Possible Utilization of the Water Hyacinth in Nutrition and Industry

Abstract

The water hyacinth (Eichhornia crassipes [Mart] Solms-Laubach) is a large, free-floating, tropical aquatic plant with attractive lavender flowers and shiny, bright green leaves on long petioles. It reproduces principally by vegetative means; daughter plants are produced by stolons that grow laterally below the water surface from the central rhizome, and the interconnected plants form enormous mats of vegetation. Propagation by seeds may contribute to the spread of water hyacinths and can be a potent source of reinfestation. Water hyacinth populations increase rapidly. Holm et al. (1) reported that in one experiment, two parent plants produced 30 offspring after 23 days, and 1,200 at the end of four months. Some authors report weight gains of 4.8 per cent per day (see ref. 2). Penfound and Earle 13) reported that the number of water hyacinth plants doubled every 11.2 to 15 days in a field observation. Standard densities of 300 to 442 tons per hectare have been found. In the tropics, Lareo found a duplication of population each seven days and an annual productivity of between 930 and 2,900 tons per hectare (4). Water hyacinths grow most rapidly in water temperatures from 28° to 30° C and at a pH from 4.0 to 8.0. They cease to grow when water temperature is above 40° or below 10° C, and the pH range for growth is between 4.0 and 10.0. With these characteristics, the water hyacinth has become a major ecological and economic problem in this century in the tropics and subtropics; yet, in these same regions, production of nutrients for human and animal consumption has become an even more serious problem than that caused by hyacinths obstructing river transporation by blocking waterways in some areas: Guyana, the Nile, the Congo, Florida, some places in Colombia, Brazil, etc. It also blocks irrigation canals. The first step adopted for the solution of this problem was control by known methods. Many years ago various kinds of herbicides such as 24-D, Dalapon, Diquat, and others were used in some places. The ecological problems created by these herbicides were obvious. The water could not be used for irrigation or human consumption for long periods of time, and the fauna in the ecosystem were seriously affected (5, 6). Biological control of the hyacinth was studied with several kinds of animal viruses, bacteria, and fungi (7), as well as with manatees (8), insects (9), herbivorous fish such as grass carp and tilapia,, ducks, geese (10, 11), turtles, snails (12), and other animals. However, the results were disappointing, perhaps because of defense mechanisms in the plants. For example, the larger plants form 2.5 or more leaves for each one destroyed by pathogen attack (13). The last form of control attempted was mechanical destruction. This was costly in time, money, and energy, and several of the procedures used damaged the ecology, affecting all animal life in the ponds infested by the hyacinth. However, negative features are only one aspect of the water hyacinth; there is another side to it. On many occasions it has been demonstrated that this weed is an excellent water depollutant for domestic water wastes. In studies where the ability of the water hyacinth to remove lead, cadmium, and mercury was tested, the plant removed approximately 65 per cent of lead, 50 per cent of cadmium, and 65 per cent of mercury from water polluted with 10 ppm of lead and 1 ppm of mercury and cadmium (14). One hectare of water hyacinth plants is potentially capable of removing 160 kg of phenol per 72 hectares from water polluted with this chemical (15). Combinations of microorganisms with water hyacinths must be seriously considered in developing filtration systems for removing toxic trace chemicals, such as heavy metals and carcinogenic materials (16). The results of several studies show that plants such as the water hyacinth use appreciable amounts of the inorganic forms of nitrogen and phosphorus found in domestic sewage. In so doing, they effect a significant reduction in the concentration of materials that are major contributors to the inorganic nutrient enrichment of natural waters (17).