dùng thực vật trong làm sạch nước thải 2
TRANSCRIPT
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Temperature
inuence on nitrogen removal in a hybrid constructedwetland
system
in
Northern
Italy
Anna Mietto, Marco Politeo, Simone Breschigliaro, Maurizio Borin *
Department of Agronomy, Food, Natural Resources, Animals and Environment –DAFNAE, Agripolis, University of Padova, Viale Dell’ Università 16, 35020
Legnaro, Padova, Italy
A R T I C L E I N F O
Article history:
Received 7 March 2014Received in revised form 16 September 2014
Accepted 9 November 2014
Available
online
xxx
Keywords:
Vertical ow (VF)
Horizontal ow (HF)
Temperature
VF operation mode
Phragmites australis
Canna indica
A B S T R A C T
The objective of this research was to investigate the ef ciency and seasonal performance of a full-scale
hybrid constructed wetland system (HCW) in reducing total nitrogen (TN), ammonia nitrogen (NH4-N)and nitrate nitrogen (NO3-N). HCWwith a total areaof about 130m
2 and hydraulic load of 2m3/day was
composed of three subsurface ow vertical systems (VF), working in parallel and one horizontal (HF)
connected in series. The system was loaded daily with synthetic wastewater having an average
concentration of TNof 250mg/L (about125mg/L of NH4-Nand125mg/L ofNO3-N). Watersamples were
collected and analyzed from May to July 2011 and from January 2012 to July 2012. Variations were
observed in nutrient removal performance related to temperature.
During thewhole monitoringperiodmedian reduction ef ciency (RE)in the HCWwas TN95%, NH4-N
95% and NO3-N 93%, although three sub-periods characterized by different performances have been
observed.Duringthe rstperiod (fromMayto July 2011) theRE waspositive for the three nitrogen forms
considered, whereas from January to the end of March 2012 the RE was lower, particularly for TN and
NO3-N. From April 2012, when the temperature rose above 14.8C, there was an increase in the
performance that reached the 2011 values.
Internal production of NO3-N was observed, mainly in the VF systems between January and March
2012. The median removals ofmass pollutants per m2 of HCW per day were TN3.1g/m2/d, NH4-N 1.5g/
m2
/d, NO3-N 1.5g/m2
/d. Segmented regression analysis identied a breakpoint at 14.2
C forwastewatertemperature thatcaused variationsin TNand NO3-N concentrationreductionperformances.According to
this approach the abatementwasalways positivelycorrelatedwith temperature, butdifferent regression
slopes were obtained below and above the breakpoint. In particular, with lower
temperature the
abatementof NO3-N and TN increased by 1.7 and2.0%per C of temperature increase;with temperature
higher than 14.2C the increase in abatement due to increased temperaturewas sharper, especially for
NO3-N.
ã 2014 Elsevier B.V. All rights reserved.
1. Introduction
Wetland technology emerged in the 1950s and the use of
controlled wetland environments for wastewater treatment hassince been developed. The major nitrogen removal mechanism is
achieved by biological processes that convert the organic and
ammonia nitrogen to nitrate in an aerobic environment (nitrica-
tion) and then reduce the nitrate to nitrogen gas in an anoxic
environment (denitrication) (Leverenz et al., 2010). Volatiliza-
tion, absorption and plant uptake play a much less important role
in CWs (Kadlec and Wallace, 2009). The use of vertical-subsurface
ow constructed wetland (VF) systems became very popular in
Europe in the 1990s compared to the horizontal system (HF) due to
their enhanced ability to oxidize ammonia nitrogen (Stefanakisand Tsihrintzis, 2012). Single-stage CWs cannot achieve high
removal of total nitrogen because of their inability to provide both
aerobic and anaerobic conditions at the same time.
The design of hybrid constructed wetland systems (HCW)
(combination of vertical and horizontal ow systems) has been
proposed to exploit the anoxic areas within the horizontal bed for
denitrication (Cooper et al., 1999; Kadlec and Wallace, 2009;
Molle et al., 2008). HCW systems have been used to treat domestic
or municipal sewage (Brix et al., 2003; Canga et al., 2011), and more
recently for many other types of wastewater including agro-* Corresponding author. Tel.: +39 0498272838; fax: +39 0498272839.
E-mail address: [email protected] (M. Borin).
http://dx.doi.org/10.1016/j.ecoleng.2014.11.027
0925-8574/ã 2014 Elsevier B.V. All rights reserved.
