Ecological Engineering 41 (2012) 8–12
Contents lists available at SciVerse ScienceDirect
Ecological Engineering
journal homepage: www.elsevier.com/locate/ecoleng
Short communication
Vertical oxygen distribution trend and oxygen source analysis for vertical-flow
constructed wetlands treating domestic wastewater
Jianfeng Ye a,∗ , Liang Wang b , Dan Li a , Wei Han c , Chao Ye c
a
Shanghai Academy of Environmental Sciences, Shanghai 200233, China
Shanghai Advanced Research Institute, the Chinese Academy of Sciences, Shanghai 201210, China
c
College of Environment, Hohai University, Nanjing 210098, China
b
a r t i c l e
i n f o
Article history:
Received 29 June 2011
Received in revised form 9 November 2011
Accepted 10 December 2011
Available online 1 February 2012
Keywords:
Constructed wetland
Oxygen concentration
Atmospheric reoxygenation
Domestic wastewater
Vertical-flow constructed wetland (VFCW)
a b s t r a c t
To improve nitrogen removal performance in vertical-flow constructed wetlands (VFCWs) and promote
their application in domestic wastewater treatment in China, it is important to identify the reoxygenation
sources, their contribution weightings and the oxygen distribution profiles in the VFCW substrates. This
study indicated that dissolved oxygen (DO) levels rise at first and then drop in the VFCWs vertically from
top to bottom in the VFCWs treating domestic wastewater. There were other sources of oxygen supply
to the wastewater in the VFCWs, especially in the upper portion of the VFCWs. In treating domestic
wastewater, atmospheric reoxygenation is the main oxygen source for the VFCWs, contributing more
than 99.9% of the total oxygen supply. Approximately 50% of atmospheric reoxygenation is supplied to
0–10 cm below the wastewater distribution system in the VFCWs. Over 99.8% of the oxygen consumed is
used for organics degradation and nitrification. In this study, oxygen was mainly consumed in organics
degradation in the upper 0–40 cm of the VFCWs and nitrification was dominant in the lower part.
© 2012 Published by Elsevier B.V.
1. Introduction
Constructed wetland technology was developed in 1970s as an
emerging wastewater treatment technology (Kadlec et al., 2006).
This technology has been widely applied to domestic wastewater
treatment in rural areas and medium-small towns in China due
to its easy maintenance, low cost, and energy savings (Verhoeven
and Meuleman, 1999; Kivaisi, 2001; Li et al., 2009). Vertical-flow
constructed wetland (VFCW), a type of constructed wetlands, has
gained popularity in China owing to its compact size and enhanced
pollutant removal capacity (Brix and Arias, 2005).
However, unsatisfactory nitrification performance has been
reported for single-stage VFCWs usually after 1–3 years’ running
(Claudiane et al., 2006; Li et al., 2008). It has been concluded that
insufficient oxygen supply to the wetland substrate due to configuration design impairs the ammonia oxidation reactions (Yan
et al., 2006; Ke et al., 2008; Lee et al., 2009; Allen et al., 2010).
This hypothesis has not been validated by any actual experiment
data. Nevertheless, according to many studies on Radial Oxygen
Loss (ROL) and wetland plant root zoon reoxygenation sources
(Brix, 1987; Wolfgang et al., 1992; Sasikala et al., 2009), oxygen
∗ Corresponding author. Tel.: +86 21 64085119; fax: +86 21 64085119.
E-mail addresses: [email protected], [email protected] (J. Ye).
0925-8574/$ – see front matter © 2012 Published by Elsevier B.V.
doi:10.1016/j.ecoleng.2011.12.015
transportation is an active energy-consuming act of plants and the
oxygen transporting rate by ROL is not negligible (Shimp et al.,
1993; Dunbabin et al., 1998). Studies indicate that if organic pollutant concentrations of the wastewater are elevated, the oxygen
concentration will decrease substantially in constructed wetlands
without plants, but remain within the aerobic zone in wetlands
with plants. The oxygen transported by plants is able to sustain
aerobic microbes’ metabolism (Kadlec et al., 2006), which contradicts with the hypothesis that ammonia oxidation reactions are
impaired by insufficient internal oxygen supply.
