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Design and performance of a coarse media, high hydraulic
load polishing wetland for steel industry wastewater
Viet Anh Nguyen , Minh Phuong Nguyen, Karin Tonderski ,
Hai Do Thi and Anh Thi Kim Bui
ABSTRACT
This paper presents the design of a constructed wetland (CW) system in an area with limited land
availability, resulting in high hydraulic loads. The CW was constructed to act as a buffering/ polishing
step after stabilization ponds for steel industry wastewater post-treatment. A pilot test with two
different filter media (50–100 mm vs 40–60 mm diameter) indicated that a flow rate increase from
49.5 m
3
/h to 122.4 m
3
/h would lead to a head loss increase from 2.9 cm to 8.7 cm, and more than
double that for the finer gravel. This was substantially higher than the calculated theoretical values,
though the relation with flow rate was similar. Four full scale wetland cells (CW1, CW2, CW3 and
CW4) were constructed using the coarser gravel. A design value of total head loss of 1.01 m over the
total system length, with a design flow of 36,000 m
3
/day, was expected based on pilot test results.
During the first operation year (September 2017 to July 2018), the pond-CW system has received
wastewater already meeting required discharge standards. The effluent from the CWs had
consistently lower concentrations of all measured variables, and met the predicted values for
biochemical oxygen demand (BOD
5
), total nitrogen (TN) and NH
4
þ
-N. Highest removal efficiencies
were achieved for NH
4
þ
-N (>90%), Mn (>60%) and Fe (45%) with removal efficiencies for TN (14%),
BOD
5
and chemical oxygen demand (COD) (around 30%). Concentrations of phenol, CN
and Cr
6þ
were below 10, 4 and 3 μg/l, respectively, in in- and outflows. An appreciated benefit of the wetland
was the ‘green element’in the industrial landscape.
Viet Anh Nguyen (corresponding author)
Institute of Environmental Science and Engineering
(IESE),
Hanoi University of Civil Engineering (HUCE),
55 Giai phong Rd, Hanoi,
Vietnam
E-mail: vietanhctn@gmail.com
Minh Phuong Nguyen
Department of Environmental Technology,
Faculty of Environmental Sciences,
VNU University of Sciences,
334 Nguyen Trai, Thanh Xuan, Hanoi,
Vietnam
Karin Tonderski
Department of Physics, Chemistry and Biology,
Linköping University,
Sweden
Hai Do Thi
Hanoi University of Mining and Geology (HUMG),
No. 18, Pho Vien Street, Duc Thang Ward, Bac Tu
Liem District, Hanoi,
Vietnam
Anh Thi Kim Bui
Institute of Environmental Technology (IET),
Vietnam Academy of Science and Technology,
18 Hoang Quoc Viet, Hanoi,
Vietnam
Key words |filter media, free water surface, head loss estimate, horizontal subsurface flow
INTRODUCTION
Wastewater from the steel industry often contains organics,
colour, and toxicants such as phenol, cyanides and sulphur
compounds. Constructed wetlands (CWs) have been
shown to be a cost-effective and environmentally friendly
technology for pollutant removal from various types of
wastewater, including from the steel industry (Xu et al.
;Yang & Hu ;Vymazal ;Huang et al. ).
In a laboratory scale vertical flow CW with manganese ore
substrate, Xu et al. ()reported that about 98% of iron
and manganese concentrations in steel wastewater were
removed, and attributed this to the presence of iron and
manganese oxidation bacteria. The concentration reduction
of chemical oxygen demand (COD), turbidity, N-NH
4
þ
and
total phosphorus were 55%, 90%, 67% and 93%, respectively.
Similar rates were observed in a pilot-scale wetland with
gravel mixed with only 4% manganese ore, providing further
support for the role of biological oxidation. In another study,
the mean reduction rates were 77% for COD and N-NH
4
þ
,
96% for turbidity and iron and 92% for manganese in a
pilot scale subsurface gravel/manganese ore CW treating
wastewater from a steel industry (Huang et al. ).
Although several studies of laboratory scale or pilot
scale CWs treating steel industry wastewater show promis-
ing results, there are few, if any, examples of full scale
application of the technology. This paper presents the initial
experiences from a hectares scale constructed wetland treat-
ing wastewater from a steel industry in Vietnam.
