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. Low-capital intensive processes for eliminating pathogens include sunlight, time, and an intermediate plant or animal such as algae or duckweed, which is then used as organic fertilizer or animal feed. The second approach to managing the problem of pathogens focuses on selecting crops or animals that are less prone to absorb, retain, and transmit pathogens. Many cities use waste water to grow forest crops for fuel, forage for livestock, construction materials and other non-food crops.