11th International Conference on Wetland Systems for Water Pollution Control

Modelling CWs for CSO treatment – reasonable balancing between
detailed description and practicable handling
D. Meyer*, U. Dittmer** and T. G. Schmitt*
ABSTRACT
Simulation models are an important tool in planning urban drainage systems, soenhanced treatment devices need to be implemented in the computing
procedures. In combined sewer systems an enhanced treatment of combined
sewer overflow (CSO) is required when augmented water quality requirements
are demanded or conventional CSO treatment does not meet environmental
quality standards. Constructed wetlands (CWs) have become the most common
method to provide enhanced treatment of CSO in Germany.
Balancing between detailed description and practicable handling in modelling
CWs for CSO treatment, also known as retention soil filters (RSFs), depends onmodelling aims. To calculate average emissions of these CWs simple
approaches like a RSF module in long-term pollution-load model KOSMO areadequate (Kaufmann & Schmitt, 2005). To describe and investigate operational
conditions complex biokinetic reaction models like CW2D (Langergraber &Šimůnek, 2005) can be used, but they are not applicable as optimization tools
in practical use.
To optimise operation and design the model RSF_Sim has been developed.
Within the model a reasonable balance between detailed description andpracticable handling has to be found. First versions of the new model are able
to describe basic processes appropriately. Simulation results for single eventsshow a good match of hydraulic conditions and effluent concentration curves(Meyer et al., 2008). Long-term simulation runs show proper results calculating
average emissions, but also indicate needs of more detailed descriptions within
the black box under varying operational conditions. Within the paper
requirements, methods and prospects of modeling RSFs for practical use are
given and discussed.
Keywords
CWs for CSO treatment; detailed simulations; CW2D; modelling for operationand design

INTRODUCTION
During rain events discharges in combined
sewer systems often exceed the capacity of
WWTPs. A part of rainfall runoffs can be stored
in storm water tanks but in case of hard or longrain events combined sewer overflows (CSOs)
can not be avoided. To reduce the ecologicalimpact of these overflows on receiving waters
reed planted vertical flow constructed wetlands(CWs) for treatment of CSOs, also known asretention soil filters (RSFs), are widely appliedin Germany.
Constructed wetlands for CSO treatment are
generally designed as vertical flow filters with a

D. Meyer*, U.
Dittmer** and T. G.
Schmitt** Institute of Urban
Water Management,
Technical University of
Kaiserslautern, PaulEhrlich-Strasse, 14-316,
D-67663 Kaiserslautern,
Germany(dmeyer@rhrk.unikl.de)** Institute of Sanitary Engineering,
Water Quality and Solid Waste
Management, University of Stuttgart,
Bandtaele 2, D-70569 Stuttgart, Germany,(E-mail: isiawtud@iswa.uni-stuttgart.de)detention basin on top of the filter layer. Middle
sand (0/2 mm) has proved to be the mostefficient filter material for this kind of treatment.
The filter itself is planted with reed (phragmites
australis). Fig. 1 shows a cross section of awetland plant. During operation particulate
substances are retained at the filter surface. Theybuild up an additional sediment layer on top of
the original sand filter. The filtration rate iscontrolled by a throttle in the outlet structure.
RSF are operated under constant filtration rates.
Recommended values are 0.2 or 0.1 l/(s*m²)
depending on the filter material.

Fig 1: Schematic cross section of a constructed wetland for CSO treatment

November 1-7, 2008 Indore, India851

11th International Conference on Wetland Systems for Water Pollution Control

The filter layer is completely drained after every rain event to ensure maximum aeration and aerobic
degradation. Field studies have shown that dry periods between the loading events are also essential toprevent clogging (Uhl & Dittmer, 2005). To reduce frequency and duration of loading events upstream
storm water tanks are applied. The inflow of this type of CW therefore consists in the CSO fromconventional storm water tanks (figure 2). The loading regime of the CW depends on local rainfall
characteristics and on the volume of the upstream storm water tank.
outflow control 2
inflow structure
outflow control 1
storm water tank

