Constucted wetlands for waste water treatment

Seminar report 2019-20 Constructed wetlands for water treatment
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ABSTRACT
Constructed wetlands are characterized by specific conditions enabling simultaneous
various physical and biochemical processes. This is the result of specific environment for
the growth of microorganisms and hydrophytes (aquatic and semi aquatic plants) which
are capable of living in aerobic, anaerobic and facultative anaerobic conditions. Their
interaction contributes to the intensification of oxidation and reduction responsible for the
removal and retention of pollutants. These processes are supported by sorption,
sedimentation and assimilation. Thanks to these advantages, treatment wetland systems
have been used in communal management for over 50 years. In recent years, thanks to its
advantages, low operational costs and high removal efficiency, there is growing interest
in the use of constructed wetlands for the treatment or pre-treatment of various types of
industrial wastewater. The study analyzes current use of these facilities for the treatment
of industrial wastewater in the world. The conditions of use and efficiency of pollutants
removal from readily and slowly biodegradable wastewater, with special emphasis on
specific and characteristic pollutants of particular industries were presented. The use of
subsurface horizontal flow beds for the treatment of industrial wastewater, among others
from crude oil processing, paper production, food industry including wineries and
distillery, olive oil production and coffee processing was described. In Poland
constructed wetlands are used for the treatment of sewage and sludge from milk
processing in pilot scale or for dewatering of sewage sludge produced in municipal
wastewater treatment plant treating domestic sewage with approximately 40% share of
wastewater from dairy and fish industry. In all cases, constructed wetlands provided an
appropriate level of treatment and in addition the so-called ecosystem service.

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TABLE OF CONTENTS
DESCRIPTION PAGE No:
ACKNOWLEDGEMENT 3
ABSTRACT 4
LIST OF FIGURES 6
Chapter 1 INTRODUCTION 7
Chapter 2 CONSTRUCTEDWETLANDS 9
Chapter 3 CONCLUSION 22
REFERENCE 23

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LIST OF FIGURES
Sl No TITLE PAGE No
1. Root zone of wetlands 8
2. Natural wetland 10
3. Layout of constructed wetland 11
4. Constructed wetland 12
5. Surface Flow CWL 12
6. Sub Surface Flow CWL 13
7. Vertical Flow CWL 14

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1. INRODUCTION
Constructed wetlands (CWs) are engineered systems that have been designed and
constructed to utilize natural processes involving wetland vegetation, soils and the
associated microbial assemblages to assist in treating wastewaters. They are designed to
take advantage of many of the processes that occur in natural wetlands but do so within a
more controlled environment. The basic classification is based on the type of macrophytic
growth (emergent, submerged, free floating and rooted with floating leaves), further
classification is usually based on the water flow regime (surface flow, sub-surface vertical
or horizontal flow). Recently, the combinations of various types of CWs (so-called hybrid
systems) have been used to enhance the treatment effect, especially for nitrogen. The first
attempts to use the wetland vegetation to remove various pollutants from water were
conducted by K. Seidel in Germany in early 1950s. The first full-scale free water surface
(FWS, surface flow) CW was built in The Netherlands to treat wastewaters from a
camping site during the period 1967–1969. Within several years, there were about 20
FWS CWs built in The Netherlands. However, FWS CWs did not spread throughout the
Europe but constructed wetlands with horizontal sub-surface flow (HFCWs) became the
dominant type of CWs in Europe. The first full-scale HF CW was built in 1974 in
Othfresen in Germany. The early HF CWs in Germany and Denmark used predominantly
heavy soils, often with high content of clay. These systems had a very high treatment
effect but because of low hydraulic permeability, clogging occurred shortly and the
systems resembled more or less FSW systems. In late 1980s in the United Kingdom, soil
was replaced with coarse materials (washed gravel) and this set-up has been successfully
used since then. In the 1980s, treatment technology of constructed wetlands rapidly
spread around the world. In 1990s, increased demand of nitrogen removal from
wastewaters led to more frequent use of vertical flow (VF) CWs which provide higher
degree of filtration bed oxygenation and consequent removal of ammonia via nitrification.

