Nature sanitizes more than 40 million tons of human waste a year

Nature plays a major role in sanitizing cities worldwide, according to a new study published by Cell Press. A team of researchers in the UK and India investigated the role of ecosystems in sanitation, and found that nature provides at least 18 percent of these services in 48 major cities.

The experts estimate that more than two million cubic meters of human waste is processed each year without engineered infrastructure across the study sites. For example, pit latrine waste is gradually filtered and cleaned as it moves through the soil before it reaches groundwater.

Study co-author Alison Parker is a senior lecturer in International Water and Sanitation at Cranfield University in the United Kingdom. 

“Nature can, and does, take the role of sanitation infrastructure,” said Parker. “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.”

Wastewater treatment infrastructure serves as an important promoter of human health by converting human feces into harmless products. By 2017, however, more than 25 percent of the world’s population did not have access to basic sanitation facilities, while 14 percent used toilets that disposed of waste onsite. 

Previous research has suggested that this potentially hazardous waste may be naturally treated by natural wetlands and mangroves. For example, the Navikubo wetland in Uganda processes untreated wastewater from more than 100,000 households, which protects the Murchison Bay and Lake Victoria from harmful contaminants. 

In the United States, coastal wetlands in the Gulf of Mexico remove nitrogen from the Mississippi River, noted the researchers.

“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 study co-author Simon Willcock. “But the role for nature was largely unrecognized.”

The team used Excreta Flow Diagrams to quantify sanitation ecosystem services in 48 cities containing about 82 million people. The study was focused on all diagrams that were available on December 17th, 2018, specifically those coded as “fecal sludge contained not emptied.” 

The researchers estimate that nature processes 2.2 million cubic meters of human waste per year within these 48 cities. Since more than 892 million people worldwide use similar onsite disposal toilet facilities, they further estimate that nature sanitizes about 41.7 million tons of human waste per year before the liquid enters the groundwater. These services are collectively valued at about $4.4 billion per year, but the study authors note that these estimates likely undervalue the true worth of sanitation ecosystem services.

The experts hope their findings will shed light on an important but often unrecognized contribution that nature makes to many people’s everyday lives, which may inspire the protection of ecosystems such as wetlands that protect communities from wastewater pollutants downstream.

“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,” said Parker.

The study is published in the journal One Earth.

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By Chrissy Sexton, Earth.com Staff Writer