Wetland - An introduction
Wetland can be understood as an ecological system lie on a continuum between uplands, where excessive water is not a factor for plant growth, and deeply flooded lands, or aquatic systems, where flooding excludes rooted emergent vegetation. Figure 1 shows how to describe this wetland concept
WETLAND - AN INTRODUCTION …
Le Anh Tuan
Catholic University of Leuven, Belgium
December, 2003
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I. WHAT IS WETLAND ?
Wetland can be understood as an ecological system lie on a continuum between
uplands, where excessive water is not a factor for plant growth, and deeply flooded
lands, or aquatic systems, where flooding excludes rooted emergent vegetation. Figure
1 shows how to describe this wetland concept.
Arbitrary Arbitrary
Upland Wetland Aquatic
High water table
High water
Low water table Low water
Seasonally
saturated Seasonally
soils flooded
Figure 1. Wetland in general meaning (Kadlec & Knight, 1996)
More than 100 years before, in many cities, towns and villages around the whole
world, natural wetlands have been used as convenient wastewater discharge sites as
long as sewage has not been collected (Kadlec and Knight, 1996). There is over 6
percent of land surface of the world, or 8.6 million km2, is wetland (Bazilevich et al,
1971, Maltby & Turner, 1983). In natural wetlands, the 5-day Biochemical oxygen
demand (BOD5) from wastewater can be removed by the support of a large and
diverse population of bacteria which grow on the submerged roots and stems of
aquatic plants. These plants have also the ion exchange or adsorption capacity in
wastewater. In addition, the wastewater solids will be accumulate in the wetland bed
by their quiescent water conditions. Other aspects of wetlands are considered as
determining factors on climatic stable and balance. Wetlands are residence places for
many kinds of wildlife as birds, retiles, amphibians, and so on.
Over the past 20 years, the application of constructed wetlands (CW) for domestic and
municipal pollution control has been rediscovered and has gradually developed in
many European countries and many parts of the world today. The most advantage of
CW is their simple construction, low energy process requiring minimal operational
cost. However, constructed wetland treatment may be economical relative to other
options only where land is available and affordable. Their performance may be less
consistent than in conventional treatment. Otherwise, the biological components are
sensitive to toxic chemicals, such as ammonia and pesticides, flushes of pollutants or
surges in water flow may temporarily reduce treatment effectiveness .
The water and environmental scientist found that CW could be applied as a useful
domestic wastewater treatment system for small village communities with populations
of 50 – 1000 person equivalents. Wetland treatment systems use aquatic plant species
and shallow, flooded or saturated soil conditions to provide various types of
wastewater treatment. There are two basic types of CW treatment systems; i.e.
constructed surface flow (SF) wetlands and constructed subsurface flow (SSF)
wetlands. According to IWA (2000), SF wetlands are densely vegetated by a variety
of plant species and typically have water depths less tan 0.4 m. Open water areas can
be incorporated into a design to provide for the optimization of hydraulics and for
wildlife habitat enhancement. SSF wetlands use a bed of soil or gravel as a substrate
for the growth of rooted emergent wetland plants. The bed depth in SSF wetlands is
typically between 0.6 and 1.0 m, and the bottom of the bed is sloped to minimize
water flow overland.
According to Melbourne Water, 2002, There are three main elements to wetland
systems – pre-treatment, inlet zone and macrophyte or wetland zone. An Ephemeral
Zone may also be required if the pre-treatment element is insufficient. Each has
performance criteria that are described below and illustrated in Figure 2.
Figure 2: Schematic representation of a constructed wetland system
(Melbourne Water, 2002)
During the period 1960 – 1980, Seidel and her co-workers at the Max Planck Institute
in Germany has developed the origins of SSF wetland technology (Seidel, 1976;
Kickuth, 1977). Numerous studies on water and wastewater treatment with wetlants
plants have been published from 1955 through that lat 1970s. In mid-1985, the British
Water Research Centre first proved the potential of horizontal flow through the reed
bed treatment systems in improvement the quality of wastewater. Between 1985 and
1990, Weyerhaeuser Company began two separate pilot studies of SSF wetland
systems to treat pulp and paper mill effluents. Treatment troughs were planted with
cordgrass (Spartina cynosuroides), cattail (Typha latifolia) or common reed
(Phragmites australis) (Thus, 1989, 1990 a,b, 1993); Since 1985 to now, hundreds of
CW systems were built around the world, special in European countries (Austria,
Belgium, Denmark, France, Germany, Sweden, Switzerland, the Netherlands, United
Kingdom), North America, Australia and Asia (China and India). In September 1990,
the International Conference on Constructed Wetlands was held in Cambridge for
presenting the European Design and Operations Guidelines for Reed Bed Treatment
Systems (Cooper & Findlater, 1990). SSF wetlands that use gravel substrates have
been used extensively in the United States (Reed, 1992). From last 10 years to now,
there many scientists in many different fields have a great and wide research for
several solutions concerning with the wetland systems. Besides many progress
measure achievements in hydrology, ecology, chemistry, bio-environment, natural
resources management,… many mathematical and physical models for wastewater
flow in constructed wetlands have been established.
