© IWA Publishing 2015 Water Science & Technology | 71.7 | 2015

965

Design-support and performance estimation using
HYDRUS/CW2D: a horizontal ﬂow constructed wetland forpolishing SBR efﬂuent
Tamás Gábor Pálfy, Zoltán Gribovszki and Günter Langergraber

ABSTRACTThe 4,000 PE (700 m3/d) wastewater treatment plant at Balf, Hungary was based on sequencing
batch reactor technology with phosphorus precipitation as the tertiary step. Its efﬂuent met quality
thresholds on average, with above-threshold peaks mainly in winter. The HYDRUS/CW2D model
package (PC Progress s.r.o.) was used to simulate the treatment performance of a horizontal ﬂow
constructed wetland for polishing efﬂuent. The goal of this study was to provide design-supportive
information about the suitability of the proposed wetland and to prove the applicability of thecomputational tool used to gain that information. The simulations showed that the wetland with theproposed layout could not tackle peaks in NH4-N. Other efﬂuent thresholds could be met, including
chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total nitrogen, total inorganic

Tamás Gábor Pálfy (corresponding author)Günter LangergraberInstitute of Sanitary Engineering,
University of Natural Resources and Life Sciences
Vienna (BOKU University),
Muthgasse 18,
A-1190 Vienna,
AustriaE-mail: palfy.tamas@gmail.com
Tamás Gábor Pálfy
Zoltán Gribovszki
Institute of Geomatics and Civil Engineering,
University of West Hungary Bajcsy-Zs. 4.,
H-9400 Sopron,
Hungary

nitrogen and total phosphorus. The tool was unable to simulate COD and BOD5 removal in periods
when the electron acceptors were depleted because anaerobic processes are not modelled. Using atool of such complexity for designing carries excessive work demands and involves many
uncertainties. The simulation study highlighted that the model used could still facilitate the design ofan effective system by showing the weaknesses of a test scenario as it was demonstrated.
Key words | constructed wetland, CW2D, design-support, polishing treatment, process-basedmodel, sequencing batch reactor

INTRODUCTION
The wastewater treatment plant (WWTP) at Balf, Hungary
was based on sequencing batch reactor (SBR) technology
and designed for 4,000 PE (capacity 700 m3/d, average
180 m3/d). The plant was located in the basin of Lake
Fertő/Lake Neusiedl, an important natural site, including
areas protected by strict national and international laws.
The advanced treatment chain included phosphorus precipitation after mechanical and biological treatment. The plant
met the efﬂuent quality thresholds set for concentrations ofchemical oxygen demand (COD), biochemical oxygen
demand (BOD5), total nitrogen (TN), total inorganic nitrogen(TIN), NH4-N and total phosphorus (TP) on a long-term average. However, the thresholds were breached for short periods
due to the variability of the inﬂow rate and concentrations,and these periods of non-compliance were more frequent
during winter (ÉDKTVF ; Gribovszki et al. ).
Extending the treatment chain by a horizontal ﬂow (HF)
constructed wetland (CW) was one of the possible solutions
doi: 10.2166/wst.2015.052

for improving the efﬂuent quality of Balf SBR. Other solutions considered were pumping either the treated or theuntreated wastewater out of the watershed of the lake. An
HF CW has advantages compared to these solutions,
namely it would not negatively affect the water balance of
the astatic steppe lake, because the water stays in thebasin and it is a low-technology solution meaning low-maintenance costs and work demands (Gribovszki et al. ). It
was assumed that an HF CW is the most robust solution thatis capable of ﬂattening occasionally occurring peaks to
below legislative thresholds by the combined effect ofhydraulic buffering (including a buffer pond), adsorption
and biological degradation processes.
The applicability of HF CWs for the tertiary treatment ofvarious types of wastewater has already been conﬁrmed.
These systems are capable of polishing treated efﬂuents ofdomestic wastewater origin (Ayaz , Vymazal ,
Ghosh & Gopal ). The CW2D (Langergraber &966

T. G. Pálfy et al. | Design-support and performance estimation using HYDRUS/CW2D

Šimůnek ) biokinetic model implemented in the
HYDRUS program package (Šimůnek et al. ; Langergraber & Šimůnek ) (HYDRUS/CW2D) is a tool that had
already been used to simulate polishing treatment in pilotscale wetlands. The results ﬁtted well, measured COD andNO3-N, and were slightly above the measured NH4-N concentrations regardless of the low temperatures (Toscano
et al. ).
The aim of our study was to show the applicability of amechanistic computational tool, i.e., HYDRUS/CW2D to
support the design of an HF CW. This was to be achievedby simulating the polishing treatment performance of an
HF CW with ﬁxed parameters, following practical design
considerations. Simulations might help, not only thedesign, but also provide decision support.

