J. Hydrol. Hydromech., 66, 2, 227–231
DOI: 10.1515/johh-2017-0053

Simulating vertical flow wetlands using filter media with different grain sizes
with the HYDRUS Wetland Module
Bernhard Pucher*, Guenter LangergraberInstitute of Sanitary Engineering and Water Pollution Control, University of Natural Resources and Life Sciences Vienna (BOKU),
Muthgasse 18, A-1190 Vienna, Austria.
*
Corresponding author. E-mail: bernhard.pucher@boku.ac.at

Abstract: In this study, the simulation results of four vertical flow wetland systems using the HYDRUS Wetland Module are presented. The four wetland systems comprise three single-stage pilot scale systems and one full-scale two-stage
system. The main difference between these systems is the filter media used, referred to as fine media, i.e., sand with agrain size distribution 0.063–4 mm, or coarse media, i.e., sand with grain size distributions between 1–4 mm, respectively. The water-flow simulation of each system is carried out using the single porosity van Genuchten-Mualem model. A
good match between measured and simulated volumetric effluent flow rates could be achieved for all wetland systems.
For reactive transport simulations, the CW2D biokinetic model was applied. First, simulations were run using the standard CW2D parameter set. For some systems, adjustments of the parameter set were needed in order to avoid unlimited
bacteria growth. To better fit measured COD, NH4-N, and NO3-N effluent concentrations, adjustments of few parametersof the standard parameter set were required. The results show that for the VF wetlands with fine sand, no adjustments of
the CW2D standard parameter set were needed, while for systems with coarser filter media as the main layer, the standard parameter set had to be adjusted to match simulated and measured effluent concentrations.
Keywords: Treatment wetlands; Vertical flow; French VF wetland; HYDRUS Wetland Module; CW2D.
INTRODUCTION
In order to improve the understanding of water flow and solute transport, including biological and chemical processes
within the soil matrix, the use of software tools like HYDRUS
is steadily increasing. With the basic description of water flow
in variably-saturated conditions coupled with solute transport,
adsorption/desorption processes, geochemical reactions, andbiokinetic degradation, a very broad range of applications isavailable and furthermore extended by various scientific communities (Šimůnek et al., 2016). One implemented module, the
HYDRUS Wetland Module (Langergraber and Šimůnek, 2012),
was developed in order to simulate the governing biologicaland chemical transformation and degradation processes ofwastewater constituents in subsurface flow treatment wetlands(SSF TW). The field of TW simulation grew consistently overthe last decades and has evolved from looking at such systems
as a black box to the use of process-based models describing
the simultaneous occurring processes responsible for the degradation of various pollutants in the wastewater (Langergraber,2017; Meyer et al., 2015).
For the treatment of domestic wastewater, SSF TWs are
generally applied. Furthermore, SSF TWs can be divided into
horizontal flow (HF) and vertical flow (VF) wetlands, whereby
in HF wetlands mainly anaerobic and in VF wetlands mainly
aerobic processes occur (Kadlec and Wallace, 2009). Twobiokinetic model formulations are implemented in the HYDRUS Wetland Module: CW2D (Langergraber and Šimůnek,2005) that describes aerobic and anoxic processes and is mostly
used for simulating VF wetlands and CWM1 (Langergraber etal., 2009a) that additionally describes anaerobic processes andis also applicable for HF wetlands. Their mathematical formulation is based on the IWA Activated Sludge Models (Henze etal., 2000).
In this study, simulation results of four different VF TWs arepresented and discussed. Two systems are classical single-stage

VF TWs (pilot-scale), one is a full-scale two-stage VF wetland
system, and one is a French single-stage VF TW system (pilotscale).
The two-stage VF wetland system is intermittently loadedwith mechanically pre-treated wastewater. The first stage consists of a coarse filter media (sand 1–4 mm) for the main layer
and has an impounded drainage layer, while the second stage
uses a finer sand (0.063–4 mm) as the main filter layer. Thissystem is designed for full nitrification and high nitrogen elimination. The design is based on the specific area needed with
1 m² per people equivalent (PE) for each stage, i.e., in total
2 m² per PE (Langergraber et al., 2008; Langergraber et al.,2009b; Langergraber et al., 2014).
The French VF wetland is also a two-stage VF wetland system but is intermittently loaded with raw wastewater. The main
layer of the first stage consists of gravel. The loading of thefirst stage filter lasts for three to four days and is followed by aresting period twice this time. This is important for the degradation of organic matter and the drying of the sludge layer. The
main layer of the second stage consists of sand and treats thepercolating water from the first stage (Molle et al., 2005; Paing
and Voisin, 2005).
Simulation for the four systems is carried out following thesame procedure. Simulation results are compared to measured
data and the need for adjustments of model parameter is discussed in relation to the different wetland parameters, i.e., sandused for the main layer, loading regime, etc.
MATERIALS AND METHODS
VF wetlands
A general description of the systems modeled is given in
Table 1. Systems 1, 2, and 3 are based on the Austrian system
where System 1 (Langergraber, 2001) and 2 (Pucher, 2015)
represent the first stage of a 1m² pilot-scale system operated in
the technical lab hall at the Institute of Sanitary Engineering at

