I.
WHAT IS A CONSTRUCTED
WETLANDS
Constructed wetlands (CWs) are planned and built systems that use wetland vegetation to help
treat wastewater in a more controlled environment than natural wetlands.
According to Hammer (1990), constructed wetlands are built, artificial complex of saturated
substrate, emergent and submerged flora, animal life, and water that replicate wetlands for human
purposes and benefits.
The pollutants removed by CW’s include organic materials, suspended solids, nutrients,
pathogens, heavy metals and other toxic or hazardous pollutants.
In wastewater treatment, the main objectives of constructed wetlands are to optimize the contact
of microbial species with substrate and to promote the bioconversion to carbon dioxide, biomass
and water.
II. TYPES OF CONSTRUCTED WETLANDS
1. Free Water Surface (FWS) System
Figure 1. Emergent macrophytes treatment system with surface flow
In FWS systems, the flow of water is above the ground, and plants are rooted in the sediment
layer at the base of water column.
It is very appropriate for polishing secondary and tertiary effluents, and for providing habitat.
Has an aerobic environment at the surface which promotes microbial film growth on all available
plant surfaces. This microbial film growth is the main mechanism of pollutant removal
Has more biodiversity compared to subsurface flow system
2. Subsurface Flow (SSF) System
Figure 2. Emergent macrophytes treatment system with sub-surface flow
In SSF systems, water flows through a porous media such as gravels or aggregates, in which the
plants are rooted.
SSF systems are best suited for treating primary wastewater, because there is no direct contact
between the water column and the atmosphere.
It is particularly useful for treating septic tank effluent or grey water, landfill leachate and other
wastes that require removal of high concentrations organic materials, suspended solids, nitrate,
pathogens and other pollutants.
Has an anaerobic environment that cause microbial biofilm reproduction directly on the roots and
rhizomes of plants.
SSF systems are good for nitrate removal (denitrification), but not for ammonia oxidation
(nitrification), since oxygen availability is the limiting step in nitrification.
There are two types of SSF systems: horizontal flow SSF (hSSF) and vertical flow SSF (vSSF).
III. ADVANTAGES AND DISADVANTAGES of
CONSTRUCTED WETLANDS
Advantages of constructed wetlands include:
Relatively inexpensive to construct and operate
Easy maintenance
Provide effective and reliable wastewater treatment
Relatively tolerant of fluctuating hydrologic and contaminant loading rates (optimal size for
anticipated waste load
Provide indirect benefits such as green space, wildlife habitats and recreational and educational
areas
Disadvantages of constructed wetlands include:
Land requirements (cost and availability of suitable land)
Current imprecise design and operation criteria
Biological and hydrological complexity and our lack of understanding of important
process dynamics
The costs of gravel or other fills, and site grading during the construction period
Pest problems
IV. ZONES and COMPONENTS of
CONSTRUCTED WETLANDS
1. Inlet Zone
Component: Inlet structure & splitter box
Function: Flow distribution across the full width at a minimum
of 3 – 5 m interval
2. Macrophyte Zone
Component: Porous bed/substrate, open water, vegetation, island,
mixing baffles, flow diversion
Function: To provide the substrate with high hydraulic
Conductivity, to provide surface for the growth of
Biofilm, to aid in the removal of fine particles by
sedimentation or filtration and to provide suitable support
for the development of extensive root and rhizome
system for emergent plants.
3. Deep Water Zone
Component: Usually deeper, non-vegetated plants
Function: Reduce short circuiting by re-orienting flow path,
reduce stagnant areas by allowing for mixing by
wind and enable UV disinfections of bacteria and other
pathogens; provide habitat for waterfowl.
4. Littoral Zone
Component: Littoral Area
Function: Littoral vegetation protects embankment from
erosion and serves to break up wave action.
5. Outlet Zone
Component: Collection devices, spillway, weir, outlet structures
Function: Control the depth of the water in the wetland, gather
the effluent water without creating of dead zones in
the wetlands and provide access for sampling and flow
monitoring.