The narrative was initiated by an emergency discharge of
poorly treated steel industry wastewater and pipe cleaning
solutions that resulted in a marine life disaster in central Viet-
nam in 2016. This prompted the Vietnamese government to
request the Formosa Steel Corporation to improve their
59 © IWA Publishing 2019 Water Science & Technology |80.1 |2019
doi: 10.2166/wst.2019.244
wastewater treatment with enhanced physico-chemical and
biological processes, and to divert the treated flow
(1,500 m
3
per hour) through a buffering pond system before
discharge to the ocean. In response to this, it was decided
to construct a pond and wetland system on an approximately
10 ha area. Besides serving as a polishing and bio-indicator
step in the treatment system, the wetland cells should also
serve to prevent wash out of algae to the ocean, and create
a green element in the factory landscape. It was decided to
build a combined pond and wetland system, consisting of
two emergency tanks and three stabilization ponds in series
followed by subsurface and free water surface CWs.
For subsurface flow wetlands, the substrate composition
is an essential component, and the substrate also makes up a
substantial part of the total construction costs. The trade-off
between achieving a high removal of particles and efficient
sorption processes, and the desire to minimize the use of fil-
tering substrate, causes a dilemma in the design phase. The
available area at FHS company also posed a challenge with
respect to the resulting high hydraulic load on the wetland.
To address this, a pilot experiment was set up to select
appropriate, locally available media, assess the head loss
to be expected depending on choice of media material and
to determine the design parameters for the full scale subsur-
face flow (SSF) wetland cells at FHS. Full scale construction
of the wetland cells (CW1, CW2, CW3 and CW4) was
initiated in March 2017 and they have been in operation
since August 2017. This paper presents the results from
the pilot experiments, the design calculations and the first
treatment results from the full scale wetland system.
MATERIAL AND METHODS
Pilot experiments
Two parallel pilot CW cells were designed to study the
hydraulic conductivity and assess the head loss for two
filter media with different grain sizes, aiming at gaining
data for design of the full scale wetland. The CW cell dimen-
sions were: length ×(width on top; width in bottom) ×
depth ¼31.5 ×(4.5; 4) ×1.0 m. Each cell was filled with
1 m of one of the two filter media to allow for maximum
root system depth development (Grace ;Davis ;
Borst et al. ). Cell A was filled with 10 cm of coarse
sand at the bottom for protection of the HDPE liner;
60 cm of crushed limestone (50–100 mm) as the main filter
media; 10 cm of peanut gravel (5–10 mm) for preventing
top sand from entering the main media layer; and a top
20 cm of coarse sand for plant establishment (Figure 1).
Cell B was filled identically except that the 60 cm crushed
local limestone layer had a grain size of 40–60 mm.
Figure 1 |Filter media layers in cell A and cell B of the pilot CW at Formosa Steel Company, Vietnam (not to scale).
60 V. A. Nguyen et al. |Coarse media polishing wetland for steel industry wastewater Water Science & Technology |80.1 |2019
The same hydraulic load (flow rate) was set for both cells;
that is, the expected rate for the full scale wetland. A design
flow of 36,000 m
3
/day and a first horizontal flow wetland
cell with an inflow width of 74 m and a 0.6 m effective
depth of filter media would result in a horizontal flow velocity
of 33.8 m/h. Based on this, the flow rate to the pilot CW cells
was regulated at various rates between 50 and 120 m
3
/h,
using valves on the discharge pipe of the feeding pumps. In
addition, the CW cells outflow, Q
out
(m
3
/h), was measured
as the accumulated volume of water V (m
3
)intheoutflow
well over a certain operation time T (h): Q
out
¼V/T. The
observed head losses at different flow rates were compared
with theoretical head loss values, calculated using the
Carmen–Kozeny formula (Equation (1); Quasim et al. ).
hL¼f
F
1e
e3
L
d
v2
g(1)
f¼friction factor; f ¼150*(1-e)/N
R
þ1.75.
N
R
¼Reynolds number; N
R
¼d*ν*ρ
w
/m.
ν¼kinematic viscosity (¼0.028 m
2
/s).
ρ
w
¼density of water (¼1,000 kg/m
3
).
m¼absolute viscosity (¼1.518*10
3
N-s/m
2
).
F¼particle shape factor (usually 0.85 to 1.0).
e¼porosity ratio, defined by experiment, measured volume-
trically in the laboratory.
