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Nitrogen Cycling in Constructed Wetlands as Related to Swine

Wastewater

G. B. Reddy and Richard PhillipsDepartment of Natural Resources and Environmental Design

P. G. Hunt, M. Poach, and Terry MathenyUSDA-ARS, Coastal Plains Soil, Water, and Plant Research Center

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INTRODUCTION• Animal production is a major component of US agriculture for food consumption and nation’s

economy

• NC ranks second in the nation in swine production

• Managing 42 billion pounds of swine manure per year environmentally friendly is challenging

• Traditionally swine operators flush the manure from houses into an anaerobic lagoon and spray from the lagoon to the land.

• Due to continuous application on land, overspills from lagoons, and unexpected hurricanes, the surface and ground water are being contaminated.

• Many farmers have limited land

• NC new regulations mandate that lagoons need to be closed and new technologies to treat swine waste need to be implemented in coming years.

• Of all the technologies available, constructed wetlands are cost effective, passive, easy to maintain, and low cost to operate.

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Swine wastewater contains following pollutants:

• N (total – N, NH4, NO3)

• P (total –P, Ortho Phosphate)

• Total solids

• Suspended solids

• COD

• Fecal coliforms

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OBJECTIVES

• To determine N removal rates in MPM constructed wetlands treated with different loads of N in swine wastewater.

• To investigate denitrification enzyme activity in the marsh sediments of MPM constructed wetlands

• To determine uptake of N by vegetation

• To quantify the contribution of NH3 volatilization to the overall N removal of MPM system.

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METHODOLOGY

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NC A&T State University Swine Unit

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Wastewater was flushed in to an anaerobic primary lagoon and overflows into a secondary lagoon

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Nitrogen Loading Rates: 5 to 50kg/ha/day

Hydraulic loading rate: 7 to 12. 5m3 day

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75 cm

15 cm

10 m 20 m 10 m

Inflow Outflow

Marsh-Pond-Marsh

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Wastewater Flow Diagram

11Roots/Rhizomes

Water

Sediment/Organic Matter

Soils

Plants

Sedimentation and Sorption(5-day BOD,TSS, N & P) Diffusion

(N & P)

Volatilization(C & N)

IncomingWastewater

OutgoingWastewater

MicrobesT

rans

form

atio

ns

Nitrogen Cycle

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Sampling

• Six auto samplers were installed.

• The water sampler combined daily samples into weekly composites.

• A tipping bucket wired to an electronic totalizer (cycle counter) was installed at the inflow and at the outflow of each wetland cell.

Monitoring Equipment

ISCO 2700 auto samplers were used for sampling on a daily basis combined into weekly composition

Tipping Bucket

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Data Logger

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Plant Analysis • Cattails and bulrushes were sampled in an area of 0.25 m2. The plants

were dried at 60 0C for 48 hours or until constant weight obtained, ground and total-N determined by using C-H-N analyzer (Perkin-Elmer model 2400)

Ammonia volatilization measurements • A special open-ended enclosure was used to measure NH3

volatilization from each section of wetland cell.

Waste Water Analysis• Waste Water Analysis Total-N, NH4 and NO3 were analyzed by

using USEPA methods 351.2, 351.2, and 353.1, respectively.

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Denitrification enzyme activity

• Sediment samples were collected at the 0-2.5 cm depth from marsh 1 and 2 of each wetland cell. DEA was measured by the acetylene inhibition method (Tiedje, 1994).

• 10-15 g of field moist sediment was placed in 60 ml serum bottles (five replications).

• Each bottle received one of the following amendments

– 5 ml chloramphenicol (1 g /L)– 5 ml chloramphenicol with NO3 (200 mg NO3-N/L)– 5 ml chloramphenicol with glucose (2 g glucose-C/L)– 5 ml chloramphenicol with NO3 and glucose

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CALCULATIONS

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Weekly N Load EquationL = [(C/106) • F]/Aw

Where L = weekly N load (kg ha-1 day-1) C = weekly nutrient concentration of liquid manure (mg L-1) F = weekly average of daily flows of liquid manure (L day -1), and Aw = wetland area (ha)

Effn = [(Li –Lo)/Li] x 100

Where Effn = N removal rate (kg ha -1 day -1) Li = mean N load at the inlet (kg ha -1 day -1), and Lo = mean N load at the outlet (kg ha -1 day -1)

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Hourly rate of NH3 volatilization equation

Va = [ dA • Fd / Ap / D] • [ 1mg / 100g]

Where Va = NH3 volatilization

dA = difference in NH3-N captured by outlet and inlet gas-

washing bottles in mg,

Fd = Air flow through enclosure in L min-1 divided by the air

sampling rate of 6 L min-1,

Ap = Plot area of 4m2, and

D = duration of the test in hours

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RESULTS

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Nitrogen removal rate vs. N loading rate in M-P-P constructed wetlands

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Ammonia volatilization in marsh and pond sections of constructed wetlands. Loaded with

different N rates

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Denitrification Enzyme Activity (DEA) as Influenced by N Loading Rate

0

1

2

3

4

5

6

0 10 20 30 40

Nitrogen Loading Rate (kg N ha-1 d-1)

DE

A (

g

N2O

-N g

-1 S

oil

hr

-1)

Control Nitrate Glucose N + C

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NH3 Volatilization, plant uptake, and % N removal in M-P-M constructed Wetlands.

NitrogenLoaded In

Kg ha-1 day-1

Ammonia –NVolatized

Marsh-Pond

Nitrogen Removed By Plants

Kg ha-1 day-1

Nitrogen Removed

By Wetlands

Nitrogen % Removed

37 11 2.2 18.0 49

31 9 3.0 18.0 58

28 8 2.6 15.0 53

17 3 2.6 8.0 47

14 1 2.8 8.0 57

4 0 2.0 1.0 25

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CONCLUSIONS

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Nitrogen removal vs. Nitrogen load

• Wetlands were less effective in removing N when loaded at > 28 kg ha day.

• As N load increased, pond sections exhibited a significantly higher rate of NH3 volatilization (p < 0.001) than the marsh sections.

• Ammonia volatilization was greater than 36 mg NH3-N m2

h, when pond section received greater than 15 kg N ha day.

• The MPM wetlands are NO3 limited systems. However, partial denitrification existed in the wetlands.

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IMPACT• This research focused on resolving some fundamental

problems in constructed wetlands treated with swine wastewater and finally to provide a low-cost, low operation, and efficient system that small to medium hog operators can use to treat their swine waste.

• This technology will be environmental and ecological

friendly and esthetically appealing. Also, in future we are providing a technology where, farmers not only removing pollutants, but also can recover P to use or sell as fertilizer.

• Once the whole technology is developed, it will be demonstrated to the hog operators and extension personnel.