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Constructed Wetlands for Water Pollution Control - Processes Parameters and Performance.

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Dev. Chem. Eng. Mineral Process. 12(.5/6),pp. 491-504, 2004
Constructed Wetlands for Water Pollution
Control - Processes, Parameters and
Performance
M. Greenway
School of Environmental Engineering, GrfBth University, Nathan,
Brisbane, Queensland 41 11, Australia
constructed wetlands are now recognisedas an ecologically sustainable optionfor water
pollution control. Natural wetlands are biologically diverse ecosystem. Theyprovide an
array ofphysical, biological and chemical processes tofacilitate the removal, recycling,
transformation or immobilisation ofsediment and nutrients. Most of thL.se p rocesses are
facilitated by the wetland vegetation, associated biofilms and micro-organisms. Wetland
ecosystems are complex and the interactions between abiotic and biotic components are
jrndamental to an understanding of the treatment processes. Coristructed wetlands must
therefore be designed to have the attributes of natural wetland ecosystems.
The treatment efficiency of a wetland system requires a balance between pollutant
loading rate and hydraulic retention time, which is also affected by the water quality
and quantity of wastewater efluent or stormwater runoff: The size of a wetland will
depend upon the volume of runoff; pollutant characteristics, desired level of treatment
and the extent to which the wetland is expected to function as aflood retention basin.
Water depth and extent of inundation will determine the types and species of aquatic
plants. A combination of emergent, submerged andfloating species should be selected.
Pretreatment and detention times are crucial parameters to maximise pollutant
removal eflciency. Sedimentation ponds are important in stormwater wetlands to remove
particulates, but dense vegetated macrophyte zones are essentinl to enhance the removal
of suspended solids and nutrients.
Ecologists and engineers need to work together to maximise the treatment efficiency
of constructed wetlands. Planners and landscape architects' must become involved to
ensure that stormwater wetlands have a multi-functional role in the urban setting.
Constructed wetlands offer the ideal challenge to environmental engineers allowing
f o r the integration of engineering and ecological principles tofind the technical solution
to fit both nature and society.
8
* Author f a r correspondence (m.greenway@gr$fith. etiu.au).
491
M. Creenway
I 11 trod uct ion
The use of natural wetlands to assist in water purification has been in existence in many
parts of the world for centuries, however the functional processes were not understood
until ecological research focused on the nutrient dynamics of wetland systems in the
1960s and 1970s [l]. The use of constructed wetlands that are specifically designed for
the treatment of wastewater (municipal, industrial, urban and agricultural) has become
widely accepted over the past 20 years [2, 31.
Wastewater treatment systems have traditionally been the realm of civil engineers for
structural design and chemical engineers for the processes. However, wetland systems
are complex ecosystems where an understanding ofthe interactions between abiotic and
biotic components is fhdamental for effective treatment processes. Constructed wetland
design involves the integration of engineering and ecological principles.
Wetland Ecosystems: Natural wetlands are areas that are permanently or periodically
inundated or saturated by surface or groundwater and support the growth of aquatic
vegetation. Wetlands can be either saltwater (e.g. mangroves, salt marshes) or freshwater
(e.g. sedgelands, reed beds, swamp forests and shallow lagoons). Wetlands are
strategically placed in the catchment where they can intercept runoff andor flood waters,
thus water levels fluctuate with rainfall. Many wetlands completely dry up during the dry
season.
Plants: Wetland plants, known as macrophytes can be classified into two fbnctional
types: (i) rooted plants and (ii) floating plants. Rooted plants can be further classified
into trees and shrubs (e.g. Melaleuca, mangroves located at the l a d w a t e r margin);
emergent macrophytes (i.e. roots in the sediment and emergent stems and leaves, e.g.
reeds, bulrush, sedges); submerged macrophytes (i.e. stems and leaves submerged, e.g.
pond weeds (Potumogeton, Cerutophyllum)); floating leafed macrophytes (i.e. sterns
submerged and leaves floating, e.g. water lilies (Nyphaea, Nymphoides), nardoo
(Mnrseliu)).
