Saqib Mukhtar, PhD, (foreground) Texas AgriLife Extension Service engineer, and MD Borhan, PhD, Texas AgriLife Research scientist, pose with a lab-bench scale test of a process that can extract 50% of the dissolved ammonium in liquid manure in 20 days. Photo: Robert Burns |
Though it may not sound very glamorous, a new
method of extracting ammonium from liquid animal manure could be exciting news
for both confined animal operations and environmental groups, according to a
Texas AgriLife Extension Service engineer.
The method uses gas-permeable membrane
technology that tests have shown could remove 50% of the dissolved ammonium in
liquid manure in 20 days. The removed ammonium is “not scrubbed but captured,”
says Saqib Mukhtar, PhD, AgriLife Extension engineer and interim associate
department head of the Texas
A&M University
department of biological and agricultural engineering.
By captured, Mukhtar means, the ammonium is
concentrated as ammonia sulfate compound, which as commercial fertilizer could
potentially offset the cost of the removal process.
Though still in the lab-bench test stage, the
technology shows great promise to solve a long-standing, expensive
well-documented problem that confined-animal feeding operations such as dairies
and feedlots face daily, Mukhtar says.
“Excessive ammonia emissions from animal feeding
operations are considered a source of odor and environmental pollution,”
Mukhtar says. “Once emitted, ammonia may contribute to formation of fine
airborne particulates in the presence of certain acidic compounds in the
atmosphere.”
Also, ammonia emissions
from improperly managed manure systems may contaminate groundwater and cause
excessive vegetative growth in lakes and reservoirs, he says.
“And it may even be a
constituent of nitrous oxide, a potent greenhouse gas,” he says.
There are other methods
of mitigating ammonia emissions from manure storage and treatment facilities,
including acidic solution-sprayed scrubbers and biofilters, and chemicals such
as acidified clays and sodium hydrogen sulfate, Mukhtar says.
“Several of these
methods have been promising, but high costs, lack of ‘staying power’ of
chemicals and other additives, lack of ammonia recovery for beneficial uses,
and the complex operation and management of some of the technologies have restricted
their extensive use in animal agriculture,” he says.
In comparison, the
membrane technology Mukhtar and his associates have been testing is relatively
simple.
Gas-permeable tubing is
submersed in a tank of liquid manure. A very dilute solution of sulfuric acid
is pumped through the tubing, which has a porosity of only 2 microns. To put
this in perspective, a typical human hair is 70 microns in diameter.
The method takes
advantage of a property of dissolved gases described by Fick’s first law of diffusion.
A high concentration of a dissolved gas, such as ammonia, will migrate to
regions of lower concentration. As the concentration of ammonium is high in the
liquid manure and low to zero in the permeable tubing, the ammonium is drawn
into the tubing and out of the liquid manure.
Also, the migration is
enhanced by ammonium being a base and chemically attracted to the acid in the
tubing.
The name of the tubular
membrane they used is “expanded polytetrafluoroethylene, which is usually
abbreviated ePTFE,” Mukhtar says.
The product has several uses including blood
filtration and synthetic blood vessel and even dental floss, he said, and once
was prohibitively expensive. But with the expiration of several patents for
this material and its uses, the cost has dropped dramatically, allowing its use
for other applications.
Mukhtar says the next
step is to scale up from the small bench model to a large tank, perhaps 100
gallons, he said. The team also wants to experiment with how little tubing can
be used, and how dilute the acid solution can be, while still capturing about
50% of the ammonium within a reasonable amount of time.
They are also looking
ahead to learn how to economically scale up the process for use on the farm.
“Obviously, we can’t use
a ‘gazillion’ feet of tubing in a large manure lagoon,” Mukhtar says. “Potentially, what we could do is divert some of the flushed manure in a much
smaller basin and apply membrane technology to extract ammonia from it.”
The manure from which
the ammonia has been extracted would then be transported back into the large
lagoon, he says.
“By doing this repeatedly, we could concentrate
ammonia as a relatively high pH solution of ammonium sulfate,” Mukhtar says.