An area calling out for application of the recycling principle is the release of phosphates and nitrates by urban systems into natural waters. Urban areas are second only to intensive, advanced agroecosystems in the concentration of these "waste" nutrients released into the environment. The phosphorous budget of Hong Kong, for instance (which grows much of its own vegetables) consists of imports of 5434 kg/day of phosphorous for food, 1247 kg/day for fertilizer, and 3145 kg/day for animal feed. The city discards 7300 kg/day into the environment, contaminating Hong Kong harbour and adjacent waters. Recycling this phosphorous internal to the urban system could reduce a great deal of Hong Kong's phosphorous import needs.