Rooted emergent macrophytes are restricted to shallow water from a few centimeters
to a maximum depth of about 1.5 m. Most emergent sedges (Cyperaceae) and rushes
(Junaceue)prefer water depths of less than 30 cm [4] with relatively few species growing
in water deeper than 50-60 cm. The depth of distribution of submerged plants is restricted
by turbidity and availability of light.
492
Constructed Wetlands - Processes, Parameters anti Performance
Floating plants have surface leaves and stems and their roots hang down into the
water. They can be hrther classified as free floating (e.g. duckweed (Lemna), Azolla) or
aquatic vineskreepers (e.g. water primrose (Ludwigia), water couch (Paspalum)).
Micro-organisms: Micro-organisms, although inconspicuous, are the most abundant and
diverse group of living organisms in wetland systems. They include autotrophic and
heterotrophic bacteria, fungi, unicellular and filamentous algae and protozoans. Microorganisms occur in the water column, or attached to surfaces as biofilms. Anaerobic
bacteria occur in low oxygen environments in the sediment.
Aquatic Orgarrisms: Wetlands support a diversity of aquatic animals including
crustaceans (shrimps, crayfish), insects (water bugs, water boatman, dragonflies, beetles),
frogs, and fish. These organisms are a crucial component of wetland ecosystems
providing invaluable food web linkages between plants, micro-organisms and other
animals. Predator-prey relationships are important in the control of mosquitoes [ 5 ] .
Hydrologic conditions influence the biotic components, in particular the distribution
and types of aquatic plants. The hydroperiod is the duration of flooding or saturated soil
conditions. The water regime is the combination of water depth and flooding duration.
Wetlands that are permanently flooded will support different plant species compared to
wetlands that are only seasonally flooded. Greenway [4] has identified 52 native species
of aquatic plants growing in surface-flow constructed wetlands receiving secondarytreated effluent in Queensland. In most natural wetlands the hydro-periods are extremely
variable and stochastic.
Constructed Wetlands
There are basically two types of constructed wetlands - Free Water Surface Systems
(FWS) and Subsurface Flow Systems (SSF), as shown in Figure 1. A FWS consists of
channels or free form shallow basins, with a natural or constructed base of clay or
impervious geotechnical material to prevent seepage, and a layer of suitable substrate to
support rooted emergent macrophytes. Water depth can vary to suit the plant species
used; lagoon configurations can also support floating aquatic plants. Substantial areas of
land may be required to establish successful FWS.
FWS are more suitable in
subtropicaYtropica1 conditions where year round plant growth occurs and a wide range of
plant species can be used [ 5 ] .
493
M. Greenway
Surface
I
Surface
Subsurface flow
,
Lagoon
Figure I . Scheinatic constructed wetland system showing combined lagoon,
free water surface and sub-surface flow systems.
A SSF consists of trenches with impermeable liners and a substrate of gravel andor
soil supporting emergent macrophytes. The systems can be designed to allow the
wastewater to flow horizontally through the root zone maximising filtration and sorption
in the substrate, nutrient uptake by plants and micro-organisms, and microbial
degradation. Vertical flow systems and layered gravel-sand reed beds are typically dosed
intermittently from the top with wastewater. The wastewater drains vertically down, the
bed is then allowed to aerate before the next dosing. The absence of standing water
precludes the use of many truly aquatic plant species. In Europe Phragmites, Phalaris
and Typha are commonly used. In Australia Phragmites has been used. Recently
Baumea articulata, Carex fasicularis. Phylirirum languinosum and Schoenoplectus
mucronatn have been trialled [6].
Wastewater from municipal sewage, urban stormwater, agriculture and industrial
processes contains a variety of pollutants (sediment, suspended solids, BOD, nutrients,
metals, hydrocarbons, pathogens) which can have a detrimental impact on aquatic
organisms and ecosystem health. The effectiveness of water quality improvement is
dependant upon an array of complex and interacting processes which can broadly be
classified in 3 categories
- physical,
biological and chemical. Most processes are
facilitated by the wetland vegetation and micro-organisms.
