, but it is only a rational choice if the benefits of using it outweigh the costs,” said Jennie Popp, professor of agricultural economics and agribusiness and associate dean of the University of Arkansas Honors College.
Popp noted more research is needed to determine the economic feasibility of electrochemically precipitated struvite in row-crop agriculture. However, their economic analysis of the experiment station’s field study data shows electrochemically precipitated struvite showed “real promise” on corn when they price similar products, like the chemically precipitated struvite Crystal Green, to estimate a price. The results were mixed on the other crops, Popp said.