Sewage Sludge Incinerator Ash as a Phosphorus Fertilizer for Corn and Soybean

The Metropolitan Council Environmental Services (MCES) operates the Metropolitan Wastewater Treatment Plant in downtown St. Paul, Minnesota, which treats an average of 172 million gallons of wastewater a day. Solids from this plant are incinerated to reduce sludge volume and produce energy, which offset energy needs within the plant. This process produces around 38 tons of sewage sludge incinerator ash (SSA) a day, which is currently landfilled. However, this ash contains significant amounts of phosphorus (P), an essential nutrient for healthy plant and animal development that is currently mined for in the US, China, and Sub-Saharan Africa. In this project, SSA from the Metro plant was applied to corn and soybean fields as a phosphorus source to assess its overall fertilizer viability and effects on agronomic response and soil chemical conditions. This research project is complete, and the findings could apply to any utility that incinerates their solids. Phosphorus is an integral part of every living thing, forming nucleic acids, metabolic energy transfer units (ATP/ADP), and cell membranes. In plants, P is responsible for healthy root growth and fruit development and is regularly applied as an annual fertilizer in agricultural systems. P-deficient soils can lead to 15% biomass losses in crop biomass and a 40% decrease in carbohydrate development in grain. The SSA produced in the Twin Cities is 11% total P (25.3% P2O5) and has been shown to be a viable source of P in past incubation and greenhouse studies. However, there are still several remaining questions regarding SSA applied as a P source in realistic conditions and its impacts on crops and the soil environment. We conducted a three-year corn and soybean field study at the University of Minnesota's Rosemount Research and Outreach Center. Our approach was threefold; We wanted to understand 1) whether SSA could be a source of P for agricultural crops, 2) how SSA compared to other P sources, including commercially-available residual products, and 3) the effect on metals concentrations and microbial communities in the SSA-amended soil and crops. While SSA is not a biosolid, the metals outlined in the 40 CFR Part 503 Biosolids Rule that regulates biosolids management in the US were used for comparison. The experiment was conducted on 1.4 hectares (3.5 acres) of Waukegan silt loam soil in Rosemount, Minnesota. Treatments were SSA, conventional P fertilizer (triple superphosphate), dried and pelletized biosolids from MCES' Blue Lake Wastewater Treatment Plant in Shakopee, MN, and commercial struvite. Each P source was applied and incorporated in the spring before planting corn at 20, 40, 60, and 80 kg P ha-1, with a zero-P control included for each P-source. Soil samples were taken throughout the season and were analyzed for available plant nutrients, pH, and total EPA 503 metal concentrations. Biomass samples (grain, when available, and stover) were also taken throughout the season, and analyzed for total P and 503 EPA metals. Corn yields in 2017 and 2018 were high for all treatments overall and only responded specifically to TSP amendment in 2017. In 2019, however, corn yields increased with increasing applications of all P sources, eventually plateauing at the highest rates of P application. These results indicate that in 2019, SSA performed similarly to the other P sources applied in the experiment. Though corn yield results were inconsistent in the first few years, many other measures of P uptake, including 2018 and 2019 soybean yields, P concentration and P uptake in corn grain and soybean among others, did show significant increases with increasing rates of SSA applied. At every sampling point following the initial pre-season sampling date, chemical analyses of the top 15 cm of soil demonstrated an increase in available P with increasing rates of any P source. Soil available zinc and copper increased with increasing applications of SSA and biosolids but not with the application of other P sources. Concentrations of copper in plant biomass did not respond to the application of any P source, whereas concentrations of zinc were higher in plants grown with SSA and biosolids. Total concentrations of soil mercury did statistically increase with SSA application, though the overall increase was low and was not reflected in the plant biomass concentrations. Otherwise, there were no detectable increases in soil or plant material of any metals of concern monitored by EPA 503 regulations. Results from this study indicate that SSA can be a viable source of P in an agricultural system with minimal short-term negative effects on soil or plant metal concentrations. Further investigation should include field studies with differing soil types, crops, and climates as well as a long-term study to monitor metal loading and slow-release P behavior. Available data from all three years (2017-2019) will be presented at the conference. If SSA is found to be an effective P fertilizer with minimal environmental impacts, MCES will have found an opportunity to beneficially reuse another wastewater residual product and recycle an invaluable natural resource.