Ecological Engineering 75 (2015) 291–302
Contents
lists
available
at
ScienceDirect
Ecological
Engineering
journal homepage: www.else vie r .com/locate/e coleng
mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://www.sciencedirect.com/science/journal/09258574http://www.elsevier.com/locate/ecolenghttp://www.elsevier.com/locate/ecolenghttp://www.elsevier.com/locate/ecolenghttp://www.elsevier.com/locate/ecolenghttp://www.elsevier.com/locate/ecolenghttp://www.elsevier.com/locate/ecolenghttp://www.elsevier.com/locate/ecolenghttp://www.elsevier.com/locate/ecolenghttp://www.sciencedirect.com/science/journal/09258574http://dx.doi.org/10.1016/j.ecoleng.2014.11.027http://dx.doi.org/10.1016/j.ecoleng.2014.11.027mailto:[email protected]://crossmark.dyndns.org/dialog/?doi=10.1016/j.ecoleng.2014.11.027&domain=pdf
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industrial (Comino et al., 2011), agricultural (Borin et al., 2013;
Hunt and Poach, 2001; Kantawanichkul et al., 2003) and landll
leachate (Mæhlum et al., 1999).
As reported in several previous studies (Akratos and Tsihrintzis,
2007; Kadlec and Wallace, 2009; Kotti et al., 2010; Kuschk et al.,
2003; Vymazal, 1999) temperature is one of the principal variables
that mainly inuences biological activity and so the seasonal
performances of constructed wetlands. Hill and Payton (1998)
reported that the ef ciency of treatment in a constructed wetland
decreases at low temperature primarily due to reduced biotic
activity. Kadlec and Reddy (2001) studied the temperature
dependence in surface ow wetlands on removal of contaminants.
They concluded that the performance of wetlands in treating
wastewater is seasonally cyclic and the biotic reactions are reduced
at temperatures lower than the optimal range (20 to 35 C). Kadlec
(2006) pointed out three reasons for the importance of water
temperature in treatment wetlands: (1) temperature modies the
rates of several key biological processes, (2) temperature is
sometimes a regulated water quality parameter, and (3) water
temperature is a prime determinant of evaporative water loss
processes. Several biogeochemical processes that regulate the
removal of nutrients in wetlands are affected by temperature, thus
inuencing the overall treatment ef ciency (Kadlec and Reddy,
2001).The goals of this study are (1) to investigate the ef ciency of a
full-scale hybrid constructed wetland system (HCW) in reducing
total nitrogen (TN), ammonia nitrogen (NH4-N) and nitrate
nitrogen (NO3-N); (2) study the effects of temperature on nitrogen
forms abatement; (3) conrm or identify a wastewater tempera-
ture breakpoint that causes variation in nutrient removal perform-
ances; (4) study the effects on vertical ow cells performance in
relation to different vegetation, medium and operational mode.
2.
Materials
and
methods
2.1. Hybrid constructed wetland con guration and characteristics
The HCW was located at a private pig farm in Carmignano diBrenta, Padova, in Veneto Region, NE Italy, (E: 11419.58 ; N:
453745.16; 46 m a.s.l).
It was built in 2008 and designed to provide tertiary treatment
of 2 m3/d of pre-treated liquid fraction of pig slurry ef uent. The
design guidelines provided by APAT were based on municipal
wastewater treatment wetlands (APAT, 2005).
The HCW system is composed of three vertical-subsurface ow
wetlands (VF1–VF2–VF3) in parallel with a total area of 21 m2,
followed by one horizontal-subsurface ow wetland (HF) con-
nected in series (105 m2) (Fig. 1).
The entire system was designed for a hydraulic retention time
(HRT) of 7 days as minimum. Each VF unit was built in concrete
(length: 10 m; width: 1 m; depth: 0.7 m). Three different plastic
sheet liners were placed inside each cell to prevent leakage and
contact of wastewater with groundwater. The layers from bottom
to top were: nonwoven geotextile sheet with a basic weight of
400–800 g/m2 and thickness 1 mm; interlayer EPDM geo-mem-
brane; nonwoven geotextile sheet with a basic weight of 400–
800 g/m2 and thickness 1 mm. The rst two cells were lled with
washed gravel: grain size 10–20mm (d10= 8.5 mm; d60= 9.7 mm)
with porosity of 40%. The rst one (VF1) was vegetated with Canna
indica L., the second (VF2) with Phragmites australis (Cav.) Trin. Ex
Steud (common reed). The third (VF3), planted as VF2 cell, was
lled with a 0.10m deep gravel layer (grain size 10–20 mm)
overlying a 0.10m deep coarse sand (grain size 3–5mm) and
zeolite (grain size 5–10 mm) transition layer and a 0.30 m deep
gravel drainage layer (10–20 mm in size). The main components of
the zeolite were: chabasite 60%, K-feldspar 13%, phillipsite 5%, mica
5% and augite 2%. The synthetic wastewater was distributed evenly
over the surface of the VF beds by a pressurized PVC distributionpipe 75 mm in diameter that ran along the VF wetland units. Three
lters with interchangeable cartridges were placed in series at the
inlet of VF system. The lters were used to remove particles larger
than 1 mm from the feeding tank ef uent, and were installed to
minimize the accumulation of solids in the inuent distribution
pipe. At the inlet of each VF wetland units a water meter with ve
digit mechanical counter was attached at the distribution pipe to
measure the incoming wastewater quantity delivered to each cell.