r2
filte
filter 1

emergency overflow

overflow 2
perforated
pipes

wet weather flow
by-pass

perforated
pipes

overflow 1

2 x dry weather flow
to WWTP

Fig. 2 Scheme of the full scale plant
Typical overflow frequencies of storm watertanks in Germany are 40 to 60 events per yearwith an accumulated duration of 200 to 300
hours per year (long term mean values). Runoff
volume and pollution loads vary within a wide
range. Flow rates and pollutant concentrations
also vary in the course of each event (e.g. firstflush effects). Dry periods can last up to several
months.
Dimensioning of CW for CSO treatment isbased on long term rainfall-runoff simulation.
The main criterion is the annual hydraulic load
[m³/a] per filter area [m²]. The long term average
shall not exceed 40 m³/(m²*a).
PROCESSES IN RSFS
The main processes within the filters are (1)detention and reduction of peak flows, (2)reduction of suspended solids, (3) retention ofsoluble and particulate pollutants by adsorptionand (4) subsequent biological degradation(especially of ammonium nitrogen). The
processes in CWs for CSO treatment have been
analysed in several research project including
field studies and lab scale experiments. The
main results of these activities are summarized
below.
Hydraulic processes
The hydraulic processes within the filter are
determined by the characteristics of the filter
material (middle sand) and by the operational
conditions (mostly inflow >> outflow). Bothfavour saturated flow, as soon as the retainedwater volume exceeds the effective pore volume

November 1-7, 2008 Indore, Indiaof the filter. In case of smaller loading rates
and/or durations the filter works underunsaturated or partly saturated conditions. The
filter layer is completely drained after loadingrain events. A more detailed description of thehydraulic processes is given in Meyer et al.
(2008).
Retention and degradation processes forpollutants
The elimination of ammonium is based on a
two-step process: during infiltration ammonium
is absorbed by biofilms within the filter layer.
Concentrations are reduced to a backgroundlevel. Nitrification of the retained nitrogenmainly takes place during the following dry
period. Nitrification activity is highest
immediately after the draining and re-aeration of
the filter layer. Nitrification of the retained
ammonia regenerates the sorption capacity.
Long term loading and high ammonia loads canlead to a breakthrough of the inflow
concentration. Adsorption capacity is highest in
the sediment layer and in the upper parts of thesand filter. A high number of small loading
events can also lead to an inhomogeneous
horizontal distribution with higher sorption
capacity close to the inflow structure. Biofilm
growth in new-built RSFs takes several months,
but established biofilm can keep its sorption
capacity over dry periods up to a year.
As one result of nitrification NO3-N
concentrations in the filter layer rise during dry
periods. This leads to nitrate wash-outs at the
beginning of the next events. The adsorption852

11th International Conference on Wetland Systems for Water Pollution Control

capacity for ammonium usually is regenerated
completely within a few days.
Organic carbonates – represented by COD –show a different behaviour. Most part of theparticulate fraction (size > 0.5 µm) is retained at
the surface of the filter layer and oxidized during
dry periods. For soluble COD compounds thefilter shows an almost constant removal rate.
Oxygen consumption during infiltration
indicates
immediate
degradation.
Ammonification of organic nitrogen during dry
periods shows that there is also delayed
degradation of organic substances that havepreviously been retained by absorption.
MODELLING PROCESSES IN RSFS
Simulation studies using CW2D
To simulate the performance of lab-scale filter
columns in detail the multi-component reactive
transport model CW2D (Langergraber &Šimůnek, 2005) was used. CW2D originally wasdeveloped to simulate the treatment of municipal
wastewater in subsurface flow constructedwetlands. It is applied to simulate thesaturated/unsaturated flow, transport anddegradation processes occurring in a constructedwetland for the treatment of CSOs. Simulation
results showed that the biokinetic reaction modelCW2D is generally applicable for the simulationof RSFs (Dittmer et al., 2005), but additional
components have to be implemented. Especiallythe filtration process has to be taken into
account, because filtrated organic solids are thesource of nutrient uptake for microorganisms
after degradation of sorbed nutrients (Meyer etal., 2006).
The description of purification processes in
CW2D is more detailed than the actual
knowledge on RSF processes. For using themodel several assumptions have to be made, thatcan not be experimentally verified (e.g. influent
COD
fractionation,
concentration
anddistribution of microorganisms, organic matter
degradation and nitrification). This applies
especially to the description of COD degradationin dry periods between the loading events(Henrichs et al, 2007)
Simple model for long-term pollution-load
simulation
Pollution-load simulations are generally used todescribe the long term impact of urban drainage
systems on receiving waters. Essential
parameters are annual loads of COD andsuspended solids. For ammonia the focus lies on
the frequency of critical concentrations.
Based on the results of a field study a simple
module for balancing constructed wetlands has
been developed and implemented in thepollution load simulation model KOSMO(Kaufmann & Schmitt, 2005). Figure 3 shows afunctional diagram of the module.