Gerald A. Moshiri, 1993, have listed major events notable first include the use of
constructed wetlands for treating:
• 1956 - livestock wastewaters - experimental; Seidel
• 1975 - petroleum refinery wastewaters - operational; Litchfield
• 1978 - textile mill wastewaters - operational; Kickuth
• 1978 - acid mine drainage - experimental; Huntsman
• 1979 - fish rearing pond discharge - operational; Hammer and Rogers
• 1982 - acid mine drainage - operational; Pesavento
• 1982 - reduction of lake eutrophication - experimental; Reddy
• 1982 - urban stormwater runoff - operational; Silverman
• 1983 - pulp/ paper mill wastewaters - experimental; Wolverton
• 1985 - seafood processing wastewaters - experimental; Guida and Kugelman
• 1988 - compost leachate - operational; Pauly
• 1989 - sugar beet processing plant wastewaters; Anderson
• 1989 - reduction of lake eutrophication - operational; Szilagyi
• 1990 - harbor dredged materials - experimental; Pauly
• 1991 - pulp/paper mill wastewaters - operational; Thut
Concerning mathematical and physical models for describing treatment wetlands, we
may get many published technical papers. Jerald L. Schnoor (1996) have published to
introduce groundwater contaminant solute transport equations. Datta (2002) have
considered convection-dispersion equations in a flowing stream and in a saturated
porous solid. R.H. Kadlec (2002) have reported, in Elsevier Science Journal, his
observations on effects of pollutant speciation in treatment wetlands design, including
potential effects of distributions of detention times and first order removal rate
constants. Lin, et al. (2003) have performed their hydraulic tracer tests in the Predo
Wetlands, Riverside County, California, USA and evaluated by comparing the
breakthrough curve (BTC) of Rhodamine WT® and bromide in the determination of
hydraulic characteristics of constructed wetlands.
II. CONSTRUCTED WETLAND DESIGN PROCEDURE
II.1. Types of constructed wetland
Constructed wetlands can be classified by their treatment functions. Figure 3
shows the types of constructed wetland.
CONTRUCTED
WETLAND
Free water surface Subsurface Flow
(FWS) (SSF)
treatment wetland treatment wetland
FWS wetland with
Horizontal-flow systems
emergent macrophytes
FWS wetland with
Vertical -flow systems
free floating macrophytes
FWS wetland with
floating-leaved,
bottom-root macrophytes
FWS wetland with
floating mats
FWS wetland with
submersed macrophytes
Figure 3. Types of constructed wetland
Based on the ecological engineering of natural wetlands, the constructed free water
surface (FWS) wetlands were build for wastewater treatment purposes mainly. They
are considered as the mimic hydrological regime in small-scale shallow basins
constructed of soil and aquatic plants with their water balance: water flows in and out
over the soil surface and losses to evapo-transpiration and infiltration within the
wetlands. Although not all wetland species are suitable for wetland treatment (R.H.
Kadllec et al, 2000), but we can find the common emergent macrophytes such as:
common reed (Phragmites australis), bulrushes (Scripus spp.), spikerush (Eleochris
spp.), cattail (Typha spp.), …; floating plant species: water hyacinth (Eichhornia
crassipes), duckweed (Lemma spp.), water spinach (Ipomoea aquatica), …; floating-
leaved, bottom-rooted macrophytes such as: water lilies (Nymphaea spp.), lotus
(Nelumbo spp.), cowlilies (Nuphar spp.), …; floating mats such as: cattail (Typha
spp.), common reed (Phragmites australis), …; and submerged aquatic plants as:
waterweed (Elodea spp.), water milfoil (Myriophyllum spp.), naiads (Najas spp.), …
Depending on the characteristics of constructed soil, water and kind of conspicuous
plants (macrophytes), we can distinguished following the figures 4 below:
Inlet pipe Outlet weir/pipe
Low-permeability soil
(a) FWS wetland with emergent macrophytes
Inlet pipe Outlet weir/pipe
Lined basin
(b) FWS wetland with floating plants
Inlet pipe Outlet weir/pipe
Lined basin
(c) FWS wetland with rooted, floating leaf plants
Inlet pipe Outlet weir/pipe
Low-permeability soil
(d) FWS wetland with floating emergent macrophyte mats
Inlet pipe Outlet weir/pipe
Low-permeability soil
(e). FWS wetland with submerged macrophytes
Figure 4: Types of Free water surface treatment wetlands
The SSF wetlands are designed as a basin or channel with a boundary to prevent
seepage and a suitable depth bed of porous media that support the emergent plants.
