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. The most subtle and challenging hurdle to cross in the use of urban waste water for agriculture may be cultural. Irrigation with soiled water is often considered taboo and unsafe.
Obstacles also exist among traditional cultures and religions.
In Muslim countries, for example, there is particular reticence to using waste water for food production. As the Middle East runs out of fresh water, it may need to be in the front-line of waste water agricultural research. The transformation of waste water from a pollutant to an input is likely to be gradual. The conversion from the 19th century disposal system to the 21st century reuse system may take a generation, being phased in over the useful life of the old system or according to the capacity of urban agriculture to absorb it. It can often be introduced first in areas with non-existent or poorly developed sewage systems.