In the Twin Cities, sewage sludge is incinerated and in the process 5 MW of power is generated per day. Incineration produces significant amounts of ash (38 tons/day) which contains nearly 30% total phosphate (P2O5). Currently, sewage sludge incinerator ash (SSA) is landfilled at a cost to taxpayers, but previous studies have shown that the ash has the potential to be a source of phosphorus (P) for crop production. Additionally, P is a limited and difficult-to-renew resource.
What did we do?
To determine the viability of SSA as a P fertilizer, we are in the middle of conducting a 3-year corn and soybean field study comparing this ash to other P sources including conventional P fertilizer (triple super phosphate), dried pelletized biosolids, and struvite. Each P source was applied in the spring at 40, 80, 120, and 160 lb P2O5/ha, with a zero-P control included. Soil and plant samples were taken throughout the season and after harvest and then were analyzed for available P and EPA 503 metals (elements monitored by the EPA in biosolids land application). In-situ probes that act as a proxy for ions in soil solution were also analyzed.
What we have learned?
After two years of field studies, there is evidence to show that SSA can be a viable phosphorus fertilizer as well as a source of copper (Cu) and zinc (Zn), although additional years of study on a more P-responsive soil are required to draw definitive conclusions. While soils amended with biosolids or SSA have higher levels of DTPA-Cu and DTPA-Zn compared with TSP- or struvite-amended soils, only grain concentrations of zinc reflected this difference in source.
While Cu and Zn are both elevated in biosolids and SSA soils, in-situ probes show that these elements are significantly more available in the biosolids-amended soils. This may be due to the difference in matrices and requires more study. However, our results demonstrate that land-application of ash and biosolids for crop production are two potential options for resolving sewage solids disposal and beneficial reuse of nutrients that go through our food systems. While land-applying biosolids is a good source of carbon and nitrogen where SSA is not, SSA has the benefit of having minimal organic contaminants (pathogens, pharmaceuticals) due to the incineration process. Additionally, biosolids are applied to meet a crop’s nitrogen requirements whereas SSA is applied to meet a crop’s phosphorus requirements. Because of this, the total amount of SSA-amendment added is significantly less than biosolids and thus results in lower or equal amounts of EPA 503 metals.
Future plans:
We will continue our study through a third field season (2019) and will continue to monitor P and 503 metals concentrations in soils and uptake by plants. We are also analyzing soil samples for changes in microbial populations due to SSA application. However, an early proof-of-concept incubation showed no significant effects of SSA when applied at agronomic P-rates.
Authors:
Persephone Ma1, phma@umn.edu
Carl Rosen1
1Department of Soil, Water, and Climate, University of Minnesota, Twin Cities
Additional information:
Bierman P, Rosen C. 1994a. Phosphate and trace-metal availability from SSA. JEQ. 23(4):822-830. Bierman P, Rosen C. 1994b. SSA effects on soil chem-prop and growth of lettuce and corn. Comm in SS and Plant Analysis. 25(13-14):2409-2437. Crants, J., C. Rosen, C. Blake, and M. McNearney. “Is SSAa Safe and Effective Phosphate Fertilizer?” U of Minn, 2015. Abs. Syn in Sci: Partn .for Sol. ASA, CSSA, SSSA, 16 Nov. 2015. Jasinski, S. “Phosphate Rock Statistics and Information.”Phosphate Rock Stat. and Info., USGS, 2017. Walker, J., et al. A Plain English Guide to the EPA Part 503 Biosolids Rule , USEPA, Off. of Ww. Mgmt, 1994.
Acknowledgements:
Metropolitan Council, Rosen Lab field crew, Rosemount Research and Outreach Center field crew, Department of Soil, Water, and Climate field crew
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