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In late 1990s, the inability to produce simultaneously nitrification and de-nitrification in a
single HF or VF CWs and thus remove total nitrogen lead to the use of hybrid systems
which combine various types of CWs. The concept of combination of various types of
filtration beds was actually suggested by Seidel in Germany in the 1960s but only few
full scale systems were built in 1980s and early 1990s. At present, hybrid CWs is
commonly used throughout Europe as well as other parts of the world. VF–HF
combination is the dominant set-up but HF–VF combination is also used and FWS CWs
are commonly used in hybrid systems. In 1970s and 1980s, constructed wetlands were
nearly exclusively built to treat domestic or municipal sewage. Since 1990s, the
constructed wetlands have been used for all kinds of wastewater including land fill leach
ate, runoff (e.g. urban, highway, airport and agricultural), food processing (e.g. winery,
cheese and milk production), industrial (e.g. chemicals, paper mill and oil refineries),
agriculture farms, mine drainage or sludge dewatering.
Fig. 1. Possible interactions in the root zone of wetlands for wastewater treatment.
Source: www.google.com

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2. CONSTRUCTED WETLANDS
2.1 WETLAND
Simply stated, wetlands are parts of our landscape that are defined by the presence of
water. More specifically, wetlands are areas where the presence of water determines or
influences most, if not all, of an area's biogeochemistry—that is, the biological, physical,
and chemical characteristics of a particular site.
Many wetlands are transitional zones between upland and aquatic ecosystems, although
others are scattered across the landscape in upland depressions that collect water or in
zones where groundwater comes to the surface.
The amount of water present in a wetland can vary greatly. Some wetlands are
permanently flooded, while others are only seasonally flooded but retain saturated soils
throughout much of the unflooded period. Still other wetlands may rarely flood, but
saturated soil conditions still are present long enough to support wetland-adapted plants
and for hydric soil characteristics to develop. Hydric soils develop when chemical
changes take place in the soil due to the low-oxygen conditions associated with
prolonged saturation.
Different plant communities may be found in different types of wetlands, with each
species adapted to the local hydrology (the quantity, distribution, and movement of water
throughout a given area). Wetland plants are often referred to as hydrophytes because
they are specially adapted to grow in saturated soils. Many birds, insect, and other
wildlife species are completely dependent on wetlands for critical stages in their life
cycles, while many other species make use of wetlands for feeding, resting, or other life
activities. They are of mainly two types.

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1. Natural wetlands
2. Constructed wetlands
2.2 NATURAL WETLANDS
Natural wetlands are important for maintaining aquatic ecosystem biodiversity and
should be considered as part of an effective ecosystem management strategy. There are
four major groups of natural wetlands:
1. Fringe wetlands, which include salt marshes and lakeside marshes in which water
typically flows in two opposite directions, influenced by lunar and/or storm tides.
2. Riverine wetlands, which occupy floodplains, are usually characterized by water
flowing in one direction.
3. Depressional wetlands, such as prairie potholes, which usually receive much of
their water from runoff and/or groundwater seepage rather than from surface
water bodies, so that water residence times are longer.
4. Peat lands also have long water residence times, but the accumulated peat creates
a unique hydrologic regime that differs from the previous three types of wetlands
Fig.2 Natural Wetland
Source: http://i.livescience.com

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2.3 CONSTRUCTEDWETLAND
A constructed wetland is a type of sustainable wastewater treatment system that is
designed to look and function as a natural wetland does. Constructed wetlands are created
for the purpose of treating wastewater from small, rural communities in an
environmentally-friendly way before allowing it to return to the water system safely.
Constructed wetlands are usually made up of a primary settlement tank where wastewater
from the community is collected and from that, several ponds follow which are planted
with wetland plants including reeds, rushes and sedges. The ponds are usually gently
sloped towards a river to allow water to flow very slowly through the wetland before
flowing away. Any particles that have been carried in the water will settle on the bottom
and the plants and natural microorganisms (e.g. bacteria, algae and fungi) in the wetlands
will break down and remove certain pollutants and elements e.g. phosphorus in the water.
Integrated constructed wetlands are carefully planned to integrate into the natural
surrounding landscape and are built using natural materials like native plants, trees, soil,
sand and stones.
Fig:3 layout of an integrated constructed wetland.
Source: https://www.water.ie

