What affects the amount of waste in cities

City

Waste, wastewater and greenhouse gases do not increase at the same rate as a city grows. © Elisa Heinrich Mora

Every single one of us produces waste every day. The global amount of waste is increasing rapidly, posing challenges to societies and ecosystems. A study now shows how the size of cities influences the amount of waste, wastewater and greenhouse gases generated. Analysis of more than a thousand cities worldwide suggests that the amount of solid waste increases linearly with city size. Large cities, on the other hand, are more efficient when it comes to greenhouse gases and more wasteful when it comes to wastewater. The results help to uncover underlying mechanisms and develop measures for greater sustainability.

The human population is growing and more and more people live in cities. Together with changes in lifestyle and consumption habits, this means that there is more and more waste. While waste products in natural systems are usually used by other living beings - be it microorganisms that break down feces or plants that use CO2 - large amounts of waste created by humans accumulate and pose challenges to ecosystems and societies.

Waste grows with cities

“Until now, however, it was still unclear what connections exist between waste generation and social changes such as population growth and urbanization,” explains lead author Mingzhen Lu from Stanford University in California. “The key question is whether waste is produced more efficiently or less efficiently as the size of the systems increases and what the recycling effort is as a result.”

Together with his team, Lu analyzed how much solid waste, wastewater and greenhouse gases they produce for over a thousand cities around the world. The researchers related these values ​​to the size of the cities. They used so-called scaling theory - a method commonly used in biology to describe how the physiology of organisms changes with their body mass. “Scaling theory allowed us to extract overarching patterns and overcome the individuality of each city,” explains Lu.

Size, wealth and environmental factors

The result: “Solid waste increases linearly with city size,” reports the team. The amount of wastewater, however, grows disproportionately as the size of the city increases. “In many cases, the increased wastewater volume can be explained by increased economic activity in larger cities,” said Lu and his colleagues. When it comes to greenhouse gases, however, large cities are more economical than small ones relative to their size. One reason for this is that growth usually brings with it a more efficient energy and transport infrastructure and thus helps to save greenhouse gases.

In addition to the size, the wealth of a city also plays a role in the amount of waste it produces. Overall, the richer a city, the more waste it produces. “An example is Seattle in the USA and Lilongwe in Malawi,” said the team. “The much richer Seattle produces eight times more waste than Lilongwe, despite having a smaller population.” The same can be seen with greenhouse gases. The rich Dutch city of Rotterdam emits many times more greenhouse gases than the much larger but poorer Indian city of Bandung. When it comes to the amount of wastewater, natural conditions also play a role, as the researchers explain. For example, cities in drier areas produce less wastewater than cities in areas with higher rainfall.

For a more sustainable future

“Our work provides a framework to better understand current and future human waste production,” write Lu and his team. “We need ways to decouple the connection between growing economic prosperity and waste production per capita.” Cities like San Francisco could provide a role model. “San Francisco produces less waste than any other major city in the USA,” say the researchers. In order for a city to produce less waste than would be expected based on its size and wealth, structural and cultural characteristics as well as political measures play a role. “Our approach provides a systematic framework to uncover these underlying mechanisms, which could be key to reducing waste generation and achieving a more sustainable future.”

Source: Mingzhen Lu (Stanford University, California, USA) et al., Nature Cities, doi: 10.1038/s44284-023-00021-5

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