L¼length of filtration module (m).
d¼media grain diameter, m.
v¼filtration velocity (m/s).
g¼acceleration due to gravity (¼9.81 m/s
2
).
For comparative purposes, the equation was applied
only to the 60 cm layer with limestone, ignoring the influ-
ence of the bottom layer with coarse sand, assuming a
mean value for d ¼0.075 m for the A cell and d ¼0.05 m
for the B cell. The measured porosity (e) was 0.45 for the
coarse material (cell A) and 0.4 for the material in cell B.
Full scale wetland design
The full scale constructed wetland system consisted of 4
cells in series, with a total wet area of 4.32 ha, preceded
by open ponds. The entire system of ponds and wetlands
was dimensioned based on literature data; for common vari-
ables like biochemical oxygen demand (BOD
5
), total
nitrogen (TN) and NH
4
þ
-N, the Tanks-in-Series approach
was used (Equation (2); Kadlec & Wallace ).
Cout C¼Cin C
(1 þk=N×q)N (2)
where N ¼number of tanks in series, q ¼hydraulic load
(m/yr), C* ¼background concentration of the variable con-
cerned, and C
out
and C
in
are concentrations in the outflow
and inflow. CW1, CW2 and CW3 were subsurface horizon-
tal flow CWs, planted with a mixture (multi-culture) of
Phragmites australis Cav., Typha angustifolia L. and
Cyperus tegetiformis. The final cell, CW4, was a free water
surface wetland planted with P. australis Cav. and
T. angustifolia L. P. australis (Cav.), T. angustifolia L. and
C. tegetiformis were chosen because they are most tolerant
to the local environment conditions, and the plants could
be found locally. For design purposes, a conservative N-
value of 3 was used for the entire system (to represent free
water surface wetlands rather than subsurface flow).
Water sampling and analytical methods
From September 2017 to July 2018, water samples were
taken weekly in the influent and effluent of each wetland
cell (2 samples per site [morning and afternoon] ×5
sampling sites ×40 weeks) and water quality analysed fol-
lowing standard methods (APHA AWWA WEF ). pH
was measured according to method 9040C, colour was
measured by the platinum–cobalt method at 455 nm
(method 110.2), COD was measured by the potassium dichro-
mate colorimetric method (method 410.1), BOD
5
was
measured according to method 405.1, Total suspended
solids (TSS) were measured gravimetrically (method 160.2)
and Cr
6þ
was measured by ion chromatography (method
218.7). Mn, Fe, Cd, Hg were measured by atomic absorption
spectrometry (method 7000B), TN, NH
4
þ
-N and cyanide was
measured by semiautomated colorimetry (method 351.2, 350
and 335.4) and phenol was measured by flame ionization
detector gas chromatography (method 604). Total grease
and fat was measured by hexane extractable gravimetry
according to method 1664 (APHA AWWA WEF ).
RESULTS AND DISCUSSION
Determining hydraulic design parameters for the full
scale wetland cells
The results from the pilot experiments showed significant
differences in head loss between cell A and B. In cell B,
with smaller grain sizes, the head loss increased from
6.6 cm to 16.3 cm when the flow rate was changed from
49.2 m
3
/h to 82.7 m
3
/h. In cell A, a flow rate increase
from 49.5 m
3
/h to 122.4 m
3
/h caused an increase in head
61 V. A. Nguyen et al. |Coarse media polishing wetland for steel industry wastewater Water Science & Technology |80.1 |2019
loss from 2.9 cm to 8.7 cm. The measured head losses
exceeded the theoretical calculations using the Carmen–
Kozeny formula but the relation with flow rate was similar
(Figure 2), suggesting that the experiments gave reliable
results regarding this aspect. Several assumptions in theor-
etical models of flows through packed beds can often not
be met in real soil filtering systems, which has implications
for the system design, as discussed by, for example, Schöpke
(). One important reason that the measured head loss
was higher than the theoretical in the current study was
most probably that the filter media was not washed (i.e.
contained finer particles), and was therefore neither homo-
geneous nor consolidated. This was important to consider
when selecting and filling the media into the wetland cells.
