Aquaculture, or fish farming, is one
of the fastest growing sectors of agriculture in the world today. However,
farmers in the United States who wish to capitalize on this momentum face
regulatory hurdles when dealing with fish waste. But new research shows that a
simple, organic system can clean aquaculture wastewater effectively and
inexpensively. Researchers built bioreactors-long
containers filled with wood chips-to treat wastewater from a fully operational
recirculating aquaculture system in West Virginia. The idea is simple: water
from the fish tank enters the bioreactor at one end, flows through the wood
chips, and exits through a pipe at the other end. Along the way, solids settle
out and bacteria housed in the wood chips remove nitrogen, a regulated
pollutant. Laura Christianson, assistant professor of
water quality at the University of Illinois and lead author of the study, is a
bioreactor expert. Her research has shown just how effective they are at
removing excess nitrogen from tile-drained agricultural fields across the
Midwest. But this project was a different kettle of fish. "The bioreactors that we usually promote in
Illinois are for taking nitrogen out of tile drainage," Ms Christianson
explains. "Wastewater from a fish farm is a lot gunkier. It looks brown and can
be smelly. We wanted to see if we could get a bioreactor to take the nitrogen
out of that kind of water without the bioreactor clogging up with solids." The team set up four identical bioreactors,
varying only in retention time, or the amount of time it takes for water to
travel from end to end. "Retention time varied from 12 to 55 hours
in the four bioreactors. If you're trying to treat a lot of water, you want a
lower retention time so you can keep it moving through. But the more time you
give those bacteria to take the nitrogen out, the more effective they are. We
were trying to find a balance between moving water through quickly and making
sure it's staying in there long enough to get treated properly," Ms
Christianson explains. Solid waste in the water presented another
complication. At high flow rates, more solids were entering the system,
settling out, clogging the spaces between wood chips, and impeding flow. The
researchers found that the optimal retention time to both treat the water and
avoid immediate clogging was 24 hours. "The long and the short of it is that the
bioreactors worked great," Ms Christianson says. "They worked as a filter for
the solids and took nitrates out. But for systems that need to move a lot of
water in a short amount of time, we recommend an additional microscreen filter
to settle some of the solids out before they enter and clog up the bioreactor." At face value, a study about clogging
potential of aquaculture bioreactors might not seem revolutionary, but the
results could play a part in the evolution of the agriculture industry. "In the US, we import more than 80 per cent
of our seafood-mostly from southeast Asia and China-so it's an important
industry. If we want to increase our food security, especially around this
great source of protein, we should raise more fish domestically. But to do that
in an environmentally responsible way, dealing with the wastewater from fish
farms will be really important," Ms Christianson says. The article, "Denitrifying bioreactor
clogging potential during wastewater treatment," is published in Water Research and can be downloaded here.
| 