227

Bernhard Pucher, Guenter Langergraber
Table 1. General description of the simulated systems.
SizeSingle-stage VF wetlands
System 1
Pilot-scale
System 2
Pilot-scaleTwo-stage VF wetland
System 3
Full-scale
French VF wetland
System 4
Pilot-scale

Wastewater

Filter media

Depth*

Grain size*

Area

DrainagePrimary treatedPrimary treated

Sand
Sand

50cm
50cm

0.063–4 mm
1–4 mm

1 m²
1 m²

Free
Impounded

Primary treated

Sand

50cm

2–3 mm50 m2

ImpoundedRaw

Pea gravel

40cm

2–4mm

2.3 m²Free* of the main layer

Table 2. CW2D processes (Langergraber and Šimůnek, 2012).
Heterotrophic bacteria
1
Hydrolysis
2
Aerobic growth of XH on CR
3
Anoxic growth of XH on CR
4
Anoxic growth of XH on CR
5
Lysis of XH
Autotrophic bacteria
6
Aerobic growth of XANs on NH4-N
7
Lysis of XANs
8
Aerobic growth of XANb on NO2-N
9
Lysis of XANb

BOKU University. The third system (Langergraber et al., 2014)is a full-scale two-stage VF wetland system located on a mountain. System 4 (Ruiz et al., 2016) is a single-stage wetland
based on the French system.
HYDRUS Wetland Module
HYDRUS (Šimůnek et al., 2016) numerically solves variably saturated water-flow and solute-transport equations. The
water-flow equation incorporates a sink term to account forwater uptake by plant roots. The solute-transport equations
consider convective-dispersive transport in the liquid phase,
diffusion in the gaseous phase, as well as nonlinear nonequilibrium reactions between the solid and liquid phases(Šimůnek et al., 2016). Version 2 of the HYDRUS Wetland
Module includes two biokinetic model formulations to simulate
the biochemical transformation and degradation processes in
TWs: CW2D and CWM1 (Langergraber and Šimůnek, 2012).
Up to now, only the single-porosity water-flow model can beused together with the HYDRUS Wetland Module althoughdual-porosity water-flow models are available within the standard HYDRUS software.
For the simulation of VF wetlands with aerobic conditions,the CW2D biokinetic model is commonly applied(Langergraber, 2017) and therefore CW2D is also chosen forthis work. The mathematical formulation of CW2D includes
12 components and nine processes. The components are
dissolved oxygen (O2), organic matter (OM) divided into
readily available (CR), slowly available (CS), and inert organic
matter (CI), heterotrophic microorganisms (XH), autotrophic
microorganisms (XANs, XANb), nitrogen compounds asammonium (NH4-N), nitrite (NO2-N), and nitrate (NO3-N)
nitrogen, and nitrogen gas (N2), as well as inorganic
phosphorous (PO4-P) (Langergraber and Šimůnek, 2005). The
nine processes described in CW2D are related to the activitiesof the heterotrophic and autotrophic bacteria groups, respectively (Table 2).

228

Conversion of CS into CR
Mineralization of OM
Denitrification on NO2-N
Denitrification on NO3-N
Production of OM
First step of nitrification
Production of OM
Second step of nitrification
Production of OM