METHODS
Measured SBR efﬂuent quantity and quality data (2007–2013) were obtained for simulations in HYDRUS/CW2D.
Volumetric data were available for every day and water quality data were available from grab samples taken once a
week. The measured water quality parameters were COD,
BOD5, TN, TIN, NH4-N, NO3-N, NO2-N and TP. The concentrations were low because the outﬂow was the efﬂuent
after a tertiary treatment; however, all threshold values(there was no threshold for nitrate and nitrite) were breached at least once. For the numerical simulations,continuous data series for one winter were needed to lookat performance in the most critical months. Days with
only volumetric data were ignored because quality parameters were necessary for a continuous model input.
Days with both quality and volumetric data were groupedby the month of the sampling, giving a realistic data seriesof an average 26 days for each month. This series is highly
reliable compared to randomly assigning quality parameters
for every 6 out of 7 days.
The measured quality parameters were slightly different
from the input parameters required by CW2D. In the model,
COD was deﬁned as three different fractions: slowly (CS)and readily (CR) biodegradable, and inert (CI) COD (Langergraber & Šimůnek ). CR was estimated to be 26%
of COD, which is the average proportion of BOD5 and
COD, CI was set to a constant value of 9 mg/L (Ghosh &Gopal ) and the rest of the COD was allocated as CS.
TP was allocated between the organic phosphorus (org. P)
and PO4-P components of the model, based on an estimated
0.1% P content of the organics after the amendment of

Water Science & Technology | 71.7 | 2015

ferrous sulfate at the tertiary step. Organic nitrogen was
not measured and the basic composition parameter setting(Langergraber & Šimůnek ) was used for calculations.
No temperature series were recorded for the wastewater,
its value was set to 9 C for December and February and
8 C for January based on single measurements made by
the operator.
The selection of the ﬁlter material and the dimensioning
were preliminarily based on literature data and on practicalconsideration, but it was necessary to set up simulations thatcan give more speciﬁc knowledge of whether the design
could or should be improved. The parameters for the ﬁlter
material were selected based on literature data from thework of Salvato et al. (): ø 0–4 mm sand, 86 m/dsaturated hydraulic conductivity, θ ¼ 0.3 for porosity. The
adsorption parameters were taken from the literature andare summarized in Table 1. The values are representative
for sand media and were set up for the model components
CS, NH4-N and PO4-P.
The total volume of the ﬁlter material (1,350 m3) was
selected based on the average daily hydraulic load and on
a targeted nominal retention time of 2.25 days, which is typical for HF CWs (Kadlec & Wallace ). The depth of thedomain was set to one meter to avoid soil frost, and no plant
effects were considered. Eight rectangular beds of 15 ×11.25 m horizontal dimensions with an 0.075 m inclination
would enable an optimal arrangement and they were inaccordance with the length to width ratio proposed by
Kadlec & Wallace ().
The water release from the SBR took only 1 hour, which
exceeds the intake capacity of the ﬁlter beds limited byhydraulic conductivity. A levied buffer pond was added tothe theoretical system, which stores the SBR efﬂuent if too
many high loadings arrive at the HF CW in a row. The
pond has a capacity of 810 m3 and an outﬂow limitationof 270 m3/d. The outﬂow limitation was selected near tothe peak hydraulic intake capacity of the ﬁlters and limits
the lowest possible retention time to 1.5 days. A complete
W

W

Table 1 | Parameters used in the simulations to describe the adsorption capacity of theﬁlter material
Isotherm parameters
Component

Isotherm type

Kd

Nu

Beta

Alpha

CS

Linear

0.575

0

1

2.4

NH4-N

Langmuir

0.9

0.003

1.2

2

PO4-P

Langmuir

9

0.044

0.77

1

The values were taken from the literature. CS: Mburu et al. (2012), NH4-N: Pálfy & Langergraber (2014), and PO4-P: Xu et al. (2006).

