Performance of Constructed Wetland Systems for
Nitrogen Removal
Soeprijanto
Department of Chemical Engineering, Sepuluh Nopember Institute of Technology
(ITS), Surabaya, Indonesia.
Email: [email protected]
Liu, J.C
Department of Chemical Engineering, National Taiwan University of Science and
Technology (NTUST), Taipei, Taiwan, Republic of China.
Email: [email protected]
Abstract
Wetlands are natural wet ecosystems with diverse and complex roles in nature and
fundamentally all wetlands are at least intermittently flooded with water depths that
support the growth of emergent vegetation such as Cattail, reeds, sedges, bulrushes,
rushes, and grasses. The vegetation provides surfaces for the attachment of microbial
films, aids on the filtration and adsorption of wastewater constituents, transfers
oxygen into the water column, and controls the growths of algae by restricting the
penetration of sunlight. The extensive root systems serve as large surface areas for
the development of microorganisms and enable filtration as well as adsorption of
sediment matter. The purpose of this study was to evaluate the performance of
nitrogen removal from domestic wastes in a constructed wetland to improve the water
quality effluents. The systems consisted of three wetland cells in series, i.e.,
subsurface flow effluent (SSF), free water surface1 (FWS1), and free water surface2
(FWS2). Each of which had a surface area of 80 m2, 90 m2, and 150 m2, respectively.
These experiments were conducted during the period of September to
November 2005. The results showed that the average removals for the parameters
monitored in the constructed wetland cells were as follows: CODfilter 85.56%, TKN
99.36%, NO3 -N 89.16 %, and TN 99.30 %; and COD, TKN, NO3-N and TN were
reduced to levels below the maximum permissible levels required for direct
discharge.
Keywords: Constructed wetland; Domestic wastes; Free water surface; Nitrogen
removal; Subsurface flow;
1
Constructed wetlands have also
been successfully used on global basis
as secondary and tertiary treatment of
effluent in US (Martin et al., 1994),
Netherlands (Schreijer et al., 1997),
Norway (Maehlum et al.,1995),
Australia (Mitchell et al., 1995).
The purpose of this study was to
evaluate the nitrogen removal from
septic thank effluents in a constructed
wetland to improve the water quality
effluents.
1. Introduction
Nitrogen plays a significant role
in the quality of water. Nitrogen
contaminated groundwater can affect
the safety of water for human
consumption and degrade the quality of
natural waters. Nitrate concentrations
above 10 mg N /L have been reported to
cause
the
conditions
methemoglobinemia
in
sensitive
individuals. Areas experiencing heavy
nitrogen inputs to surface or ground
waters are currently required to remove
nitrogen
during
the
wastewater
treatment process to prevent further
degradation of water quality.
Nitrogen can be removed from
wastewater using a two-stage biological
nitrification-denitrification process. The
first stage, nitrification uses aerobic
bacteria (Nitrosomonas bacteria) to
convert ammonia-nitrogen to nitritenitrogen and then to nitrate - nitrogen
by Nitrobacter bacteria. Nitrification
occurs only after the carbonaceous
biochemical oxygen demand (CBOD)
of the wastewater has been sufficiently
depleted.
Organic matters in wastewaters
which may be in soluble and suspended
forms
are
removed
through
microbiological
degradation.
The
microorganisms
responsible
for
degrading the organic matter are
generally associated with biofilms
which are developed on the surfaces of
soil, vegetation, and litter. Natural
systems are generally operated to
maintain aerobic conditions, therefore,
the removal of the organic matters is
predominantly conducted by the aerobic
microorganisms. Also, the capacity of
degradation of the organic matter
aerobically is limited by the oxygen
transfer from the atmosphere. Thus, the
system must be designed such that the
organic loading rate applied is less than
the estimated rate of oxygen transfer to
the systems.
2. Fundamental
Wetlands are natural wet
ecosystems with diverse and complex
roles in nature (Droste, 1997).
Definitions of wetlands vary (Mitsch
and Gosselink, 1992 in Droste, 1997),
however fundamentally all wetlands are
at least intermittently flooded with
water depths typically less than 2 ft (0.6
m) that support the growth of emergent
vegetation (plants) such as Cattail
(Typha spp.), reeds (Phagmites spp.),
sedges (Carex spp.), bulrushes (scirpus
spp.), rushes (Juncus spp.), and grasses
(Metcalf and Eddy, 1991; Mitsch and
Gosselink, 1992 in Droste, 1997). The
vegetation provides surfaces for the
attachment of microbial films, aids on
the filtration and adsorption of
wastewater
constituents,
transfers
oxygen into the water column, and
controls the growths of algae by
restricting the penetration of sunlight
(Metcalf and Eddy, 1991). The
extensive root systems serve as large
surface areas for the development of
microorganisms and enable filtration as
well as adsorption of sediment matter
(Biddlestone et al., 1991).
