In July of 2011, a denitrifying bioreactor was installed in the Hewitt Creek watershed in northeast Iowa. A denitrifying bioreactor is installed to reduce nitrate delivery from agriculture fields.

1. Denitrification Bioreactors:
A Practical Introduction
Laura Christianson
Dr. Alok Bhandari
Dr. Matt Helmers

2. Nitrate in
tile-drainage
Denitrifying
bioreactor
with by-pass flow
Reduced nitrate
loading to
surface waters
nitrate
+
organic fill
bacteria nitrogen
gas
Tile-drained field

3. How do Bioreactors Work?
• Denitrification: a natural biochemical process
that happens in all soils by “good” bacteria
• Denitrifier bacteria need:
– Anaerobic conditions: low oxygen (under water)
– Food: carbon source (the woodchips)
– Something to breathe: nitrate, not oxygen
– Place to live: on woodchips

5. Sizing a bioreactor correctly requires a balance
• Treatment of peak drainage flow
– Not practical to treat 100% of maximum flow
– Design to treat approximately 20% of peak flow rate
which means the majority of total annual flow will be
treated
• Retention time
– Enough time to allow bacteria to remove nitrate
– Not so long to allow other unwanted processes to
occur
• Practical size considerations
• Depth is set by the depth of tile

6. Drain Flow Analysis
• 38 ac Drainage Area
• 363 gpm Max Flow Rate
• 20% of Peak Flow rate = 72.6 gpm
• 41% of total flow volume occurred under 20% peak flow rate
• 86% of flow measurements occurred under peak flow rate

7. Drain Flow Analysis
• 27 ac Drainage Area
• 327 gpm Max Flow Rate
• 20% of Peak Flow rate = 65.4 gpm
• 71% of total flow volume occurred under 20% peak flow rate
• 94% of flow measurements occurred under peak flow rate

8. Denitrification Bioreactor Info
• Flow reduction will not occur in the drainage
system because bypass flow is incorporated
into the design
• The life of a bioreactor is estimated to be 10
to 20 years, though this is still being
researched
• Can be incorporated into existing grass buffer
• Surface area of bioreactor roughly estimated
to be about 0.1% of drainage area

9. Operation and Maintenance
• Leaching of dissolved organic carbon during the
start-up phase: This loss of carbon represents a
reduction in carbon available for the
denitrification process as well as a concern for
the receiving water body.
• Sulfate reduction: if retention time is too
long, bacteria other than the denitrifiers use the
woodchips for food and produce hydrogen sulfide
gas
• Incomplete denitrification product: nitrous oxide

10. Denitrifying Bioreactor
Installation
in
Hewitt Creek Watershed
The bioreactor was designed by ISU Extension and
installed by Al Wente and Jeff Pape
With funding from the:
Iowa Watershed Improvement Review Board

12. The bioreactor was placed
in a buffer area along a
small stream.

13. Inlet control structure –
Incoming water flows
into the bioreactor
until the water level
reaches a set
height, about 1.5 – 2 Ft.
Half of the bioreactor was
lined with plastic to prevent
water seeping out and keep
the stream bank stable.
Bypass
In

14. Inlet control structure with two sets of baffles.
To Bioreactor
inlet tile
Water from
drainage tile
Bypass
Leave 1.5 to 2 ft at bottomRemove all baffles

15. Looking North
InletOutlet
During high flow periods excess water
can exit through the overflow bypass.
Water flow

16. Outlet control structure to allow water samples
to be taken and, if desired, reduce the rate of
flow from bioreactor.
Outlet From bioreactor

17. Water enters the
bioreactor through
the T-shaped tile to
spread the water
evenly across the
bioreactor.
After filtering through
the woodchips water
exits through
perforated tile.
Enter
Exit

18. Inlet control
structure
Woodchips were
spread evenly with
a tracked
skidloader.

19. Permeable geotextile
fabric was placed over
the woodchips to keep
soil from filtering into
the woodchips and
slowing the system.

20. Soil was replaced over the
bioreactor and will be
seeded.
Finished Product