Soil filters and sanitizes about 42 million tons of human waste in pit latrines and septic tanks every year, a service that prevents pathogens and other contaminants from entering groundwater and is worth at least $4.4 billion.
Published Thursday by U.K. and Indian scientists in One Earth, an assessment of 48 cities around the world is the first global calculation of the value humans receive from nature’s treatment of their waste — though it is almost certainly an undercount. Their investigation was intended to recognize nature’s role in sanitation systems and to better inform the development of new sanitation infrastructure.
Soil and the living things it hosts can “clean” human feces before it reaches groundwater. Soil filtration removes pathogens by physically filtering them out, starving them of nutrients or exposing them to predators, while plants and microorganisms in the soil consume nitrates. Insufficiently sanitized waste can contaminate groundwater and pose health risks to nearby populations.
Nature plays a larger sanitizing role for the many people worldwide who do not have access to modern sewage systems or other forms of treatment infrastructure. Two billion people don’t have any access to toilets, latrines or other basic sanitation facilities, according to the World Health Organization, and another 1 billion use toilets or latrines where feces are disposed of onsite.
“We realized that no one had really thought about sanitation ecosystem services before,” said Alison Parker, one of the study’s lead authors and a senior lecturer in international water and sanitation at Cranfield University in England. “So we conceptualized the idea [of] how we thought that nature was providing sanitation services currently, and then we found a way of quantifying it.”
The researchers said nature can provide sanitation services by acting as a pipeline, such as a river moving human waste away from human populations, or acting as a treatment plant, filtering waste and harmful bacteria before it reaches groundwater. Wetlands, for instance, are known to process untreated wastewater from about 100,000 households in Uganda and remove nitrogen from the Mississippi River as it enters the Gulf of Mexico.
But given restrictions on available data as of 2018, the team assessed only one ecosystem sanitation service: the processing of human waste by soil in pit latrines and septic tanks. The analysis was limited to sites that do not contaminate groundwater and either empty by seeping into the soil or are safely abandoned once full. These conditions were ensured by each city’s excreta flow diagrams, which track the pathways and disposal of human waste.
The study covered 48 cities which are home to 82 million people, including 27 cities in South and Southeast Asia, 15 in Africa, four in the U.S. and two in South America. The assessed populations ranged in size from 27,400 people in Bure, Ethiopia to 16.7 million people in Delhi, India.
Across these cities, researchers found that nature processes about 2.2 million cubic meters of human waste per year, or about 18% of sanitation. They calculated the service to be worth about 57 cents per person per year, or about $4.4 billion per year for the world’s total population.
The resulting value is “conservative and underestimates the value of sanitation ecosystem services,” the study’s authors wrote, because of its limited scope of 48 cities and only one known pathway of human waste. Nature likely plays a larger role in rural areas of the world, they said, and the other ways the ecosystem is involved in sanitation would have been difficult to confidently quantify.
The researchers urged further investigations into the scope of sanitation measures provided by nature, to understand how they can and should interact with human infrastructure and to ensure that they are not overloaded.
“At the moment we're really highlighting the gap in engineered infrastructure and the role that nature is playing,” Parker said. “We think that there needs to be more collaboration between the ecologists and engineers to develop systems that use the best of both approaches.”
The article, “Nature provides valuable sanitation services,” was published Feb. 19 in One Earth. The authors of the study were Simon Willcock and Indunee Welivita, Bangor University; Alison Parker, Tim Brewer, Sarah Cooper, Dolores Rey and Paul Hutchings, Cranfield University; Charlotte Wilson, Cranfield University and Durham University; Dishaad Bundhoo and Kenneth Lynch, University of Gloucestershire; Sneha Mekala, Fresh Water Action Network South Asia; and Prajna Mishra and Kongala Venkatesh, University of Hyderabad. The lead authors were Simon Willcock and Alison Parker.