A drainage pipe (diameter 75 mm and length 10 m) was located on
the bottom of each VF cell in order to facilitate ef uent collection.
The drainage pipe was connected on one side to a 100 mm
diameter collection pipe that discharged the ef uent from the bed
to a manhole that had a water level control structure equipped
with a siphon pipe where a timer-controlled pump was placed(Fig. 2).
The siphon maintained water level at 0.30 m from the surface in
each VF cell. A 200 watt power submersible pump installed in the
same manhole was used to drain the porous media and transport
the leachate to separate sumps (OUT VF1, OUT VF2, OUT VF3). The
wastewater discharged from each VF sump was collected in a
Fig. 1. HCW system dimensions in overhead view. Sampling points: (1) inuent, (2) VF1 ef uent, (3) VF2 ef uent, (4) VF3 ef uent, (5) inuent to HF, (6) HF ef uent (nal
ef uent).
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common sump (OUT VF) (length: 1.2 m; width: 0.8 m; depth:
0.66 m). The VF ef uent was then transferred with a submersible
sump pump with an integrated oat switch to the horizontal
subsurface ow wetland. A water meter with vedigit mechanical
counter attached to the submersible pump outlet measured the HF
incoming wastewater volume.
The horizontal-subsurface ow wetland (HF) was a basin 25 m
long, 4 m wide and depth 0.7 m with a bottom slope of 1%. The
bottom and sides of the basin were waterproofed with the same
plastic sheets used for VF cells. The inlet and the outlet sections
were lled using two strips of coarse-rock material (grain size 50–
100 mm) along two opposite edges of the basin, with washed
gravel: grain size 10–20mm (d10= 8.5 mm; d60= 9.7 mm) with
porosity
of
40%
placed
in
the
central
area.
At
the
HF
inlet
adistributor pipe was buried immediately below the surface
(diameter 100 mm), placed horizontally and perpendicular to
the direction of ow. At the outlet, a similar collector pipe was
buried at the bottom. The HF ef uent volumes were measured by a
water meter, collected in an interred tank and recycled for cleaning
the piggery. The wetland was planted in April 2008 with
Phragmites australis (Cav.) Trin. Ex Steud. (common reed).
2.2.
System
mode
operation
From May 2011 the HCW system was loaded daily with
synthetic wastewater. The wastewater was prepared daily in the
feeding tanks just before the feeding by dissolving ammonium
nitrate fertilizer 26% N (13% nitrate and 13% ammonia) in 1.7 m3 of
fresh water. The average concentration was 250.3 (3.7) mg/L of
TN, 124.5 (2.5) mg/L of NO3-N and 124.9 (3.2) mg/L of NH4-N.
The feeding tank (pump chamber) consisted of a 5 m 0.7 m
concrete tank 0.7 m deep, with a submersible water pump inside
used to load the wastewater to each VF unit. The pump was
controlled by two programmable timers in series. One timer
dictated the time of loading cycles and was set to work once per
day, with one loading cycle in the morning from 10 am to 12 am.
The other timer dictated the loading time per cycle (one minute)
and the time between pumping events within each loading cycle
(every 4 min). Daily wastewater ow rate of 1.7m3 was evenly
distributed to all three VF cells with average ow rate of 565 L/day
per cell.
Two programmable timers in series controlled the submersible
water pump placed inside the water level control structure of each
VF. The rst timer dictated the time of discharging cycles and was
set to work once per day, the other timer dictated the discharging
time per cycle (one minute) and the time between pumping events
within each discharging cycle (every 4 min).
To provide suf cient oxygen transfer for nitrication, load and
discharge cycles of VF unit were set with anoxic/oxic (A/O) steps.
The A/O stages were generated with rapid water ow through the
lter media, the phenomenon called passive air-pump (Greenet al., 1998). During the investigation period two different daily
operation modes (DOM) of the vertical-ow system were tested
(Table 1). We chosen this loading scheme to manage the VF cells
with alternation of period of saturation and unsaturation as
describe below. The rst (DOM 1) was applied from May to July
2011 and from January to 11th July 2012 with a feeding strategy
consisting of 2 h of wastewater inow, followed by 6 h of
completely full cell, 2 h of discharging and 14 h of completely
empty cell in order to assist the oxidation. This “intermittent
feeding” mode was chosen to provide good oxygen transfer to the
water phase. DOM 2 was applied from 11th to 25th July 2012,
programming 14 h of fully lled cells and 6 h of empty cells.