Wetlands may consist of natural
and constructed wetlands. In natural
wetlands, vegetation is a function of the
type of wetlands and its location.
Constructed wetland systems have two
types that have been developed for
2
wastewater treatment, i.e., subsurface
flow (SSF) systems (Figure 1) and free
water surface (FWS) systems (Figure
2). Using these types, a variety of plant
species plants can be used for vegetative
coverage. These systems can provide
mineral cycling and the attachment of
niches
for
the
microbiological
populations that in turn is improving the
water quality (Kadlec and Knight,
1996). The typical SSF systems treat
water by allowing the wastewaters to
flow laterally through the media and
contact the microbiological populations
which are responsible for nitrificationdenitrification. The application of these
systems has been rapidly expending to
treat the municipal and industrial
wastewaters in recent years (Zachritz
and Fuller, 1993).
Figure 1. Schematic Diagram of Subsurface Flow (SSF)
Fgure 2. Schematic Diagram of Free Water Surface (FWS)
China. The systems consisted of three
wetland cells in series, i.e., subsurface
flow effluent (SSF) (Figure 4), free
water surface1 (FWS1) (Figure 5), and
free water surface2 (FWS2) (Figure 6).
Each of which has a surface area of 80
m2, 90 m2, and 150 m2, respectively.
3. Methods
Site Descriptions
The constructed wetland (cw)
systems (Figure 3) is located in 100acre area behind of Administration
Building of National Taiwan Normal
University (NTNU), 88, Sec. 4, TingChou Road, Taiwan 116, Republic of
Figure 3. Schematic Diagram of Wetland Systems.
3
Figure 4. Subsurface Flow (SSF)
Figure 5. Free Water Surface1 (FWS1)
Figure 6. Free Water Surface2 (FWS2)
wavelength of 275 nm to determine
interference due to dissolved organic
matter; TKN was analysed using the
macro-Kjeldahl
method
(APHA,
AWWA and WEF, 1995).
Sampling and analysis
Water samples were weekly
collected from September 30 to
November 29, 2005 at the inflows and
outflows of the three wetland cells.
Average temperature in September to
November 2005 was approximately
between 23 to 25o C. Physical-chemical
parameters (dissolved oxygen (DO) and
temperature) were measured in situ
using
portable
WissenschaftlichTechnishe
Werkstatten
(WTW)
microprocessor probes and meters.
The samples were filtered
through a membrane filter paper with
pore diameter of 0.45 µm and the
filtered liquid was analysed for
chemical oxygen demand (COD),
nitrate (NO3-N). Unfiltered sample was
analysed for TKN. The samples were
stored at temperature 7oC if not used
immediately. The analysis of the water
samples was done in laboratory of
environmental engineering, NTUST.
The methods used for the
analysis were as follows: COD was
measured using HACH COD methods
and colorimetric determinations were
using
a
HACH
DR/4000U
spectrophotometer; nitrate-N were
determined
spectrophotometrically
using a Shimadzu Spectrometer Model
UV-160A at a wavelength of 220 nm to
obtain NO3–N reading and a
4. Results and Discussion
COD Removal
Figure
7
shows
COD
concentrations at each constructed
wetland effluents during time period of
October to November 2005. The results
showed that the average COD
concentration in the influent of the
constructed wetlands was 77.65 mg/l
and the effluent of the system was 10.56
mg/l with the removal efficiency of
85.56 %. These average value
concentration effluent could meet, the
concentration demand in Taiwan, COD
< 250 mg/l. The reduction of COD in
the whole constructed wetland systems
was because of the participation of the
microorganisms
in
the
aerobic
decomposition of organic matter. The
oxygen was supplied from the
atmosphere and from the plants. Since
the aquatic vegetations in this system
played an important role for the
diffusion of oxygen from roots creating
conditions required for the development
of microorganisms.
4
Concentration of COD (mg/l)
100
80
77.65
60
40
25.62
14.36
20
10.56
0
Septic Effl
SSF Effl
FWS1 Effl
FWS2 Effl
September - November 2005
Figure 7. Concentrations of COD at each constructed wetland effluents.
utilised organic
compounds
for
nitrification process that were indicated
by reducing COD concentration in the
system from 77.65 to 25.62 mg/l
(Figure 7).
