Table Of ContentTreating Urban Runoff in Australia Using Floating Wetlands
Christopher WALKER1,2, Darren DRAPPER3, Peter NICHOLS2, Kristin REEVES2, and
Terry LUCKE2
1Covey Associates Pty Ltd, 124 Duporth Avenue, Maroochydore, Queensland, 4558 Australia
2University of the Sunshine Coast, School of Science and Engineering, 90 Sippy Downs
Drive, Sippy Downs, Queensland 4556 Australia
3SPEL Environmental, 96 Cobalt St, Carole Park, Queensland 4300
*Corresponding author’s e-mail: [email protected]
ABSTRACT
Overview
It is widely accepted that Australia's increasing levels of urbanisation will result in increased
stormwater runoff and higher levels of pollution. Water sensitive urban design (WSUD) has
risen in prominence as a consequence of the need to address stormwater issues in relation to
urban development. Integrating urban planning and management practices to protect and
conserve the urban water cycle and ensuring that urban developments are sensitive to natural
hydrological and ecological processes are key principles of WSUD. Constructed wetlands and
bioretention systems have been used previously to replicate and enhance the environmental
benefits and ecosystem services provided by natural buffer systems (e.g. natural wetlands).
This paper describes the performance evaluation of an innovative solution to treating
stormwater in the form of an artificial floating wetland. Floating Wetlands Treatment (FWT)
systems have the potential to provide greater stormwater pollution removal rates per unit area
compared to constructed wetlands and other treatment systems. A research study is underway
at Bribie Island in Southeast Queensland (SEQ) to assess the ability of FWT to treat
stormwater. The experimental design of the study incorporates an array of impermeable baffles
which directs all of the catchment stormwater runoff through the FWT system, eliminating
short circuiting and allowing the efficacy of the system to be assessed in variable flow
conditions. This approach has successfully eliminated the short circuiting issues experienced in
previous studies. This approach makes the FWT an on-line treatment system and enables real-
time water sampling to be undertaken to accurately evaluate the FWT pollution removal
performance.
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Objectives
The objectives of this research project include:
⦁ To evaluate the overall pollution removal performance of the SPEL Waterclean FWT
system in treating;
⦁ Total nitrogen (TN)
⦁ Total phosphorus (TP)
⦁ Total suspended solids (TSS)
⦁ To determine the treatment performance in various flow conditions;
⦁ To evaluate the pollution removal performance of Carex appressa plants by analysing
sediment mass collected on plant roots and by analysing root length and mass.
⦁ To establish the optimal ratio of contributing catchment area to treatment area to
enable FWT systems to be sized effectively.
Method
The sampling and handling procedures were in accordance with the Queensland Department of
Environment and Heritage Protection (DEHP)’s Monitoring and Sampling Manual(2009) and
the National Association of Testing Authorities (NATA) requirements. All analyses are
completed in the NATA accredited laboratory at the USC. The FWT system was installed in
June 2013 with instrumentation for data collection installed in February 2014, allowing for an
8 months plant establishment timeframe. It was originally proposed to install the data collection
instruments in November, however, site activity delayed this. To date, two storm events have
been captured, however, ongoing issues with the flow meter have caused problems with
triggering the sampling protocols in rain events. In two events, samples were collected based on
time, rather than flow, as the flow meter was not registering flow even though flow was
evident. As part of a troubleshooting exercise, water from the outlet of Lake 1 was pumped
into the inlet of Lake 1, directed at the floating wetlands. This served to test the flow meter and
simulate an event. A number of samples were collected during this exercise, with samples
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collected on 5 and 3 minute intervals at flow rates of 100L/s and 200L/s, respectively. These
flow rates equated to approximately half of a Q event (100L/s) and a Q event
3-month 3-month
(200L/s) for the contributing catchment. While it is recognized that the existing results are
collected on the basis of time, rather than flow rate, the flow rates in the pumping test were
known.
Results
The preliminary results to date show substantial promise, particularly as the FWT system
represents 0.1% of the contributing catchment. In an event collect on 14 May 2014, the FWT
demonstrated pollutant concentration reductions from the inlet to the outlet of 56.1% for TSS,
27.1% for TN and 83.33% for TP. The results from the pump test sample collection found that
the average pollutant concentration reductions from the inlet to the outlet of the system in a
100L/s flow rate were 96% for TSS, 41% for TN and 91% for TP. In the 200L/s flow rate, the
average pollutant concentration reductions from the FWT inlet to outlet were 68% for TSS,
13% for TN and 63% for TP.