Simulation procedure
For the two-stage system, each plot was simulated on its
own. The simulated effluent concentrations of the first stage
have been used as influent concentrations of the second stage(Hochfeldt, 2017). All four VF wetland systems have beencalibrated following the same workflow. The water flow wasmodeled using the standard van Genuchten-Mualem soil hydraulic model (Mualem, 1976; van Genuchten, 1980).
Measured soil hydraulic parameters, namely the hydraulicconductivity Ks, and the saturated water content θs, as well as
the measured boundary flux of each system were used todetermine the missing parameters, namely the residual watercontent θr and the form parameters α and n, using the inverse
solution implemented in HYDRUS, leading to the calibration of
the water-flow model.
The reactive transport simulation, including the biokinetic
model, was carried out first using the soil and solute specificparameters proposed by Langergraber and Šimůnek (2011) andthe standard parameter set of the CW2D module (Langergraberand Šimůnek, 2005). No adsorption processes were considered.
The effects of plants, like nutrient uptake, were neglected and aroot model was not implemented within the Wetland Module.
The influent concentration of COD was fractionalized as follows: CI (inert organic matter) was set to the measured effluent
COD concentration while the ratio of readily to slowly biodegradable organic matter, i.e., CR:CS, was 2:1 (Toscano et al.,2009). The adjustment of the biokinetic model parameters wasdone as follows:(a) The first adjustment of the parameter set addresses theunlimited growth of the two bacterial groups, XH and
XANs/XANb, as no growth limiting function is
implemented. Therefore the lysis rates [d–1] (bh, bANs, bANb)
were increased proportionally until a steady state within themodel domain was reached (Pálfy et al., 2016; Pucher et al.,2017).
(b) In order to fit the simulated and measured NH4-N effluent
concentrations, the maximum aerobic growth rate µANs [d–1]of XANs was adjusted.

Simulating vertical flow wetlands using filter media with different grain sizes with the HYDRUS Wetland Module(c) In the impounded drainage layer, denitrification occurred.
To fit the measured and simulated NO3-N effluent
concentrations, two parameters needed to be changed. First,
the hydrolyses rate Kh [d–1] was decreased so that less OM
had been degraded in the main layer and more was available
for denitrification in the impounded layer as electron donor.
Secondly, the maximum denitrification rate µDN [d–1] waschanged addressing the growth of XH in the impoundedlayer under anoxic conditions.
RESULTS AND DISCUSSION
For all systems, a good match between measured and simulated flow data could be reached (results not shown here but
can be found in Hochfeldt (2017), Langergraber (2017), andPucher et al. (2017)).
Table 3 shows measured and simulated influent and effluent
concentrations for all four VF wetland systems. In the first step,
simulations were carried out using the standard parameter set of

the CW2D biokinetic model (Langergraber and Šimůnek,2005). These results are referred to as the "standard" parameter
set in Table 3. When the match between the simulated andmeasured effluent concentrations was not sufficient, adjustments to the biokinetic model parameter set were carried out.
The adjusted parameters are listed in Table 4 and simulation
results are referred to as the "adjusted" parameter set in Table 3.
For system 4, no parameter adjustments were performed. For
more information on System 4, readers are referred to Pucher etal. (2017).
The fitting procedure was carried out using a trial-and-error
approach starting with the growth of the three bacterial groups,
namely XH and XANs/b, in order to get steady-state conditions(Figure 1). System 1 only needed to be adjusted for XH, while
System 2 was adjusted for all three. For System 3, no adjustment to the lysis rate was needed (Figure 2). This is explainedby the low hydraulic loading based on the event driven operation (Langergraber et al., 2014).

Table 3. Measured and simulated influent and effluent concentrations as mean values; *measurements taken only on the third day of loading.
Parameter set
System 1

Influent
Effluent

System 2

System 3

Measured
Measured
Simulated
Simulated
Measured
Measured
Simulated
Simulated
Measured
Measured
Simulated
Simulated
Measured
Simulated

Influent
Effluent

Influent
Effluent Stage 1

Effluent Stage 2

Simulated
System 4*

Influent
Effluent

COD[mg.L–1]300
20
29Standard
AdjustedStandard
AdjustedStandard
AdjustedStandard
Adjusted as for stage
1

Measured
Measured
Simulated
SimulatedStandard
Adjusted

NO3-N[mg.L–1]
0.1
37
56

394
35
29
35
351
42
33
32
<10
7

NH4-N
[mg.L–1]60
0.1
0.1
No adjustment required
93
14.5
0.1
13.6
52.9
14.1
0.9
16.3
<0.03
0.09

7

12.9

<0.01

850
171
141

94
30
3
No adjustments performed

2
38
83

0.59
70.8
85.0
83.6
< 0.1
0.6
54.7
5.0
12.2
12.9

XH [mgCOD.kg-1]

5000
4500

Standard lysis rate

4000

Adjusted lysis rate

3500
3000
2500
2000
1500
1000
500
0
0

20

40

60

80

100

120

140

160

180

200

Time [Days]

Fig. 1. Effect of the lysis rate adjustment for XH of System 2.

229

Bernhard Pucher, Guenter Langergraber
25

XH [mgCOD.kg-1]

20

15

10

5

0
0

50

100

150

200

250

Time [Days]

Fig. 2. Steady state conditions of XH growth in Stage 1 of System 3 with the standard value for the lysis rate bH.