2.3. Sampling, chemical analysis and data elaboration
Water samples were collected and analyzed from May to July
2011 and from January to July 2012 from inow (1; IN), outow of
VF1 (2; OUT VF1), outow of VF2 (3; OUT VF2), outow of VF3 (4;
OUT VF3), inow of HF (5; IN HF), outow of HF and nal ef uent
(6; OUT HF) (Fig. 1). IN sample was taken at the beginning of each
cycle, after synthetic feed preparation, OUT VF of each cell and IN
HF were taken at the end of the daily feeding period of the cycle
(after 6 h from inlet), and OUT HF sample was taken according to
the retention time (after 7 days from the start of the cycle).
Monitoring consisted of thirty seven weekly samplings during the
whole investigation period, collecting 211 samples.
In situ, measurements of pH, electrical conductivity (EC),
dissolved
oxygen
(DO),
wastewater
temperatures
(T )
and
redox-potential (E h), were taken with a Hach Lange HQD 40d multi-
parameter meter with interchangeable probes according to
standard methods (APHA, 1998). Before testing, each probe was
carefully calibrated according to the manufacturer's procedures.
Samples were collected, preserved at 4 C and then analyzed
within a short time.
Total nitrogen (TN), ammonia-nitrogen (NH4-N) and nitric-
nitrogen (NO3-N), were determined photometrically using a Hach-
Lange DR-2800 spectrophotometer and adequate cuvette test kits
(cuvette-tests LCK 338, 302, 340), (Hach-Lange,1989), according to
DIN (1985). Adequate sample dilutions were made with a stock
supply of deionized water.
Air temperature, humidity, solar radiation, rainfall volumes,
wind
speed
and
direction
were
recorded
every
day
from
the
Fig. 2. Schematic representation (not to scale) of water level control structure of
each VF cell.
Table 1
Daily
operation
modes
of
the
vertical-ow system. DOM1: from May to July
2011 and from January to 11th July 2012; DOM2: from 11th to 25th July 2012.
DOM 1 DOM 2
Phase VF unit cycle Duration
(h)
Schedule Duration
(h)
Schedule
I loading cycle 2 10 a.m.–12 a.
m.
2 10 a.m.–12 a.
m.
II wet period 6 12 a.m.–6 p.
m.
14 12 a.m.–2 a.
m.
III
unloading
cycle
2
6
p.m.–8
p.m.
2
2
a.m.–4
a.m.
IV dry period 14 8 p.m.–10 a.
m.
6 4 a.m.–10 a.
m.
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beginning of the experiment by a meteorological station installed
at the site. Evapotranspiration (ETO) was calculated with the FAO
56 Penmam-Monteith equation (Allen et al., 1998) for short
reference crop ETos (Allen et al., 2005).
The data series of the parameters did not follow normal
distribution even after transformations. Thus, statistical analyses
were carried out with the Kruskal–Wallis nonparametric test to
compare the six sampling positions (IN, OUT VF1, OUT VF2, OUT
VF3, IN HF, OUT HF) and Box and Whiskers were used to display the
variability.
In this study, the performance comparison of each HCW unit
was based on three different approaches:
(a) concentration percentage abatement (A), calculated on the
rst quartile (Q1), second quartile (Q2; median) and third quartile
(Q3) concentration values as (Eq. (1)):
A% ¼ Cin Coutð Þ
Cin
100 (1)
where Cin is inow concentration (mg/L) and Cout is outow
concentration (mg/L);
(b) Reduction ef ciency (RE) calculated as (Eq. (2)):
RE% ¼ Cin Vinð Þ Cout Voutð Þ
Cin
Vinð
Þ
100 (2)where Cin is inow concentration (mg/L), Vin is average inow
volume of synthetic wastewater applied (m3/d) with daily rainfall
volume (mm/d) included, Cout is outow concentration (mg/L),
Vout is outow volume detected at the outlet of the unit (m3/d);
(c) Areal load reduction (ALR), expressing the removed
pollutants mass per m2 of CW and time (g/m2/d). ALR is a useful
parameter to assess system ef ciency (Stefanakis and Tsihrintzis,
2012).
In addition, segmented linear regression analysis (or broken-
line regression) with a non-parametric approach developed by
Pettitt (1979) was used to identify a change-point of wastewater
temperature that caused variation in nutrient percentage abate-
ment. For this purpose, the Flat Steps method (Bai and Perron,
2003)
was
used,
implemented
in
the
Strucchange
library
of
R software (Zeileis et al., 2003). Partial F -test in one-way analysis of
variance was used to determine any signicant differences at
p < 0.05.