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Fig:4 Constructed wetland
Source: http://enacademic.com
2.3.1 Types of Constructed Wetlands
In general one can distinguish three types of constructed wetlands;
2.3.1.1. Surface flow constructed wetlands
Surface flow constructed wetlands appear similar to natural swamp area's in which plants
are rooted in a submerged layer of sand or gravel. Aeration of the sediment takes place by
the unique property of helophyte plants which act as oxygen pumps providing dissolved
oxygen with their roots to a wide variety of micro organisms. We apply surface flow
constructed wetlands generally when flow rates are highly unpredictable (run-off from
roads) and when anaerobic pre treatment in a septic tank or biodigester is not required,
this because of the odor nuisance it would cause. The design is mainly dependent on
spatial limitations, ambient temperatures, matrix characteristics, and organic and
hydraulic load.
Fig:5 Surface flow constructed wetland.
Source; http://kilianwater.nl

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2.3.1.2 Horizontal subsurface flow constructed wetlands
This type of constructed wetland is most commonly used for aerobic post treatment of
domestic wastewater and can take a higher hydraulic load than a surface flow constructed
wetland. In order to dissolve solid organic matter anaerobic pre treatment in a septic tank
or biodigester is required. A thick layer of gravel above the aquifer holds a layer of
stagnant air and prevents odor nuisance in the vicinity. Aeration takes place as in surface
flow constructed wetlands. The wastewater is however forced to pass thorough the matrix
ensuring intensive contact between wastewater and the bacteria in the rhizosphere (root
zone of the plants). In this manner all wastewater is treated as no short circuit flow is
possible. Horizontal subsurface flow constructed wetlands, when accurately designed;
provide an extremely reliable low cost aerobic post treatment solution which is applicable
all over the world.
Fig:6 Subsurface flow constructed wetland.
Source: http://kilianwater.nl
2.3.1.3 Vertical flow constructed wetlands
The desire to further reduce the size of constructed wetlands led to the development
of vertical flow constructed wetlands. Anaerobic pre treated wastewater coming from a
septic tank or biodigester is intermitted pumped on top of the constructed wetland. By
trickling down the wastewater effectively sucks air in the constructed wetland whenever
the pump stops, forcing aeration of the rhizosphere. This increases the aeration capacity
up to approximately twenty times compare to horizontal subsurface flow constructed
wetlands. Apart from that no short circuit flows are possible and due to lower levels of
oxygen deeper in the matrix nitrate is removed under anoxic conditions. We can, for
instance, adjust the level of the aquifer and the depth of the matrix as design parameters.

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Fig:7 Vertical flow constructed wetland
Source: http://kilianwater.nl
2.3.2 Processes in constructed wetlands
Processes in Sub-surface Flow Constructed Wetlands (SSFCW) Reference: Design
Manual on Waste Stabilization Ponds and Constructed Wetlands, UNEP-IETC with the
Danish International Development Agency. Wetland can effectively remove or convert
large quantities of pollutants from point sources (municipal, industrial and agricultural
wastewater) and non-point sources (mines, agriculture and urban runoff), including
organic matter, suspended solids, metals and nutrients. The focus on wastewater
treatment by constructed wetlands is to optimize the contact of microbial species with
substrate, the final objective being the bioconversion to carbon dioxide, biomass and
water. Wetlands are characterized by a range of properties that make them attractive for
managing pollutants in water. These properties include high plant productivity, large
adsorptive capacity of the sediments, high rates of oxidation by micro flora associated
with plant biomass, and a large buffering capacity for nutrients and pollutants.
2.3.2.1. Biological processes
There are six major biological reactions involved in the performance of constructed
wetlands, including photosynthesis, respiration, fermentation, nitrification, de-
nitrification and microbial phosphorus removal. Photosynthesis is performed by wetland
plants and algae, with the process adding carbon and oxygen to the wetland. Both carbon
and oxygen drive the nitrification process. Plants transfer oxygen to their roots, where it
passes to the root zones. Respiration is the oxidation of organic carbon, and is performed
by all living organisms, leading to the formation of carbon dioxide and water. The
common microorganisms in the CW are bacteria, fungi, algae and protozoa. The

