A new study shows that without nature, humanity may drown in its own filth. 

The research finds that the ecosystem provides at least 18 per cent of the sanitation services in 48 cities worldwide.

It estimates that more than 2 million cubic meters of the cities' human waste is processed each year without engineered infrastructure. This includes pit latrine waste that gradually filters through the soil - a natural process that cleans it before it reaches groundwater.

“Nature can, and does, take the role of sanitation infrastructure,” says Dr Alison Parker, one of the authors of the study. 

“While we are not marginalising 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.”

The international research team was able to quantify sanitation ecosystem services in 48 cities containing about 82 million people using Excreta Flow Diagrams, which involve a combination of in-person interviews, informal and formal observations, and direct field measurements to document how human fecal matter flows through a city or town. 

The researchers assessed available diagrams to find instance of “fecal sludge contained not emptied” (FSCNE), in which the waste is contained in a pit latrine or septic tank below ground but does not pose a risk to groundwater, for example, because the water table is too deep.

Conservatively, 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 - a service worth about $4.4 billion per year. 

However, the authors note that these estimates likely undervalue the true worth of sanitation ecosystem services, since natural processes may contribute to other forms of wastewater processing, though these are harder to quantify.

The team says it hopes to shed light on an important but often unrecognised contribution that nature makes to many people's everyday lives, inspiring the protection of ecosystems such as wetlands that protect downstream communities from wastewater pollutants.

The study is accessible here.