3. Results and discussion
3.1. Wastewater, air temperature and evapotranspiration
During the rst monitoring period (May– July 2011), slight
differences in air and wastewater temperatures were found at
different HCW sampling points (Fig. 3). Inlet temperature values
ranged between 20.5 and 22.3 C. These values result as being more
ef cient for biological N removal (range of 20–25 C) as reported by
Sutton et al. (1975) since ambient temperature positively
inuences microbial activity and diffusion rates (Phipps and
Crumpton, 1994).
During the second period (January– July 2012) air temperature
vs inlet wastewater showed two different trends: in cold months
(January–March) air temperature (average 6.3 C) was lower than
the inlet wastewater (average 11.9 C) whereas in the warm period
(from April to July) an opposite tendency was observed, especially
in June– July. The freshwater temperature used to prepare synthetic
wastewater (average 14.2 C) showed less variation than the other
sampling points during the second part of the monitoring period,
ranged between 9.2 and 17.7 C. Until the beginning of March
2012 wastewater temperature measured from VF unit (average
8.1C) was lower than inlet (average 11.1 C), whereas higher
values (average 19.8 C) were observed in VF with respect to theinlet (average 15.3 C) till the end of the experimental period. HF
temperature followed the same tendency observed for VF
wastewater (Fig. 3).
The daily ETO results were in accordance with the data found in
the literature for similar conditions (size, latitude and measure-
ment method) (Borin et al., 2011). The time pattern of the
cumulative ETos in the rst period, from May to September 2011,
showed an average daily ETO of 5.90 mm; from October 2011 to
March 2012, plants consumed 1.97mm/day on average. In the
winter-spring period, daily ETO was 5.04 mm.
3.2. On site parameters
During rst monitoring period (May– July 2011), electricalconductivity (EC) of the inuent wastewater was higher than
1600 mS/cm and increased after passage through VF cells. HF
slightly increased this to a median value of 1840 mS/cm (Fig. 4).
Fig.
3.
Mean
air
and
wastewater
temperature
at
the
sampling
points
of
the
HCW
system
during
the
monitored
period.
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Fig. 4. Variation of EC, pH, DO, E h at the sampling points of the HCW system during the monitored period.
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Fig. 5. Nitrogen forms concentration at the sampling points of the HCW system during the monitoring period.
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During the second monitoring period (January– July 2012), the
incoming EC did not exceed 1600 mS/cm until 28th March, then
rose to higher values until the end of monitoring: this is probably
related to the increasing water temperature that promoted
dissolving ammonium nitrate in water. The same trends as the
previous monitoring period were observed for VF and HF, mainly
due to substrate biolm interaction that may result in soluble salt
release from the media to the water (Stefanakis and Tsihrintzis,
2012). EC seasonal variations were observed during summer each
year probably due to increased evapotranspiration (May–Septem-
ber 2011 and April– July 2012) and plant growth, as reported by
Hench et al. (2003).
The median inuent pH during the rst and second monitoringperiod was neutral or slightly alkaline 7.64 0.04. HF unit
exhibited alkaline pH values from 21st March 2012 till the end
of the monitoring period (Fig. 4). As nitrication proceeds
optimally at pH between 7.5 and 8.5 (Platzer, 1996), the pH values
were optimal in all three VF beds. However pH values measured in
the HF unit were not optimal for denitriers that operate best in
the range between 6.5 and 7.5 (Paul and Clark, 1996). Furthermore
ammonia nitrogen loss through volatilization was negligible since
it generally requires a pH of 9.3 ( Jing and Lin, 2004).
Median dissolved oxygen concentration value at the inlet was
constant (4.17 0.05 mg/L) during the whole investigation period.
DO concentration above 1.5 mg/L is essential for nitrication to
occur (Ye and Li, 2009). During rst monitoring period DO
concentration increased after passage through VF cells to the
median value of 4.57 mg/L probably due to aerobic conditions
provided by the oxic stage during DOM 1 of the vertical-ow
system. The DO detected at the ef uent of HF cell varied from
0.14 to 0.28 mg/L. The same trends were observed during the
second monitoring period in VF unit: from the beginning of March
to 11th July 2012 a slight increase appeared in ef uent DO
concentration, possibly due to plant growth and enhanced oxygentransfer to the plant roots. From 11th July 2012 to the end of
monitoring average DO concentration from VF unit ef uent
decreased from 4.18 to 2.74 mg/L due to the longer anoxic stage
promoted by DOM 2. HF ef uent DO concentration was very low
during end winter-early spring, from May to 4th July 2012 it
slightly rose with the common reed re-activation (Fig. 4).
