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United Nations Environment Programme
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Freshwater Management Series No. 7

Phytotechnologies

A Technical Approach in Environmental Management

III. Examples of Environmental Applications of Phytotechnology >

D. Floating Macrophytes and Root Zone Plants for Improving Water Quality

Large open pond

Improving water quality through the use of floating macrophyte species is another phytotechnology application. For example, different types of duck weed and water hyacinths have been used as an alternative to waste stabilization ponds. The inorganic nitrogen and phosphorus contained in wastewater and decompos ed from organic pollutants by microorganisms are absorbed by the water hyacinths. The water hyacinths with microorganisms and organic materials attached or coagulated on root surfaces can then be harvested as feed for fish culture ponds and animal farms. However, use of water hyacinths requires strict control, as they can easily spread becoming a nuisance and also because they tend to incorporate and store heavy metals.

Root zone plants can also be effectively used for the treatment of small volumes of municipal wastewater, particularly where construction of a sewage collection system to an adjacent wastewater treatment facility would be prohibitively expensive. The decomposition of organic matter and denitrification usually do not cause any problem, provided that the plant is 5 to 10 square meters per person equivalent (depending on climatic conditions). The efficiency of phosphorus removal is usually no more than 10 to 20% for a root zone plant. However, this efficiency can be increased to 80% or more wit h the addition of iron chloride which allows the precipitation of iron phosphate. The application of this method appears to be attractive for recreational areas, where the density of population is low, but where wastewater loadings can have a significant impact.

E. Combined Waste Stabilisation Ponds and Wetlands

Combining the use of artificial wetlands with waste stabilization ponds and root zone plants is an attractive wastewater treatment method for developing countries, recreational areas adjacent to lake ecosystems, and areas with low population density. The reasons for this are:

Wetlands can provide, through the use of filter media (i.e., sand, gravel or peat), a significant reduction of suspended matter from maturation pond effluent.
Wetlands can buffer the pH value of the effluent from a waste stabilization pond.
Effluents from waste stabilization ponds most often need a post treatment polishing step and the use of wetlands offers an excellent cost-effective solution to this problem.
The use of waste stabilization ponds and wetlands, in combination, offers a higher level of certainty as an effective pollution abatement and aquatic ecosystems management strategy.

F. Use of Plantations for Wastewater Treatment

Specialized, fast-growing plantations that require water and nutrients offer an efficient, inexpensive way to recycle wastewater. Trees absorb water for transpiration and metabolize the chemical constituents through timber biomass production. This has many advantages for some communities relative to other conventional methods of wastewater treatment. Tree plantations operate outside of the food chain, require little energy and have relatively low operating costs. However, additional health and safety measures are required for these open environment systems.

A plantation recycling system is composed of a forest plantation, a storage lagoon and a distribution network (i.e., pumps, pipes and sprinklers). It can be located at low cost on marginal agricultural land. To prevent groundwater contamination, this type of effluent recycling should be practised only during dry spells. When conditions are favourable, such as in sunny and/or windy weather, the recycling system should be loaded to its full capacity.

The process that determines the effectiveness of fast growing plantations for wastewater treatment is evapo-transpiration, which is largely dependent on weather. In temperate zones where this phytotechnology has been applied, hybrid poplars have been used because they quickly develop large foliage that stays on trees throughout the growing season. Due to advection, greatly enhanced by air circulation in a well-spaced plantation, this type of recycling system can achieve very high evapo-transpiration performance. Practical operational experience in applying this phytotechnology has enabled researchers to combine an innovative, environmentally sound effluent recycling system with intensive silviculture. This approach has considerable potential for recycling wastewater in crowded and growing cities in developing countries.

G. Use of Floodlands for Water Quality Improvement and Eutrophication Control

Another large open pond

The application of ecohydrological principles for eutrophication and phosphorous control starts from the top of the catchment. The first stage involves the enhancement of nutrient retention within the catchment by reforestation, the creation of ecotone buffering zones, such as artificial flood lands, and the optimisation of agricultural practices. The buffering zones at the land water interface reduce the rate of groundwater flux due to evapotranspiration along the river valley gradient. Nutrient transformation into plant biomass in ecotone zones may further reduce the supply of nutrients into the river. The wetlands in the river valley buffering zone reduce the mineral sediments, organic matter and nutrient load transported by the river during floods periods through sedimentation. Also nitrogen load can be reduced significantly by regulation the water level to stimulate denitrification through anaerobic processes. The properties of large scale systems are difficult to predict and therefore should be assessed continuously at every stage of implementation and adjusted to maximise potential synergistic effects.

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