The wastewater that goes down drains and toilets will likely be repurposed far more frequently for crop irrigation and other uses as water supplies become more constricted across the globe, according to a new U.N.-backed study.

In North America, for example, 3.8 percent of treated wastewater is reused. This percentage is predicted to grow in the coming years, said study co-author Manzoor Qadir, senior research fellow at the United Nations University's Institute for Water, Environment and Health in Hamilton, Ontario.

"Water scarcity is the major driving force in using treated wastewater," Qadir said, pointing out that water-strained California has started to use notable amounts of its wastewater for agriculture and watering lawns.

But even as the need to repurpose wastewater becomes more pressing, the same study shows that there is a paucity of data on global wastewater generation, treatment and use. Out of the 181 countries analyzed, 55 had complete information on this potentially critical resource, while 57 had no available information at all.

In order for the world to capitalize on the full potential of available wastewater -- which in the United States alone equals more than 22 trillion gallons each year -- the huge gaps of data on the topic must be filled.

A large data void

Qadir said that it was initially surprising how difficult it was to collect information on how much wastewater different countries produce and use.

In several of the developing countries he studied in the past, Qadir said, officials were leery of investigating just how much wastewater was being used because there was neither the money nor the political will to build sufficient water treatment facilities.

Even in the United States, the most recent water volumes available for generated and treated wastewater were collected in 1995.

"It's very hard, actually, to find numbers," Qadir said. "Wastewater has been historically taken as a waste, not a resource."

But despite the lack of data, the study points out several trends in global wastewater use.

High-income countries have the resources to treat far more of their wastewater -- about 70 percent -- but tend to repurpose less. The United States reused about 4 percent of its treated wastewater in 1995, while the United Kingdom reused 4.3 percent in 2008.

Drought-plagued Australia, on the other hand, is more efficient and reused almost 20 percent of its wastewater in 2008.

Developing countries tend to use far more wastewater, but about 8 percent of wastewater in the low-income countries that were studied is treated. This becomes a major public health concern because most repurposed wastewater is used for crop irrigation.

In Vietnam, approximately 9,500 hectares of croplands is irrigated with untreated wastewater, and in Pakistan, most of the wastewater used to irrigate 32,500 hectares of crops is also untreated. The practice is also common in neighboring India, the study said.

An expanding health problem

Of the 48 countries in sub-Saharan Africa examined, three had complete information on wastewater availability, treatment and use. Previous studies, however, have shown that untreated wastewater is commonly used to water crops in Ghana, Senegal, Kenya and Zimbabwe. In these countries, this polluted water often leads to diseases like diarrhea and cholera.

The study projects that the demand for wastewater as a source of irrigation will only increase in arid or semiarid regions of poorer countries, likely at a faster pace than the safe distribution of water that can be achieved.

"Thus, 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," it states.

Despite these challenges, Qadir and his co-authors see great opportunity in wastewater reuse, even beyond its capacity to stand in for the global supply of fresh water. The use of wastewater can stand in for fertilizer, he said, providing farmers with a free source of phosphorous and other nutrients. It also takes far less energy to use wastewater than to pump water out of aquifers.

There are also opportunities for wastewater beyond crop irrigation. Wastewater can be used to irrigate agroforestry systems and recharge aquifers. Also, Qadir said, "there are growing examples where wastewater is now considered as a potential source of energy," possibly even offsetting the energy needed for treatment.

"The common person, without a little background about the potential of this resource, would just see this as dirty stuff," he added. But among experts, Qadir said, "there is an increasing awareness of this type of water as a very useful resource."

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