Redox potential (E h) variations within a range of a few hundred
mV from reduced (0 mV) to moderately oxidized redox conditions
(+270 mV) were observed (Fig. 4). During the rst period, median
wastewater redox potential value of 269 decreased to 206 mV after
VF cells passage. This suggests that the VF system provided
conditions suitable for nitrication, but not for denitrication. The
HF outow E h decreased drastically in the range from +10 to 0 mV
probably inducing favorable (anaerobic) conditions suitable formicrobial denitrication. The second period followed the same
trend as the rst. From 11th July 2012 to the end of monitoring E hfrom VF unit ef uent decreased drastically, possibly due to the
prolonged period of saturation of the cell promoted by DOM 2, as
demonstrated by DO values. In the HF cell Eh patterns differed
between the warm and cold periods probably affected by
wastewater temperature. Increase of temperature during the
warm period accelerated biochemical processes including bacteria
activity (Kadlec and Reddy, 2001). Moreover, oxygen solubility in
Fig. 6. Box-plot diagrams of nitrogen forms concentration (mg/L), at the sampling points of the HCW system during the monitoring period. Different letters indicate
signicant differences at p = 0.05 by Kruskal–Wallis test.
Table 2
Slope and R2 for the linear relationships among wastewater temperature and
nitrogen forms abatements and for values below and above breakpoint.
Nitrogen
form
Linear
regression
Values
below
breakpoint
Values
above
breakpoint
Slope R 2 Slope R 2 Slope R 2
TN 3.63 0.88 2.01 0.38 2.9 0.7
NH4-N 0.62 0.66 0.36 0.4 0.44 0.23
NO3-N 6.5 0.83 1.7 0.1 6.6 0.62
Fig. 7. Wastewater temperature and percentage abatement correlation charts with breakpoints for nitrogen forms.
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water decreases with increasing temperature, so as a result, Eh
decreased, as also reported by Dušek et al. (2008) (Fig. 4).
3.3. Nitrogen forms concentrations
From May to July 2011, TN concentration decrease was clear and
constant accomplished by VF unit with median value of 173.4 mg/L and HF unit with 105.6 mg/L. With low temperatures measured
during the rst part of the second period (January–March 2012,
average temperature 10.4 C), TN concentration abatement was
negligible. Then, from the end of March to the rst ten days of May
following an increase in temperature from 15.5 to 20.4 C the
concentrations at the outlets of VF and HF progressively decreased.
Finally, when mean wastewater temperature of 20.2 C was
reached, TN outlet concentrations showed similar values as the
year before in the same period (Fig. 5).
The VF cells provided outlet values statistically lower than the
inlet ones; differences were also found among cells, with lower
values in VF3 with respect to VF1, probably related to the combine
effect of different plant and porous media types. The performance
of VF2 and VF3 units, both planted with P. australis, were similarand the statistical analysis showed no signicant differences. The
HF system lowered the inlet median value from VF unit from
146 mg/L to a nal discharge concentration of 114mg/L and slightly
increased the variability (Fig. 6).
Ammonia median concentration measured from VF and HF unit
showed a decreasing trend in both monitored periods. From May to
July 2011 median NH4-N concentration at VF outlet was 68.3 and
48.6 mg/L in HF. From January to July 2012 it was 68.4 and 50 mg/L,
respectively. The different wastewater temperature did not
inuence ammonia values at the outlet of wetland cells. No
statistically signicant differences were found between VF1 and
VF2 even though different plant species were used. Signicant
differences were found between VF3 and the rst two VF cells
(VF1
and
VF2)
probably
due
to
composition
of
the
medium
layerwhich comprised zeolite stones that could absorb NH4
+ (Nguyen
and Tanner, 1998) (Fig. 6).
During the rst period, NO3-N trend concentrations followed
the same temporal pattern as the other nitrogen forms, with
decreasing values passing through VF and then HF. During the
second period, in the cold season ef uent concentrations from VF
and HF unit exceeded inuent with median values of 179.7 and
182.7 mg/L respectively. With increasing wastewater temperature
during the warm season, measured concentration decreased
reaching values found in the rst period (Fig. 5). Statistical
differences were found only between inlet and outlet of HCW
system (Fig. 6). As reported by Kuschk et al. (2003), the microbial
activities related to nitrication and denitrication can decrease
considerably
at
water
temperature
below
15
or
above
30
C.
In
particular, the activity of denitrifying bacteria in CWs is generally
more robust in spring–summer seasons than in autumn–winter
(van Oostrom and Russel, 1994).
3.4.
Effect
of
wastewater
temperature
In our study, correlation analysis was performed betweenpercentage abatement (A%) for nitrogen forms calculated on a
weekly basis for all 37 weeks of monitoring and the corresponding
average wastewater temperatures during the whole monitoring
period. We focus on the results obtained for the whole HCW.
First, linear regressions between A% and temperature were
calculated for the nitrogen forms, obtaining signicant relation-
ships, with higher R2 for TN and NO3-N (Table 2). The regression
slope for NO3-N suggested a sharp inuence of temperature on
abatement, which increased by 6.5% with every 1 C. On the
contrary, the effect of temperature on NH4-N was less marked, as
suggested by the lower R2 and regression slope.