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T. G. Pálfy et al. | Design-support and performance estimation using HYDRUS/CW2D

mixing calculated in the pond but any other potential qualitychanging effects were ignored.
A 2D domain was simulated in HYDRUS/CW2D, representing the longitudinal section of one of the 15 × 1 m ﬁlter
beds with 121 × 20 (length × depth) ﬁnite elements. The ﬁrst
simulations were repeated six times with data from theNovember months, with a 10 C inﬂow in order to get an
established bacterium distribution in the domain. The simulations continued, with the winter data for evaluation
representing a 77-day period when the performance of thedomain was expected to be lowest. The parameters of
CW2D (Langergraber & Šimůnek ) were unchanged,
except for the half-saturation coefﬁcients for dissolved
oxygen from the ammonia- and the nitrite-oxidizing bacteria. These were both decreased to 0.0026, based on thework of Pálfy & Langergraber ().

Water Science & Technology | 71.7 | 2015

also 0.1 mg/L SO (dissolved oxygen). This induced an endless growth in CW2D, as described by Samsó & Garcia() for CWM1. At this stage of growth, the bacterium concentrations were in a stable range in the other parts of theﬁlter and concentrations were not detectably effected by
the anomaly.

W

RESULTS AND DISCUSSION
General remarks
The simulation results predicted the performance of the conceptualized HF CWs for polishing the release of Balf SBR.
The measured quality of the SBR efﬂuent (hereafter referred
to as ‘inﬂuent’) and the simulated wetland efﬂuent (hereafterreferred to as ‘efﬂuent’) concentrations were compared byeach pollutant.
The microbial concentration was at steady-state at theobservation points during the simulations, despite the fact
that, at the three inﬂow nodes, the propagation had not
stopped and did not seem to converge to a constant value.
The microbes have an optimal environment at these inlet
points because the inﬂow contains enough nutrients and

COD and BOD5
The efﬂuent concentrations exceeded the legislative
threshold in a few instances. The ideal application ﬁeld for
CW2D is vertical ﬂow CWs, which are aerobic systems; inaddition, the CW2D was also listed for usage with lowloaded HF CWs (Langergraber & Šimůnek ). Even
though the inﬂuent had low concentrations, it had peaks
with 225 mg/L COD. Figure 1 shows the quality thresholds
for COD and BOD5 and the effect of the simulated HF CW
domain on efﬂuent quality. CW2D does not consider
anaerobic processes, so when the electron acceptors weredepleted, the biological removal of COD ceased and thedomain’s only ﬂattening effect was due to adsorption andmixing. Cross-checking the COD with the NOx-N values
reveals that where the COD values are above the threshold,
NOx-N was not present in the system. The domain turned
anaerobic, and CW2D reached its limits because it lacks
anaerobic degradation processes. Therefore, these concentration peaks have to be interpreted with care and they
provide an example of why CW2D has to be applied withcaution on any HF CWs. The concentrations only slightly
exceeded the threshold, even without anaerobic degradation, and therefore these results were rather positive.
The same is true for the BOD∞ which added up as thesum of the readily biodegradable (CR) and slowly biodegradable (CS) COD in the efﬂuent. BOD∞ is a safe basis for

Figure 1 | COD and BOD∞ at the HF CW domain: legislative threshold on concentration, simulated outﬂow concentrations using HYDRUS/CW2D, and inﬂow concentrations and solute ﬂuxes
representing measured efﬂuent data from Balf WWTP. The threshold on BOD is for BOD5. The gaps at days 16 and 50 separate the December, January and February data sets.

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T. G. Pálfy et al. | Design-support and performance estimation using HYDRUS/CW2D

comparison to use with BOD5 thresholds as it represents thetotal COD minus the non-biodegradable COD (9 mg/L).
Nitrogen forms
Figure 2 shows the quality thresholds for TN, TIN, NOx-N
and NH4-N and the effect of the simulated HF CW domainon the efﬂuent quality. TN, TIN and NOx-N concentrations followed a similar trend in the efﬂuent. NOx-N
was almost exclusively nitrate and was the main form of
nitrogen in periods with increased nitrogen concentrations. TIN and TN breached thresholds after a load of
about 60 mg/L nitrate at the beginning of the simulation
period but it was predicted that the HF CW would cut
all other peaks, keeping them below or at the legislative
limits.
The case of ammonium nitrogen was different. The predictions showed a low overall removal rate for the cold
period. The main effect of the CW was to rather ﬂatten thepeaks, which would be a signiﬁcant improvement in an ecological sense but would be unsatisfying regarding thelegislative limits. The issue could be targeted by mixing amaterial with high NH4-N adsorption capacity with theWater Science & Technology | 71.7 | 2015

