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1.2 WATER QUALITY IMPROVEMENT TECHNOLOGIES
1.2.1 Artificial Wetlands for Wastewater Treatment
Constructed wetlands are artificial, shallow excavations which are
designed, built and operated to emulate the natural functions of wetlands.
They contain a bed of porous soil, gravel or ash of about 0.3 to 1.0 m in
depth, and are constructed with a peripheral embankment at least 0.5 m
above the bed to contain storm conditions and the accumulation of
vegetation and influent solids. Emergent aquatic vegetation, such as
Typha, is planted within these basins to emulate the structure and
functions of swamps, marshes and wetlands (Figure 23).
Water quality improvement is accomplished through physical, chemical and
biological processes operating independently, and through the interaction
of the plants, media and microorganisms. The vegetation physically
obstructs flow and reduces water velocity, thereby enhancing sedimentation
and deposition of absorbed elements and contaminants. The root systems
improve soil permeability, transfer oxygen into the surface sediments, and
provide an increased surface area for aerobic decomposition of organic
compounds by microorganisms and the chemical oxidation of many metal ions.
An example of this interactive process is the co-precipitation of
phosphate by iron, aluminium and calcium, which drastically reduces
phosphate levels across the wetland profile.
Artificial wetlands may take various forms but generally include a
screening stage to separate settleable solids (which are sent to a drying
bed prior to land disposal), a wetland cell, and an aeration and
disinfection stage as shown in the schematic layout below.
Extent of Use
There are projects using this technology in Zimbabwe, Zambia, Kenya,
Uganda and Mozambique. Significant research on this technology has been
undertaken in South Africa (Wood and Pybus, 1992).
Operation and Maintenance
Without separation of particulates, the subsurface flow systems
experience severe clogging after two to three years of operation. When
this occurs, the systems are suitable only for nutrient removal from an
otherwise clear effluent. Subsurface flow systems have a further
disadvantage in that they are frequently unable to maintain adequate
dissolved oxygen levels to effect ammonia removal to the level necessary
to satisfy most water quality standards and criteria.
During the first growing season, during the period when the wetland the
vegetation is initially being established, water levels should not overtop
the new growth. At a minimum, routine weekly inspections are essential to
properly manage flows within the system. The following aspects require
regular checking: piping, vegetation health and vigour, water levels in
each component, and dykes and flow control structures. Short-circuiting
should be prevented through (re-)planting vegetation or blocking any
channels. There are no moving parts, and so the system can be managed
easily with readily available tools and staff.
Figure 23. Layout and functions of a constructed wetland.
Level of Involvement
This is a relatively simple technology, therefore, can be constructed
and maintained by untrained personnel, unlike conventional treatment
plants. Initial design, however, must assure adequate capacity to prevent
transmittal of disease and creation of odours.
Costs comprise annual and unit costs for the system components, land
costs, labour costs, supervisory costs, and costs of technical training.
Costs associated with the operation of conventional, mechanical-type
plants in the United States, which may be considered as high relative to
wetland treatment systems, have been calculated at between $0.011 to
$0.057/m3/day. There are no equivalent cost estimates for Africa.
Effectiveness of the Technology
Wetland treatment is at least as effective as conventional treatment
methods with respect to the removal of bacteria, and the reduction of
common nitrogen compounds and suspended solids (Sinclair, 1995). Depending
on the type of vegetative material used, wetlands may also remove toxic
elements, with some types of plants removing these elements preferentially
and at a much lower costs than using chemically-based techniques.
Wetland treatment is ideal for small communities of up to 5 000 people,
and work well in isolated settlements like rural schools, leisure resorts,
housing complexes, flats and office buildings. Wetland systems are also
suitable for treating wastewater from towns and small cities, mine
drainage, urban stormwater runoff, runoff from livestock production
facilities, landfill leachate, and wastewaters from paper mills,
tanneries, food processing plants, petroleum refineries and several other
industrial sources. Wetlands may also be used to replace failed septic
tank fields, providing a semi-natural alternative for small institutions
in areas where there is a high water table and no central sewerage system.
However, these conditions can lead to a risk of groundwater contamination
This technology can be aesthetically pleasing, depending on the type of
plants chosen. Wetland provide a habitat for a wide range of birds,
plants, reptiles, and invertebrates. Where they are properly designed,
they can also be used for recreational and educational purposes. However,
the environment may not be not completely free of pathogens.
Wetlands utilise indigenous, microbial communities to breakdown harmful
waste products. Wetland treatment systems are not dependant on external
energy or chemical inputs, and require little maintenance. They therefore
have low operation and maintenance costs. Wetland treatment systems offer
a viable option in cases where soakaways fail due to unfavourable soils,
and they can be landscaped into a feature like a water garden. Effluent
from the wetland can be used for lawn or non-edible crop irrigation with
or without further treatment. Most countries require some degree of
post-treatment if the effluent is directly used for edible crop
There is a possibility of increased salinity in the water due to the
discharge of leachate from the wetland and biogeochemical transformations
of pollutants within the soil media. Leachate discharge may occur
seasonally during periods of reduced plant growth (i.e., during winter).
There is no direct handling or reuse of the wastewater, but it is
important to ensure local acceptance of this technology before promoting
the technology in specific communities.
Further Development of the Technology
More experimental and operational systems would help in the accumulation
of relevant data for use in the design and operation of future wetland
Clearwater Revival, Post Office Box 413, Marondera,
Zimbabwe, Tel. 263-79-23619, Fax 263-79-24279.
I. Sinclair, Nature experiences (Pvt) Ltd, Post Office
Box CH 69, Chisipite, Harare, Zimbabwe.
University of Makerere, Kampala Uganda.
Hammer, D.A. 1991. Constructed Wetland can Replace Conventional
Wastewater Treatment. Water and Wastewater International, p.
Jesperson K. 1995. Constructed Wetland Project is Nature's Classroom.
Small Flows, 9(3).
Sinclair, I. 1995. Constructed Wetland. Connections, 3(3):93.
Wood, A. and P. Pybus 1992. Artificial Wetland Use for Wastewater
Treatment - Theory, Practice and Economic Review. WRC Report No.
232/93, Water Research Commission, Pretoria.