If a well sieved, uniform filter media cannot be used, a
larger ‘safety factor’for head loss increase must be
accounted for in the design. In this case, the pilot exper-
iments results indicated that the unit head loss in cell A
was 22.8 cm/100 m. Translating this to a full scale wetland
with a total flow length of 450 m, the head loss over the
system could reach 1.01 m at the maximum design flow of
36,000 m
3
/day. This was an important factor when deter-
mining the elevation of the four full scale wetland cells.
In cell B, the unit head loss was 51.8 cm/100 m at a flow
rate as low as 82.7 m
3
/h, translating to 2.33 m over a 450 m
long wetland system. Hence, with the high hydraulic design
load for the FHS CW system, this filter media (diameter
40–60 mm) was deemed unsuitable. A too high head loss
would lead to the need for a significantly increased elevation
of the first ponds with substantially more earth work and an
increase in the pump head. An alternative solution could
be to decrease the flow velocity through the wetland cross
sections, but this could only be done through an increase
in the media depth or wetland cell area. Besides the possibly
negative effect on plant growth and the biogeochemical
conditions in the cells (more anaerobic conditions), this
would increase the total media volume and/or required wet-
land area and lead to an increase in project costs.
Treatment performance of the wetland cells
During the first year of operation, the effluent from the
wastewater treatment system preceding the wetland system
had a neutral pH, with little variation, and concentrations
of potentially toxic compounds were, for several substances
below the detection limit (Table 1). This effluent quality
met the Vietnamese standards for industrial wastewater dis-
charge to both inland waters (class A) and the sea (class B).
Hence, the wetland system functioned as a polishing step
during this monitoring period. As predicted, the hydraulic
load was high, resulting in relatively high load of substances
despite the low inflow concentrations. Concentrations were
further reduced in the wetland system leading to very low
levels of critical variables, such as NH
4
þ
-N, Fe and Mn, in
the outflow from the system (Table 2). Concentrations of
phenol, CN
and Cr
6þ
were below the respective detection
limits (10, 4 and 3 μg/l) also in the outflow from the system
(values not shown).
The concentration reductions achieved agreed rela-
tively well with the design model (Equation (2)) for
BOD
5
and TN. There was a tendency to improved per-
formance in the second period, when the observed
concentrations of both BOD
5
and TN were lower than
the predicted (Table 3). No effort was made to account
for the possible influence of the approximately 20% lower
hydraulic load on the removal rate constant in period 2,
though a positive relation between hydraulic load and
k-values has been observed when applying first order rate
models to monitoring data (Kadlec ). Regarding
NH
4
þ
-N, the wetland system performed better than pre-
dicted with the model (Table 3), and this is probably a
result of the initial influence of plant uptake. The k
20
-
value used in the design model was conservatively set to
the lower 0.2 percentile of the indicated range for lightly
loaded ‘agronomic’wetlands, i.e. wetlands with an annual
NH
4
-N load <0.33 g/(m
2*
d) (Kadlec & Wallace ) simi-
lar to the load to the FHS wetland. If instead setting the
k
20
-value to 0.55 m/d (above the 0.8 percentile), the pre-
dicted outflow concentrations would have been 0.23 and
0.1 mg/l for period 1 and 2, respectively; this is still
higher than the observed values. As the plants were in
the establishment phase, they would have competed with
nitrifying and heterotrophic bacteria for available nitrogen,
resulting in the observed high NH
4
þ
-N removal efficiency
Figure 2 |Head loss versus flow velocity values, experimental and theoretical values in
pilot wetland cell A and cell B at the Formosa Steel Company, Vietnam.
62 V. A. Nguyen et al. |Coarse media polishing wetland for steel industry wastewater Water Science & Technology |80.1 |2019
(>90%; Figure 3). Those results corroborated well with pre-
vious studies on NH
4
þ
-N treatment efficiency from various
industrial wastewaters: 84–94% (Vymazal ) and were
higher than data reported by Xu et al. ()and Huang
et al. ()studying wetlands treating steel wastewater.
