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 While nitrogen can literally be pulled out of the air,
phosphorus has to be mined from finite phosphate rock reserves
and treated to be made available to plants. Most of the world’s
phosphate rock is in Morocco, and at some point these reserves
will run out.
Additionally, some of the phosphorus applied to agricultural
fields is lost through erosion, carried away with soil particles
into waterways, where it joins phosphorus products expelled from
wastewater treatment plants, livestock operations, and other
point sources. When it builds up in receiving waterways,
phosphorus can degrade water quality and contribute to “dead
zones” that no longer support aquatic life.
A new Journal of Environmental Quality report from the
University of Illinois describes the feasibility of recycling
phosphorus on a regional scale in the Midwest, simultaneously
solving the problem of an uncertain global supply and minimizing
loss from agricultural fields.
“There's a lot of phosphorus being moved around, from fertilizer
all the way to sewage. Right now it's mostly a one-way street.
It ultimately ends up in waste streams, like septic tanks or
effluent from point sources like wastewater treatment plants.
But there's a lot of value in that phosphorus. We can capture it
and reuse it as a fertilizer,” says Andrew Margenot, assistant
professor in the Department of Crop Sciences at Illinois and
lead author of the report. “We're trying to make circular
something that is currently a one-way trajectory.”
The concept of recycling phosphorus in the Midwest isn’t
entirely new. A 2016 study out of Canada first proposed the idea
that the U.S. Corn Belt could be fully self-sufficient in its
agricultural phosphorus needs if all waste-stream phosphorus was
recycled. But the U of I report represents the first time
biogeochemists, economists, and engineers have come together to
consider the true feasibility of a holistic phosphorus recycling
system in the Midwest.
The researchers considered all the waste sources of phosphorus
in the United States, and honed in on several that were
overlooked in previous thought experiments, including corn
ethanol and soybean processing plants. They then focused on the
forms of phosphorus flowing through the system, another key
aspect not fully captured in previous studies.
Phosphate, a common form of phosphorus in waste streams, is
highly soluble in water. To avoid losing it in waste streams,
the water can be treated with agents to make phosphate settle
out. However, these agents also make phosphate insoluble and
therefore less available for uptake by plants.
“There's an inherent tension between the ability to recover
phosphorus from wastewater, which renders it insoluble, and its
utility as a fertilizer. Insoluble forms won’t dissolve as
readily, so they are less immediately useful. That's an
agronomic reason why we can't re-use 100% of what’s flowing
through waste streams. But we can play with the chemistry of
recovery to match the right soil type and crop type,” Margenot
says. “We need to consider the context we’re dealing with. We
believe we can overcome these challenges by drawing on expertise
across scientific disciplines.”
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One of the biggest roadblocks to establishing a
circular phosphorus economy in the Midwest is the current cost of
fertilizer. Pound for pound, phosphorus fertilizer is more expensive
than nitrogen, but at the moment, it’s much cheaper than the current
cost of recycled phosphorus.
A few Midwestern wastewater treatment facilities are
already recovering phosphorus, but it will take major investment and
commitment to build up the infrastructure needed to overhaul the
entire system. And that doesn’t include the costs required to
transport recycled phosphorus from point sources to fields,
especially in heavy, wet forms like phytin – an organic-matter-rich
sludge left over from soybean processing.
The researchers say phosphorus trading schemes, similar to carbon
credits, could be part of the solution.
“In the article, we talk about the possibility of point sources
paying farmers to take recovered phosphorus off their hands. Our job
as researchers is to show farmers that these alternative phosphorus
sources won't compromise crop yields and could save them money on
phosphorus inputs” Margenot says.
He points to ongoing field experiments by his lab showing that
struvite – an inorganic, granular, slow-release form of phosphorus
recoverable from multiple waste streams – can almost completely
substitute traditional phosphorus fertilizers without yield loss for
multiple crops. The low water solubility of struvite makes it less
likely to move from the field into surrounding waterways. These
potential water quality benefits are the subject of ongoing study as
part of Margenot’s Illinois Nutrient Research and Education
Council-funded research project.
Ultimately, the researchers believe a circular phosphorus economy is
feasible for the Midwest.
“I think we've overlooked phosphorus a bit in Midwestern systems,
where it is often all about nitrogen. But in many ways, phosphorus
is easier to deal with than nitrogen. We can trap it and force it to
flow through the agricultural production chain in ways that are
beneficial to farmers and the environment. Phosphorus is low-hanging
fruit,” Margenot says.
The article, “Toward a regional phosphorus (re)cycle in the U.S.
Midwest,” is published in the Journal of Environmental Quality [DOI:
10.2134/jeq2019.02.0068]. Authors include Andrew Margenot, Dianna
Kitt, Benjamin Gramig, Taylor Berkshire, Neha Chatterjee, Allen
Hertzberger, Sammy Aguiar, Aliza Furneaux, Navneet Sharma, and
Roland Cusick, from the departments of crop sciences; agricultural
and consumer economics; and civil and environmental engineering at
the University of Illinois. The departments of crop sciences and
agricultural and consumer economics are in the College of
Agricultural, Consumer and Environmental Sciences at Illinois.
[Source: Andrew Margenot,
News writer: Lauren Quinn] |
