Multicriteria approach for the selection and a rational use of aquatic plants for wastewater treatment
Uncontrolled invasion of water bodies by aquatic plants, due to eutrophication, is a major environmental concern, especially in tropical countries. Most aquatic plants exhibit water-cleansing properties, but only some of them can be exploited as highly valuable sources of proteins and antioxidizing agents. In this regard, a rational exploitation of judiciously selected aquatic plants can provide low-cost technologies that combine increased biomass productivities and effective depolluting capacity, in a sustainable development context. In the present paper, we have considered some aquatic plants that usually grow in tropical countries, namely water hyacinth, duckweed, water lettuce, common papyrus and water spinach. To achieve a rigorous methodology that allows the rational exploitation of aquatic plants, attempts were made through a multicriteria analysis method to select the most adequate plant for this purpose. Furthermore, many criteria were selected and ranked, according to their impact upon the ecosystem in which the technology is to be implemented. Among all the plants examined, water hyacinth displays the most interesting features, namely, a marked ability to remove pollutants from water (metallic cations, phosphate, nitrate, organic matter, etc.), and high contents in proteins and antioxidizing agents. It also displays an increased resistance towards eutrophic sites, along with an appreciable adaptability to environmental changes. Under suitable conditions, water hyacinth can also afford a considerable biomass productivity of about 200 tons dry weight/ha/year. Nevertheless, the main criterion in favour of this species is undoubtedly the high number of ways in which the water hyacinth biomass can be exploited. As a result, a rational exploitation of water hyacinth, and to lesser extent, of water lettuce or of duckweed, can be regarded as being a very convenient strategy. The main constraint in this regard must be a sustainable development that includes both environmental and socio-economical considerations. In conclusion, the choice of an appropriate aquatic plant to depollute wastewaters in small settlements in tropical countries, using the satisfaction analysis method, seems to be a suitable tool for obtaining sustainable solutions, without major investments. Due to its high protein level, water hyacinth can be rationally exploited on the surfaces of eutrophic water bodies, by implementing a suitable control of the biomass excess, reducing thereby the environmental impact of such culture. The biomass excess can be optimized, regularly harvested (2 or 3 times per month) and then processed to obtain valuable protein extracts. In this way, the continuous eutrophication process of the water body will contribute to feed the plant growth.