Fig. 3: Functional diagram of the modelling module for RSFs.
Constructed wetlands are represented in the model as a control structure with a defined storage volume
and a default removal efficiency of pollution loads. The storage volume consists of the detention basin
on top of the filter layer and the volume of the effective pores within the soil filter (15 %). The module
calculates the removal processes in a simplified way: either a constant outflow concentration ofpollutants or a constant removal rate (η) can be chosen. These values should represent annual meanvalues. Descriptions of specific processes for suspended solids (SS), filterable COD (COD_X),
dissolved COD (COD_S), ammonium and nitrate are given in table 1. In the case of ammonia theconstant concentration (first phase) and constant removal rate (last phase) are combined.

November 1-7, 2008 Indore, India853

11th International Conference on Wetland Systems for Water Pollution Control

Table 1: Description of specific processes in KOSMO
Parameter
Process
EquationsSSconstant c_eff
c_eff = 3 mg/l
COD_X
constant c_eff
c_eff = 3 mg/l
COD_S
constant ηηCSB_S = 50 %NH4-N
sorption capacity (available at the beginning of sorpNH4 = 4 g/m2
every loading event)phase 1: constant c_eff
c_eff = 0.1 mg/l
phase 2: linear transition ph.1 to ph. 3 over 36 h c_eff = 0.1 + c⋅(1-η)⋅tS-end/36
after exhausting sorption capacity
tS-end < 36 h;phase 3: constant ηc_eff = 0,1 + CRBF⋅(1-η)ηNH4-N = 10 %c_in = inflow concentration; c = concentration in detention basin; c_eff: effluent concentration
Discharges and pollutant loads from filter
overflow and outflow of the constructed wetland
are balanced as emitted volume and load
respectively. Further simplifications andpresumptions of the model are:•draining starts instantaneously with the
beginning of the loading of the filter,•the conditions of the soil (e.g.
saturation) are not considered,•
only pore volume of sand layer is
included as additional detention
volume,•interdependencies/interactions between
different pollutants are disregarded,•
presumption of complete degradation ofretained substances during dry periods.
For these simulations the CW has been modelledwith constant effluent concentrations or
pollutant retention efficiencies.
This model only describes the emissions from
RSFs. The internal processes are not modelled.
As sorption and degradation kinetics are notconsidered it is only applicable for a constantreactiontime.
Assuming
predominantly
saturated flow, reaction time is kept constant by
the control of the filtration rate. Thus the model
is not able to predict the effect of variations inoperation mode or plant layout.
Development of new model RSF_Sim anddiscussion of further improvements
In spite of a good approximation on annual
balances the KOSMO approach seemed to beinefficient to reach the aim as an optimisation
tool for design and operation due to themultitude of influencing factors. In contrast to
the previously described approach the newmodel RSF_Sim (Retention Soil Filter
Simulation) is not embedded into the KOSMO
simulation program. It is an independent tool
which can be used for manual data input as well
as for dealing with input data from KOSMO via
a software interface. For easy handling agraphical user interface has been implemented
for data input.