In order to improve nitrogen removal performance in VFCWs
and promote their application in domestic wastewater treatment
in China, it is critical to identify the reoxygenation sources, their
contribution weightings and the tendency of oxygen distribution
in the VFCW substrates. Data gained in this research can be used to
improve the wetland configuration and system designs.
2. Materials and methods
2.1. VFCW reactors
Six identical reactors simulating VFCWs were divided into two
groups (Group A and B) in this study. Both groups are composed of
three duplicate reactors. The dimensions of the reactors are 1.5 m
(L) × 0.5 m (W) × 1.4 m (H). The reactors were filled with a coarse
sand substrate to 120 cm in height. The d10 and d50 diameters of the
substrate are 0.25 mm and 0.60 mm, respectively. The uniformity
coefficient (Cu ) of the substrate is 3.2 and the permeability coefficient is 1.27 × 10−2 cm s−1 . Wastewater distribution systems were
installed at 10 cm below the surface level. Wastewater harvesting
systems were laid at 0.5 cm above the reactor bottoms, which were
40 cm below the distribution systems. Wastewater distribution and
harvesting systems were made of serrated PVC pipes of 20 mm in
diameter with holes for wastewater distribution/harvesting every
12.5 cm along the pipes. Each reactor has six wastewater and substrate sampling points at 0 cm, 10 cm, 20 cm, 40 cm, 60 cm and
80 cm below the wastewater distribution systems. The height of
the wastewater distribution systems is set to be 0 cm in this paper
hereafter. Phragmites australis were planted on top of the substrate
in the six reactors with a density of 4 stems·m−2 .
Wetlands height / cm
J. Ye et al. / Ecological Engineering 41 (2012) 8–12
9
0
0
20
20
y=94.39-42.62x
2
R =0.86(n=6)
40
60
60
y=104.90-21.83x
2
R =0.92 (n=6)
80
3
2.2. Experiment settings and sampling method
2.3. Influent water quality and hydraulic loads
Domestic wastewater was used as influent for the VFCW reactors in this study. The influent was introduced continuously
without extra pressure. The hydraulic load was 0.5 m3 m−2 d−1
for Group A reactors, and 0.3 m3 m−2 d−1 for Group B reactors. The water quality values were measured daily in influent
water: DO is 0.27 ± 0.19 mg L−1 (range: 0.07–0.7), pH is 7.37 ± 0.24
(range: 6.9–7.9), SS is 113 ± 25 mg L−1 (range: 64–192), COD is
225 ± 72 mg L−1 (range: 133–310), TP is 1.80 ± 0.49 mg L−1 (range:
1.2–2.7), NH4 + -N is 25.0 ± 6.63 mg L−1 (range:13.0–39.7).
80
-2
-1
0.5m .m .d
3
-2 -1
0.3m .m .d
100
The experiment was conducted from June to November 2009.
The VFCW reactors were placed outdoor where temperature
ranged from 24.0 to 37.3 ◦ C, and sunlight intensity was about
1480–4450 lx. The reactors were run for one month to stable status
before wastewater and substrate samples were taken. Wastewater samples were collected via vacuumed hoses connected to
wastewater sampling points. Substrate samples were taken at
each sampling points by using a Luoyang shovel, which should be
inserted into the reactors at a 45 cm distance. Each sample should
be mixed up completely.
40
100
120
120
0
1
2
3
4
5
6
DO levels in wastewater/mg.L-1
Fig. 1. DO distribution in wastewater in VFCW reactors.
spectrophotometer(Shimadzu). The oxygen supply volumes can
be determined by the difference between the ␣-naphthylamine
concentrations of the two sample batches excluding the values of
blank duplicates (Colmer, 2003). The biofilm mass was tested by
an ultrasonic and chemical stripping method (Ye et al., 2008). The
weight loss during the ignition is considered as the volatile biofilm
mass. Biological oxygen demand (BOD) concentrations were tested
by a multiple dilution method. Ammonia nitrogen concentrations
were tested using the Nesslers reagent colorimetric method. Nitrite
nitrogen levels were tested by spectrophotometry. Nitrate nitrogen levels were tested by ultraviolet spectrophotometry. DO levels
were tested with a HACH Portable DO Meter. All samples were analyzed according to applicable Chinese national standards (MEPC,
2002).