In agreement with the model predictions, the reduction
of TN was the lowest of all the variables (mean 14%). It
is well known that the % removal is depending on the
inflow concentrations as a first order rate removal model
often describes the concentration reductions in CWs
(Kadlec & Wallace ). Hence, a higher removal could
have been expected in our study since the inflow contained
around 10 mg/l TN, but the hydraulic load counteracted
this as also confirmed by the predictions from the design
model. Although nitrification-denitrification processes
usually are the main nitrogen removal mechanisms in CWs
(accounting for 60–96% of TN removal) (Chen et al. ),
other authors have showed that macrophyte uptake can
Table 1 |Quality of the wastewater entering the CWs at the Formosa Steel plant, Vietnam, and concentration limits in the Vietnamese standards for industrial wastewater discharges to
different recipients (class A and B; QCVN 40:2011/BTNMT)
Variable measured Unit Mean value ±SD (min-max)
Standard for
industrial
wastewater, class A
Standard for
industrial
wastewater, class B
pH –7.3 ±0.18 (6.8–7.8) 6–9 5.5–9
Color Pt/Co 27.8 ±4.9 (20–39) 50 150
COD mg/l 23.1 ±2.6 (14–30) 75 150
BOD
5
mg/l 9.4 ±2.3 (5 –15.8) 30 50
TSS mg/l 17.4 ±2.1 (14–26) 50 100
Mn mg/l 0.43 ±0.12 (0.08 –0.75) 0.5 1
Fe mg/l 0.33 ±0.18 (0.01 –0.86) 1 5
TN mg/l 10.5 ±2.8 (6.1 –19.2) 20 40
NH
4
þ
-N mg/l 0.57 ±0.19 (0.04 –2.1) 5 10
Cr (VI) mg/l <0.003 0.05 0.1
Cd mg/l <0.0007 0.05 0.1
Hg mg/l <0.0003 0.005 0.01
Phenol mg/l <0.01 0.1 0.5
CN
mg/l <0.004 0.07 0.1
Grease and fats mg/l <0.3 5 10
Arithmetic means ±SD for the period September 2017–July 2018.
Table 2 |Mean (n¼80 per sampling site) water quality of the effluent from wetland cells CW1, 2, 3, 4 at Formosa Steel work, Vietnam, for the period September 2017 to July 2018.
Arithmetic mean ±SD (min-max) are shown and units are as in Table 1
Variables measured
Wetland cells
CW1 CW2 CW3 CW4
Color 20.6 ±4.5 (18–30) 19.6 ±2.8 (17–28) 17.9 ±2.9 (14–25) 16.7 ±2.6 (12–21)
COD 20.2 ±2.9 (15–29) 19.9 ±2.0 (14–25) 17.4 ±3.0 (11–23) 16.7 ±3.3 (10–24)
BOD
5
7.7 ±1.6 (6.0–11) 7.8 ±1.1 (5.6–10.1) 6.7 ±2.0 (4.9–9.1) 6.5 ±0.9 (4.2–9.3)
TSS 15 ±1.5 (10–22) 14.4 ±2.5 (8–20) 13.0 ±2.5 (5–17) 13.4 ±2.4 (7–18)
Mn 0.27 ±0.06 (0.22–0.61) 0.22 ±0.05 (0.1–0.32) 0.17 ±0.06 (0.03–0.41) 0.16 ±0.08 (0.03–0.55)
Fe 0.22 ±0.03 (0.06–0.49) 0.25 ±0.16 (0.04–0.56) 0.18 ±0.09 (0.03–0.71) 0.17 ±0.10 (0.02–0.7)
TN 9.2 ±1.5 (9.2–17) 9.5 ±2.4 (8.3–16) 9.2 ±2.1 (6.5–14.9) 8.9 ±2.0 (6.1–15.8)
NH
4
þ
-N 0.29 ±0.08 (0.13–0.67) 0.15 ±0.03 (0.09–0.42) 0.08 ±0.02 (0.02–0.69) 0.04 ±0.01 (0.01–0.14)
63 V. A. Nguyen et al. |Coarse media polishing wetland for steel industry wastewater Water Science & Technology |80.1 |2019
account for a considerable TN reduction (14–52%) in wet-
lands receiving low loads of N (Wu et al. ). Uptake in
P. australis and T. orientalis accounted for about 27 and
40% of TN removal in that study. In a systematic review of
CWs receiving either non-point source water or secondary
or tertiary treated wastewater, Land et al. ()found that
the average removal efficiency was 39%. However, in a
study by Bulc et al. (), TN removal efficiency in CWs
treating textile wastewater was also only 5%, suggesting that
the nitrogen was bound in recalcitrant compounds. This
may also be the case in the FSH CW and, in that case, the
plant uptake would play little role in the efficiency, though
it was likely a significant process for the NH
4
þ
-N removal.