Physical Processes: In FWS, emergent macrophyte vegetation enables the sedimentation
of particles by decreasing water velocity especially around plant stems due to viscous
drag, and by reducing wind effects. Both submerged and emergent vegetation is
particularly effective in removing fine particles which will adhere directly onto the plant
494
Constructed Wetlands - Processes, Parameters arid Performance
surface due to the sticky nature of biofilms. The vegetation also distributes the flow and
reduces turbulence thereby allowing settlement of particles. The root system binds and
stabilises deposited particles. Leaf litter and vegetation reduces resuspension.
Biological Processes: Plants and photosynthetic micro-organism remove soluble
inorganic nutrients (ammonium, nitrite, nitrate, phosphate) and heavy metals by direct
uptake. Rooted macrophytes remove these nutrients from the sediment, whereas
submerged and floating macrophytes, and algae (phytoplankton,periphyton and biofilms)
remove the nutrients directly from the water column. These inorganic nutrients are
converted into organic matter and rendered relatively unavailable. Submerged
macrophytes, algae and phytoplankton also improve overall water quality by producing
oxygen during photosynthesis which diffuses into the water column.
Emergent
macrophytes have aerated stems, some of this oxygen is translocated to the rhizomes and
roots where it diffuses into the sediment to produce an aerobic microenvironment around
the root zone (rhizosphere) [7].
Microbial processes of significance to the removal and transformation of nitrogen
are ammonification, nitrification and denitrification. Amrnonification of dead organic
matter occurs under both aerobic and anaerobic (without oxygen) conditions.
Ammonium ions can either be assimilated by plants and algae, or nitrified under aerobic
conditions by nitrifying bacteria to nitrites and nitrates. Sedimentsbeing waterlogged are
often anaerobic, therefore nitrification cannot proceed and ammonium ions dominate.
However in aerobic microenvironments around the rhizosphere nitrification occurs.
These nitrates can then be taken up directly by the plant roots.
The denitrification process occurs under anaerobic conditions, usually in deeper
sediments. Nitrates are reduced to gaseous nitrogen which diffuses into the water and is
ultimately lost to the atmosphere. Denitrification is the only significant long-term
process for nitrogen removal from wetland system. Thus, the alternation of nitrification
and denitrification niaximises nitrogen removal.
Micro-organisms remove inorganic phosphate from the water column or sediment
porewater and convert this to organic microbial biomass. However mineralization of
organic phosphorus can also occur, thus releasing phosphate ions.
Microbially mediated processes are important for the removal of metals. Metal
oxidising bacteria in the aerobic zone, and sulphate reducing bacteria in the anaerobic
49s
M.Greenway
zones, cause precipitation of metal oxides and sulphides respectively. Hydrocarbon
degrading micro-organisms assist in the bioremediation of oils and fuel, and in the
treatment of wastewaters generated by the petroleum industry.
Chemical Processes: Chemical processes facilitate adsorption and desportion of
phosphorus and metals onto and from sediment particles [ 11. Diffusion of oxygen from
the roots of emergent macrophytes maintains an oxidised sediment surface layer and
microenvironment around the root zone. This modifies the sediment redox conditions as
well as facilitating aerobic microbial processes including nitrification. Chemical
processes assist in hydrocarbon degradation, due to photochemical oxidation and
pathogen degradation caused by natural UV radiation.
Wetting and Drying Cycles: Flooding and drying of wetlands especially in the
subtropics/tropics is a natural occurrence and necessary to maintain wetland vegetation
and maximise nutrient cycling. Under waterlogged (flooded) conditions, organic
degradation of plant material is slow and peat accumulates. This represents long term
storage of organically bound nutrients in plant biomass. Denitrification occurs under
anaerobic waterlogged conditions resulting in the loss of nitrogen from the system.