The SSF effluent entering the
FWS1 and FWS2, the remaining nitrate
underwent degradation to 2.78 mg/l and
0.09 mg/l, respectively (Figure 8). This
indicated that the nitrification did not
occur although aerobic conditions were
achieved in these systems (as seen in
Figure 9 with DO value > 4 mg/l); the
nitrate degradation was predominantly
absorbed by the aquatic vegetations for
their lives and growths rather than by
the nitrifying bacteria. This also
happened to the reduction of TKN
(Figure 10), the nitrifying bacteria did
not take part to convert TKN to nitrate;
however, the reduced TKN was
predominantly absorbed by the aquatic
vegetations and other microorganisms
for
their
lives
and
growths.
Nitrate - N Removal
Nitrogen removal in constructed
wetlands
mainly
occurred
by
nitrification-denitrification bacteria. In
the nitrification process, ammonia either
originally available in the influent or
produced by organic degradation, were
oxidised to nitrate. The nitrate was
subsequently reduced to nitrogen gas in
denitrification process.
Profile of nitrate concentrations
in constructed wetlands is shown in
Figure 8, during time period of October
to November 2005. The average nitrate
concentration in the influent was 0.83
mg/l. The results showed that the
nitrification occurred in SSF, as the
increased concentration of nitrate was
found to be 3.81 mg/l in the SSF
effluent. This was suggested that the
nitrifying bacteria required oxygen that
was supplied from the atmosphere and
was diffused from the root of the
aquatic vegetations. Also, these Bacteria
5
NO3-N (mg/l)
5
3.81
4
2.78
3
2
1
0.83
0.09
0
Septic Effl
SSF Effl
FWS1 Effl
FWS2 Effl
September - November 2005
Figure 8. Concentrations of NO3-N at each constructed wetland effluents.
6
Dissolved Oxygen (mg/l)
10
9.08
7.49
8
6
4.47
4
2.16
2
0
SSF Effl
Midle FWS1
Effl
FWS1 Effl
FWS2 Effl
September - November 2005
Figure 9. Concentrations of dissolved oxygen at each constructed wetland effluents.
140
TKN (mg/l)
120
113.96
100
80
60
34.07
40
20
3.35
0.60
FWS1 Effl
FWS2 Effl
0
Septic Effl
SSF Effl
September - November 2005
Figure 10. Concentrations of TKN at each constructed wetland effluents.
adsorbed by the plants for their growths
in the SFF constructed wetland
(reduction from 114.78 mg/l to 37.88
mg/l). In the FWS1 and FWS2, the TN
concentration gradually decreased up to
6.13 mg/l and 0.69 mg/l, respectively.
The removal of TN was predominantly
caused by the adsorption of the plants,
as nitrogen was used as a nutrient by the
aquatic vegetations for their lives and
growths.
Total Nitrogen Removal
Total nitrogen (TN) was
calculated by the sum of total Kjeldahlnitrogen (TKN) and nitrate-nitrogen.
The TN effluent of each wetland cells
was showed in Figure 11 during time
period of October to November 2005.
The average value of TN influent to the
constructed wetland was 114.78 mg/l.
During the process, a part of TKN was
converted to nitrate, and the others were
7
Total Nitrogen (mg N/l)
140
120
114.78
100
80
60
37.88
40
20
6.13
0.69
0
Septic Effl
SSF Effl
FWS1 Effl
FWS2 Effl
September - November 2005
Figure 11. Concentrations of Total Nitrogen at each constructed wetland effluents.
5. Conclusions
The average removals for the
parameters monitored in the wetland
cells from September to November
2005 were as follows: CODfilter 85.56%,
TKN 99.36%, NO3 -N 89.16 %, and TN
99.30 %; and COD, TKN, NO3-N and
TN were reduced to levels below the
maximum permissible levels required
for direct discharge.
List of Abbreviations
Symbols
CBOD
Carbonaceous Biochemical
Oxygen Demand
COD
Chemical Oxygen Demand
CW
Constructed wetland
DO
Dissolved Oxygen
FWS
Flow Water Surface
SFF
Subsurface Flow
TKN
Total Kjeldhal Nitrogen
TN
Total Nitrogen
Acknowledgement
This work was funded by
Taiwanese Government through the
collaboration programme ‘‘Lecturer
Exchange’’ between the Department of
Chemical Engineering, National Taiwan
University of Science and Technology,
Taipei, Taiwan and Sepuluh Nopember
Institute
of
Technology
(ITS),
Surabaya, Indonesia.
I would also like to thank
Professor Liu who always constantly
provided assistance for this work.
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