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maintenance of Removal mechanisms in Wetlands for the Contaminants in Wastewater.
Fermentation is the decomposition of organic carbon in the absence of oxygen, producing
energy-rich compounds (e.g., methane, alcohol, volatile fatty acids). This process is often
undertaken by microbial activity. Nitrogen removal by nitrification/de-nitrification is the
process mediated by microorganisms. The physical process of volatilization also is
important in nitrogen removal. Plants take up the dissolved nutrients and other pollutants
from the water, using them to produce additional plant biomass. The nutrients and
pollutants then move through the plant body to underground storage organs when the
plants senesce, being deposited in the bottom sediments through litter and peat accretion
when the plants die. Wetland microorganisms, including bacteria and fungi, remove
soluble organic matter, coagulate colloidal material, stabilize organic matter, and convert
organic matter into various gases and new cell tissue. Many of the microorganisms are
the same as those occurring in conventional wastewater treatment systems. Different
types of organisms, however, have specific tolerances and requirements for dissolved
oxygen, temperature ranges and nutrients.
2.3.2.2. Chemical processes
Metals can precipitate from the water column as insoluble compounds. Exposure to light
and atmospheric gases can break down organic pesticides, or kill disease producing
organisms. The pH of water and soils in wetlands exerts a strong influence on the
direction of many reactions and processes, including biological transformation,
partitioning of ionized and un-ionized forms of acids and bases, cation exchange, solid
and gases solubility.
2.3.2.3 Physical processes
Sedimentation and filtration are the main physical processes leading to the removal of
wastewater pollutants. The effectiveness of all processes (biological, chemical, physical)
varies with the water residence time (i.e., the length of time the water stays in the
wetland). Longer retention times accelerate the remove of more contaminants, although
too-long retention times can have detrimental effects.

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2.3.3 Removal mechanism of pollutants
2.3.3.1 Mechanisms of suspended solids removal
The two types of wastewater wetlands offer different approaches at which suspended
solids are removed from the system. It is important to know the removal mechanisms for
both systems starting with the free water surface wetland. The FWS wetland removes
suspended solids primarily by flocculation/sedimentation and filtration/interception.
Settling by gravity can be divided into discrete settling and flocculent settling. Both of
these processes are influenced by particle size, shape, specific gravity, and properties of
the fluid medium. Discrete settling is when a particle settles independently on its own
with no contact from other particles. Flocculent settling cannot be as easily determined as
discrete settling and must be found experimentally. Flocculent settling involves the
interacting of particles changing size and characteristics. The formation of larger
flocculants results from charge imbalances on the surface of the particles. Smaller
particles would take about 200 days and would need 11,000m of wetland to settle out.
The larger particles would be removed in the primary part of the wetland whereas the
smaller particles may be flocculated by varying velocity gradients imposed by plant
stems. Filtration does not typically play a large part in suspended solids removal of FWS
wetlands since the plant stems of plants are too far apart. Interception and adhesion to
plant surfaces play an important part in solid removal.
2.3.3.2 Mechanisms of organic matter removal
Organic removal is different for FWS and VSB wetlands but both share some of the same
principles. First these principles will be addressed and the specifics of each system
looked at more closely. Organic material is made up of about 50% carbon which
microorganisms use an energy source. The vast arrays of microorganisms are adapted to
aerobic surface waters or anaerobic soils. The aerobic microorganisms consume oxygen
to breakdown organics which provides energy and biomass for the microorganism.
Anaerobic bacteria breakdown organic matter to produce methane. Biological oxygen
demand is used to measure how much oxygen microorganisms are consuming to break
down organics. It is important that there is enough oxygen in the water after the wetland
so that plants and animals can survive. Wastewater wetlands are also capable of storing
organic carbon in plant biomass thus making wastewater wetlands natural consumers of

