Forests and scattered trees in drylands provide a wide range of environmental services, such as soil amelioration and protection from soil erosion and desertification. Forests and scattered trees can supplement farmer incomes as sources of woodfuel, fibre products and fodder. Trees also provide shade for livestock and, when planted in shelterbelts, protect agricultural crops by reducing evapotranspiration and acting as windbreaks.

Trees require soil moisture to grow, and they are often seen as competitors of agricultural crops. With their extended root systems, however, they are able to draw water from deeper in the soil profile, to some extent occupying different environmental niches to annual agricultural crops. Moreover, many trees are less demanding than agricultural crops in the quality and quantity of water they require to survive and grow.

In many regions worldwide, the disposal of wastewater produced by industries, agriculture and urban settlements poses significant challenges. Depending on its origin, wastewater may contain pathogens harmful to human health, such as intestinal worms, bacteria and viruses, as well as salts, heavy metals and poisons.

According to FAO’s Aquastat database, only 52 percent of the municipal wastewater produced globally is recycled. Many developing countries have inadequate systems for treating wastewater, and there is a lack of sewage networks and wastewater treatment plants. As a result, large quantities of untreated wastewater are discharged into rivers, seas or, after dispersal on land, into groundwater, causing pollution, soil salinity and a reduction in water quality. In countries with limited freshwater supplies and where effluents are unmanaged, the risk of contaminating soils and groundwater and harming agricultural and forestry production is very high. Conversely, the effective treatment of wastewater can transform a potential environmental threat into an important source of additional water while reducing pollution and improving nutrient recycling. Treated wastewater can be used to increase the production of wood, biomass and food.

To minimize or eliminate the need for pumping (and therefore reduce energy costs), treated wastewater should be used immediately downstream of the wastewater treatment site from which it is discharged. Therefore, the use of treated wastewater in forestry and agroforestry should be integrated into urban and peri-urban areas, where it can contribute to the greening of landscapes and help support local farmers in the production of goods and environmental services.

Planted forests irrigated with wastewater may be established for the production of lumber, pulpwood or woodfuel or for environmental purposes (e.g. soil protection), or integrated with agriculture to produce wood, provide shade and fodder for animals, and protect crops from damaging winds. Not all tree species and tree crops are suitable for wastewater irrigation, and special care must be taken when planting trees for fruit production.

Effective wastewater treatment is essential for maintaining high standards of public health. Untreated wastewater should never be discharged into the environment, and, especially in drylands, the safe use of treated wastewater makes environmental sense. Irrigation with treated wastewater may be expensive because of the costs involved in the establishment and maintenance of wastewater treatment plants and the energy required to treat wastewater. The cost of producing wastewater suitable for use in irrigation is likely to be more than offset, however, by the environmental and public-health benefits of eliminating the discharge of polluted water into the environment and by the increased productivity of irrigated lands.