There are three basic standards of wastewater treatment. Primary treatment involves the removal of settling materials as well as floating oils, greases and light solids. Secondary treatment uses microorganisms to process the majority of organic matter and nitrogen and significantly reduce the concentration of pathogens. In tertiary treatment, secondary-treated water is disinfected chemically or physically (e.g. through the use of ultraviolet lamps) to make it suitable for landscape irrigation (e.g. recreational parks and golf courses) and to meet commercial and industrial water needs. Water subject to tertiary treatment may also be used for groundwater recharge or agricultural purposes; if subject to natural soil filtration (e.g. filtering through sandy soils), tertiary-treated wastewater can be used in the drinking supply. The tertiary treatment of wastewater tend­s to be energy-intensive, however.

Most wastewater is treated to the secondary level; secondary-treated water can be used for certain agricultural crops and for the production of fodder and woody crops (but it cannot be used for horticultural irrigation). Trees are well-suited to irrigation with low-quality water: they can work as additional filters to purify polluted water and to mitigate soil or water pollution.

Tree irrigation with treated wastewater can be implemented close to human settlements in urban and peri-urban areas; indeed, close proximity is preferred because it reduces the cost of pumping. A treatment plant serving a village of 5 000 inhabitants is likely to produce about 700 cubic metres of treated wastewater per day. In drylands with high rates of evapotranspiration, this volume is sufficient to irrigate about six hectares of a high-density tree plantation and more than 15 hectares of a low-density tree plantation.

An important issue to consider in drylands is the seasonality of the irrigation requirements of different crop species. The area of a grass crop, for example, should be limited to the extent that it can be fully irrigated during evapotranspiration peaks in summer. In contrast, woody crops can withstand periods of water stress and therefore a larger area can be irrigated.

The inappropriate use of treated wastewater poses risks for humans and ecosystems, and the use of treated wastewater may also be culturally unacceptable. The World Health Organization, the Food and Agriculture Organization of the United Nations and the International Water Management Institute promote and provide guidance on the safe use of wastewater in agricultural systems in the form of guidelines, handbooks and field projects (see Tools).

The use of wastewater should be made as safe as possible to maximize the nutritional and food-security benefits for rural communities whose livelihoods depends on wastewater irrigation. It is essential, therefore, to ensure that all hazards are identified and that preventative measures are taken to minimize the risk of contamination.

According to FAO’s Aquastat database, global freshwater withdrawals worldwide are allocated as follows: agriculture (69 percent), industry (19 percent) and urban settlements (12 percent). Much of the water used in agriculture is unrecoverable because it feeds back into the global water cycle through evaporation and infiltration (although an estimated 32 percent is recoverable from agricultural drainage). It is relatively easier to collect and reuse wastewater produced by industries and urban settlements where sewage networks exist. However, most wastewater produced by industries contains pollutants, including heavy metals, and requires additional treatment prior to discharge into ordinary treatment wastewater plants. Urban settlements constitute the most common source of treated wastewater for forestry and agroforestry, although only about 52 percent of urban water is currently recycled.

Wastewater to be used for agriculture and forestry should be treated to at least the secondary level to avoid the risk that foresters and farmers, and consumers of harvested products, are exposed to pathogens present in wastewater effluents, most of which are capable of persisting in soils and contaminating crops. In situations where wastewater is unsuitable or unacceptable for food crop production, its use in forestry and agroforestry systems could be a viable option for reducing risks to human health and recovering some of the cost of wastewater treatment.

It should be noted that, often, unused secondary treated wastewater (as well as untreated wastewater) is discharged into natural freshwater bodies such as rivers and lakes, from which water for agricultural irrigation is extracted. This constitutes a highly risky use of potentially contaminated water.

Treated wastewater is an unconventional resource that may be available even in drylands with scarce surface water and groundwater resources. It can be used, for example, to establish and maintain intensive productive planted forests (for wood production) or for environmental, agricultural and social purposes through the irrigation of, for example, recreational areas, orchards, urban greenbelts, windbreaks, and shade and fodder trees.