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organic carbon.FWS wetlands can remove organic matter by physical means and
biological means. Physical removal is similar to suspended solid removal in that the
mechanisms are similar and it is not uncommon for effluent to have similar
characteristics. The separation processes of organics include sorption and volatilization.
The biofilms located on plant surfaces offer pathways for plants to break down organics.
Although the amount volatile organic compounds entering wastewater wetlands is fairly
low, the removal rate of VOCs are in the 80-96% range. The biological breakdown of
organic matter is a very important one. Organisms will break down organic matter in
order to produce new biomass, reproduce, and sustain life. Energy is a key element in any
biological system and it can be in many forms. The main types of reactions with organic
matter include aerobic, anoxic, and anaerobic. In an aerobic environment, oxygen is
present and serves as the terminal electron acceptor. This is the most efficient conversion
of starting material to end products. In an anoxic environment nitrates, sulfates, and
carbonates serve as the terminal electron acceptor which are reduced to form oxides.
Anoxic reactions are less efficient than aerobic reactions. Anaerobic environments use
the organics as the terminal electron acceptor and donor. The reactions the organisms use
to break organics into energy are reactions that yield energy for the organism. These
include oxidation and reduction reactions, hydrolysis, and photolysis. These reactions
produce methane and are the least efficient of the three reactions. Macrophytes are
aquatic plants located on top of the surface of the water which play an important role in
producing oxygen to the water.
2.3.3.3 Mechanism of nitrogen removal
One of the important issues when treating wastewater is the removal of nitrogen. There
have been many environmental and health problems associated with high amounts of
nitrogen in water. High concentration of nitrates in drinking water can cause “blue baby”
syndrome in infants. Ammonia that is not ionized can be toxic to marine organisms and
aquatic life. High amounts of nitrogen also contribute to eutrofication in which nutrients
promote excessive plant growth where plants deplete oxygen in the water. The need for
proper nitrogen removal is very important. Nitrogen exists in many forms such as
inorganic and organic forms. Inorganic nitrogen includes nitrates, nitrites, and
ammonium. In natural environments where oxygen is in surplus nitrogen usually exists as

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nitrates and nitrites. In environments that lack oxygen, nitrogen is available as
ammonium which is the case in wetland soils. As nitrogen containing material settle in
wetlands, the matter is either taken up by plants or broken down by microorganisms.
Plants use nitrates and ammonium as nutrients which can be stored as organic nitrogen.
When plants die the organic nitrogen present accumulates as peat as a long term storage
mechanism. Microorganisms break down inorganic nitrogen mostly by de-nitrification
which converts nitrate to nitrogen gas. If nitrogen is in the form of ammonium then this
must be converted to nitrate by nitrification. Nitrate removal in wetlands is usually very
high. The removal of nitrogen involves a number of processes all which act on different
types of wastewater wetlands. These processes include ammonia volatilization,
ammonification, nitrification, nitrate ammonification, de-nitrification, fixation, plant and
microbial uptake, ammonia adsorption, etc. These are the major nitrogen mechanisms
some of which occur in different types of wastewater wetlands. The following will go
into more detail on these mechanisms and in which types of wetlands the mechanisms are
present .As noted before the most important forms of organic nitrogen found in wetlands
are ammonium, nitrate, and nitrite. These various forms of nitrogen are required for
biological life to function in the wetland. The processes that transform various forms of
nitrogen are all necessary for wetlands to function successfully.
2.3.3.4 Mechanism of phosphorus removal
Another important nutrient that causes eutrofication in water is phosphorus. Removal of
phosphorus tends not to be as high as nitrogen removal in wastewater wetlands. This is
because wetlands do not provide the direct metabolic pathway to remove phosphorus.
Wetlands use physical, chemical, and biological means to reduce phosphorus.
Phosphorus exists as phosphates as inorganic and organic forms. The predominant form
is in the form of orthophosphate which can be used by algae and macrophytes. Inorganic
phosphorus can also be found as polyphosphates. Organic forms include phospholipids,
nucleic acids, nucleoproteins, and phosphorylated sugars. These forms are primarily
known as easily decomposable phosphorus and there other forms called slowly
decomposable organic phosphorus which contains phytin.The major phosphorus
transformations in wastewater wetlands are done by physical/chemical means and
biological means.

