Wastewater reuse for aquaculture has been practiced in many countries for a considerable period of time. It has the potential of wider application in the tropics. There is great diversity of systems involving cultivation of aquatic species, (mainly fish) and plants (mainly aquatic vegetables such as water spinach). Farmers and local communities have developed most reuse systems; the primary motivating factor has been reuse of nutrients for food production rather than wastewater treatment, and with scant attention to either waste treatment or to public health. In most aquaculture systems, wastewater is not reused directly in aquaculture and the nutrients contained in the wastewater are used as fertilizer to produce natural food such as plankton for fish. These nutrients, mainly nitrogen and phosphorus, are also taken up directly by large aquatic plants such as duckweed, which is cultivated for animal feed, and aquatic vegetables such as water spinach and water mimosa cultivated for human food.

There are a number of constraints to wastewater-fed aquaculture and they need to be considered where the practice is considered an option. They include:

• lack of knowledge of aquaculture as a technical option in wastewater treatment and reuse.
• limited available sites in peri-urban areas where wastewater is available for reuse
• rapid urbanization in developing countries threatens the existing wastewater-fed systems
• rapid eutrophication from both urbanization and industrialization
• improved sanitation reduces the availability of night soil for agriculture and aquaculture.
• rapid industrialization contaminates nutrient-rich domestic wastewater with industrial wastewater.
• social and cultural acceptance of wastewater-fed
• climate - wastewater-fed aquaculture involves the farming of warm water organisms

Despite the constraints listed above, there is considerable potential for the reuse of wastewater in managed aquaculture in the tropics. A correctly managed system would limit public health risks and wastewater should never be reused without prior treatment if the produce (fish or aquatic vegetables) is intended for direct human consumption.

(UNEP 2000)