UN-backed experts have forecast a sharp increase in treated wastewater use in irrigation amid growing competition for freshwater from industry and cities.

A study led by Japan's Tottori University and UN University's Canadian-based Institute for Water, Environment and Health (UNU-INWEH) has  forecast "a rapid increase in the use of treated wastewater for farming and other purposes worldwide.”

A rising world shortage of potash, nitrogen and phosphorus is also cited as reason for predicted increase in uptake of wastewater reuse.

However, research shows that treated wastewater is mostly unused and, in many nations, not even quantified.

Of 181 countries studied, only 55 have information on three key aspects of wastewater: generation, treatment, and reuse. Another 69 countries have data on one or two aspects, 57 countries show no information on any aspect.

In countries where data does exist it is mostly outdated: almost two thirds (63%) of the numbers are five or more years old, according to the study.

As water supplies fall and stress rises in many areas, the potential resource of wastewater is being widely recognised, says the study, published 5 September by Elsevier journal Agricultural Water Management. Water demands already exceed supplies in regions with more than 40% of the world's population and in just 12 years as much as 60% of the world's people may confront water scarcity.

Synthesizing what data there are on wastewater treatment, the study shows that, on average, high income countries treat 70% of the generated wastewater, upper­middle­income countries treat 38%, and lower­middle­income countries treat 28%. Just 8% of wastewater generated in low­income countries undergoes any kind of treatment.

In North America, of the estimated 85 cubic kilometres of wastewater generated each year, 61cubic kilometres (75%) is treated. Annually, however, just 2.3 cubic kilometres or 3.8% of that treated wastewater is used. For a comparison, annual treated wastewater in North America roughly equals the volume of water flowing over Niagara Falls.

Tables in the study detail the wastewater generation, treatment and reuse ­­ and how up to datethe numbers are ­­ in individual countries around the world.

"From the earliest of times, most wastewater has truly been wasted. However, it is a vast resource if we reclaim it properly, which includes the separation of municipal from industrial wastewater," says UNU­INWEH Director Zafar Adeel.

"Another way of envisioning the volume of the resource potentially available worldwide each year is to imagine 14 months watching the outflow from the Mississippi River into the Gulf of Mexico."

It has been reported that wastewater today irrigates between 1.5% and 6.6% of the global irrigated area of 301 million ha (1.2 million sq. miles) and that about 10% of world food is produced using wastewater. However, according to the study, there is little data to support such claims.

In developing countries, particularly in water scarce countries, wastewater volumes are thought to have increased substantially in recent years due to rural­urban migration.

Many farmers in water scarce developing countries irrigate with wastewater because:

• It is the only water source available for irrigation year­-round
• Wastewater irrigation reduces the need for purchasing fertilizer
• Wastewater irrigation involves less energy cost if the alternative clean water source is deep groundwater
• Wastewater enables farmers in peri­urban areas to produce high value vegetables for sale in local markets.

Lead author Toshio Sato of Tottori University, Japan, says:

"Given the growing importance of wastewater management to the health of people and economies at local and national levels, having up-to-date basic insights into wastewater generation, treatment and reuse is an essential investment."

"The key point underlined throughout this report is the need to invest the time and resources to fill the global data gap," adds author Manzoor Qadir of UNU­INWEH. "Better data will enable the research and policy community to enhance understanding and craft effective solutions that will benefit millions of producers and consumers worldwide."

Irrigation with treated wastewater likely will expand in developed countries, particularly in arid and semi-arid areas, where competition for freshwater supplies will continue to increase, the study says. Technical solutions and public policies generally are adequate in developed countries to accommodate increases in the treatment and use of wastewater. The same is not true for many developing countries, where treatment facilities already are inadequate, and much of the wastewater used by farmers is not treated.

The study says it is likely that the demand for wastewater as a source of irrigation will increase in arid and semi-arid areas of developing countries at a faster pace than the development of technical solutions and institutions that might ensure the safe distribution and management of wastewater.

Therefore, the key technical and policy questions in developing countries include those pertaining to better methods for handling untreated wastewater on farms and in farm communities; better recommendations regarding the crops and cultural practices most suitable for settings in which wastewater is the primary source of irrigation; better methods for protecting farm workers and consumers from the potentially harmful pathogens and chemicals in wastewater; and capacity development of relevant professionals to tackle the complex issues arising from the agricultural use of wastewater.

“The country level information aggregated at the regional and global levels would help in identifying the gaps in pertinent data availability and assessing the potential of wastewater in food, feed, and fish production at different scales, “ the study says. “Better data will enable the research and policy community to enhance understanding and craft effective solutions that will benefit millions of producers and consumers, worldwide.”