Conclusions
The interim study results suggest that the SPEL Waterclean FWT systemtrialled in this study
may offer an innovative approach to urban stormwater treatment. The modular nature of the
device allows for flexibility in design and construction, and easy maintenance. Providing study
results continue in a positive trend, this new system may establish itself to be a valuable
addition to the diversity in approaches available to land developers, and ultimately local
governments, suitable for at-source stormwater treatment.
KEYWORDS
Floating wetlands; stormwater pollution; urban catchment runoff; nutrient removal
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INTRODUCTION
Urban development in Australia has increased substantially and it is widely accepted that as
urban development continues, stormwater runoff volumes and pollutant loads will increase in
tandem. Water Sensitive Urban Design (WSUD) has become a prominent approach to
managing and mitigating the effects of urban runoff. The key principles of WSUD are to
integrate urban planning and management practices to protect and conserve the urban water
cycle and ensuring that urban developments are sensitive to natural hydrological and ecological
processes. Constructed wetland, bioretention and other vegetated/detention systems (e.g.
vegetated swales and sediment basins) have been the principal approaches used to treat
stormwater runoff, though in recent years, proprietary devices (e.g. gross pollutant traps and
filter cartridges) have also been utilised to treat stormwater runoff. This paper describes a
research study that is underway in SEQ to assess the potential for floating wetland treatment
(FWT) systems to be a new means of treating urban runoff in Australia
FWT systems have been used in aquatic enhancement projects for over 20 years internationally
to treat effluent and to provide and/or improve water habitats (Burgess and Hirons, 1992;
Kerr-Upal et al., 2000; Headley and Tanner, 2008; Sukias et al., 2011). A number of early
FWTs provided habitat for aquatic waterfowl (Kerr-Upal et al., 2000), while other projects
focused on the removal of total suspended solids (TSS) pollutants from mine tailings (Burgess
and Hirons, 1992; Smith and Kalin, 2000).
FWT systems simulate the naturally occurring water treatment processes that take place in
natural floating wetland islands. Natural floating wetlands develop when large quantities of
floating organic matter, sediment, and wetland plants combine to form a buoyant island. This
provides an ideal surface for rooted macrophyte species plants to grow. Artificial FWTs are
supported by a floating medium, typically comprised of woven plastic, matting, or fibreglass,
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where plant roots grow directly into the water column, similar to a hydroponic system. As the
plant roots grow through the floating medium and into the water below, they provide an
extensive surface area for biofilm to grow on the root hairs (Figure 1). Biofilm coverage is an
essential requirement for the removal of nutrients from stormwater (Borne et al., 2013;
Winston et al., 2013), as it helps remove nutrients (particularly nitrogen) from the water
through nitrification/denitrification processes, and is ultimately taken up by the macrophytes.
Phosphorus can be retained through binding processes that occur within the biofilm (e.g.
adsorption) and uptake of orthophosphates is achieved by vascular macrophyte species.
Figure 1 -Floating wetland schematic (Image courtesy of SPEL Waterclean)
Research in the United States (Stewart et al., 2008) and New Zealand (Sukias et al., 2011) has
found that FWT can provide an effective, low cost and low maintenance means of treating
domestic and agricultural wastewater. Sukias et al. (2011) found that FWT were capable of
reducing TSS by up to 81%, total nitrogen (TN) by up to 34%, and total phosphorus (TP) by
up to 19%. However, the number of studies on the performance of FWT in treating urban
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stormwater runoff is limited.
There is a distinct lack of knowledge regarding the ability of FWT systems to improve urban
stormwater runoff quality. Although a number of recent studies have looked at using FWT to
treat runoff from urban, non-residential settings (e.g. major roadways and parking lots) (Borne
et al., 2013; Winston et al., 2013), there have been limited studies on using FWT systems to
treat TSS and nutrient loads associated with urban residential stormwater. Filling this
knowledge gap is critical to expanding options when it comes to urban stormwater treatment
and advancing the principles of WSUD. In Australia, stormwater treatment systems can often
take up substantial areas of valuable development land. This is compounded when
detention/retention systems are required. Stormwater treatment systems are not credited within
infrastructure agreements in Australia (Leinster et al., 2009; Leinster et al., 2010), so the area
required for stormwater treatment is essentially removed from the development footprint, thus
reducing the potential economic yield per hectare.
