Denitrification Bioreactors: A Practical Introduction

Laura ChristiansonDr. Alok BhandariDr. Matt Helmers

Nitrate in

tile-drainageDenitrifying

bioreactorwith by-pass flow

Reduced nitrate

loading to surface waters

nitrate

+ organic fill

bacteria nitrogen

gas

Tile-drained field

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

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

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

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

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

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

Denitrifying Bioreactor

Installationin

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

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

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

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

Looking North

InletOutlet

During high flow periods excess water can exit through the overflow bypass.

Water flow

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

Outlet From bioreactor

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

Inlet control structure

Woodchips were spread evenly with a tracked skidloader.

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

Soil was replaced over the bioreactor and will be seeded.

Finished Product