In a second step a segmented regression analysis was conducted
according to Bai and Perron (2003) to identify any presence of a
breakpoint of wastewater temperature that caused variations innutrient abatement performances. The segmented analysis model
was signicant with respect to the linear regression for TN and
NO3-N and evidenced breakpoints at 14.2C for both nitrogen
forms (Fig. 7).
According to this approach the abatement was always
positively correlated with temperature, but different regression
slopes were obtained below and above the breakpoint. In
particular, with lower temperature the abatement of NO3-N and
TN increased by 1.7 and 2.0% per C of temperature increase; with
temperature higher than 14.2 C the increase in abatement due to
increased temperature was sharper, especially for NO3-N (6.6%
every 1 C) (Table 2).
Nitrogen pollutants abatement statistics were calculated for
wastewater
temperatures
below
and
above
the
breakpointidentied with segmented regression analysis (Table 3). As
previously mentioned, higher performance was observed at
temperature above breakpoint for both TN as NO3-N. In particular,
NO3-N median abatement that was negative below 14.2C, became
consistent above this critical temperature.
3.5. Nitrogen RE and ALR
Median TN RE during the rst period (from May to July 2011)
was 63%, 83% and 96% for VF, HF unit and SIF respectively (Fig. 8).
During the second period from January to the end of March
2012 the RE was lower, with values between 11% and 40% for VF
unit and the entire system. Considering the same period negative
reduction
values
were
found
for
HF
unit
(ranging
from
0.7
and
Table 3
Nitrogen pollutant percentage abatement (A) statistics for wastewater temperatures above and below identied breakpoint.
Temperature below break point Temperature above break point
VF1 VF2 VF3 HF HCW VF1 VF2 VF3 HF HCW
TN (A%)
Minimum 3.5 6.1 4.4 8.9 5.3 16.2 7.8 22.9 7.4 22.7
First quartile (Q1) 1.2 3.6 3.8 8.7 0.5 18.8 29.5 37.8 14.7 53.8
Median (Q2) 5.3 3.8 7.1 4.9 5.1 21 34.4 42.6 19.9 57.5
Third quartile (Q3) 12.4 8 10.8 2.3 8.1 24.3 36.4 46.7 29.2 60.9
Maximum 17.7 26 35 13.1 41.8 27.1 42.1 50.4 39 63.7
NO3-N (A%)
Minimum 54.4 49.8 61.6 2.7 55.7 28.5 22.3 20.7 1.8 27
First quartile (Q1) 50.6 46.7 51.2 2.1 51.3 10.4 16.7 18.2 14.6 35.6
Median (Q2) 47.8 41.8 47.1 0.2 47.3 13.4 26 26.5 25.3 45.4
Third quartile (Q3) 44.3 39.9 39.9 1.2 45.6 14.4 28.7 30.4 30.6 59.3
Maximum 3.2 3.3 2.6 2.9 1.8 22.1 33.8 46.2 52 68.4
298 A. Mietto et al. / Ecological Engineering 75 (2015) 291– 302
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Fig. 8. Nitrogen forms RE (%) at the sampling points of the HCW system during the monitoring period.
A. Mietto et al. / Ecological Engineering 75 (2015) 291– 302 299
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6%). From April 2012, when the temperature rose above14.8 C,
there was an increase of TN reduction in the system that reached
the 2011 values (Fig. 8).
VFand HF system provided a NH4-N reduction pattern similar to
that found for TN during 2011,with values ranging from 57% to 94%
in the entire system during the whole monitoring period (Fig. 8).
Fig. 9. Nitrogen forms ALR of the HCW during the monitored period.
300 A. Mietto et al. / Ecological Engineering 75 (2015) 291– 302
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VF reduction was slightly higher during the warm period of 2012
(median value 71%) than cold period of the same year (median
value 43%). The HF system performed better during the last
monitoring period, with median reduction ranging between 35
and 87%.
Median NO3-N reduction ef ciency during 2011 was higher forHF (85%) than VF (57%) (Fig. 8). During the cold period of
2012 negative reduction values were observed for VF unit and in
the SIF. The negative RE value (30%) rose from the end of March
until reaching, at the end of the investigation period, a value higher
than 60% in VF.
The median removals of mass pollutants per m2 of HCW per day
were TN 3.1 g/m2/d, NH4-N 1.5 g/m2/d and NO3-N 1.5 g/m
2/d
(Fig. 9). ALR removals of nitrogen forms showed a different trend
during the rst monitoring period of 2012. In particular, from
January to March 2012 the data reveal a low treatment capability,
probably related to temperature (Stefanakis and Tsihrintzis, 2012).
