Impact hotspots of reduced nutrient discharge shift across the globe with population and dietary changes

Abstract

Reducing nutrient discharge from wastewater is essential to mitigating aquatic eutrophication; however, energy- and chemicals-intensive nutrient removal processes, accompanied with the emissions of airborne contaminants, can create other, unexpected, environmental consequences. Implementing mitigation strategies requires a complete understanding of the effects of nutrient control practices, given spatial and temporal variations. Here we simulate the environmental impacts of reducing nutrient discharge from domestic wastewater in 173 countries during 1990-2050. We find that improvements in wastewater infrastructure achieve a large-scale decline in nutrient input to surface waters, but this is causing detrimental effects on the atmosphere and the broader environment. Population size and dietary protein intake have the most significant effects over all the impacts arising from reduction of wastewater nutrients. Wastewater-related impact hotspots are also shifting from Asia to Africa, suggesting a need for interventions in such countries, mostly with growing populations, rising dietary intake, rapid urbanisation, and inadequate sanitation.

Fig. 1

Spatial patterns of nitrogen and phosphorus flows from domestic wastewater management in the three regimes. a Box-whisker plots show the range of global total N and P flows. b Circles in the maps indicate 173 countries. Circle sizes indicate N emissions (Gg N yr⁻¹), and their colours indicate P emissions (Gg P yr⁻¹). c Column heights represent per capita N discharge, and column areas denote total N emissions from their respective regions. Mid-range outputs are used to generate the charts of b and c

Fig. 2

Temporally and spatially differentiated normalised environmental impact hotspots. a Nitrogen cycle; b Phosphorus cycle; c Climate change; d Stratospheric ozone depletion; e Atmospheric aerosol loading; f Chemical pollution; g Biodiversity loss; and h Freshwater use. To acquire these normalised results of the annual regional-scale trajectories of environmental impacts from wastewater treatment, the scores for each impact category, year, and country are divided by the global impact scores for the same year, providing a dimensionless ratio. The colour scale ranks the magnitude of the impacts (with magenta denoting a significant impact hotspot) and reveals the shifts in pressures (from navy-blue to magenta)

Fig. 3

Sensitivity analysis for different driving factors. Circle size indicates RSI. The larger the circle size, the more sensitive the model output is to the input parameter. Circle colour indicates the relationship between model output and input (orange represents negative while blue represents positive). The sensitivity of aggregated impacts is determined as the average of the RSI of relative individual impacts

Fig. 4

Implications of wastewater management trajectories in terms of global thresholds on planetary boundaries. a Circles represent the overall global effects attributed to wastewater services, and their size represents the planetary impact ratio. b Ring diagrams trace the contributions of the continents to the effects. The following reported thresholds for the planetary boundaries are used to normalise the effect scores into dimensionless ratios: 62 Tg N yr⁻¹ (NC); 11 Tg P yr⁻¹ (PC); + 1.0 W m⁻² (CC); 14 DU yr⁻¹ (OD); 14.57 Tg PM₁₀-eq (AL); 4.36 million DALYs (CP); 280,729 species yr yr⁻¹ (BL); and 4000 km³ yr⁻¹ (FU)