Keywords: Constructed,
Wetland, Sustained.
Definition: Constructed
wetlands are actually the artificial wetlands designed to treat wastewater or
storm water runoff which provides a substrate (leaves, stems and roots) for
microorganisms to grow as they breakdown organic materials. Constructed
wetlands involves the natural purification processes of vegetation, soils and
microbes to remove contaminants from the discharge by making use of technology
which is low in cost to improve water safety and access, making it important
for climate change adaptation. Additionaly the green spaces created by wetlands
produce habitats for wildlife and may also improve recreational value.
Types: There are
two main types of constructed wetlands: Subsurface flow and Surface flow.
1.Subsurface
flow wetlands. They filter and purify water under the surface of the soil, and
are therefore filled with porous soils and sand. Water in this type is either
purified vertically through the soil and is collected in pipes in the
underlying basin, or goes through the soil layer in a more diagonal direction
due to a slant, after which it gets collected in pipes and sent to an external
reservoir. Subsurface flow wetlands can treat a variety of different
wastewaters, such as household wastewater, agricultural, paper mill wastewater
mining runoff, tannery or meat , processing wastes, storm water.
2. Surface flow wetlands consist of more
impregnable, silty soils that keep water above the soil and water moves slowly
in a horizontal pattern through the vegetation and top soil, removing sediment
and contaminants before it gets collected in pipes at the wetland’s end.
Pathogens are destroyed by natural decay, Sedimentation, predation from higher
organisms, and UV irradiation as the water is exposed to direct sunlight. The
soil layer below the water is anaerobic but the roots of the plants release
oxygen around them which facilitates complex biological and chemical reactions
required for the process.
Surface flow wetlands can be supported by a wide
variety of soil types such as bay mud and other silty clays. Commonly used
plant species for this purpose are water hyacinth (Eichhornia crassipes)
and Pontederia spp.
However, surface flow constructed wetlands may
encourage mosquito breeding but they may also have high algae production that
lowers the effluent quality and due to open water surface mosquitos and odours,
it is more difficult to integrate them in an urban neighbourhood.
Site selection: Site
selection usually includes a low-lying area so that discharge can be easily
collected for example, next to a road, near municipal water-storage tanks, or
similar locations. Construction activities include placing underlying basin at
the site, topping the basin with soil and planting vegetation. Vegetation
in a wetland provides a substrate (leaves, stems and roots) upon which microorganisms can
grow as they break down organic materials. This community of
microorganisms is often known as the periphyton.
The periphyton and natural chemical processes are responsible for approximately
90 percent removal of pollutants and
waste breakdown. The plants remove about seven to ten percent of
pollutants, and act as a source of carbon for the microbes when they
decay. Different species of aquatic
plants have got different rates of heavy metal uptake, thus
accordingly plant selection is done in a constructed wetland used for water
treatment.
Preference
is usually given to the native species, which has tolerance for high moisture
and possess good ability to retain contaminants. Monitoring and maintenance of
the wetland is done by removal of invasive species, clearing clogs, and
monitoring water flow and water quality.
Benefits and Suitability for Developing Countries
1.
Constructed wetlands provide green areas, wildlife habitats and recreational
and educational areas.
2.
These are cost effective, ecologically sustainable, simple but powerful, low
maintenance and low energy technology which provides a sustainable technology
option in developing countries.
3.
Land tenure issues could provide a barrier as the wetlands require large areas
to operate.
4.
As constructed wetlands operate at community level,therefore they are mostly
located close to communities.
5.
They provide water purification facility and are also helpful im biological
control and improves water quality.
6.
They provide water regulation in extreme conditions, such as during drought
conditions.
7.
Constructed wetlands provide not only aesthetic and educational but also
recreational value for the local populations.
8.
Constructed wetlands reduces the cost of water treatment.
10.
They also improve the adaptations to climate changes .
Drawbacks
11.
Constructed wetlands need to have relatively large areas, which make it
difficult and costly to gain land tenancy rights.
12.
Wastewater contains very high volumes of contaminants, particularly metals
which have harmful impact on flora and fauna and thereby making it unsuitable
for wetland treatment.
13.
Surface wetland systems can also provide breeding grounds for mosquitos which
lead to great risk of vector-borne diseases.
14.
Occurrence of unexpected diseases or some invasive species may also disrupt
habitat functions.
References
1.
Rehabilitation of Wetland Cell and Replanting of Wetland Plant at Tasik NAHRIM,
Malaysia
2.
Wai Bulabula Wetlands Treatment Systems in Fiji
3. Abdelhakeem,
Sara G.; Aboulroos, Samir A.; Kamel, Mohamed M. (2016-09-01). "Performance of a vertical
subsurface flow constructed wetland under different operational
conditions". Journal of Advanced Research. 7 (5):
803–814. doi:10.1016/j.jare.2015.12.002. ISSN 2090-1232.
4.
UNEP-DHI Partnership- Constructed
wetlands for water treatment Vymazal, J and Kröpfelová, L. 2008. Wastewater
Treatment in Constructed Wetlands with Horizontal Sub-Surface Flow. Springer
Science and Business Media B.V.
5.
E., Ulrich, L., Lüthi, C., Reymond, Ph., Zurbrügg, C. (2014): Compendium of
Sanitation Systems and Technologies – (2nd Revised Edition). Swiss
Federal Institute of Aquatic Science and Technology (Eawag), Duebendorf,
Switzerland. ISBN 978-3-906484-57-0
6. Dotro,
G.; Langergraber, G.; Molle, P.; Nivala, J.; Puigagut Juárez, J.; Stein, O. R.;
von Sperling, M. (2017). "Treatment
wetlands". Volume 7. Biological Wastewater
Treatment Series. London: IWA Publishing. ISBN 9781780408767. OCLC 984563578.
7.
Maiga, Y., von Sperling, M., Mihelcic, J. 2017. Constructed
Wetlands. In: J.B. Rose and B. Jiménez-Cisneros, (eds) Global Water Pathogens Project. (C. Haas,
J.R. Mihelcic and M.E. Verbyla) (eds) Part 4 Management Of Risk from Aragones,
David G.; Sanchez-Ramos, David; Calvo, Gabriel F. (2020). "SURFWET: A
biokinetic model for surface flow constructed wetlands". Science
of the Total Environment. 723: 137650. Bibcode:2020ScTEn.723m7650A. doi:10.1016/j.scitotenv.2020.137650. PMID 32229378. S2CID 214748458.
