Floating Plant Pond
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A floating plant pond is a modified maturation pond with floating (macrophyte) plants. Plants such as water hyacinths or duckweed float on the surface while the roots hang down into the water to uptake nutrients and filter the water that flows by.
Water hyacinths are perennial, freshwater, aquatic macrophytes that grow especially fast in wastewater. The plants can grow large: between 0.5 to 1.2 m from top to bottom. The long roots provide a fixed medium for bacteria which in turn degrade the organics in the water passing by.
Duckweed is a fast growing, high protein plant that can be used fresh or dried as a food for fish or poultry. It is tolerant of a variety of conditions and can significantly remove quantities of nutrients from wastewater.
Contents
Design Considerations
Locally appropriate plants can be selected depending on their availability and the characteristics of the wastewater. To provide extra oxygen to a floating plant technology, the water can be mechanically aerated but at the cost of increased power and machinery. Aerated ponds can withstand higher loads and can be built with smaller footprints. Non-aerated ponds should not be too deep otherwise there will be insufficient contact between the bacteria-harbouring roots and the wastewater.
| Advantages | Disadvantages/limitations |
|---|---|
| - Water hyacinth grows rapidly and is attractive - Potential for local job creation and income generation |
- Requires a large land (pond) area - Some plants can become invasive species if released into natural environments |
Appropriateness
A floating plant pond is only appropriate when there is a sufficient amount of land (or pre-existing pond). It is appropriate for warm or tropical climates with no freezing temperatures, and preferably with high rainfall and minimal evaporation. The technology can achieve high removal rates of both BOD and suspended solids, although pathogen removal is not substantial.
Harvested hyacinths can be used as a source of fibre for rope, textiles, baskets, etc. Depending on the income generated, the technology can be cost neutral. Duckweed can be used as the sole food source for some herbivorous fish.
Health Aspects/Acceptance
Water hyacinth has attractive, lavender flowers. A well designed and maintained system can add value and interest to otherwise barren land. Adequate signage and fencing should be used to prevent people and animals from coming in contact with the water. Workers should wear appropriate protective clothing. WHO guidelines on wastewater and excreta use in aquaculture should be consulted for detailed information and specific guidance.
Operation & Maintenance
Floating plants require constant harvesting. The harvested biomass can be used for small artisanal businesses, or it can be composted. Mosquito problems can develop when the plants are not regularly harvested. Depending on the amount of solids that enter the pond, it must be periodically de-sludged. Trained staff is required to constantly operate and maintain it.
References
- Crites, R. and Tchobanoglous, G. (1998). Small and Decentralized Wastewater Management Systems. WCB/McGraw-Hill, New York, US. pp. 609-627. (Book. Comprehensive summary chapter including solved problems)
- Iqbal, S. (1999). Duckweed Aquaculture. Potentials, Possibilities and Limitations for Combined Wastewater Treatment and Animal Feed Production in Developing Countries. Eawag (Department Sandec), Dübendorf, CH.
- McDonald, R. D. and Wolverton, B. C. (1980). Comparative Study of Wastewater Lagoon with and without Water Hyacinth. Economic Botany 34 (2): 101-110.
- Reddy, K. R. and Smith, W. H. (Eds.) (1987). Aquatic Plants for Water Treatment and Resource Recovery. Magnolia Publishing Inc., Orlando, FL, US. (Book)
- Skillicorn, P., Spira, W. and Journey, W. (1993). Duckweed Aquaculture. A New Aquatic Farming System for Developing Countries. The World Bank, Washington, D.C., US. (Comprehensive manual)
- U.S. EPA (1988). Design Manual. Constructed Wetlands and Aquatic Plant Systems for Municipal Water Treatment. EPA/625/1-88/022. U.S. Environmental Protection Agency, Cincinnati, OH, US.
- WHO (2006). Guidelines for the Safe Use of Wastewater, Excreta and Greywater. Volume 3: Wastewater and Excreta Use in Aquaculture. World Health Organization, Geneva, CH.
Acknowledgements
The material on this page was adapted from:
Elizabeth Tilley, Lukas Ulrich, Christoph Lüthi, Philippe Reymond and Christian Zurbrügg (2014). Compendium of Sanitation Systems and Technologies, published by Sandec, the Department of Water and Sanitation in Developing Countries of Eawag, the Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland.
The 2nd edition publication is available in English. French and Spanish are yet to come.