Mean measured effluent concentration

1
0.9
0.8

µANs [d-1]

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0

2

4

6

8

10

12

14

16

18

20

NH4-N [mg.L-1]

Fig. 3. Effect of decreasing µANs on the simulated NH4-N effluent concentration during the fitting procedure of System 2.
Table 4. Comparison of the parameter adjustments required for the CW2D biokinetic model parameters.
Parameter

Description

Standard

Kh

Hydrolysis rate

[d–1]

bh

Lysis rate for XH

–1

[d ]

0.4

bANs/b

Lysis rate for XANs/b

[d–1]

0.15

–1

System 1

3
0.9

System 2

System 3
Stage 1

–

0.59

0.96

–

0.216

–

µANs

Maximum aerobic growth rate of XANs

[d ]

0.9

0.25380.18µDN

Maximum denitrification rate of XH

[d–1]

4.8

–

60

The fitting of measured and simulated NH4-N was carriedout by decreasing the maximum nitrifier growth rate µANs. The
effect of this adjustment is illustrated in Figure 3 for System 2
as an example.
The simulation results for System 1 show that, for systems
using the fine-filter material, sand with grain size distribution
0.063–4 mm, no parameter adjustments are needed, while for
System 2, using the coarser sand (grain size distribution
1–4 mm), the CW2D standard parameter set overestimates thetreatment performance. The same effect can be observed within
System 3 using coarser sand with grain size distribution
2–3 mm. When applying the adjusted parameter set from thefirst stage of System 3 to its second stage (using fine sand withgrain size distribution 0.063–4 mm), the simulated treatment
performance is underestimated. By using the CW2D standard

230

parameter set, a good fit can be achieved for the second stage ofSystem 3.
The difference in the simulation performance between fineand coarse filter media can be explained by the available waterflow model when using the HYDRUS Wetland Module. A
comparison between the classical equilibrium flow model and a
non-equilibrium model (dual-porosity model) was carried out
by Morvannou et al. (2013). When simulating a tracer experiment for the first stage of a French VF wetland system, thedual-porosity model fitted the experiment well, while with thestandard equilibrium water-flow model, the tracer couldn’t befitted. These results can be directly compared with the simulation of the treatment performance carried out in this study, as
for coarse media a higher performance was simulated than
measured. This is explained by the longer contact time between
wastewater and biofilm due to the equilibrium flow model.

Simulating vertical flow wetlands using filter media with different grain sizes with the HYDRUS Wetland Module

CONCLUSION
Based on the simulation results the following conclusions
can be drawn:• Despite differences in construction and operational
conditions of the presented systems, the simulation using
the HYDRUS Wetland module gives good results when
adjustments to the standard parameter set of the CW2D
biokinetic model (Langergraber and Šimůnek, 2005) havebeen made.
• The most limiting factor in achieving good simulation
results using the CW2D standard parameter set is related to
the grain size of the filter material used. For fine filter
media, the CW2D standard parameter set provides
sufficient results, while for coarser media, the treatment
performance is overestimated.
• As only single-pore water-flow models can be used with the
HYDRUS Wetland Module, only equilibrium-matrix flow
is described. Preferential flow paths occurring in coarser
media cannot be described and therefore the simulated
hydraulic retention time (HRT) is too high, resulting in an
overestimation of the pollutant removal.
• It is expected that when coupling a dual-porosity water-flow
model with the HYDRUS Wetland Module, better
description of the actual water flow, especially preferential
flow path and simulated HRT, can be achieved and thus
excessive changes of the biokinetic model parameters can
be avoided. Finally, it should become possible to simulate
measured effluent concentrations for VF wetlands withcoarse sand as the main layer using the standard parameterset of the biokinetic model.
REFERENCES
Henze, M., Gujer,W., Mino, T., Loosdrecht, M., 2000.
Activated Sludge Models ASM1, ASM2, ASM2d andASM3. IWA Publishing, London, UK
Hochfeldt, V., 2017. Numerical simulation of a full-scale twostage constructed wetland system. Master Thesis. Universityof Applied Sciences Höxter, Germany and University of
Natural Resources and Life Sciences, Vienna (BOKU University), Austria.
Kadlec, R.H., Wallace, S.D., 2009. Treatment Wetlands. 2nd
ed. CRC Press, Boca Raton, FL, USA.
Langergraber, G., 2001. Development of a Simulation Tool for
Subsurface Flow Constructed Wetlands. Ph.D. Thesis. University of Natural Resources and Applied Life Sciences, Vienna (BOKU University), Austria.
Langergraber, G., 2017. Applying process-based models for
subsurface flow treatment wetlands: recent developments
and challenges. Water, 9, 1, 5.
Langergraber, G., Šimůnek, J., 2005. Modeling variably saturated water flow and multicomponent reactive transport inconstructed wetlands. Vadose Zone Journal, 4, 4, 924–938.
Langergraber, G., Šimůnek, J., 2011. HYDRUS Wetland Module Manual - Version 2. HYDRUS Software Series 4. Department of Environmental Sciences, University of California Riverside, Riverside, CA, USA, 56 p.
Langergraber, G., Šimůnek, J., 2012. Reactive transport modeling of subsurface flow constructed wetlands using the HYDRUS Wetland Module. Vadose Zone Journal, 11, 2.
Langergraber, G., Leroch, K., Pressl, A., Rohrhofer, R., Haberl,R., 2008. A two-stage subsurface vertical flow constructedwetland for high-rate nitrogen removal. Water Science andTechnology, 57, 12, 1881–1887.

