Struvite fertilizer recovered from wastewater is a high-quality product that slowly releases nutrients into the soil. Image: Fraunhofer IGB

Sewage
sludge, wastewater and liquid manure are valuable sources of fertilizer
for food production. Researchers at the Fraunhofer Institute in Germany
have now developed a chemical-free, eco-friendly process that enables
the recovered salts to be converted directly into organic food for crop
plants.

   

Phosphorus
is a vital element not only for plants but also for all living
organisms. In recent times, however, farmers have been faced with a
growing shortage of this essential mineral, and the price of
phosphate-based fertilizers has been steadily increasing. It is
therefore high time to start looking for alternatives. This is not an
easy task, because phosphorus cannot be replaced by any other substance.
But researchers at the Fraunhofer Institute for Interfacial Engineering
and Biotechnology IGB in Stuttgart have found a solution that makes use
of locally available resources which, as unlikely as it might seem, are
to be found in plentiful supply in the wastewater from sewage treatment
plants and in the fermentation residues from biogas plants: a perfect
example of the old saying “from muck to riches”. The new process was
developed by a team of scientists led by Jennifer Bilbao, who manages
the nutrient management research group at the IGB.

“Our process precipitates out the nutrients in a form that enables them to be directly applied as fertilizer,” she explains.

Mobile pilot plant for field tests

The
main feature of the patented process, which is currently being tested
in a mobile pilot plant, is an electrochemical process that precipitates
magnesium-ammonium phosphate—also known as struvite—by means of
electrolysis from a solution containing nitrogen and phosphorus.
Struvite is precipitated from the process water in the form of tiny
crystals that can be used directly as fertilizer, without any further
processing. The innovative aspect of this method is that, unlike
conventional processes, it does not require the addition of synthetic
salts or bases. Bilbao: “It is an entirely chemical-free process.”

The
2-meter-high electrolytic cell that forms the centerpiece of the test
installation and through which the wastewater is directed contains a
sacrificial magnesium anode and a metallic cathode. The electrolytic
process splits the water molecules into negatively charged hydroxyl ions
at the cathode. At the anode an oxidation takes place: the magnesium
ions migrate through the water and react with the phosphate and ammonium
molecules in the solution to form struvite.

Energy-saving, chemical-free process

Because
the magnesium ions in the process water are highly reactive, this
method requires very little energy. The electrochemical process
therefore consumes less electricity than conventional methods. For all
types of wastewater tested so far, the necessary power never exceeded
the extremely low value of 70 watt-hours per cubic meter. Moreover,
long-duration tests conducted by the IGB researchers demonstrated that
the concentration of phosphorus in the pilot plant’s reactor was reduced
by 99.7% to less than 2 mg per liter. This is lower than the maximum
concentration permitted by the German Waste Water Ordinance (AbwV) for
treatment plants serving communities of up to 100,000 inhabitants.

“This
means that operators of such plants could generate additional revenue
from the production of fertilizer as a sideline to the treatment of
wastewater,” says Bilbao, citing this as a decisive advantage.

Struvite
is an attractive product for farmers, because it is valued as a
high-quality, slow-release fertilizer. Experiments conducted by the
Fraunhofer researchers have confirmed its effectiveness in this respect:
crop yields and the uptake of nutrients by the growing plants were up
to four times higher with struvite than with commercially available
mineral fertilizers.

The
scientists intend to spend the next few months testing the mobile pilot
plant at a variety of wastewater treatment plants before starting to
commercialize the process in collaboration with industrial partners
early next year.

“Our
process is also suitable for wastewaters from the food-industry and
from the production of biogas from agricultural wastes,” adds Bilbao.
The only prerequisite is that the process water should be rich in
ammonium and phosphates.

       

Source: Fraunhofer Institute