The wastewater will flow in a high level site of the wetland and then flow through the
porous media of sand soil and plant rhizosphere. The wastewater is treated by the
physical-chemical and biochemical complex processes of filtration, sorption and
precipitation processes in the soil and by microbiological degradation. Finally, the
treated wastewater flow out in the bed that remain below the top of the
gravel/stone/rock media. The SSF systems have also several other names, such as:
vegetated submerged bed, root zone method, microbial rock reed filter, and plant-rock
filter systems.
The SSF wetland types have several advantages if compared with the FWS wetland
types. It is found that in the constructed SSF wetland, the available of wastewater
treatment is better than the constructed FWS one. Wastewater flowing subsurface
media may also avoid of the little risk of heavy odors, dark-color exposure and insect
vectors effects. The area application for SSF wetland can be smaller than a FWS
system with the same wastewater withdraw conditions.
Constructed soil-and gravel based subsurface flow (SSF) wetlands are common used
to treat mechanically pretreated municipal wastewaters in many places in the world.
There are two types of constructed SSF wetlands, i.e. horizontal-flow systems and
vertical-flow systems (Figure 5). As their names, this classification is based on the
flow direction to the soil-and gravel layers. This technology is generally limited to
systems with low flow rates and can be used with less than secondary pretreatment
(Kadllec et al, 2000).
In most of the systems in the United States, the flow path is horizontal, although some
European systems use vertical flow paths (USDA - NRCS, EPA - Region III).
The Mekong delta (MD), the most downstream part of the Mekong river, is draining
an area of 600,000 km2 along a course of 3,650 km through China, Miamar, Thailand,
Laos, Cambodia before entering Vietnam. In topographical speaking, the MD is
typified by its flatness, especially in the lower part, the average slope of land is about
1/100,000 (1 cm/km). In high flood period, haft of the Delta is partly submerged. The
inundation has recorded that 2.5 m depth near the border of Cambodia - Vietnam to
0.7 m northwest of My Tho. Along the 600 km-coastal areas, the tide effects strongly
the hydrological regime of the MD. Generally, there are two kinds of tidal variations:
the semidiurnal tide (twice daily) in the East Sea and the diurnal tide (daily) in the
Gulf of Thailand. These tides result in a complex movement to the rivers and canals
and also an intrusion of saline water in the downstream part of the delta. These
hydrological characteristics make the Delta become a wetland continually.
In the Mekong River Delta, it seems fitting with the horizontal-flow systems more
than vertical one due to its upper groundwater level is too high, nearly remains below
the land surface a few decimeters. However, there is a great lack of wetland hydraulic
research in Vietnam, especially in the Mekong Delta. From the general view, natural
wetlands have been used as convenient floodwater and wastewater discharge sites for
as long as drainage systems have not been collected.
Plants Water level maintained
Big stones
Big stones
Inlet flow
Impermeable liner Outlet flow
Medium (gravel, sand, crushed stones) Outlet collector
(a) Longitudinal section of a constructed wetland with horizontal SSF (Vymazal, 1997, modified)
Solid pipe Feed dosed intermittently over whole surface
Perforated pipe
(~110 mm o.d.)
25 cm
~ 8 cm "sharp" sand
6 mm-washed
~ 15 cm pea-gravel
12 mm-round
~ 10 cm
washed-gravel
30-60 mm-round
~ 15 cm washed-gravel
Free-draining outlet
LDPE liner 1% slope Large stones
Network of agricultural drainage pipes
(b) Typical arrangement of a vertical reed bed system (Cooper, 1996, modified)
Figure 5: General arrangement for the constructed wetland with
(a) a horizontal flow and (b) a vertical flow
II.2. Design procedures
In general speaking, the constructed SSF wetlands can be designed and built
almost anywhere that the emergent plant species can be planted and grown up. In the
winter freezing areas, the application of the constructed wetlands may be limited.
When designing a constructed wetland, some considerations should be noted:
• Site selection
+ Topography
+ Soil types and their permeability
+ Hydrological factors
+ Water and land use rights
+ Social, environmental and public health considerations
• Treatment expectations
+ BOD5 removal
+ Suspended solids (SS) removal
+ Nitrogen removal
+ Phosphorus removal
+ Heavy metals removal
+ Refractory organics removal
+ Bacteria and virus removal
• Process variables
+ Design objectives
+ BOD5 loading rates
+ Hydraulic loading rates
+ Water depth (in FWS systems)
+ Detention time
• Pre-application treatment
• Vegetation/ plants
+ Emergent plants
+ Submerged aquatic plants
• Physical design factors
+ System configurations
+ Distribution system
+ Outlet structures
+ Vector control (in FWS wetland cases)
+ Vegetation harvesting
• Costs and maintain
+ Study plan and permits
+ Land acquit
+ Construction
+ Access ways
+ Operating and monitoring costs
+ Other expenditures
