World cities grouped by phosphorus yield. Graphic: McDonald and Shemie, 2014 / The Nature Conservancy

18 November 2014 (Urban Water Blueprint) – Impacts on water quality are not limited to sedimentation rates. As watersheds are exploited for agricultural purposes, and as agriculture turns intensive, the use of fertilizers increases and more fertilizers end up in the water. The two most common nutrients that cause problems are excessive phosphorus and nitrogen, which come primarily from agriculture and pastureland. In practice, phosphorus and nitrogen loading—hereafter “nutrient pollution”—are highly spatially correlated, meaning that if one occurs, it is likely that the other will as well. This report includes information for phosphorus due to space limitations.

More than 384 million urbanites (46 percent of all people living in the 100 largest cities) get their drinking water from watersheds with high nutrient pollution. This analysis divides the water sources of the world’s large cities into three categories, based upon their level of nutrient yield. As with sediment, the task of raw water quality maintenance seems harder for the developing world than for the developed (Figure 2-4).

If current trends continue, nutrient pollution will worsen over the next decade. For instance, agricultural area is forecast to increase by 70 million hectares by 2030. Perhaps more significantly, fertilizer use is forecast to increase by 58 percent globally over the same time period. Overall, the cities that are likely to have the biggest increase in nutrient loading from agriculture are located in Brazil, Argentina, and parts of sub-Saharan Africa.

While human wastewater is a minor part of the overall nitrogen and phosphorus cycle in many water sources, in rivers such as the Ganges, wastewater from multiple cities (often released without treatment) becomes the drinking water source for other cities. In these basins, increased access to sanitation and the installation of basic treatment for wastewater is needed to prevent a further decrease in raw water quality.

As with sedimentation, high nutrient levels leads to higher O&M costs for water treatment. Our analysis finds that a 10 percent reduction in nutrients on average reduces treatment costs by 2 percent. Higher nutrient concentration is associated with a greater frequency and intensity of algae blooms and higher organic matter content. Both lead to more frequent filter cleaning and additional treatment processes to remove unwanted colors or odors from the water.

In extreme cases, nutrient levels have even led to plant shutdowns. High nutrient levels in source water also generate more wastewater, which in turn increases the cost of treating effluent exiting a plant. The use of chlorine, for example, as a disinfectant in the presence of organic matter can lead to unwanted disinfection byproducts, some of which can have negative health effects [18].

Higher levels of nutrients are also associated with more complex treatment technologies and hence higher capital costs. See Appendix C for a quantitative look at this trend.

Blueprint for Urban Water Security

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