Nitrogen Removal in Integrated Constructed

Wetland Treating Domestic Wastewater

Mawuli Dzakpasu1, Oliver Hofmann2, Miklas Scholz2, Rory Harrington3, Siobhán Jordan1, Valerie McCarthy1

1 Centre for Freshwater Studies, Dundalk Institute of Technology, Dundalk, Co. Louth, Ireland. 2 Institute for Infrastructure and Environment, School of Engineering, The University of Edinburgh, Edinburgh EH9 3JL. 3 Water Services and Policy Division, Department of Environment, Heritage and Local Government, Waterford, Ireland.

2nd Irish International Conference on Constructed Wetlands for

Wastewater Treatment and Environmental Pollution Control

1st – 2nd October 2010

Presentation outline

• Introduction

o Background

o Aim and objectives

• Case study description

• Materials and methods

• Results

• Conclusions

• Acknowledgements

Background

• Constructed wetlands used to remove wide

range of pollutants

• High removal efficiency (70% up) recorded

for several pollutants e.g. COD, BOD5, TSS

• Nitrogen removal efficiencies usually low and

variable

Background

Integrated Constructed Wetlands (ICW) are:

• Multi-celled with sequential through-flow

• Free water surface flow wetlands

• Predominantly shallow densely

emergent vegetated

Background

ICW

concept Biodiversity enhancement

ICW conceptual framework

Landscape fit

Water treatment

Background

• Application of ICW as main unit for large-scale

domestic wastewater treatment is novel

• Limited information to quantify nitrogen removal

processes in full scale industry-sized ICW

Background

Nitrogen biogeochemical cycle in wetlands

Research aim and objectives

Aim

• To evaluate the nitrogen (N) removal performance of a full scale ICW

Objectives

• To compare annual and seasonal N removal efficiencies of the ICW

• To estimate the areal N removal rates and determine areal first-order kinetic coefficients for N removal in the ICW

• To assess the influence of water temperature on N removal performance of the ICW

Case study description

Location map of ICW site

Case study description

• Design capacity = 1750 pe.

• Total area = 6.74 ha

• Pond water surface = 3.25 ha

• ICW commissioned Oct. 2007

• 1 pump station

• 2 sludge ponds

• 5 vegetated cells

• Natural local soil liner

• Mixed black and grey water

• Flow-through by gravity

• Effluent discharged into river

Case study description

Process overview of ICW

• Automated composite

samplers at each pond inlet

• 24-hour flow-weighted

composite water samples

taken to determine mean

daily chemical quality

Materials and methods

Wetland water sampling regime

Materials and methods

Water quality analysis

• Water samples analysed for NH3-N and

NO3-N using HACH Spectrophotometer

DR/2010 49300-22

• NH3-N determined by HACH Method 8038

• NO3-N determined by HACH Method 8171

• Dissolved oxygen, temperature, pH, redox

potential, measured with WTW portable

multiparameter meter

Materials and methods

Wetland hydrology

• 𝑄𝑖 − 𝑄𝑜 + 𝑄𝑐 + (𝑃 − 𝐸𝑇 − 𝐼)𝐴 =𝑑𝑉

𝑑𝑡

• Onsite weather station measures

elements of weather

• Electromagnetic flow meters and allied

data loggers installed at each cell inlet

Data analysis and modelling

𝑅𝑒𝑚𝑜𝑣𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 𝐶𝑜 − 𝐶𝑒

𝐶𝑜× 100 (1)

𝐴𝑟𝑒𝑎𝑙 𝑅𝑒𝑚𝑜𝑣𝑎𝑙 𝑅𝑎𝑡𝑒 = 𝑞 × 𝐶𝑜 − 𝐶𝑒 (2)

𝑤ℎ𝑒𝑟𝑒:

𝑞 =𝑄

𝐴 and 𝑄 = 𝑄𝑖𝑛 + 𝑃 − 𝐸𝑇 − 𝐼 𝐴

Co = influent concentrations (mg-N/L)

Ce = effluent concentrations (mg-N/L)

q = hydraulic loading rate (m/yr.); Q = volumetric flow rate in

wetland (m3/d); A = wetland area (m2); Qin = volumetric flow rate

of influent wastewater (m3/d); P = precipitation rate (m/d);

ET = evapotranspiration rate (m/d); I = infiltration rate (m/d)

Data analysis and modelling

𝐼𝑛𝐶𝑒 − 𝐶∗

𝐶𝑜 − 𝐶∗= −

𝐾

𝑞 (3)

𝐾(𝑡) = 𝐾(20)𝜃(𝑡−20) (4)

log 𝐾 𝑡 = log 𝜃 𝑡 − 20 + log 𝐾 20 (5)

C* = background concentrations (mg/L);

K = areal first-order removal rate constant (m/yr.)