Langergraber, G., Rousseau, D.P.L., García, J., Mena, J. 2009a.
CWM1: A general model to describe biokinetic processes insubsurface flow constructed wetlands. Water Science andTechnology, 59, 9, 1687–1697.
Langergraber, G., Leroch, K., Pressl, A., Sleytr, K., Rohrhofer,
R., Haberl, R., 2009b. High-rate nitrogen removal in a twostage subsurface vertical flow constructed wetland. Desalination, 246, 1–3, 55–68.
Langergraber, G., Pressl, A., Haberl, R., 2014. Experiencesfrom the full-scale implementation of a new two-stage vertical flow constructed wetland design. Water Science andTechnology, 69, 2, 335–342.
Meyer, D., Chazarenc, F., Claveau-Mallet, D., Dittmer, U.,
Forquet, N., Molle, P., Morvannou, A., Pálfy, T., Petitjean,A., Rizzo, A., Samsó Campà, R., Scholz, M., Soric, A.,
Langergraber, G., 2015. Modelling constructed wetlands:
Scopes and aims – a comparative review. Ecological Engineering, 80, 205–213.
Molle, P., Liénard, A., Boutin, C., Merlin, G., Iwema, A., 2005.
How to treat raw sewage with constructed wetlands: an
overview of the French systems. Water Science and Technology, 51, 9, 11–21.
Morvannou, A., Forquet, N., Vanclooster, M., Molle, P., 2013.
Which hydraulic model to use for vertical flow constructedwetlands? In: Šimůnek, J., van Genuchten, M.T., Kodešová,R. (Eds.): Proc. 4th Int. Conf. HYDRUS Software Applications to Subsurface Flow and Contaminant Transport Problems. Dept. of Soil Science and Geology, Czech Universityof Life Sciences, Prague, pp. 247–257.
Mualem, Y., 1976. A new model for predicting the hydraulicconductivity of unsaturated porous media. Water Resources
Research, 12, 3, 513–522.
Paing, J., Voisin, J., 2005. Vertical flow constructed wetlands
for municipal wastewater and septage treatment in French
rural area. Water Science and Technology, 51, 9, 145–155.
Pálfy, T.G., Molle, P., Langergraber, G., Troesch, S., Gourdon,R., Meyer, D., 2016. Simulation of constructed wetlands
treating combined sewer overflow using HYDRUS/CW2D.
Ecological Engineering, 87, 340–347.
Pucher, B., 2015. Simulation of vertical flow filters for thetreatment of domestic wastewater using sand and zeolite as
filter materials. Master Thesis. University of Natural Resources and Applied Life Sciences, Vienna (BOKU University), Austria.
Pucher, B., Ruiz, H., Paing, J., Chazarenc, F., Molle, P., Langergraber, G., 2017. Using numerical simulation of a one
stage vertical flow wetland to optimize the depth of a zeolite
layer. Water Science and Technology, 75, 3, 650–658.
Šimůnek, J., van Genuchten, M.T., Šejna, M., 2016. Recent
developments and applications of the HYDRUS Computer
Software Packages. Vadose Zone Journal, 15, 7, 25 p.
Toscano, A., Langergraber, G., Consoli, S., Cirelli, G.L., 2009.
Modelling pollutant removal in a pilot-scale two-stage
subsurface flow constructed wetlands. Ecological Engineering, 35, 2, 281–289.
van Genuchten, M.T., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil
Science Society of America Journal, 44, 5, 892–898.
Received 29 June 2017
Accepted 7 September 2017231