November 1-7, 2008 Indore, India

Further improvements compared to the internal
KOSMO approach are more detailed
mathematical process descriptions for each kindof pollutants. The new model RSF_Sim shouldbe accurate enough to describe the performance
under different conditions. On the other hand
handling should be as easy as possible(Langergraber et al., 2008). Proper complex
descriptions and simplifications have to be
discussed for hydraulic conditions and typicalpollutants.
Hydraulic model: In a first step of model
development RSFs are described as a sequenceof three stirred reactors with one-dimensional
water flow and variable water contents.
According to the real layer structure the wholefilter is divided into retention space, filtration
layer and drainage space. Duration of water
passage through the filter layer has been
implemented to take the hydraulic retention time
into account. First simulation studies showproper results. Detailed model descriptions andfirst simulation results are given in Meyer et al.
(2008).
In more detailed development steps the filter
layer itself is divided into horizontal and/or
vertical sub-layers to implement some of thefollowing discussed improvements of pollutant
retaining processes. Calculating processes inhorizontal sub-layers are identical to the main
layer, in the same way a sediment layer can be
defined. For vertical sub-layers the filter space is
described as parallel parts which are feed after
each other according to water contents in the
single retention layer. Water exchange between
different parallel parts is not considered due tothe prediction of part width >> part heights.
Effluent concentrations are stirred in thedrainage layer.
Suspended solids: SS have to be mainly taken
into account to calculate RSF emissions. To
calculate retention of SS the original KOSMO
approach is taken over due to high quality ofsimulation results. Accumulation of SS could854

11th International Conference on Wetland Systems for Water Pollution Control

indicate the risk of occurring mechanicalclogging effects. Therefore the main value had
to be fractioned into different particle sizes. Up
to now investigation of clogging processes do
not give proper data bases to estimate generaleffects. Detailed descriptions would also lead toa number of estimated input values andcomputation steps compared to the simple flow
model. Therefore detailed mathematicaldescriptions should be tried first in models like
CW2D using Finite-Element-Meshes anddifferential equations.
Organic carbonates: The enormous number oforganic carbonates is represented by COD. In
research programs on CWs measurements ofsolute COD compounds are accomplishedadditionally to total COD. In this way filterable
COD can be calculated so that databases are
given for two different fractions. Usually longterm pollution-load models also consider this
distinction. Due to these facts calculations of
two different COD fractions as input values havebeen implemented in the first model version of
RSF_Sim.
COD_X: In contrast to KOSMO simulations
RSF_Sim does not calculate filtration processes
with constant effluent values as backgroundconcentrations. Exact measurements of low
COD_X concentrations (< 5mg/l) are notpracticable in practice of RSF investigations, so
low measurement values only indicate very highfiltration rates and can be calculated in that way.
To be able to consider accumulation processes
RSF_Sim deals with two different filtration
rates. Between retention and filtration layer asurface filtration is described as accumulative
process by addition of retained masses. Smaller
particles can reach deeper regions of the filter
layer. They are supposed to be filtrated andaccumulated in the filter layer space only as asmall part of the total filtration process. The
addition of both filtrations leads to a total
removal rate close to 100 %. Accumulation is
described as the addition of retained masses.
Descriptions of accumulation processes give
options to calculate COD_X degradation in latermodel versions. The distinction of COD_X
retention on the surface and in filter space isneeded to consider processes in the sediment andfiltration layer separately. The surface filtration
is supposed to be the basic process forestablishing a sediment layer. Accumulation ofparticles is usually more intensive close to the
inflow structure, so vertical sub-layers can beused to take distribution into account.
Degradation of small particles within the filter
layer will have an influence on the amounts of
COD_S and ammonium as source term. Due toNovember 1-7, 2008 Indore, India

the lack of knowledge of processes within thesediment simulation results have to provepracticability.
COD_S: In the current model version recurring
processes for COD_S are immediate high-ratedegradation as well as absorption and delayed
degradation only in the dry period. In that way
simulated outflow curves could be calibrated on
measured single events with different outflow
limitation rates, but validation on long-term
measurements has failed. In a next step longterm results have been slightly improved byassuming
lower
filterperformances
corresponding to durations of dry periods anddescribing degradation rates with temperaturedependency.
Future model versions will consider the
reduction of oxygen demand corresponding toavailable oxygen. Measurements of dissolved
oxygen distribution within the filter layer(Wozniak et al., 2007) indicate proper
estimations of degrading conditions. In that way
RSF_Sim could be able to describe critical COD
overloads. Instead of describing the filter
performance depending only on the duration ofdry periods a description of slow degradation
processes for all filtrated organic particles will
be used as an indication for current filter
performances. The more degradable organic
compounds are available in long dry periods themore nutrients for microorganisms are available
and the higher the filter performances should be
on following loading events.
Ammonium nitrogen: Ammonium is supposed tobe adsorbed during passage of the filter layerand degraded after drainage. To calculate
sorption processes different mathematicaldescriptions were tested. The use of Freundlichisotherms with empirical parameters k and n(Csorbed = k * Cdissolved ^ n) is not adequate
to the simple character of RSF_Sim because
iteration computation is required. Calculation
over linear isotherms (Csorbed = k *Cdissolved) is not able to describe
breakthroughs of inflow concentrations. Either
beginning breakthroughs right after the
beginning of outflows (small k-values) oroverestimated sorption capacities (high k-values)
have to be chosen. Therefore a calculation
method with two linear isotherms valid over two
different concentration ranges in combination
with degradation was developed (Meyer et al.,2008).
Compared to the KOSMO model the new
approach is able to calculate mass variations inthe solid phase depending on dissolved
concentrations. This allows finding actual
concentrations in the liquid and solid phase for855