However, inorganic phosphorus can be released from sediments after 4-6 weeks of
inundation and become bio-available again [7,8]. Repeated wetting and drymg converts
iron oxides and absorbed phosphorus to progressively less available forms. Microbial
decomposition of organic matter is optimal under aerobic moist conditions, but is slow
when the wetland is completely dry. Constructed wetlands can be designed for both
permanently wet and seasonally dry areas.
The role of wetlands in improving water quality is summarised in Table 1. A
thorough understanding of physical, biological and chemical processes is vital for
optimising wastewater treatment using wetland systems. Thus environmental engineers
involved in designing constructed wetlands need to appreciate the ecological basis of the
wetland function in order to achieve water quality improvement. Design must be
focussed on establishing an ecologically sustainable system, and on how we can optimise
the biogeochemical processes in order to maximise the efficiency of the pollutant
removal.
4Y6
constructed Wetlands - Processes, Parameters and Performance
Table 1. Role of constructed wetlands in improving water quality.
Pollutant
Suspended
Solids and
BOD
Nutrients
Metals
Hydrocarbons
Pathogens
Role of the Wetland
Sedimentation is facilitated by the vegetation. Finer particles adhere
to the biofilm surfaces of the vegetation or the gravel substrate.
Microbial degradation of organic particulates.
Direct uptake by plants and micro-organisms. Inorganic nutrients
converted to organic biomass. Microbial processes facilitate the
removal and transformation of nutrients, especially nitrogen removal.
Plant uptake and bioaccumulation. Microbial bioremediation.
Immobilisation by adsorption onto sediments or by precipitation.
Microbial hydrocarbon degradation.
Natural UV disinfection. Natural biocontrol by microbial predators
in the wetland ecosystem. Adsorption to fine particles and
sedimentation. Natural death and decay.
Parameters
The design of constructed wetlands will depend on the desired outcomes with respect to
water pollution control, including any legislative requirements on water quality standards
for the protection of downstream waterways and aquatic ecosystems. As ecologically
sustainable systems they must be designed to be structurally and functionally optimal for
the type of wastewater being treated. Parameters to be considered during the design are
shown in Table 2; and vary depending upon the type of wastewater being treated.
The treatment efficiency of a wetland system requires a balance between pollutant
loading rate and hydraulic retention time. Constructed wetlands for the treatment of
eMuent wastewater generally have fairly steady hydraulic loading rates (and pollutant
loadings), with the most effective retention time for removal of suspended solids and
nutrients in Queenslands pilot wetlands being around 7 days [9]. By contrast, stormwater
runoff is highly variable in hydraulic and pollutant loading due to the erratic nature of
storm events in both intensity and duration [lo]. Thus, the size (area and volume) of a
stormwater wetland will depend upon catchment size, the volume of runoff, pollutant
characteristics of the runoff, the desired level of water treatment and the extent to which
the wetland is also expected to hnction as a retention basin. Stormwater wetlands should
be designed for a storage volume of between 0.5-5% of the catchment area, and a
detention time of at least 10-15 hours for good treatment efficiency. Tertiary-treatment
SSF wetlands designed at 1 m2per PE (person equivalent) will achieve < 5 mg BOD/L
and < 10 mg TSS/L, and substantial nitrification [ 111.
497
M.Greenway
Table 2. Parameters which need to be considered in the design of constructed wetlands
- a comparison between secondary treatedsewage effluent and urban stormwater runoff
Parameter
characteristics e.g.
TSS, TVS, BOD
nutrients; coliforms
Treatment
performance
expectations and
legislative
requirements
Pollutant loading
Hydraulic loading
Detention time
Catchment size
Sewage effluent
Generally higher TVS, BOD,
nutrients, coliforms
Urban stormwater
II
subdivision, rainfall intensity.
Generally higher TSS, metals,
I
hydrocarbons.
I Reuse standards
I Water Quality Guidelines.
