Water hyacinth (E. crassipes) is one of the most prominent aquatic weed plant found throughout the tropical and subtropical areas of the world. Hyacinths are one of the most productive photosynthetic plants in the world. It has been estimated that 10 plants could produce 600 000 more during an 8-month growing season and completely cover 0.4 ha of a natural freshwater surface. This rapid growth is the reason that hyacinths are a serious nuisance problem in southern waterways, but these same attributes become an advantage when used in a wastewater treatment system. Hyacinths on the water surface of a pond create a totally different environmental conditions in the water as compared to an exposed water surface. The dense canopy of leaves shades the surface and prevents algal growth thus limiting the production of oxygen. This in turn maintains the liquid pH at near neutral levels. The mass of plants on the surface also minimizes wind-induced turbulence and mixing, as well as surface re-aeration, and moderates water temperature fluctuations. As a result, the near-surface water tends to be low in oxygen and the benthic zone is usually anaerobic even in shallow ponds (Figure 14).
The extensive root system of the water hyacinth provides a huge surface area for attached microorganisms, thus increasing the potential for decomposition of organic matter. In order to ensure optimum contact opportunities between the wastewater and the attached microbial growth a relatively shallow reactor and a relatively low flow velocity are recommended. For secondary treatment systems, water depth <0.9 m, hydraulic loading rate
<8 cm retention time >6 days, and organic loading <100 kg BODjha^d-1 are recommended. The aspect ratio (length:width) of the cells should be >3:1 in order to ensure as much as possible plug-flow conditions. It is also recommended to construct several cells and due to limited oxygen concentrations it is possible to aerate the system in order to enhance the treatment effect. However, aeration makes the operation of these systems quite expensive.
Frequent harvests are considered necessary to keep plants at the optimum growth stage to ensure optimum phosphorus removal. Wet weight plant densities from 12 to 22 kgm~2 were recommended for optimum treatment with loosely packed plants with 80-100% surface coverage. With intensive harvesting, it is necessary to construct the hyacinth ponds so that harvesting can be easily accomplished. This has a tendency to increase the cost of the hyacinth system, and also develops the problem of disposing the excess material. The degree of vegetation management required depends on the water quality goals of the project and a choice between harvesting plants or frequent sludge removal. Since the hyacinth plants are about 95% water, an intermediate drying step is usually employed prior to disposal or utilization of the harvested material at the smaller systems. The dried plants can be disposed off in a landfill, or elsewhere, as permitted by the
Table 3 Treatment effect of water hyacinth-based systems local regulatory authorities. If the wastewater has very high metal concentrations it may be advisable to check the metal content of the dried plant to ensure that the levels do not exceed permit allowances for disposal or utilization. Anaerobic digestion of the plants and sludge for methane production and processing of the plants for animal feed have been shown to be technically feasible but marginally cost-effective. In Table 3, examples of treatment performance of CWs with water hyacinth are presented.
Treatment systems with E. crassipes are sufficiently developed to be successfully applied in the Tropics and subtropics. The major reason for the limited use in temperate regions or regions with even colder climatic conditions is the fact that water hyacinth is strongly damaged by frost, and the growth rate is greatly reduced at temperatures below 10 °C.
There was an explosion of research studies on the use of water hyacinth for wastewater treatment in 1970s and early 1980s. However, after this period, very less information appeared in the scientific literature, mostly because these systems are not economic due to difficult and costly management and operation.
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