11th International Conference on Wetland Systems for Water Pollution Control

different sub-layers as well as model calibrationon outflow curve measurements. Detailed
investigations of sorption processes showed thatsorption capacities can be set as equal over all
filter depths (Meyer et al., 2006), so the numberof input values does not increase corresponding
to the description of sub-layers.
Nitrate nitrogen: Due to the fact that anoxic
conditions within the filter are defined as criticaloperational
conditions
descriptionofdenitrification processes is not a goal ofmodelling RSFs. Wash-out of degradedammonium is described as strictly convective in
addition to the passage of inflow concentrations.
A comparison of simulation results withmeasured effluent nitrate values as massbalances indicates ammonification processes andcan be used for calibration.
CONCLUSIONS AND OUTLOOKModelling CWs for CSO treatment as
optimisation tool operation and design inpractical use needs to come up to long-term
pollution-load
model
requirements.
Uncertainties calculating sewer systems andpreliminary treatment in stormwater tanks within
pollution-load models are not considered in thisstudy. The new model RSF_Sim is also
practicable for manual data input and has beenused first in this way.
Reasonablebalancing
betweendetailed
description and practicable handling inmodelling CWs for CSO depends on modellingaims. To calculate average emissions of RSFs
the KOSMO approach is adequate, but in thisway operational conditions within the filter
under varying conditions can not be described.
Therefore it has been combined with simplified
ideas of complex descriptions within the
biokinetic reaction model CW2D to RSF_Sim.
First simulation studies on lab-scale CWs
showed proper results (Meyer et al., 2008), butalso indicate needs of more detailed descriptions
within the black box CW. Detailed knowledge ofoccurring processes can lead to improvements aswell as further simulation studies with extended
mathematical descriptions within CW2D.
Current aims of model development are moredetailed descriptions of filter operation during
dry periods. To consider sediment influences
accumulated CSB_X is calculated as partly
degradable as well as absorbed COD_S. Bothfractions have influence of nutrients available
for microorganisms during dry periods. Current
status of degradation is used as indication forfilter performance for the next loading event. To
consider influence distributions of sediment thefilter layer is divided horizontally into sub-layers
for sediment and filter sand. In vertical direction

November 1-7, 2008 Indore, India

higher accumulation of sediment close to the
inflow structure can be taken into account. Dueto the lack of knowledge of long-term internal
processes (esp. within the sediment) simulation
results will show practicability to calibrate input
values on long-term measurements of effluent
concentrations. For single events results of soil
investigations during filter operation (Dittmer,
2006 / Wozniak, 2007 / Meyer et al., 2006 /Schwarz et al., 2001) are used. Availabilities,
numbers and sensitivities of input parameters
have to be analysed to keep RSF_Sim applicable
as a practical tool.
As a next step of model development criticaloperation conditions will be considered.
Therefore coherences of COD and available
oxygen will be described approximately. For
single events lab-scale results of oxygen
distribution during water-saturated operational
conditions (Wozniak et al., 2007) will be used
for calibration.
Up to now RSF_Sim is able to calculate average
emissions of enhanced CSO treatment. Further
developments will show effectiveness on
optimisation of operation and design byvalidating calibrated simulation results under
specific conditions on more (given) investigation
results. The main question will be theinvestigation and the description of recurring
processes during dry periods.
ACKNOWLEDGMENT
The development and use of simulation models
for CWs for CSO treatment is funded by a PhDscholarship of the German Friedrich-EbertStiftung.
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