Sewage sludge incinerator ash as an agronomic phosphorus source

The Metropolitan Wastewater Treatment Plant in downtown St. Paul, MN incinerates sewage sludge. This generates heat and energy for the treatment plant as well as 37 tons of sewage sludge incinerator ash (SSA) every day. SSA contains significant amounts of phosphorus (P), a fundamental element required for cell metabolism. This ash is landfilled, however, resulting in the disposal of nearly 3 million pounds of P annually. Our goal was to understand the agronomic, soil chemical, and soil microbial responses to SSA land application – can SSA work as an agronomic P source and what are its other effects?

We conducted a three-year corn and soybean field study in Rosemount, MN to study SSA in comparison to three other P sources (triple superphosphate, biosolids, and struvite), applied at a range of application rates. Plant, soil, soil solution, and microbial samples were taken to observe plant yield, P uptake, and soil P concentrations, the loading and uptake of plant micronutrients and trace elements of concern, and potential microbial community responses.

SSA increased crop yields to volumes similar to the other P sources, indicating that it is a source of plant-available P. However, soil and uptake results indicate that SSA might perform best as a slow-release form of P fertilizer. SSA amendment also provided additional micronutrients (Cu and Zn) in concentrations appropriate for plant fertility. However, it did not result in any biologically significant increases in any other elements of concern or changes in microbial community composition. Ultimately, the results from this study validated SSA land application as a safe and effective method of recycling an invaluable resource and demonstrated a local opportunity to reduce the impact of human life on our natural environment.