In Peru, guinea pigs are raised in cages that hang on apartment walls. Farmer cooperatives in Calcutta produce tilapia, carp, rohu and other fish in treated sewage water, supplying one fifth of the fish consumed there. Farmers agree to maintain a tract or a body of water in exchange for the right to grow food on land they do not own. These practices do more than just keep stomachs full. According to the UNDP report, women in a cooperative in Bogota, Colombia, that produces several dozen varieties of hydroponic vegetables earn three times more than their husbands do.
Some case histories glow less brightly.
Urban farming must be carefully managed to avoid contamination of food with pollutants or raw sewage. Chile and Peru experienced cholera incidents in 1992 from untreated sewage used in irrigation. Salad leaves in Yaounde, the capital of Cameroon, often contain sump oil or sewage. Wastes can be treated, though, by exposing them to algae or to plants such as duckweed, which digest harmful microorganisms.