Many stormwater treatment systems, such as bioretention basins and constructed wetlands,
function best when ‘offline’, with extended detentions depths minimised and high flows
bypassing such systems (Water by Design, 2012a; Water by Design, 2012b). This often
requires detention/retention basins to be separate from treatment systems. FWT has the
potential to substantially reduce the footprint required for stormwater treatment compared to
other systems for two main reasons. Firstly, the hydroponic nature of root development allows
more contact between roots and polluted stormwater which provides a greater surface area for
biofilm growth. FWT systems therefore have the potential to provide significantly greater
stormwater pollution removal rates per unit area compared to constructed wetlands. Secondly,
FWTs are not affected by variations in extended detention depths, as the floating matrix rises
with the water level during storm events. This factor allows detention and treatment systems to
be combined on a large scale with minimal impact to treatment efficacy, and subject to system
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design, with virtually no impact on flood storage capacity. This could substantially reduce the
stormwater treatment footprint on residential development, thereby increasing lot yields per
hectare.
This paper describes a study that is currently underway to assess the ability of a large-scale
FWT to treat stormwater runoff from an existing urban development in SEQ. The experimental
design of the study incorporates an array of impermeable baffles which directs all of the
catchment stormwater runoff through the FWT system. This approach has successfully
eliminated the short circuiting issues experienced in previous studies. This approach makes the
FWT an on-line treatment system and enables real-time water sampling to be undertaken to
accurately evaluate the FWT pollution removal performance to be assessed in variable flow
conditions
METHODS
Site Description
The FWT monitoring site is located in an existing lake, Lake 1, (Figure 2) within the Bribie
Lakes development at Bongaree, Bribie Island, Queensland, Australia. The development site is
zoned residential and has an area of 42.3ha, which is being developed into a 600+ residential
lot development. An existing, developed urban catchment area of approximately 10 ha also
discharges into the study lake (Figure 2). The time of concentration for the external catchment
draining to the FWT is estimated to be 15 minutes. The 1-year ARI, 15 minute intensity
frequency duration (IFD) value (I ) at Bongaree is 77 mm/h (BOM, 2013). Subject to the
1y,15min
outcomes of the current research, the FWT will eventually treat runoff from the site prior to it
discharging into a larger, central lake of the development.
The study FWTs are constructed using four layers of recycled plastic fibre, with inserts of
marine grade foam to provide buoyancy. This matrix is then covered with a coconut jute mat
and the plants (Carex appressa) are installed in circular holes at a density of approximately
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three plants per square metre. Each individual module of FWT is approximately 9.2m2 and
there are a total of 11 modules, yielding a total FWT surface area of 101.2m2. Six plants
contained within metal planter boxes have been placed through the middle of the FWT. These
will be removed from the FWT at different times during the study to assess root growth and
allow for harvesting to determine biomass and pollutant retention.
The experimental design also incorporates an array of impermeable baffles (Figure 2) which
directs all of the stormwater runoff through the floating wetlands, eliminating the short-
circuiting problems experienced in previous studies. The impermeable baffles allow the
treatment performance of the FWT to be assessed under a range of flow conditions. Three
autosamplers have been installed at the inlet, middle and outlet of the floating wetland
(Samplers 1, 2, and 3, respectively). The autosamplers are triggered by flow and
rainfall.
Figure 2 -Floating Wetland Treatment System study site.
Sampling Protocol: Minimum Requirements
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In order to assess the efficacy of the FWT in treating urban runoff, the protocols and
parameters shown in Table 1 will be used to ensure qualifying stormwater flow events are
captured and standardised, and that repeatable sampling methods and procedures are utilised.
This method has been developed in conjunction with the study partners, University of the
Sunshine Coast (USC), Covey Associates and SPEL Environmental, based on the protocol
development methodology prescribed by the United States Environmental Protection Agency’s
(US EPA) Stormwater BMP Monitoring Manual (US BMP Database, 2009) and Auckland
Regional Council’s Proprietary Device Evaluation Protocol for Stormwater Quality
Treatment Devices(Wong et al., 2012).