ﬁlter media such as, e.g., zeolite. Meyer et al. ()showed extremely high retention of ammonium in a CW
with chabazite (a form of zeolite) mixed with the sandﬁlter media. There was near-zero NH4-N present in the inﬂuent on days with average inﬂuent quality. The absorbed
ammonium could be nitriﬁed on such days when the circumstances are more favorable for autotrophs, e.g.,
warmer days.
TP
Figure 3 shows the quality thresholds for TP and the effectof the simulated HF CW domain on the efﬂuent quality. TP
concentrations were predicted to be kept low by theadsorption capacity of the sand without adding any special
media. Adsorption was the main – or only – removal process for phosphorus, and the adsorption sites became
saturated with time (Kadlec & Wallace ). The presented performance represents the ﬁlter at an early stage,
the concentration of the sorbed P ranged from 5 to
1 mg/kg dry matter (inlet and outlet zone, respectively). If
the removal performance fails in the long term, the installation of a small ﬁlter bed ﬁlled with high adsorption

Figure 2 | TN, TIN, nitrate plus nitrite nitrogen (NOx-N) and ammonium nitrogen at the HF CW domain: legislative thresholds on concentration, simulated outﬂow concentrations using
HYDRUS/CW2D, and inﬂow concentrations and solute ﬂuxes representing measured efﬂuent data from Balf WWTP. There is no threshold for NOx-N. The gaps at days 16 and 50
separate the December, January and February data sets.

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T. G. Pálfy et al. | Design-support and performance estimation using HYDRUS/CW2D

Water Science & Technology | 71.7 | 2015

Figure 3 | TP at the HF CW domain: legislative threshold on concentration, simulated outﬂow concentrations using HYDRUS/CW2D, and inﬂow concentrations and solute ﬂuxes representing measured efﬂuent data from Balf WWTP. This performance is representative for the ﬁrst year of operation. The gaps at days 16 and 50 separate the December, January
and February data sets.

capacity material such as furnace slag (Xu et al. )might be necessary at the end of the treatment chain.

CONCLUSIONSHYDRUS/CW2D is a complex numerical tool and its use
for design-support has excessive work demands. However,the present study pointed out that it is possible to use it toreveal the weak points of a preliminary design and, assuch, to help to develop an optimized system for a speciﬁcsite. This kind of application of CW2D and similar models
can be rationalized in the case of large-scale projects, orfor CW projects with special characteristics, such as the
HF CW proposed for the polishing treatment of efﬂuent
from the SBR at Balf.
CW2D lacks anaerobic processes and, as such, it has tobe used with care for modeling HF CWs. The suggested
application ﬁelds of CW2D included HF systems if theloads are low. It should also be added that these domains
should exclude inﬂow concentration peaks for pollutants
that can cause anaerobic conditions in the wetland. If thesimulated domain turns anaerobic, the processes responsible for organic and ammonium transformation cease.
Anaerobic degradation would take over in real life andgrow to strength as the microbial community adapts to thechanges.
The conceptualized polishing treatment chain contained
a buffer pond and eight HF CW beds in parallel, with sandﬁlter media, a total surface area of 1,350 m2 and a depth of
one meter. The simulations have shown that adding thissystem to the existing treatment chain would be satisfying
in terms of COD, BOD5, TN, TIN and TP removal. However, the wetland would probably fail to tackle the peaksof NH4-N of the SBR efﬂuent and therefore a media withhigher NH4-N adsorption capacity should be considered.
Phosphorus removal would be highly efﬁcient until the
media reaches its adsorption capacity in the long term.
The results showed no need to mix special media to theﬁlter material to keep TP below the legislative thresholds.
However, as the simulations have been run with literature
data only, it is advised to carry out laboratory experiments
to derive exact data. The experiments would determine
ﬁlter media properties, including adsorption capacity, prior
to implementation to increase the reliability of the predicted
treatment efﬁciency.

ACKNOWLEDGEMENTS
Tamás Pálfy thanks all who have contributed to the realization of this study: colleagues at Sopron Waterworks andat the Institute of Geomatics and Civil Engineering of the
University of West Hungary, and to Bökö, Reni and Péter
Csáfordi.

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First received 28 November 2014; accepted in revised form 22 January 2015. Available online 6 February 2015