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The major removal of phosphorus is done by uptake from plant roots. The absorption
through leaves and plant parts are usually very low and thus macrophytes account for
most of removal at the beginning of the growing season. The storage of phosphorus in
plants varies between the type of plant and storage below ground is usually longer than
storage above ground. Phosphorus is released in portions at varying times throughout the
year and is cycled throughout the wetland. Phosphorus is also released after a plant dies
and begins to decay. The decaying plant matter above ground release phosphorus into the
water while decaying roots secrete phosphorus into the soil. Another important chemical
transformation is soil adsorption and precipitation. This process involves soluble
inorganic phosphorus moving from the pores in the soil media to the soil surface. With
increased clay content the soils adsorption qualities increase. The problem by physical
and chemical removal in wastewater wetlands is that the wetland only accounts for
storage in soil media or in plant tissue. Each of these will eventually reach capacity and
phosphorus removal will cease until the two are replaced. The other mechanism for
removal of phosphorus is by biological means but this process still does not allow for
much storage. The uptake of phosphorus by microorganisms is rather fast because
bacteria, fungi, and algae are able to multiply quickly. The drawback is that they are
unable to store large amounts of phosphorus. The extent at which phosphorus can be
removed or stored is dependent on the type of wetland being used. There are limiting
ways to remove and store phosphorus. These ways include sorption, storage in biomass,
and formation of new soil media. The long term solution to removing phosphorus is
through peat/accretion but will only be effective if there is lots of biomass. Adsorption of
phosphorus through soil media is mostly used in VSB wetlands but the type of media will
determine how well phosphorus is stored. In FWS wetlands the uptake from free floating
macrophytes is more important but these plants must be harvested and replaced to
maximize phosphorus removal. Removal of phosphorus by biological means is more of a
temporary solution since the phosphorus is released in the water once the organism
begins to decay. Typical phosphorus removal is in the 40-60 percent range. It is important
to recognize the processes at work and also realize that wastewater wetlands are not
capable of meeting primary phosphorus removal standards.

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2.3.3.5 Mechanism of pathogen removal
Major concerns with wastewater wetlands are their ability to remove pathogens of
helminthes, protozoan’s, fungi, bacteria, and viruses. There have been reports that
wetlands reduced total coli forms by 57% and fecal coli forms by 62%.There have also
been reports of 98% reduction of giaridia and 87% reduction of cryptosporidium. The
main process that is involved with pathogen removal is sedimentation. Sediments of
wetlands tend to accumulate as vast amounts of coli forms and bacteria. It has been found
that river mud contains 100-1000 more times more fecal coli forms than the surface
water. This is also similar to salmonella which 90% of it accumulates in sediments. These
sediments also give some bacteria the ability to survive longer. Viruses tend to attach to
colloidal material which takes longer to settle out and eventually settle out in a loose
layer above sediment. This layer can be disrupted from human activity or natural storm
events which could cause the pathogens to enter the water column. Another way to filter
out pathogens is thought to be through the root structure of plants in wastewater
wetlands. In a study done by Mohammad Karim he tested whether sedimentation played
a significant role in reducing pathogens in wetlands. He observed that there were not
large differences in the amount of fecal coli forms and coli phages in the water column
and sediments. The attachment to the root structure played a larger part with these
pathogens. The report found that giardia and cryptosporidium had concentrations two to
three times larger in sediments than in the water column. A multispecies wetland showed
73 and 58 % removal of giardia and cryptosporidium and a duckweed wetland showed 98
and 89 % prospectively. Wastewater wetlands offer promise of the possibility of
removing pathogens. Removing pathogens from the water table does not mean that
pathogens are gone for good. It is possible that pathogens can reenter the water table but
further work must be done on pathogens survivability in wetlands.
2.3.3.6 Mechanism of metals removal
There are some metals that are required for plant and animal growth but these are in very
small amounts. These nutrients include copper, selenium, and zinc. These micronutrients
are toxic at higher concentrations but other metals can be toxic at even at low
concentrations. These metals include cadmium, mercury, and lead which are typically
found in industrial wastewater. These toxic metals have no known benefit but can lead to