Table 1 -Sampling protocol
Requirements Criteria Details
Min.2mm Rainfall Depth within 30 min.
Minimum Qualifying Events 15 Min. 15 min. Storm Duration Min. 6 h
Antecedent Dry Period
Minimum of the first 60% of event Min. Composite sample 1.6L Min. 8
Hydrograph sampled
hydrograph sample aliquots
1. 300mm upstream of FWT
Three Water Sample locations Hose to extend 200mm below the base of
2. Centre of FWT modules
(Refer Figure 2) the FWT matrix.
3. 300mm downstream of FWT
To be collected within 2 hours of storm
Sample method ISCO GLS Auto-samplers
end.
Sontek IQ Plus at FWT outlet. Automatic
Flow-weighted samples Every 19,200L
logging. Calibrate annually
Plant growth analysis Measure stem height & root lengths Record and Photograph every 3 months.
Rainfall monitoring Pluviometer (0.2mm increments) Automatic logging. Calibrate annually
Flow measurement Sontek IQ Plus at FWT outlet. Automatic logging. Calibrate annually
pH & EC Handheld probe Calibrated to manufacturer’s specifications
Total Suspended Solids (TSS)
HDPE or glass bottles, Cool to 4oC, max.
& Total Dissolved Solids APHA (2005) 2540 C & D
hold time 24 hours
(TDS)
HDPE or glass bottles, Cool to 4oC, max.
Total Nitrogen & TKN APHA (2005) 4500 N
hold time 24 hours
HDPE or glass bottles, Cool to 4oC, max.
Ammonia N APHA (2005) 4500 NH3
hold time 24 hours
HDPE or glass bottles, Cool to 4oC, max.
NOx APHA (2005) 4500 NO3
hold time 24 hours
Total Phosphorous & HDPE or glass bottles, Cool to 4oC, max.
APHA (2005) 4500 P
Orthophosphate hold time 24 hours
Particle Size Distribution Laser Diffraction (Malvern Continuously stirred, without chemical
(PSD) Mastersizer 3000) dispersion or sonication
NATA registered for all parameters
Laboratory Certification --
except PSD
Random duplicates and blanks in
QA/QC accordance with the relevant --
Australian Standard.
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Sampling Protocol and Handling
The sampling procedure and handling are in accordance with the Queensland Department of
Environment and Heritage Protection (DEHP)’s Monitoring and Sampling Manual(2009) and
the National Association of Testing Authorities (NATA) requirements. All analyses are
completed in the NATA accredited laboratory, Analytical Services, at USC.
Data Analysis
To analyse the data sets, the parameters listed in Table 2 will be used.
Table 2 -Data analysis method
Requirements Criteria Details
Percent Removal based on mg/L data Arithmetic mean, and median Identify % removal for each event
table comparison of influent vs effluent AND removal efficiency based on
concentrations for: mean and median concentrations
across all qualifying events.
⦁ Sampler 1 and 3
⦁ Sampler 2 and 3
Statistical Evaluation of Data Sets Box and Whisker plots with Outlier Use recognised methods for
identification identifying and excluding outliers
Statistical Evaluation of Significance Compare raw and log-transformed
data sets for influent and effluent
samples at 90% confidence interval
CURRENT STATE OF RESEARCH STUDY
The research study site was intended to be made operational in November 2013, as the plants
had rapidly reached maturity, however, some earthworks construction was still required on the
banks of Lake1 (e.g. re-profiling of the banks, removal of tree stumps) and existing drainage
lines on site needed to be re-directed to ensure that no runoff from the existing developed
catchment bypassed Lake 1. These works were delayed by wet weather and other civil
construction activity occurring on site. Due to these delays, the site became fully operational in
February 2014.
There have been ongoing issues in the event monitoring, specifically in relation to the flow
meter (Sontek IQ Plus). To date, the flow meter has failed to record flows in four events, one of
which was a Q100 event. Due to the issues with the flow meter, two events in May and July
were captured on time-based intervals (10 minutes between samples) rather than by flow-
weighted samples as intended.
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Description:Treating Urban Runoff in Australia Using Floating Wetlands 2University of the Sunshine Coast, School of Science and Engineering, 90 Sippy Downs.
