More than half of the human population, around 3.3 billion people, now live in urban areas. And around 5 billion people are expected to be living in towns and cities by 2030. The colossal growth of urban centres across the globe is associated with a whole host of social and environmental problems, not least of which is the intensive outpouring of human waste.
Urban wastewater has been wreaking havoc with marine and freshwater ecosystems for decades. When these nitrogen- and phosphorus-rich waters leak into streams, rivers and oceans, they stimulate the growth of tiny algae that strip the ecosystems of oxygen and light. The result is dead, toxic water that is bereft of all but the most ravenous and aggressive forms of life.
Unfortunately, the problem is set to get worse. G. Van Drecht, of the Netherlands Environmental Assessment Agency, and colleagues conclude that urban wastewater emissions of nitrogen and phosphorus will increase by around two- to threefold globally by the middle of the century, compared with levels in 2000 (Glob. Biogeochem. Cycles 23, GB0A03; 2009). Their calculations are based on estimates of population and economic growth, urbanization, and the development of sewers and wastewater treatment systems.
According to their country-specific projections, nitrogen and phosphorus emissions will increase in North America, but will decline in Europe and North Asia, owing in part to the removal of nutrients from wastewater and population declines. Developing countries, on the other hand, are likely to show a uniform increase in nitrogen and phosphorus emissions through wastewater disposal. The problem will be most severe in South Asia, an area that has undergone a massive population explosion in the past few decades. Here, urban wastewater emissions are predicted to increase by up to four- to fivefold by 2050.
Under the most optimistic development scenarios assumed by the researchers, societies adopt a proactive approach to the management of ecosystems, and significant quantities of waste nitrogen and phosphorus are recycled. But even in these best-case scenarios, urban discharge of nitrogen and phosphorus is unlikely to decline before the middle of the century. The outlook for aquatic ecosystems, particularly in developing countries, is not good.
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