K(t) and K(20) = first-order removal rate constants (m/yr.);

t = temperature (oC); 𝜃 = empirical temperature coefficient

Results

Average rainfall and wastewater discharge at ICW

influent and effluent points (April, 2008 – May, 2010)

0

50

100

150

200

250

0

50

100

150

200

250

300

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Ra

infa

ll (

mm

/mon

th)

Dis

cha

rge

(m3/d

ay

)

Influent Effluent Rainfall

Materials and methods

ICW water budget

55.8 ± 11.3%

44.2 ± 11.3%

5.3 ± 2.7%

49.8 ± 23.3%

24.6 ± 12.7%

63 ± 371.3 m3 day-1

139 ± 65.7 m3 day-1 39 ± 27.9 m3 day-1

123 ± 61.8 m3 day-1

106 ± 112.2 m3 day-1

11 ± 9.4 m3 day-1

Nitrogen removal with cumulative wetland area

Results

0

1

10

100

0% 1% 15% 29% 68% 96% 100%

Influent Sludge

pond

Pond 1 Pond 2 Pond 3 Pond 4 Pond 5

Nit

rogen

(m

g-N

/L)

Ammonia Nitrate

1

10

100

Summer Autumn Winter Spring Summer Autumn Winter

2008 2009

Nit

rogen

(m

g-N

/L)

Ammonia Nitrate

* * *

* * *

*

Seasonal variations of influent nitrogen to ICW * Indicates significant seasonal variation (P < 0.01, n = 18)

Results

0

1

10

Summer Autumn Winter Spring Summer Autumn Winter

2008 2009

Nit

rogen

(m

g-N

/L)

Ammonia Nitrate

Seasonal variations of effluent nitrogen from ICW * Indicates significant seasonal variation (P < 0.01, n = 18)

* * *

* * *

*

Results

0

2

4

6

8

10

12

0

20

40

60

80

100

Summer Autumn Winter Spring Summer Autumn Winter

2008 2009

HL

R (

mm

/d)

Rem

ov

al

Eff

iien

cy (

%)

Ammonia Nitrate HLR

Seasonal variations of nitrogen removal

efficiency and hydraulic loading rate

Results

y = 0.988x - 1.551

R² = 0.99

0

600

1200

1800

0 600 1200 1800

Rem

oval

Rate

(mg m

-2 d

-1)

Loading Rate (mg m-2 d-1)

a) Ammonia

y = 0.952x - 0.111

R² = 0.99

0

250

500

750

1000

0 250 500 750 1000

Rem

oval

Rate

(mg m

-2 d

-1)

Loading Rate (mg m-2 d-1)

b) Nitrate

Areal nitrogen loading and removal rates

Results

Areal first-order nitrogen removal rate

constants in ICW

Parameter

K (m/yr) K20 (m/yr)

Mean SD n Mean SD n

Ammonia 14 16.5 120 15 17.3 101 1.005

Nitrate 11 12.5 101 10 11.3 101 0.984

n = sample number, SD = standard deviation

Results

y = -0.081x + 15.56

R² = 0.0004

0

60

120

0 5 10 15 20 25

KA (

m/y

r)

Water temperature (oC)

y = -0.098x + 11.98

R² = 0.0009

0

40

80

0 5 10 15 20 25

KN (

m/y

r)

Water temperature (oC)

Water temperature and reaction rate constants

(a) Ammonia

(b) Nitrate

Results

y = 0.05x + 2.23

R² = 0.77

0

60

120

0 500 1000 1500 2000

KA (

m/y

r)

Loading rate (mg m-2 d-1)

y = 0.09x + 4.23

R² = 0.66

0

50

100

0 200 400 600 800 1000

KN (

m/y

r)

Loading rate (mg m-2 d-1)

(a) Ammonia

(b) Nitrate

Nitrogen loading rate and reaction rate constants

Results

Conclusions

• High removal rates recorded at all times of the year

• Removal efficiency consistently > 90 %

• Removal rates slightly influenced by seasonality

• Strong linear correlations between areal loading and

removal rates: NH3-N (R2 = 0.99, P < 0.01, n = 120)

and NO3-N (R2 = 0.99, P < 0.01, n = 101)

• Low temperature coefficients are indications that

variability in N removal was independent of water

temperature

Acknowledgements

• Monaghan County Council, Ireland for funding

the research.

• Dan Doody, Mark Johnston and Eugene Farmer

at Monaghan County Council, Ireland, and

Susan Cook at Waterford County Council,

Ireland, for technical support.

We welcome your questions, suggestions, comments!

Contact:

mawuli.dzakpasu@dkit.ie

Thank you for your attention!