Gone with the Wind: Effects on Nitrogen Removal in Soil Receiving Wastewater

Laboratory of Soil Ecology and Microbiology, University of Rhode Island

Faith Anderson

• Jen Cooper, PhD

• Jose Amador, PhD – Principal Investigator

• Costal Fellows Program

• URI Honors Program

• RI Agricultural Experiment Station

• NE Onsite Wastewater Training Center

Acknowledgments

Outline

1. The Nitrogen (N) problem • Environmental effects

2. N removal in septic systems • Types and mechanisms

3. Linn & Doran (1984) revisited • Accounting for wastewater

4. Experiment #1 • N2O production from wastewater

5. Experiment #2 • 15N tracer

6. Experiment #3 • Factors limiting N2O production

Why Linn & Doran (1984)? • Study cited frequently • Many regulations have been based

on this study • Have we been applying their results

wrongly?

• N is not well removed from septic systems

• 9 kg per household per year is released into the groundwater

The Nitrogen Problem

Environmental Health

Human Health

The Resulting Problems

Last Chance to Remove N…

…In the soil

…In the drainfield…

1. Primary Treatment 2. Drainfield

WATER TABLE

Conventional System

1. Primary treatment

2. Secondary treatment

4. Controls - timers 3. Shallow Drainfield Advanced System

Septic tank effluent (STE) Sand-filtered effluent (SFE)

Septic tank effluent (STE)

NH4

+NO3- (N2O) N2O N2

Nitrification Denitrification (aerobic) (anaerobic)

- Requirements for Denitrification:

- Nitrate - Low or no oxygen - Organic C source

- Anaerobic conditions can be created by a low dissolved oxygen (DO) water (i.e. wastewater) and/or high soil moisture

Nitrification and Denitrification

- Requirements for Nitrification: - Ammonium - High oxygen

Water Table

Nitrogen (N)

Denitrification

NH4+

NO3-

Nitrification

Nitrifying bacteria O2

Denitrifying bacteria

O2 C

N2O N2

NO3-

Controls on N Removal

Type of water Type of soil Level of soil saturation

High

Low

Fine

Coarse Carbon-Poor

Carbon-Rich

Water-Filled Pore Space (WFPS)

• WFPS = percentage of soil pore space filled with water • More water less space for O2 • More water less diffusion of O2 (not easily diffusible in water)

• Linn and Doran (1984) has informed current understanding of WFPS and how it affects N removal.

Here’s What We Know…

• Linn and Doran (1984) used clean water and agricultural/surface soil horizon.

• Wastewater has organic C and varying amounts of N and O2.

• Wastewater is not dispersed in soil surface.

How Does This Apply to Wastewater?

Water Type

DO (mg/L)

BOD (mg/L)

Total N (mg/L)

NH4+

(mg/L)

NO3-

(mg/L) Septic Tank Effluent 0 200 84 47 0

Sand-Filtered Effluent 2 2 53 6 51

Deionized Water 8 0 0 0 0

How does N removal respond to changes in WFPS when we use wastewater?

The Question

Experiment #1: N2O production from wastewater

1. Optimum WFPS for denitrification will be lower for STE and even lower for SFE relative to deionized water. 2. The optimum WFPS for denitrification will be different in fine versus coarse soil.

Hypotheses:

Microcosms

Two Kinds of Soil: Fine and Coarse Three Kinds of Water: 1. Septic Tank Effluent (STE) (0 mg DO/L) 2. Sand-Filtered Effluent (SFE) (3 mg DO/L) 3. Deionized (DI) water (8 mg DO/L) Ten WFPS: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%

SFE + Soil DI + Soil STE + Soil

Experiment #1: N2O production from wastewater

Step 2: Add preliminary amount of DI water to all microcosms and incubate for 1 week

B or C B or C B or C

Step 1: Fill microcosms with 1 cm of soil done for both the B and C horizon

B or C + DI B or C + DI B or C + DI

Step 3: Add STE, SFE, or DI in varying amounts to produce 10 different WFPS ranging from 10-100%

Step 4: Fit with a septa and allow to incubate for 1 hour. Sample gases after incubation period.

SFE + Soil DI + Soil STE + Soil

N2O N2O N2O

Effects of Water Type and Soil Texture

0.0 0.2 0.4 0.6 0.8 1.0

N 2O (

L/L)

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

B horizonC horizon

DI

0.0 0.2 0.4 0.6 0.8 1.0

N 2O (

L/L)

0

5

10

15

20

25

30

B horizonC horizon

SFE

0.0 0.2 0.4 0.6 0.8 1.0

N 2O (

L/L)

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

B horizonC horizon

STE

WFPS

N2O

(μL/

L)

N2O

(μL/

L)

N2O

(μL/

L)

WFPS WFPS

Normalized Values

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

N2O

(L/

L/m

L)

-1.6-1.4-1.2-1.0-0.8-0.6-0.4-0.20.00.2

B horizonC horizon

DI

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

N2O

(L/

L/m

L)0

2

4

6

8

10

12

14

16B horizonC horizon

SFE

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

N 2O (

L/L/

mL)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

B horizonC horizon

STE

N2O

(μL/

L/m

L)

N2O

(μL/

L/m

L)

N2O

(μL/

L/m

L)

1. Optimum WFPS for denitrification will be lower for STE and even lower for SFE relative to deionized water.

2. The optimum WFPS for denitrification will be different in fine versus coarse soil.

Take Home from 1st Experiment

REJECT

SUPPORT

• Why we used it: - Most of N not lost to N2O,

instead it’s lost as N2 Cooper et al. (2016)

- Since N2 is the main

product of denitrification – we decided to measure it using an 15N tracer.