However, when comparing the predicted outflow concen-
trations with those observed, it appeared that the TN
removal improved as the plant and microbial community
developed over time. Future monitoring data will shed
more light on the N removal in this CW system.
Regarding the variables BOD
5
and COD, the values in
the inflow were close to what sometimes is considered back-
ground values for wastewater treatment wetlands receiving
raw wastewater. However, for tertiary wastewater, Kadlec
& Wallace ()suggested a C* (background) value of
1 mg/l in horizontal subsurface flow wetlands. The results
from the first months of the FSH wetland system suggest
that the wetland system was performing relatively well in
view of the high hydraulic load (Table 3 and Figure 3). Earlier
findings on COD removal rates by CWs treating steel waste-
waters ranged from 31 to 77% (Xu et al. ;Huang et al.
). Baughman et al. ()investigated the performance
of CWs treating textile wastewater and found a COD removal
efficiency ranging from 20 to 34%. The mean BOD
5
/COD
ratio in the inflow to the FHS wetland system was 0.41,
suggesting that the organic matter is not easily biodegradable.
This could mean that a longer retention time than 1 day
would be required for a substantial decomposition, or alterna-
tively a finer media to allow for more filtration and sorption
processes (Bulc et al. ). As discussed in the design section
above, this was not a viable option for the FHS wetland
system. These results correlated well with COD reduction
rates recorded in other studies of steel wastewater and, there-
fore, it is plausible that the BOD
5
removal efficiencies may
remain at the observed level.
Based on concentration reductions, the highest
removal efficiencies were achieved for Mn (>60%) and
Fe (>40%) (Figure 3). The observed Mn removal rate was
consistent with the average Mn removal rate by wetland
treatment systems previously reported by Lesley et al.
()(64.5%) despite the higher inflow concentrations in
that study –1.5 mg/l versus 0.43 mg/l. On the other
hand, in other studies, Xu et al. ()and Huang et al.
()found that treatment efficiencies of Mn by CWs treat-
ing wastewater from steel enterprises reached 92.5–95%
when influent concentrations were in the range of 0.53–
2.23 mg/l. However, in those studies, the hydraulic loads
have been lower with 1.5–2 days retention time compared
to <1 day in the present study. Regarding Fe removal, the
efficiency was 1.5 times lower than the results recorded by
Figure 3 |Removal efficiencies (concentration reductions) of constructed wetland cells
CW 1–4 at the Formosa Steel plant, Vietnam, during September 2017–July
2018.
Table 3 |Hydraulic load, observed inflow and outflow concentrations and outflow concentrations modelled using the Tanks-in-Series model (P¼3, Θ¼1.08 for temp adjustment of k
20
)
Variable Time period
Hydraulic
load m/d
Inflow
conc. mg/l
Load
g/(m
2
*d)
Remov.
rate, km/d C* mg/l
Predicted outflow
conc mg/l
Observed outflow
conc mg/l
BOD
5
Sept-Dec-17 0.58 7.5 4.3 0.235
a
1
a
5.4 6.6
Feb-Jul-18 0.47 11.4 5.3 0.235
a
1
a
7.5 6.4
TN Sept-Dec-17 0.58 10.5 6.1 0.089
b
2 9.3 9.4
Feb-Jul-18 0.47 10.4 4.9 0.089
b
2 9.0 8.5
NH
4
þ
-N Sept-Dec-17 0.58 0.72 0.42 0.056
b
0 0.66 0.06
Feb-Jul-18 0.47 0.40 0.18 0.056
b
0 0.35 0.03
a
For inflow concentrations 3–30 mg/l, 50% percentile, no temp adjustment of k(Kadlec & Wallace 2009).
b
Adjusted k-value for 25 C.
64 V. A. Nguyen et al. |Coarse media polishing wetland for steel industry wastewater Water Science & Technology |80.1 |2019
Lesley et al. (), 48% as opposed to 70%. This could be
due to the 10 times lower Fe concentrations in the influent
in the FHS wetland system. Sedimentation, filtration and
adsorption by surfaces could be important physical pro-
cesses, which helps to decrease metal concentrations in
CWs (Garcia et al. ). Metal remobilization can be
favoured by oxidation in the rhizosphere due to oxygen
released from roots of wetland plants (Garcia et al. ).