From April to the end of the monitoring period an increment was
observed: from the initially low performance, it reached the same
results
observed
during
2011
(Fig.
9).
The
combination
of
verticaland horizontal stages ensures the right conditions to obtain a
higher nitrogen removal performance. Moreover, when consider-
ing a year of operation, we can suppose that the HCW would
remove approximately 1.1Kg TN/m2.
3.6. Comparison among vertical systems
Nitrogen pollutant reduction ef ciency (RE) and areal load
reduction (ALR) of VF system measured during DOM1 and
DOM2 are presented in Table 4. The performance comparison
was made between two loading cycles of each DOM with the same
conditions (air and wastewater temperatures). In particular, we
considered the subset data from the last two weeks of DOM1 (from
27th
June
to
11th
July
2012)
and
the
data
of
the
two
weeks
of
DOM2 (from 11th to 25th July 2012). DOM 1 allowed higher RE and ALR
to be obtained, probably due to the longer empty period that
promoted better oxidative conditions. Lower average RE values for
TN (varying between 64.1 to 68.3%) and NH4-N (varying between
68.6 and 73.1%) were found with DOM 2. However RE values for
NO3-N were quite similar to those observed during DOM 1.
VF systems planted with P. australis (VF2 and VF3) showed
better performance in TN reduction ef ciency (70.3 and 70.5%,
respectively) than VF1 (63%), planted with C. indica. Average NH4-N
reduction ef ciency was 76.3 and 77.3% in VF2 and
VF3 respectively with higher ALR of 8.2 g/m2/d in VF2. NO3-N
values were lower than other nitrogen forms, best RE and ALR
performances were measured in VF2 with 65.4% and 7.1g/m2/d.
4. Conclusions
The system showed higher performance in terms of TN, NO3-N
and NH4-N reduction. Nonetheless, seasonal variations appear to
affect the HCW system performance. Lower ef uent concentra-
tions were observed during the warm period (higher temperature),
especially for TN and NO3-N, whereas the performances amelio-
rated with the increase in wastewater temperature. NH4-N
reduction ef ciency was inuenced by seasonality to a lesser
extent in the VF than the HF.
Higher RE and ALR for TN and NH4-N were obtained with the
rst daily mode of operation (DOM 1) probably related to the dry
period of 14h that seemed to promote the medium layer oxidation
and nitrication process, whilst similar ndings were obtained in
DOM 2 for NO3-N reduction ef ciency. VF systems planted with P.
australis (VF2 and VF3) showed slightly higher performance in TN
reduction ef ciency compared to VF1, planted with C. indica.
Segmented regression analysis identied a breakpoint at 14.2 C
for wastewater temperature that caused variations in TN and NO3-N
concentration reduction performances. According to this analysis
the abatement of NO3-N and TN increased by 1.7 and 2.0% per C
when temperature was below breakpoint; with a temperature
higher than 14.2 C the increase of abatement due to increased
temperature was sharper, especially for NO3-N (6.6% every 1 C)In the VF cells the mode of operation of loading/discharging
cycles induced a sharp variation in DO and E h values; both
decreased passing from 6 h of completely full cells and 14h of
empty cells to 14 h of full cells and 6 h of empty cells. As a
consequence, the NH4-N concentration at the outlet increased and
the reduction ef ciency decreased. Addition of zeolite to the
porous medium reduced the NH4-N concentration and P. australis
gave better results than C. indica.
It is important to highlight that during this study synthetic
wastewater was used and that temperature in the inuent was
affected by the environmental climate conditions. Slurry temper-
ature is usually quite constant during the year, at around 15–17 C
(Politeo,
2013) and
this
can
explain
the
lower
performanceobserved during winter compared to summer season. This has
to be taken into account when considering the effective potential
of using wetland hybrid systems to treat slurry ef uents as a higher
performance can be expected.
Acknowledgment
Research supported by Progetto AGER, grant no. 2010-2220. We
thank Luigi Guarnieri, the owner of the pig farm and the hybrid
constructed wetland, for its kind hospitality.
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Table 4
Mean values of nitrogen pollutant reduction ef ciency (RE) and areal load reduction
(ALR) measured during the two last two weeks of DOM1 (from 27th June to 11th July
2012) and DOM 2 (from 11th to 25th July 2012).
DOM1
DOM2
VF1
VF2
VF3
VF1
VF2
VF3
RE (%)
TN 63 70.3 70.5 64.1 68.3 65.5
NH4-N 73.7 76.3 77.3 68.6 72.5 73.1
NO3-N 57.1 65.4 58.8 59.4 63.7 58.4
ALR (%)
TN 13.3 15.3 12.7 13.4 14.7 12.1
NH4-N 7.7 8.2 6.9 7.2 7.8 6.7
NO3-N 6 7.1 5.3 6.2 6.8 5.4
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