84
G. Langergraber et al.
oxygen supply compared to VF constructed wetlands with intermittent feeding.
Therefore, only VF constructed wetlands can be used when nitrification is required to keep low ammonia effluent standards.
The mechanisms that are involved in improving the water quality in subsurfaceflow constructed wetlands include filtration and chemical precipitation, adsorption
and ion exchange, and breakdown, transformation and uptake of pollutants and
nutrients by microorganisms and plants. As far as known, the effect of uptake by
plants plays a minor role for common wastewater parameters compared to the degradation processes caused by microorganisms. The most important effects of
plants growing in constructed wetlands are physical effects, e.g. provision of root
surface area for attached microorganisms, and maintenance of the hydraulic properties by root growth. The vegetation cover of a constructed wetland protects the
surface from erosion and the shading of the surface prevents algae growth. Dead
plants provide an insulation layer on the wetlands surface, especially for operation
during winter in temperate climates. Plants also have other functions not directly
related to the treatment process. They provide a habitat for wildlife and give the
treatment system a more aesthetic appearance compared to conventional technical
systems.
Constructed wetlands are applied for many purposes (Kadlec et al. 2000): e.g.
for wastewater treatment as main or tertiary treatment stage, for stormwater management in urban areas, treatment of surface water and various industrial wastewater, and sludge consolidation.
Several problems in the operation of constructed wetlands are related to different climatic conditions. Within a temperate climate, the operating of constructed
wetlands under winter conditions is a major problem. For regions with hot and dry
climate, the loss of water due to evapotranspiration can be a limiting factor especially when the treated water is to be reused.
The general characteristics, such as utilisation of natural processes, simplicity
in construction, operation and maintenance, and process stability, allow the application of constructed wetlands in various regions. The experience in several countries (e.g. PR China, Nepal, Uganda, Mexico and Nicaragua) shows that constructed wetlands can be used successfully with differing quality of the influent
water and under various climatic conditions.
Although constructed wetlands are now used widely, their design is based
mostly on rules of thumb, providing a specific area requirement per person. A
simulation model for subsurface-flow constructed wetlands was developed to provide a better understanding of the processes within the black box "constructed
wetland" and to optimise the design of constructed wetlands.
2 Simulation of Constructed Wetlands
2.1 Migration Models
The simulation of a constructed wetland is more or less the simulation of migration processes. A migration model always consists of two models: the flow model
G. Langergraber et al.
oxygen supply compared to VF constructed wetlands with intermittent feeding.
Therefore, only VF constructed wetlands can be used when nitrification is required to keep low ammonia effluent standards.
The mechanisms that are involved in improving the water quality in subsurfaceflow constructed wetlands include filtration and chemical precipitation, adsorption
and ion exchange, and breakdown, transformation and uptake of pollutants and
nutrients by microorganisms and plants. As far as known, the effect of uptake by
plants plays a minor role for common wastewater parameters compared to the degradation processes caused by microorganisms. The most important effects of
plants growing in constructed wetlands are physical effects, e.g. provision of root
surface area for attached microorganisms, and maintenance of the hydraulic properties by root growth. The vegetation cover of a constructed wetland protects the
surface from erosion and the shading of the surface prevents algae growth. Dead
plants provide an insulation layer on the wetlands surface, especially for operation
during winter in temperate climates. Plants also have other functions not directly
related to the treatment process. They provide a habitat for wildlife and give the
treatment system a more aesthetic appearance compared to conventional technical
systems.
Constructed wetlands are applied for many purposes (Kadlec et al. 2000): e.g.
for wastewater treatment as main or tertiary treatment stage, for stormwater management in urban areas, treatment of surface water and various industrial wastewater, and sludge consolidation.
Several problems in the operation of constructed wetlands are related to different climatic conditions. Within a temperate climate, the operating of constructed
wetlands under winter conditions is a major problem. For regions with hot and dry
climate, the loss of water due to evapotranspiration can be a limiting factor especially when the treated water is to be reused.
The general characteristics, such as utilisation of natural processes, simplicity
in construction, operation and maintenance, and process stability, allow the application of constructed wetlands in various regions. The experience in several countries (e.g. PR China, Nepal, Uganda, Mexico and Nicaragua) shows that constructed wetlands can be used successfully with differing quality of the influent
water and under various climatic conditions.
Although constructed wetlands are now used widely, their design is based
mostly on rules of thumb, providing a specific area requirement per person. A
simulation model for subsurface-flow constructed wetlands was developed to provide a better understanding of the processes within the black box "constructed
wetland" and to optimise the design of constructed wetlands.
2 Simulation of Constructed Wetlands
2.1 Migration Models
The simulation of a constructed wetland is more or less the simulation of migration processes. A migration model always consists of two models: the flow model