|Sourcebook of Alternative Technologies for Freshwater Augmentation in Africa (International Environmental Technology Centre - United Nations Environment Programme, 1998, 182 p.)|
|Part B - Technology profiles|
|1. Agricultural technologies|
|1.2 Water quality improvement technologies|
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.
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 5000 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 (Jesperson, 1995).
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 irrigation.
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 treatment systems.
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. 17-22.
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.