3. Results and discussion
2.4. Test methods
3.1. Oxygen distribution and transformation
Test of radial oxygen loss from reed root was performed as follows: after analyzing the DO in wastewater samples, the intact reed
plants were taken out and cleaned with deionized water. Residual
deionized water on the reeds was removed with blotting paper.
Each reed plant was dipped in a separate sealed cylinder container
with a mixture of 2.0 L of 40 mg L−1 ␣-naphthylamine solution
and 2.0 L of 0.1 M phosphoric acid buffer solution. The containers were shaken and left stationary to eliminate the influence of
DO in the solution and the air in the containers. After being kept
stationary for 5 min, 2 mL of the solution was taken out from each
container and put into individual 25 mL volumetric flasks (batch 1).
The cylinder containers were then sealed with silicon, kept indoor
with temperature of 23–28 ◦ C, and shaken every hour. After 5 h,
2 mL of the solution was taken from each of the containers and
transferred to separate 25 mL volumetric flasks (batch 2). 10 mL
of deionized water, 1 mL of 1% aminobenzene sulfonic acid solution and 1 mL of 100 mg L−1 sodium nitrite solution was added into
each of the volumetric flasks for both sample batches. The volumetric flasks were then shaken for 5 min for chromo generation
of the mixed solutions and filled with deionized water. Within
20–60 min, the ␣-naphthylamine concentrations in the solutions
in all the flasks were tested with a 510 nm band using UV-1700
3.1.1. Vertical oxygen distribution in wastewater
Curves of wastewater DO levels in VFCW reactors with different
hydraulic loads are shown in Fig. 2. As shown in Fig. 1, vertically
from top to bottom, DO levels in the wastewater increased from
0 to 10 cm below the wastewater distribution systems and then
decrease further below. With various hydraulic loads in this study,
the DO levels in wastewater at 10 cm below the wastewater distribution systems were tested to be 18 times and 33 times of the
ones in the wastewater influents. However, from 10 cm below the
wastewater distribution systems to further below, the DO levels in
wastewater decreased gradually, with Group B having a rate 1.95
times that of Group A’s.
The biofilm mass and volatile biofilm mass were analyzed and
the results are illustrated in Fig. 2. Based on the above analysis,
it was found that from 10 cm below the wastewater distribution systems, DO levels in wastewater have positive correlations
to biofilm mass at the same height of the reactors. The correlation coefficient of DO levels in wastewater to biofilm mass for
Group A and Group B is 0.899* (p < 0.015) and 0.941* (p < 0.005)
respectively. The correlation coefficient of DO levels in wastewater to volatile biofilm mass for Group A and Group B is 0.709
10
J. Ye et al. / Ecological Engineering 41 (2012) 8–12
Wetlands of group B
Wetlands height (cm)
Wetlands of group A
0
0
0
0
20
20
20
20
40
40
40
40
60
60
60
60
80
80
80
80
100
100
100
100
120
120
0
40
80 120 160
120
120
0
40
80
-4
120 160
-1
Biological membrane quantity (10 mg.mLsubstrate)
0
20
40
60
0
20
40
-4
60
-1
Volatility biological membrane quantity (10 mg.mLsubstrate)
Fig. 2. Vertical distribution of the biofilm mass and volatile biofilm mass.
and 0.691, respectively. Therefore, it is concluded that beside the
oxygen contained in the wastewater influent stream, there are
other oxygen sources present (atmospheric reoxygenaration and
ROL from plant roots), especially in the top section of the reactors.