The technical feasibility of forest tree cultivation using treated wastewater has been demonstrated in various studies and afforestation programmes. Algeria, Egypt, Iran, Jordan, Morocco, Oman, Saudi Arabia, Sudan, Tunisia, the United Arabian Emirates and Yemen, among others, have all developed wastewater re-use programmes to irrigate greenbelts and woodlots or for sand-dune fixation. Commonly used species in such programmes are multipurpose and fast-growing trees such as Eucalyptus, Casuarina, Acacia, Pinus, Khaya (African mahogany) and Tamarix (see case studies for more information on these programmes).

Two methods for treating wastewater are commonly used in forestry and agroforestry: constructed wetlands, and the selective modular filtering of secondary-treated wastewater.

Bacteria, fungi, algae and plants have all been shown to store and digest composite molecules and heavy-metal pollutants. Phytoremediation uses plants such as reeds, weeds, shrubs and trees to treat polluted water and soils and to recover industrial sites contaminated by the discharge of pollutants.

Wetlands work as natural biofilters, removing water sediments through the filtering effects of reeds and marsh plants. Vegetation in wetlands provides a substratum (i.e. roots, stems and leaves) on which microorganisms can grow as they break down organic materials. Plants have been shown to remove 70–90 percent of pollutants from wastewater; they also provide a carbon source for microbes as they decay. Wetlands can remove pollutants, including heavy metals, from wastewater discharged into the environment.

Constructed wetlands are artificial swamps created to purify discharged wastewater through natural filtering. They comprise ponds that grow reeds, shrubs and trees selected for their ability to filter impurities from water. The wastewater settles in a series of connected treatment storage basins and is then discharged in a state suitable for irrigation.

Constructed wetlands are tolerant of fluctuations in hydrological and contaminant loading rates. They are relatively inexpensive to construct and operate and easy to maintain, and they have low or no energy requirements. Constructed wetlands are cost-effective, affordable and sustainable for rural communities in remote dry areas where economic circumstances do not allow the establishment of standard wastewater treatment plants. They produce secondary-treated water suitable for agroforestry systems and tree plantations to produce cash crops, reduce (or prevent) soil erosion, and provide windbreaks, shade and fodder. Constructed wetlands were pioneered in northern Europe and are now being used increasingly in rural villages in arid developing countries.

Constructed wetlands require relatively large areas of land, however, and involve a considerable level of biological and hydrological complexity; their establishment requires considerable skill. Especially when established in drylands, constructed wetlands may deliver relatively lower productivity due to evapotranspiration and infiltration losses, and they may provide habitat suitable for mosquitoes, constituting a health hazard. There is still a lack of knowledge and experience in the use of constructed wetlands in drylands, and there is a need to collect real data for chemical modelling of pollutant removal. The vulnerability of constructed wetland systems to environmental factors such as high winds and sand storms is yet to be assessed.

The ultimate goal of wastewater treatment is tertiary treatment, which produces water with a sufficiently low content of pathogenic microorganisms to be suitable for human consumption. Tertiary treatment is not always the most efficient way of managing water, however. The removal of all solids and impurities requires significant energy, produces large amounts of sludge (which must be disposed of), and involves the loss of nutrients (such as organic carbon, nitrogen and phosphorus) contained in the organic matter.

New treatment methodologies allow the production of safe secondary-quality wastewater through the selective removal of most pathogens and of part of the solids fraction and other impurities. Selective removal is implemented at the secondary stage of treatment in conventional wastewater treatment plants by using additional physical and chemical filters that remove most of the pathogens and reduce the discharge of nutrients and organic matter. Water can be filtered at different stages of the secondary treatment to increase or decrease the quantity of nutrients based on the needs of the crop and of the soil to be irrigated. Thus, selective removal is a simplified type of secondary treatment that reduces energy costs and sludge and allows the recycling of most soil nutrients and the reuse of water for both irrigation and soil fertilization through “fertigation”.

Selective removal is beneficial for soils and improves soil carbon storage and water retention. It is especially viable for non-food crops (e.g. woodlots for energy or lumber) in drylands, which are often characterized by poor soils, because it provides water and nutrients and increases water retention in soils. Fertigation requires continuous monitoring to minimize the risk of spreading disease and to enable adjustments in the level of fertilization in keeping with the needs of soils and crops.