Mexico City pumps over half of its sewage 50 miles and more to the north, where it is used to irrigate over 100,000 hectares for livestock feed. Morocco and Tunisia have modern waste-water reuse systems. Singapore is a world leader in the biological treatment of waste water for agricultural applications. Cities from China to California convert waste water safely into food. However, for each city that does so safely, there are many more cities that lack monitoring or are directly engaged in practices that spread disease through improper use.
Some obstacles confront the safe use of waste water in urban agriculture.
The first obstacle is the presence of industrial chemicals in waste streams. The use of wastes as agricultural inputs is more feasible in the urban areas of Third World countries than in those of industrial ones, as their wastes contain less chemicals and toxic materials. Careful monitoring is clearly necessary. A more serious obstacle to the use of municipal effluent as an input to food production in Third World countries is the presence of pathogens. Fortunately, waste water can be biologically treated to remove the pathogens sufficiently so that it can be safely used for irrigation and as a medium for raising fish and other aquatic crops.