The formation of iron plaque on the root surface of P. aus-
tralis has been previously found in full scale HSSF CWs
(Mantovi et al. ). The significance of metal uptake by
plants is still a matter of debate. While several authors con-
cluded that wetland plants play a significant role in metal
uptake (Khan et al. ;Maine et al. ), other authors
reported that plant uptake is negligible for total metal
removal (Stottmeister et al. ). Some of that discrepancy
can be related to load differences; plants have a limited
uptake capacity and this will account for a larger pro-
portion of the metal inflow in low loaded wetlands.
Colour removal is often an issue in treatment of steel
works wastewater. There are studies showing that CWs
can be effective in decolouration of some types of waste-
water (mean removal of up to 90%) (Moshiri ). This
depends strongly on the cause of the colour, and Bulc &
Ojstršek ()found that CWs planted with P. australis
removed 70 to 90% of the colour in textile wastewater,
which was substantially higher than in the FHS wetland
system (40%). Probably the short retention time and the
relative recalcitrant organic compounds counteracted an
efficient colour removal.
CWs are well known to remove TSS (Manios et al.
;Bulc & Ojstršek ), also from difficult wastewater
such as textile wastewater (Bulc & Ojstršek ). In this
study, removal efficiencies of TSS were only 23%, and
this was equivalent to 13.35 mg/l in the effluent. This is
lower than previous results on TSS removal in CWs treat-
ing petrochemical and refinery wastewaters (42–44%)
(Vymazal ). Kadlec & Wallace ()summarized
a large number of studies on gravel wetlands treating
domestic wastewater, and the results suggest that a
concentration around 5 mg/l TSS is achievable. It is poss-
ible that the TSS removal in the FHS wetland system will
improve as the plant root system develops and covers a
larger part of the filter media. This could improve the fil-
tration effect and support the development of a viable
microbial community. Filtration, followed by aerobic
microbial degradation on the surface or anaerobic degra-
dation within the media, could affect the effectiveness of
organic solids removal (Manios et al. ).
CONCLUSIONS
Experiments in pilot scale horizontal subsurface flow wetlands
receiving treated wastewater from a steel industry showed that
theoretically calculated values of head loss at different hydrau-
lic loading rates can substantially underestimate the observed
values. Hence, field testing is warranted when designing large
scale wetlands for high hydraulic loads. The study showed
that filter media of 50–100 mm diameter had an observed
unit head loss of 22.8 cm/100 m at a flow rate 122 m
3
/h,
equal to a total head loss of 1.01 m over the total length of
the planned full scale wetland system, with a design flow rate
of 36,000 m
3
/day. The discrepancy from the theoretical
values was probably explained by the heterogeneity of the
gravel, which was neither sieved nor washed. In case media
with such non-uniform size ranges must be used, a safety
factor for head loss increase should be included in the design.
During the first operational year, the four full scale CW
cells, with a mean retention time of 0.9 days, received a
wastewater that met the discharge standards. The inflow
concentrations were further reduced in the CW system,
and in the second half of the observation period they were
lower than predicted from the design model for BOD
5
and
TN, suggesting that the system improved as the plant
and microbial communities developed. Highest removal effi-
ciencies were observed for NH
4
þ
-N (>90%), Mn (>60%),
and Fe (48%). The removal of TN was lower at 15%, for
BOD
5
31% and for COD 28%, corresponding to 8.9, 6,5
and 16.7 mg/l, respectively, in the effluent. Those relatively
Figure 4 |Hybrid pond –wetland system for Formosa Steel plant wastewater polishing
treatment, Vietnam (design flow 36,000 m
3
/day, total area 10 ha). (1) Waste
stabilization pond cells; (2) subsurface flow constructed wetland cell CW1; (3)
subsurface flow constructed wetland cell CW2; (4) subsurface flow con-
structed wetland cell CW3; (5) free water surface flow constructed wetland
cell CW4.
65 V. A. Nguyen et al. |Coarse media polishing wetland for steel industry wastewater Water Science & Technology |80.1 |2019
low removal rates are explained by the high hydraulic load
and short retention time. Despite this, the removal of colour
amounted to 40%, whereas TSS was removed by 23%. The
results confirm that a hybrid pond and CW system can
serve as a polishing step for wastewater from the steel indus-
try, reducing the risk for discharge of potentially toxic
substances to the environment. A further benefitisthe
‘green element’created in the industrial landscape (Figure 4).
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