|
 “Phosphorus
removal structures have been developed to capture dissolved
phosphorus from tile drainage systems, but current phosphorus
sorption materials are either inefficient or they are industrial
waste products that aren’t easy to dispose of. This motivated us
to develop an eco-friendly and acceptable material to remove
phosphorus from tile drainage systems,” said study author Hongxu
Zhou, who completed the study as a doctoral student in the
Department of Agricultural and Biological Engineering (ABE),
part of the College of Agricultural, Consumer and Environmental
Sciences and The Grainger College of Engineering at U. of I.
Zhou and his co-authors used sawdust and lime sludge, byproducts
from milling and drinking water treatment plants, respectively.
They mixed the two ingredients, formed the mixture into pellets,
and slow-burned them under low-oxygen conditions to create a
“designer” biochar with significantly higher phosphorus-binding
capacity compared to lime sludge or biochar alone. Importantly,
once these pellets bind all the phosphorus they can hold, they
can be spread onto fields where the captured nutrient is slowly
released over time.
Leveraging designer biochar’s many sustainable properties, the
team tested pellets in working field conditions for the first
time, monitoring phosphorus removal in Fulton County, Illinois,
fields for two years. Like the majority of Midwestern corn and
soybean fields, the experimental fields were fitted with
subsurface drainage pipes. This drainage water flowed through
phosphorus removal structures filled with designer biochar
pellets of two different sizes. The team tested 2-3 centimeter
biochar pellets during the first year of the experiment, then
replaced them with 1 cm pellets for the second year.
Both pellet sizes removed phosphorus, but the 1-centimeter
pellets performed much better, reaching 38 to 41% phosphorus
removal efficiency, compared with 1.3 to 12% efficiency for the
larger pellets.
The result was not a surprise for study co-author Wei Zheng, who
said smaller particle sizes allow more contact time for
phosphorus to stick on designer biochar. Zheng, a principal
research scientist at the Illinois Sustainable Technology Center
(ISTC), part of the Prairie Research Institute at U. of I., has
done previous laboratory studies showing a powdered form of
designer biochar is highly efficient for phosphorus removal. But
powdered materials wouldn’t work in the field.
“If we put powder-form biochar in the field, it would easily
wash away,” Zhou said. “This is why we have to make pellets. We
have to sacrifice some efficiency to ensure the system will work
under field conditions.”
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After showing the pellets are effective in real-world
scenarios, the research team performed techno-economic and
life-cycle analyses to evaluate the economic breakdown for farmers
and the overall sustainability of the system.
The cost to produce designer biochar pellets was
estimated at $413 per ton, less than half the market cost of
alternatives such as granular activated carbon ($800-$2,500 per
ton). The team also estimated the total cost of phosphorus removal
using the system, arriving at an average cost of $359 per kilogram
removed. This figure varied according to inflation and depending on
the frequency of replacing pellets — two years appeared to be the
most cost-effective scenario.
The life cycle analysis showed the system — including returning
spent biochar pellets to crop fields and avoiding additional
phosphorus and other inputs — could save 12 to 200 kilograms of
carbon dioxide-equivalent per kilogram of phosphorus removed. Zhou
says the benefits go beyond nutrient loss reduction and carbon
sequestration to include energy production, reduction of
eutrophication, and improving soils.
“At the moment, there's no regulation that requires farmers to
remove phosphorus from drainage water. But we know there are many
conservation conscious farmers who want to reduce nitrate and
phosphorus losses from their fields,” said co-author Rabin Bhattarai,
associate professor in ABE. “If they’re already installing a
woodchip bioreactor to remove nitrate, all they’d have to do is add
the pellets to the control structure to remove the phosphorus at the
same time. And there’s something very attractive about being able to
reuse the pellets on the fields.”
The study, “Exploring the engineering-scale potential of designer
biochar pellets for phosphorus loss reduction from tile-drained
agroecosystems,” is published in Water Research [DOI:
10.1016/j.watres.2024.122500]. The research was supported by the
U.S. Environmental Protection Agency [grant no. 84008801] and the
Illinois Nutrient Research and Education Council [grant no.
2019–4–360232].
This work earned Zhou first place (Ph.D. category) in the
prestigious 2024 Boyd-Scott Graduate Research Award from the
American Society of Agricultural and Biological Engineers. He is now
a postdoctoral research associate in ISTC. Zheng is also an adjunct
faculty in ABE.
[Sources: Hongxu Zhou, Wei
Zheng,Rabin Bhattarai,
News writer: Lauren Quinn, ] |