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health hazards in humans. Removal of metals in wastewater wetlands occur by plant
uptake, soil adsorption, and precipitation. The ability of plants to uptake metals depends
on the type of plant and type of metal. There are some types of plants which are capable
of storing large amounts of metals in plant biomass and in its roots. One such plant is
duckweed which is known to store large amounts of cadmium, copper, and selenium. The
metals that pass by the root structure tend to accumulate on the structure of the root rather
than being absorbed by the plant. Wetland soils are also sources into which can trap
metals. Metals including cadmium, copper, nickel, lead, and zinc form insoluble
compounds when interacting with sulfides under anaerobic conditions in the soils of
wastewater wetlands. This minimizes the ability of these metals to resolubilize under
anaerobic conditions. Through a process called chemisorptions, metals such as
chromium, copper, lead, and zinc form strong chemical complexes with the organic
material that is present in the soil and water. Furthermore, the metals chromium and
copper can be chemically bound to clays and oxides and allowed to settle out. In a study
done by M.A. Maine studied metal uptake in a wetland containing of several plant
species. The wetland had 80% of cover by Eichhornia crassipes (water hyacinth), 14 %
cover by Typha domingensis (cattail), and four percent of Panicum elephantipes
(elephant panicgrass). The wetland removed 86% of chromium and 67% of nickel. The
concentration of zinc was below 50 micro grams per liter in most of the samples. Iron
sulfide precipitation helped in reducing the iron content by 95%.A larger wetland and a
smaller wetland were tested where the metals in the larger one were retained by
macrophytes and retained by sediment in the smaller wetland. Wastewater wetlands show
promise for removal of metals but further research is needed in this area.

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6. CONCLUSION
Constructed wastewater wetlands have shown that there is capability of treating different
kinds of wastewater. Recent research has focused on using constructed wetlands to treat
domestic wastewater. The contaminants being removed include suspended solids, organic
matter, nitrogen, phosphorus, pathogens, and metals. The removal of suspended solids is
mostly done by flocculation/sedimentation and filtration/interception. Typical suspended
solids concentrations range between 3 and 5 mg/L for constructed wetlands. The removal
of organic matter is done by physical and biological means. Physical removal is done by
sorption and volatilization and biologic removal by aerobic, anaerobic, and anoxic
organisms. Removing nitrogen is done by a number of processes, the major one by
nitrification and de-nitrification. Wastewater wetlands have the ability to reduce nitrogen
by 30 to 50 percent. Phosphorus removal is carried out by plant uptake,
adsorption/precipitation, and by biological storage in microorganisms. Typical amounts
of phosphorus removal are in the range of 40 to 60 percent in wastewater wetlands. The
slow moving water in constructed wetlands enable pathogens to settle out and thus
wastewater wetlands are capable to removing high percentages of fecal coli form,
giardia, and cryptosporidium. The ability of wastewater plants to use plant uptake, soil
adsorption, and precipitation help in the removal of metals in wastewater. By selecting
proper plant species, wastewater wetlands can achieve relatively high percentages of
metal removal. By understanding how contaminants are removed proper decisions can be
made for how to implement constructed wastewater wetlands. Research has found that
wastewater wetlands are successful in removing contaminants but sometimes may not be
the best option for primary treatment standards. They make a good secondary method for
treating domestic wastewater. They offer aesthetic pleasing environments which function
on less complicated technologies that are successful in removing many different types of
contaminants.

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