Denitrifying Bioreactors: Opportunities and Challenges for Managing

Offsite Nitrogen Losses

A. J. Gold1, L. A. Schipper2 and K. Addy1

1University of Rhode Island, USA 2University of Waikato, NZ

FAO/IAEA Int’l Symposium on Managing Soils for Food Security and Climate Change Adaptation and Mitigation

Vienna, Austria July 24, 2012

IAEA-CN-191-42

Fish Kill, Rhode

Island, USA

Excess nitrogen from agriculture:

• Stimulates algal growth; fishkills; degrades coastal habitats

• Generates a potent greenhouse gas, nitrous oxide (N2O = 300 CO2 equivalents)

• Drinking water contaminant

Offsite nitrate losses can be removed within natural denitrification sinks

NO3- NO2- NO N2O N2 • Electron donor (labile carbon) • Anaerobic conditions • Extended retention times • Appropriate temperatures

Nitrate

Organic soils, Riparian forest buffer

Nitrogen gas

from denitrification

Schipper, U. of Waikato

Shallow groundwater flow

Filling trench box with wood chips

Photo courtesy of Betty Buckley, URI Graduate School of Oceanography

Where natural sinks are missing, denitrifying bioreactors can treat nitrate-laden waters:

© THE UNIVERSITY OF WAIKATO • TE WHARE WANANGA O WAIKATO, Schipper et al.

• Requires nitrification in advance of bioreactor • High nitrate removal rates if designed properly • Labile C sources induce anaerobic conditions • Micro-organisms appear to be self-seeding

Denitrifying Walls: Rely on intercepting natural groundwater flow

Gold and Addy, 2008

• In deep aquifers, substantial nitrate can bypass wall bioreactor

Schipper et al., 2010

• Work well close to source

• Work well in shallow aquifers

15 m

Denitrification beds: Intercept nitrate from concentrated flows

Schipper, et al. 2010. Ecol. Engin.

Wood chip denitrifying bed placed into channel

(Robertson et al. 2009)

Denitrifying bioreactors: Factors controlling nitrate removal rates and denitrification

• N source : Nitrification must precede bioreactor

• Retention time: Variations in flow rate must be considered (minimum recommended > 0.25 days)

• Temperature (rates increase with high temperatures)

• Carbon source: Lability; longevity; porosity

Year

96 97 98 99 00 01 02 03 04 05

Nitr

ate (

mg N

L-1 )

0

2

4

6

8

10

12

Upslope

Downslope

Schipper and Vojvodic-Vukovic (2001)

10 years performance of denitrification wall

Nitrate removal rates with different carbon sources

Pinus

radiata

(mea

n)

Har

dwoo

d

Whe

at stra

w

Gre

en w

aste

Maize

cob

s 0

2

4

6

8

10

12

14

16

18

20

Nitra

te r

em

oval ra

te (

g N

m-3 d

-1) 24

oC

14oC

Note that temperature generally increases removal rates

Lability

Longevity

Wood chips

Cameron and Schipper, 2010

Wood chip longevity: Little decline in rates over

15 years of observation

Temperature (oC)

Ra

te (

mg

N L

-1 d

-1)

Comparison of Year 1-15 Removal Rates

0 5 10 15 20 250

4

8

12

Year 1 columns (sawdust)Year 15 column (sawdust)Year 1-7 field PRBy = 0.17 exp(0.16x), R2 = 0.96

Robertson, 2010.

15N studies to determine mechanism of NO3-

removal: Natural abundance

Length (m)

0 40 80 120 160 200

15

N

-15

-10

-5

0

5

10

15

20

25

15N-N2 headspace equilibrium

15N-N2 vacuum extraction

15N-NO3

-

Note increasing enrichment of nitrate along length of bed

Push-Pull Method: In situ denitrification capacity through 15NO3

- enrichment

Push Pull

Water Table

Introduced

plume

2 cm

mini-piezometer

• Pump groundwater • Amend with 15NO3

-

• Push (inject) into well • Incubate • Pull (pump) from well • Analyze samples for

15N2 and 15N2O

(products of microbial denitrification)

Addy et al. 2002, JEQ

15NO3-

15N2, 15N2O

Wood Chips

Push pull A

Push pull B

N2 conc. along

In vitro DR

Measured nitrate

Denitrification r

ate

(g N

m-3

d-1

)

0

10

20

30

40

50

the bed

(C2 H

2 inh.)

removal rate

Push-Pull 15N method shows denitrification is the main mechanism for nitrate removal (Acetylene-block

overestimated rates)

Warneke, et al. 2011

Adverse effects?

• Can generate greenhouse gases N2O, CH4, CO2 • Dissolved carbon leaving bed – problem at

start up

• H2S – possible health hazard

• Methyl mercury

Managing adverse effects: Requires balancing NO3

- load with retention time

Denitrifying Bioreactor: Nitrous oxide emission

0

1

2

3

4

5

6

7

April June Sept Nov Jan

N2O

flu

x in u

mol m

-2

min

-1

On average < 0.9 % of NO3--N removed emitted as N2O gas emissions

(IPCC: Groundwater N2O gas emissions as high as 1.5% of NO3—N leached)

Warneke et al., 2011. Ecol. Engin.

Future Directions and Challenges

Woodchip/wetland bed, H. Leverenz, UC Davis

Opportunities to combine carbon bioreactors with

wetlands for food or fiber

Acknowledgements: This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-51130-31120. Contribution from Joint FAO/IAEA CRP: “Strategic placement and area-wide evaluation of water conservation zones in agricultural catchments for biomass production, water quality and food security”

Selected References

Addy, K, D.Q. Kellogg, A.J. Gold, P.M. Groffman, G. Ferendo and C. Sawyer. 2002. In situ push-pull method to determine ground water denitrification in riparian zones. J. Environ. Qual. 31:1017-1024.

Cameron S.G. and Schipper, L.A. 2010. Nitrate removal and hydraulic performance of carbon substrates for potential use in denitrification beds. Ecol. Engin. 36 : 1588-1595.

Robertson, W.D. 2010. Nitrate removal rates in woodchip media of varying age. Ecol. Engin. 36: 1581-1587.

Robertson, W.D. and L.C. Merkley. . 2009. In-Stream Bioreactor for Agricultural Nitrate Treatment. J. of Environ. Qual. 38:230-237

Schipper, L.A., W.D. Robertson, A.J. Gold, D.B. Jaynes, S.C. Cameron, 2010. Denitrifying bioreactors—An approach for reducing nitrate loads to receiving waters. Ecol. Engin. 36:1532-1543.

Schipper, L.A., M. Vojvodic-Vukovic. 2001. Five years of nitrate removal, denitrification and carbon dynamics in a denitrification wall. Water Research. 35: 3473–3477.

Warneke, S.; Schipper, L.A.; Bruesewitz, D.A.; Baisden, W.T. 2011. A comparison of different approaches for measuring denitrification rates in a nitrate removing bioreactor. Water Research. 45: 4141-4151.