V. PROCESSES in subsurface FLOW
CONSTRUCTED WETLANDS
A. Mechanism of Pollutant Removal
Constructed wetlands have high plant productivity, large adsorptive capacity of the sediments,
high rates of oxidation by microflora associated with plant biomass, and a large buffering
capacity for nutrients and pollutants, making them an excellent system for managing water
pollutants
Wetlands have different mechanisms on each pollutant to remove them in wastewater which
includes:
Pollutant Removal Process
Organic material (measured as BOD) Biological degradation, sedimentation, microbial uptake
Organic contaminants (e.g., pesticides) Adsorption, volatilization, photolysis, and biotic/abiotic
degradation
Suspended solids Sedimentation, filtration
Nitrogen Sedimentation, nitrification/denitrification, microbial
uptake, volatilization
Phosphorus Sedimentation, filtration, adsorption, plant and microbial
uptake
Pathogens Natural die-off, sedimentation, filtration, predation, UV
degradation, adsorption
Heavy metals Sedimentation, adsorption, plant uptake
B. Biological Processes
The six major biological reactions involved in the performance of constructed wetlands, including
photosynthesis, respiration, fermentation, nitrification, denitrification 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.
Respiration is the oxidation of organic carbon, and is conducted by all living
Organisms such as bacteria, fungi, algae and protozoa, leading to the formation of carbon dioxide
and water.
Fermentation is the process of decomposition of organic carbon in the absence of oxygen,
producing energy-rich compounds (e.g.,
methane, alcohol, volatile fatty acids) which is initiated by microbial activity.
Nitrification first takes place, generally in the rhizosphere and in
biofilms (aerobic process). It is a two-step process catalyzed by Nitrosomonas and Nitrobacter
bacteria. In the first step, ammonia is oxidized to nitrite in an aerobic reaction catalyzed by
Nitrosomonas bacteria
The nitrite produced is oxidized aerobically by Nitrobacter bacteria, forming nitrate as follows:
In order for nitrification process to occur, the Nitrosomonas must
compete with heterotrophic bacteria for oxygen and the biochemical oxygen demand (BOD) of
the water must be less than 20 mg/l.
Denitrification is the process in which nitrate is reduced in anaerobic conditions by the benthos
to a gaseous form. The reaction is catalyzed by the
denitrifying bacteria Pseudomonas spp. and other bacteria, as follows:
Phosphorus removal in constructed wetlands is based on the phosphorous cycle, and can involve
a number of processes. Primary phosphorus removal mechanisms include adsorption, filtration
and sedimentation. Other processes include complexation/precipitation and assimilation/uptake.
C. Chemical Processes
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
D. Physical Processes
Sedimentation and filtration are the main physical processes leading to the removal of wastewater
pollutants.
E. Suspended Solid Removal
The physical processes responsible for removing suspended solids include sedimentation,
filtration, adsorption onto Biofilm and flocculation and precipitation. Wetland plants increase the
available substrate area for the development of the Biofilm. The surface area of the plant stems
also traps fine materials within its rough structure.
F. Pathogen Removal
Constructed wetlands reduces the amount of pathogen by up to five orders of magnitude from
wetland inflows. The removal mechanism of pathogens include processes such as natural die-off,
sedimentation, filtration, ultra-violet light ionization, unfavorable water chemistry, temperature
effects, and predation by other organisms
VI. Abiotic factors and their influence on
WETLANDS
A. Oxygen
Important for heterotrophic bacterial oxidation and growth
Essential component for many wetland pollutant removal processes, especially nitrification,
decomposition of organic matter, and other biological mediated processes
The oxygen-demand processes in wetlands include sediment-litter oxygen demand
(decomposition of detritus), respiration (plants/animals), dissolved carbonaceous BOD, and
dissolved nitrogen that utilizes oxygen through nitrification processes
B. pH level
Wetland waters usually have a pH of around 6-8. The biota of wetlands especially can be
impaired by sudden changes in pH
C. Temperature
It exerts a strong influence on the rate of chemical and biological processes
in wetlands, including BOD decomposition, nitrification and denitrification.
