Organic pollution of rivers: Combined threats of urbanization, livestock farming and global climate change

Abstract

Organic pollution of rivers by wastewater discharge from human activities negatively impacts people and ecosystems. Without treatment, pollution control relies on a combination of natural degradation and dilution by natural runoff to reduce downstream effects. We quantify here for the first time the global sanitation crisis through its impact on organic river pollution from the threats of (1) increasing wastewater discharge due to urbanization and intensification of livestock farming, and (2) reductions in river dilution capacity due to climate change and water extractions. Using in-stream Biochemical Oxygen Demand (BOD) as an overall indicator of organic river pollution, we calculate historical (2000) and future (2050) BOD concentrations in global river networks. Despite significant self-cleaning capacities of rivers, the number of people affected by organic pollution (BOD >5 mg/l) is projected to increase from 1.1 billion in 2000 to 2.5 billion in 2050. With developing countries disproportionately affected, our results point to a growing need for affordable wastewater solutions.

Figure 1

(a) Variables and processes affecting organic pollution of rivers, expressed as in-stream Biochemical Oxygen Demand (BOD); (b) Calculated in-stream BOD concentration profiles from headwater to river mouth for the year 2000 along the main stem of the Rhine and Yamuna-Ganges rivers. Without natural degradation and wastewater treatment (purple profiles), BOD concentrations gradually increase along the densely populated Rhine, whereas for the Yamuna-Ganges river a rapid increase near the cities of Delhi and Agra is followed by dilution with freshwater from several large tributaries. Pollutant loading from livestock farming adds significant pollution in both basins (red profiles). When natural degradation is taken into account (yellow profiles), BOD concentrations decrease significantly in both rivers, illustrating the self-cleaning capacity of natural rivers. Wastewater treatment (blue profiles) further reduces BOD concentrations, especially in the Rhine, but also near Delhi and Agra, which have higher rates of wastewater treatment than smaller cities further downstream.

Figure 2. Global patterns of computed river BOD concentrations in the years 2000 and 2050.

Figure 3. Number of people (in millions) living along polluted rivers in 10 major river basins.

Basins are listed from more to less polluted in the year 2000, with green and red arrows to the left indicating their change in ranking in the year 2050.

Figure 4. Environmental Kuznets curves for the relation between computed country-wide organic river pollution (with and without wastewater treatment) and per capita income in the year 2000.

Figure 5. Conceptual diagram of computing in-stream BOD concentrations.