TSS < 30 mg/L
e.g. Brisbane-City CouncIl
BOD < 20 mg/L
TSS < 15 mg/L
TN 5-15 mg/L
TN < 0.65 mg/L
TP 2-10 mg/L
TP < 0.07 mg/L
High
Variable
Steady
Erratic
Constant 3-7 days
Variable 5 hrs to 40 days
Population (person equivalent) Area (ha); landuse
% impervious surface
Volume and
Depends on sewer systems and Depends on rainfall and
velocity of runoff 1 combined stormwater
impervious area
Hydroperiod
I
Permanently
flooded
(FWS);
I
Variable:
wet and dry
.
.
or saturated (SSF)
conditions, depth dependent
Plant species
Tolerant of flooding
Tolerant of wetting and drying.
Rooted and floating plants
Mostly emergent rooted plants
Wetland zones
Open water
Open water
Shallow marsh (20-50 cm)
Deep marsh (30-60 cm)
Shallow marsh
Ephemeral (dries out)
Mosquito breeding Controlled by predator diversity, abundance and access.
Type of vegetation, % open water
Maintenance
Removal of weed species
Removal of trashhediment,
weed species
I
The layout or configuration of wetland zones is important for treating all forms of
wastewater. Deep ponds or lagoons are appropriate as retention basins for stormwater
wetlands or for treating wastewater effluent using algae (phytoplankton) or floating
plants such as duckweed (Lemna). Large-scale treatment systems using floating plants
require regular harvesting. Harvesting not only removes bioaccumulated nutrients (and
metals) but also provides a potential resource as fertiliser or biogas. Treatment lagoons
can also hnction as aquaculture ponds for fish.
Emergent macrophytes are an essential component of constructed wetlands and play
a major role in facilitating physical and biological processes in pollutant removal.
498
Constructed Wetlands - Processes, Parameters and Perjbrmance
However, emergent macrophyte are restricted to shallower water usually less than 50 cm
deep, and not all species can tolerate permanent flooding. Substrate type and depth in
both Free Water Surface Systems and Subsurface Flow Systems for emergent
macrophyte growth needs to be considered. A good depth of top soil is just as important
for wetland plants as terrestrial plants, and because many FWS are clay lined a layer of
top soil is particularly important. The depth will depend on the extent of root penetration
which could vary from 15 cm to 40 cm. Many SSF systems use a gravel substrate but top
soil can be added to support rapid and healthy growth.
FWS for the treatment of steady-flow wastewater streams exhibit little variation in
water levels, and are usually designed for a depth of 30-50 cm water. Thus sedges,
rushes and reeds that are tolerant of permanent inundation need to be planted. By
contrast, huge fluctuations in water levels occur in stormwater wetlands necessitating a
range of shallow (< 10 cm) and deeper (50 cm) macrophyte zones. The shallower zones
will completely dry out during low rainfall periods but will be inundated during the wet
season, therefore plant species that can tolerate wetting and drying cycles should be
selected for these areas. A diversity of vegetation zones can also enhance the overall
wildlife value of the wetland as well as the landscape amenity.
Performance
The performance efficiency of constructed wetlands depends on several variables (see
Table 2), these include the quality and quantity of effluent to be treated; the extent of
physical, biological and chemical processes functioning within the wetland; the contact
time of wastewater with sites of biological and physical activity. Reactive biological
surfaces include the plants and associated biofilms, the litter layer andor sediment, and
associated microbial communities. The flows and storage volume determine the detention
time (hydraulic retention time, HRT) and thus the opportunity for interactions between
wastewater contaminants and the wetland ecosystem. In temperate climates low
temperature can limit biological processes. Constructed wetlands as water quality
treatment systems perform within definable limits. As wetland ecosystems, primary
production usually exceeds consumption (i.e. they are more autotrophic than
heterotrophic) resulting in export of organically bound particulate matter and dissolved
organics. Thus wetland systems will also have measurable background concentrations of
499
M.Greenway
TSS (mostly TVS), BOD, TN and TP. It is unreasonable to expect outflows from
constructed treatment wetlands to have concentrations less than about the following:
BOD 2-10 mg/L, TSS 3-6 mg/L, TN 1-5 mg/L, TP 0.1-1 mg/L, faecal coliforms 50-500
FCU/100 mL.