3.1.2. Vertical distribution of oxygen transformation in
wastewater
Except for denitrification processes, the organic pollutants in
wastewater are mostly degraded by aerobic microbes. The oxygen
demands can be calculated using the following formula (Zhang,
2000):
O2 = a BODr + b P
(1)
where a
– oxygen demand for degrading every kilo of BOD5 , which
valued 1.46; BODr – decreased BOD5 value in constructed wetlands; b – oxygen demand for sustain every kilo of active biofilm,
which is 0.18 kg kg−1 ; P – active biofilm mass in every cubic meter
of wetland substrate (kg m−3 ).
Based on the biofilm mass analysis results in Fig. 2, the average oxygen demands for degrading organic pollutants in the two
groups of VFCW reactors are calculated and listed in Table 1. The
total oxygen demand for degrading organic pollutants for Group
A reactors and Group B reactors is 77.328 g d−1 and 52.706 g d−1 ,
respectively.
Removal of ammonia nitrogen is realized via nitrification. The
oxygen demands for nitrification in the VFCW reactors are calculated and the results are summarized in Table 1. The average oxygen
demand for nitrification for Group A and Group B was calculated to
be 39.429 g d−1 and 58.844 g d−1 , respectively.
3.1.3. Analysis of vertical distribution of oxygen transformation
DO in the wastewater is mainly used in degradation of organic
pollutants and nitrification. The transformed oxygen amount is
illustrated in Fig. 3. As shown in Fig. 3, oxygen demand is higher
within 10 cm below the wastewater distribution systems than
that of further below. The daily oxygen demand in Group A and
Group B reactors within top layers was 56.332 g and 61.342 g,
respectively, which accounted for 47.37% and 52.96% of their total
oxygen demand respectively. Considering the vertical distribution profiles of oxygen concentration, oxygen is mostly consumed
for organic pollutant degradation in upper portions of the VFCW
reactors, and used mainly for nitrification in the lower portions.
Oxygen consumed for organic pollutant degradation within the
40 cm below the wastewater distribution systems in Group A and
Group B reactors accounted for 75.28% and 55.92%, respectively,
of the total oxygen demand in the upper sections of the reactors, while nitrification oxygen demand accounted for 63.55% and
72.99%, respectively, of the total oxygen demand in the lower parts
of Group A and Group B reactors. The percentage is correlated with
the hydraulic loads of the wastewater influents.
3.2. Calculations of oxygen supply in VFCWs
There are three oxygen supply sources for VFCWs: (1) DO in the
wastewater influent, (2) ROL from the plant roots and (3) atmospheric reoxygenation. Oxygen losses are due to organic pollutant
degradation, ammonia nitrification and DO in the wastewater effluent. According to law of mass conservation, the oxygen supply and
consumption can be expressed with the following formula:
Oself-in + Oplant + Oair = Oself-out + Oorg + Onitri
(2)
where Oself-in – DO in influents, g d−1 ; Oplant – ROL from plant roots,
g d−1 ; Oair – atmospheric reoxygenation, g d−1 ; Oself-out – DO in
wastewater effluents, g d−1 ; Oorg – oxygen consumed in organic
pollutant degradation, g d−1 ; Onitri – oxygen consumed in ammonia
nitrogen nitrification, g d−1 .
DO in Group A and Group B reactor effluents were
0.21 ± 0.02 mg L−1 and 0.47 ± 0.07 mg L−1 , respectively. Influent DO
was approximately 0.27 mg L−1 . Therefore, the wastewater brought
J. Ye et al. / Ecological Engineering 41 (2012) 8–12
11
Table 1
Vertical distribution of average oxygen demand for organic pollutant removal and nitrification oxygen consumption.
