by ANGELY MERCADO
Cleaning up human sewage is an international issue. As growing populations grapple with how to properly process and dispose of human waste, a significant amount of sewage ends up in waterways around the world. More than 2 billion people worldwide lack any sanitation, which means that millions of tons of human waste enters into the environment untreated every year. And in wealthier countries like the US, most treated sewage is placed directly into rivers and oceans. Human waste also introduces too much nitrogen in our waterways, which causes watersheds to experience dead zones that used to house marine life.
To map this issue, postgraduate researcher Cascade Tuholske, who is now affiliated with the Columbia Climate School, and his colleagues at UC Santa Barbara mapped out the ways that human sewage has introduced extra nitrogen and pathogens into watersheds. The team used data sets to detail whether populations were urban or rural, how much protein was consumed per person, how much nitrogen was then excreted, and how much the waste was sanitized and how, says Tuholkse. The more meat a person eats, the more nitrogen is found in their waste, which means more nitrogen ends up in the waterways near them. This happens because nitrogen comes from amino acids which make up protein. The breaking down of those proteins during digestion creates the nitrogen that ends up in wastewater.
“We estimate that 25 watersheds contribute approximately 46 percent of global nitrogen inputs from wastewater into the ocean,” Tuholske said in a release. “Nearly half as much nitrogen comes from wastewater as agricultural runoff globally, which is a huge fraction.”
More than 130,000 watersheds with a fixed point on the coastline were analyzed across the world. The study also models the journey of nitrogen into the ocean to get a rough idea of where nitrogen is building up coastal ecosystems, which can hurt marine life. Tuholske tells Popular Science the uptick of nitrogen in rapidly “developing” nations, such as some countries in Asia, may be attributed to the increase of animal protein consumption as those countries become wealthier—the OECD-FAO Agricultural Outlook for the next decade is expecting a 0.8 percent increase in meat consumption every year compared to 0.24 percent in developed nations.
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