Constructed wetlands also provide suitable conditions for pathogen removal
including filtration and sedimentation, UV light, chemical oxidation, absorption,
predation, attack by lytic bacteria and bacteriophages (viruses), and natural die off.
Background faecal coliform concentrations are generally equal or less than lo3 (often
< 10’) colony forming units per 100 ml, but higher concentrations occur where a wetland
supports ducks and wading bird populations. Both FWS and SSF wetlands can achieve
up to 90% removal of faecal coliforms from primary treated sewage and 99% removal
from secondary treated sewage. Metals are also removed by a variety of chemical,
physical and biological mechanisms, such as adsorption, precipitation, filtration,
sedimentation, microbially mediated processes, and plant uptake. Metals are mostly
stored in the roots and rhizomes of emergent plant species. Floating plants can be
harvested to remove the metals.
Case Studies
Two case studies are presented here to illustrate how the differences in pollutant
characteristics and detention time effect the performance of FWS in treating secondary
sewage effluent and stormwater runoff.
(i)
Constructed Wetlandsf o r Municipal Wastewater Treatment
Between 1992 and 1994, nine pilot wetlands were constructed in Queensland as State and
Local Government joint projects. Each wetland had a different configuration and
received different quality effluent [ 12, 131. Performance efficiency varied widely
between wetlands, but the water quality discharged was consistently below 11 mg/L
BOD and 22 mg/LTSS (see Table 3). Large areas of shallow open water at the Mackay
wetland resulted in extensive algal blooms which increased both BOD and TSS [13].
Removal efficiency was highest for oxidised nitrogen (ranging from 70-98%). At
Townsville and Emu Park, nitrate concentrations were reduced from 15.8 mg/L and 14.5
mg/L to 0.3 mg/L and 0.85 mg/L respectively as the effluent flowed through the wetlands
500
Constructed Wetlands - Processes, Parameters and Performance
Table 3. Sewage eflluent concentrations (mg/L) into and out of 5 pilot constructed
wetlands in Queensland, Australia (Source: Greenway and Woolley 1999).
with a hydraulic retention time of 7 days (see Table 3). At Cairns, efficient nitrification
in an upstream oxidation ditch resulted in very low ammonium before entering the
wetland (0.5 mg NH4- NIL-’) which could not be further reduced in concentration.
Nitrate Concentrations were reduced from 5.7 mg/L to < 0.1 mg/L with HRT between 1017 days. Removal efficiency was lowest for phosphorus (less than 30%) with export in
some wetlands. A mass balance over 3 years in the Cairns wetland showed a mass
reduction of 2 1% phosphorus and 85% nitrogen, of which plant biomass accounted for
65% P removal and 47% N removal respectively [9].
(ii) Constructed Wetlandsfor Storm water Treatment
Between 2000 and 2003 a comparative water quality monitoring study of 2 stormwater
wetlands in Brisbane, namely Golden Pond [14] and Bowies Flat, having similar
catchment size (140 ha) and urban land use, but of different configurations and ages was
undertaken. Water quality data collected during and within 12 hours of storm event
(12h ww), 24 hours after a storm event (24h ww), and base flow (dry) are given in
Table 4. Prior to entering the wetland the stormwater entered a sediment basin. At
Golden Pond the sediment basin was 100 m’at standing water level, whereas at Bowies
Flat the sediment basin (Pond 1) was 2000 m’,thus greaterpre-treatment was afforded by
the sediment basin at Bowies Flat. The rectangular-shaped wetland at Golden Pond was
1550 m2(1000 m’) and densely vegetated. The “wetland” at Bowies Flat consisted of 5
oval-shaped ponds with a combined surface area of 7000 m2, however vegetation was
restricted to the margins only.
501
M. Greenway
Table 4. Stormwater runoff concentrations (mg/L) into and out of 2 constructed
stormwater wetlands in Brisbane, Queensland.