Depth in Group A reactors
the
Substrate
(cm)
Group B reactors
BOD5 a
(mg L−1 )
P (kg m−3 )
NO2 − -N NO3 − -N Oxygen
demand for
(mg L−1 )
(mg L−1 )
organic
Pollutant
removal
(g d−1 )
Nitrification BOD5 a
oxygen
(mg L−1 )
consumption
(g d−1 )
P (kg m−3 )
NO2 − -N NO3 − -N Oxygen
demand for
(mg L−1 )
(mg L−1 )
organic
pollutant
Removal
(g d−1 )
Nitrification
oxygen
consumption
(g d−1 )
0–10
10–20
20–40
40–60
60–80
80–110
102
0
20
11
7
1
0.00140
0.00235
0.00225
0.00065
0
0
0.028
0.120
0.258
0
0.010
0.026
0
4.4
12.3
0
8.0
7.8
55.864
0.032
11.011
6.040
3.833
0.548
0.012
5.363
14.960
0.000
9.665
9.430
103
25
2
25
2
0
0.00255
0.00095
0.00085
0.0014
0.0009
0
0.469
0
0
0.040
0
0.160
21.9
0
2.9
8.5
0
15.2
33.870
8.225
0.680
8.250
0.681
0
26.639
0.000
3.503
10.281
0.000
18.421
Total
141
–
0.442
32.5
77.328
39.429
157
–
0.669
48.5
51.706
58.844
a
Experiment results exclusive of the carbon consumed in denitrification processes.
0-10
Wetlands height(cm)
Wetlands height(cm)
0-10
10-20
Wetlands of group A
20-40
40-60
Oxygen demends for nitrification
Oxygen demends for organic degradation
60-80
80-110
10-20
Wetlands of group B
20-40
40-60
Oxygen demends for nitrification
Oxygen demends for organic degradation
60-80
80-110
0
10
20
30
40
50
60
0
10
Oxygen demends(g.d-1)
20
30
40
50
60
70
Oxygen demends(g.d-1)
Fig. 3. Vertical distribution of oxygen transformation in VFCW reactors.
in 0.10 g d−1 and 0.06 g d−1 , respectively, of oxygen to Group A and
Group B reactors and took out 0.079 g d−1 and 0.18 g d−1 , respectively, of oxygen from Group A and Group B reactor.
ROL test results showed that the average total ROL from reed
roots in Group A and Group B reactors was 11.6 ± 0.0029 mg d−1
and 10.5 ± 0.0024 mg d−1 , respectively. If all the ROL dissolved in
the wastewater, the ROL supplied a maximum of 0.0116 g d−1 and
0.0105 g d−1 of oxygen to the Group A and Group B reactors.
Based on Formula 2, with the hydraulic load being
0.5 m3 m−2 d−1 and 0.3 m3 m−2 d−1 , the total oxygen demand
for Group A and Group B VFCW reactors were 116.836 g d−1 and
111.730 g d−1 , respectively. The contribution of ROL, influent DO
and atmospheric reoxygenation can be calculated by backstepping
from Formula 2. Atmospheric reoxygenation accounted for over
99.90% and 99.94% of the total oxygen supply for the Group A and
Group B reactors. Therefore, the predominant oxygen source for
VFCWs is atmospheric reoxygenation.
4. Conclusion
Vertically from top to bottom, wastewater DO levels rise at first
and then drop in the VFCWs treating domestic wastewater. DO levels in the VFCWs’ wastewater are positively correlated with the
biofilm mass, which indicates there are other sources of oxygen
supply to the wastewater beside dissolved oxygen in the influent,
especially in the upper portion. This phenomenon is consistent
with the hypothesis that atmospheric reoxygenation is the main
oxygen source for the VFCWs. When treating domestic wastewater, atmospheric reoxygenation contributes more than 99.9% of the
total oxygen supply to the VFCWs. Approximately 50% of atmospheric reoxygenation is supplied to the upper portion of the
VFCWs, which is at from 0 to 10 cm below the wastewater distribution system. Over 99.8% of the oxygen consumed is used for
organics degradation and nitrification. In this study, oxygen was
mainly consumed in organics degradation in the upper 0–40 cm
of the VFCWs and nitrification was dominant in the lower part
(40–110 cm).
Acknowledgements
The authors would like to acknowledge the financial support
from the National Natural Science Foundation of China (Grant No.
51108266), the Natural Science Foundation of Shanghai (Grant No.
11ZR1430800 and 08ZR1416400) and Open Foundation of State
Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering in China (Grant No. 2010490711).
12
J. Ye et al. / Ecological Engineering 41 (2012) 8–12
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