Experiment #2: 15N Tracer

Experiment #2 : 15N Tracer

B or C

Water + 15NH4Cl

B or C B or C O2

15NH415NO3 15NO3

15N2O

Sample Gases

15N2O 15N2

15N Results B vs. C Horizon

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

15N

2 (

g/vi

al/m

L)

0

2

4

6

8

10

12

14

16

15N

2O (p

g/vi

al/m

L)

0

50

100

150

200

250

300

350

15N215N2O

DI waterC horizon

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

15N

2 ( g

/via

l/mL)

0

2

4

6

8

10

12

15N

2O (p

g/vi

al/m

L)

0

50

100

150

200

250

300

15N215N2O

SFEC horizon

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

15N

2 ( g

/via

l/mL)

0

10

20

30

40

50

15N

2O (p

g/vi

al/m

L)

0

50

100

150

200

250

300

350

400

15N215N2O

SFEC horizon

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

15N

2 (

g/vi

al/m

L)

0

2

4

6

8

10

15N

2O (p

g/vi

al/m

L)

0

20

40

60

80

100

120

140

160

15N215N2O

DI waterB horizon

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

15N

2 ( g

/via

l/mL)

0

2

4

6

8

10

12

14

16

15N

2O (p

g/vi

al/m

L)

0

50

100

150

200

250

300

15N215N2O

SFEB horizon

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

15N

2 ( g

/via

l/mL)

02468

101214161820

15N

2O (p

g/vi

al/m

L)

0

200

400

600

800

1000

15N215N2O

STEB horizon

15N

2(μg/vial/m

L)

15N

2(μg/vial/m

L)

15N

2(μg/vial/m

L)

15N

2(μg/vial/m

L)

15N

2(μg/vial/m

L)

15N

2(μg/vial/m

L)

NH4+NO3

- (N2O) N2O N2 Nitrification Denitrification

(aerobic) (anaerobic)

• If just denitrification, should see linear relationship between N2O and N2.

• If other processes are occurring, relationship will not be linear.

Nitrification and Denitrification

Relationship Between Nitrification and Denitrification

15N2O

0 50 100 150 200 250 300 350

15N

2

0

2

4

6

8

10

12

14

16DI waterC horizon

15N2O

0 50 100 150 200 250 300

15N

2

0

2

4

6

8

10

12SFEC horizon

15N2O

0 50 100150200250300350400

15N

2

0

10

20

30

40

50STEC horizon

15N2O

0 20 40 60 80 100120140160

15N

2

0

2

4

6

8

10DI waterB horizon

15N2O

0 50 100 150 200 250 300

15N

2

0

2

4

6

8

10

12

14

16SFEB horizon

15N2O

0 200 400 600 800 1000

15N

2

02468

10121416182022 STE

B horizon

• N2 gas production appears to be related to WFPS but it’s not as simple as Linn and Doran’s model.

• N2 gas production occurs even when the soil is very dry. • Both denitrification and other processes seem to contribute to N gas

production – depending on water and texture.

• Aerobic denitrification? • Robertson, L. A., T. Dalsgaard, N.-P. Revsbech, and J.G. Kuenen. 1995. Confirmation of 'aerobic denitrification' in batch

cultures, using gas chromatography and 15N mass spectrometry. FEMS Microbiol. Ecol. 18: 113-120.

Take Home from 2nd Experiment

Hypotheses:

1. The availability of organic C limits N2O production in soil amended with SFE.

2. Nitrate availability limits N2O production in soil amended with STE.

Experiment #3: Factors Limiting N2O Production

Water Type

DO (mg/L)

BOD (mg/L)

Total N (mg/L)

NH4+

(mg/L)

NO3-

(mg/L) Septic Tank Effluent 0 200 84 47 0

Sand-Filtered Effluent 2 2 53 6 51

Deionized Water 8 0 0 0 0

SFE + Acetate Addition

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

N 2O (

L/L/

mL)

-2

0

2

4

6

8

10

12+ AcetateNo additions

N2O

(μL/

L/m

L)

STE + Nitrate Addition

WFPS

0.0 0.2 0.4 0.6 0.8 1.0

N2O

(L/

L/m

L)

-0.2

0.0

0.2

0.4

0.6

0.8

+NO3

No additionsN

2O (μ

L/L/

mL)

Take Home from 3rd Experiment

1. The availability of organic C limits N2O production

in soil amended with SFE. 1. Nitrate availability limits N2O production in soil

amended with STE.

REJECT

REJECT

• Does wastewater behave like Linn and Doran’s theory?

• Nitrogen gases produced at all WFPS?

• Do NO3- and C limit N2O production?

• Should we rethink the relationship between soil moisture

and N removal?

YES

Conclusion

NO

NO

YES