Feb. 19 (UPI) -- The majority of human waste is processed by wastewater treatment infrastructure, but according to a new global survey, the sanitization services of natural ecosystems still play a significant role in protecting water supplies.
When researchers in India and Britain analyzed sanitation services in 48 cities around the world, they found nature was responsible for cleaning 41.7 million tons of human waste annually -- approximately 18 percent of the cities' sanitization services.
Researchers published the results of their survey, the first to take a global perspective on natural sanitation, in the journal One Earth on Friday.
"Nature can, and does, take the role of sanitation infrastructure," study co-author Alison Parker, senior lecturer in international water and sanitation at Cranfield University in Britain, said in a news release.
"While we are not marginalizing the vital role of engineered infrastructure, we believe a better understanding of how engineered and natural infrastructure interact may allow adaptive design and management, reducing costs, and improving effectiveness and sustainability, and safeguard the continued existence of these areas of land."
More than a quarter of the world's population doesn't have access to simple sanitation facilities, and another 14 percent of the global population uses toilets in which waste is disposed on-site.
According to the latest survey, the wastewater treatment services provided by wetlands and mangroves regularly fill in when human-built sanitizing facilities are lacking.
In Uganda, for example, the Navikubo wetland processes the waste of more than 100,000 households, preventing the contamination of Murchison Bay and Lake Victoria, important sources of freshwater.
In the United States, coastal wetlands along the Gulf Coast help capture excess nitrogen carried downstream by the Mississippi River.
"We realized that nature must be providing sanitation services, because so many people in the world do not have access to engineered infrastructure like sewers," said co-author Simon Willcock, senior lecturer in environmental geography at Bangor University in Wales.
"But the role for nature was largely unrecognized," Willcock said.
To complete the survey, scientists analyzed so-called Excreta Flow Diagrams, an international effort combing in-person interviews with field observations and direct measurements to map the ways human waste moves and flows through 48 cities and towns around the world.
More specifically, the authors of the latest study focused on diagrams identifying the use of pit latrines and below-ground septic tanks -- diagrams coded "fecal sludge contained not emptied."
Based on their analysis of the diagrams, researchers conservatively estimated natural ecosystems in the 48 cities clean 2.2 million cubic meters of human waste per year.
Because more than 892 million people around the world use pit latrines and below-ground septic tanks, researchers estimated nature cleans 41.7 million tons of human waste every year -- sanitation services worth at least $4.4 billion annually.
Researchers hope their work will help policy makers more accurately quantify the vital ecological services provided by wetlands and mangroves. Previously surveys have revealed the vital role similar ecosystems play in containing agricultural runoff and curbing the damaging effects of flooding and coastal storms.
"We would like to promote a better collaboration between ecologists, sanitation practitioners and city planners to help nature and infrastructure work better in harmony, and to protect nature where it is providing sanitation services," Parker said.