12 hww - samples collected within 12 hours of a storm event
24 hww - samples collected within 24 hours of a storm event
Diy - samples collected after no rain for 72 hours or longer
I
Golden Pond
Event
Retention
Time
TSS
TVS
rVH4-N
12h ww
c 5 hrs
In
In
out
--20f8 24f12 7f5
--5f3
5
e
3fl
--0.0%
TN
POrP
TP
0.08f
Dry
4-38days
12hww
1.5-6 days
Bowres Not
I 24h w w
2.5-7days
1
I1
Dry
> 20 days
0.06f
0.05
- --
0.09
NO,-N
24h ww
2-4days
I
0.07
0.38f 0.28f 0.98f
0.28 0.18
0.38
1.2f
1 .Of
I .2f
0.62
0.44
0.23
- -- -O.06f O.06f
0.08f
0.06 0.06 0.04
--
0.12f 0.14f 0.1If
0.06 0.08 0.04
--
During storm events there is little reduction in TSS indicating no settlement. In fact at
Golden Pond resuspension occurs. TSS entering and leaving the wetlands was well in
excess of Brisbane City Council Water Quality Objective (BCC WQO) of 15 mg/L.
Although TSS in stormwater was reduced within 24 hours, the outlet coilcentrations were
still higher. At Bowies Flat it took between 3-4 days to achieve background TSS
concentrations in the outlet pond [ 151. During dry weather TSS was consistently higher
at both wetland outlets compared to the inlets, indicating resuspension of sediment. At
Golden Pond resuspension of sediment occurred in the shallow parts of the wetland due
to wading birds and diving ducks. The higher TSS in Bowies Flat can be attributed to
both resuspension of sediment and phytoplankton blooms. Inorganic phosphate removal
occurred in both wet and dry events in both wetlands. Nitrate was the dominant form of
502
Constructed Wetlands - Processes, Parameters and Performance
soluble inorganic N in both systems and considerable reduction in concentration occurred
in both wet and dry events. BCC WQO for TP (i.e. 0.07 mg/L) and TN (0.65 mg/L) were
only achieved at Golden Pond during dry weather. At Bowies Flat the organic
N
component at the outlet was around 0.85 mg/l and may be linked to phytoplankton.
Concentrations of NO3 and PO4 were substantially reduced in both wetland systems
when the detention time was 2 2 days, suggesting direct uptake by plants, algae
and microorganisms. Ammonium was often generated during dry periods due to
ammonification of dead organic matter.
Conclusions
Constructed wetlands can be effective for water pollution control through a combination
of physical, biological and chemical processes. However, as 'natural ecosystems' they
cannot cope with over loading of contaminants. Providing the wetland has at least 70%
dense macrophytes and a good combination of emergent, submerged and floating species
then aquatic plant growth and associated periphytic biofilm can account for the annual
removal of up to 16 g P/m2and 47 g N/m2. Microbial processes account for the removal
of even more nutrients. Retention time is crucial to maximise biological removal of
nutrients from the wastewater stream, but this is a problem in stormwater wetlands
during storm events where the retention time is often in the order of magnitude of hours
rather than days.
Physical processes of sedimentation and filtration by vegetation are important for
the removal of suspended solids, but again retention time is important. Design should
therefore attempt to maximise vegetated zones, but ponds should also be incorporated
for habitat values and removal of nutrients by phytoplankton or submerged macrophytes,
e.g. Ceratophyllurn,or floating macrophytes, e.g. duck weed.
Sediment basins are a particularly important compartment preceding stormwater
wetlands to remove course sediment and debris, and dense vegetated zones are essential
for removing finer particles.
References
I.
Reddy, K R., and d'Angelo, E.M. 1997. Biogeochemical indicators to evaluate pollutant removal
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5 03
M. Greenway
2.
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Greenway, M. 2003. Suitability of macrophytes for nutrient removal from surface flow constructed
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Greenway. M., Dale P., and Chapman, H. 2003. An assessment of mosquito breeding and control in 4
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Brix, H. 1997. Do macrophytes play a role in constructed treatment wetlands. Wnr. Sci. Tech., 35: 1 1- I 7.
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Phillips, I. 1998. Phosphorus availability and sorption under alternating water logged and drying
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