Domestic wastewater is an overlooked source and quantity in global river dissolved carbon – Nature

Report on Domestic Wastewater as a Source of Global Riverine Dissolved Carbon

Introduction: Aligning Carbon Cycle Research with Sustainable Development Goals

Accurate quantification of the global carbon cycle is fundamental to achieving Sustainable Development Goal 13 (Climate Action). Riverine dissolved carbon (DC) is a critical component of this cycle, yet significant uncertainties persist in global estimates. This report addresses a major gap in current carbon budgets: the contribution of domestic wastewater. The management of wastewater is central to SDG 6 (Clean Water and Sanitation), particularly Target 6.3, which calls for halving the proportion of untreated wastewater. Furthermore, as global urbanization accelerates, understanding the environmental impact of cities, as outlined in SDG 11 (Sustainable Cities and Communities), requires a precise assessment of pollution sources like wastewater. This analysis quantifies the impact of treated and untreated domestic wastewater on riverine DC, linking urban development and sanitation practices directly to global climate and aquatic ecosystem health (SDG 14: Life Below Water).

Global Impact of Wastewater on Riverine Carbon Fluxes

The study reveals that domestic wastewater is a significant and previously unquantified source of carbon to global river systems. This finding has direct implications for monitoring progress towards multiple SDGs.

Aggregate Global Discharge

• Total Annual Discharge: Domestic wastewater contributes approximately 21.4 Tg of dissolved carbon (DC) to rivers annually.
• Contribution to Global Export: This accounts for a notable 3.13 ± 0.46% of the total global riverine DC export to oceans, impacting carbon models relevant to SDG 13 and SDG 14.

Discharge Breakdown: A Reflection of SDG 6 Progress

The composition of this discharge highlights the global challenge in achieving SDG 6.3. Untreated wastewater is the dominant source of carbon pollution.

1. Untreated Wastewater:
   • Dissolved Inorganic Carbon (DIC): 9.64 Tg per year.
   • Dissolved Organic Carbon (DOC): 4.21 Tg per year.
   • Total: 13.85 Tg per year.
2. Treated Wastewater:
   • Dissolved Inorganic Carbon (DIC): 6.42 Tg per year.
   • Dissolved Organic Carbon (DOC): 1.17 Tg per year.
   • Total: 7.59 Tg per year.

These figures demonstrate that failing to treat wastewater not only poses health and sanitation risks but also releases nearly double the amount of dissolved carbon into aquatic ecosystems, undermining efforts for both clean water (SDG 6) and climate action (SDG 13).

Regional Disparities and Urbanization Impacts

The contribution of wastewater-derived carbon is not uniform globally, reflecting disparities in economic development, urbanization, and progress towards SDG 6 and SDG 11.

Concentration in Densely Populated Regions

• High-Contribution Zones: The highest contributions are observed in densely populated regions with significant wastewater treatment capacity, such as parts of Asia, Europe, and North America. This indicates that even with treatment, the sheer volume of effluent from megacities (a focus of SDG 11) constitutes a major carbon source.
• Developing Regions: In low- and middle-income countries, particularly in South and East Asia, the high proportion of untreated wastewater discharge makes these regions hotspots for carbon pollution. This directly correlates with challenges in meeting SDG 6.3 and results in disproportionate impacts on local water quality and regional carbon cycles.

Case Study: China’s Major River Basins

Analysis of China’s seven major river basins illustrates the direct link between rapid urbanization and riverine carbon loads.

• Significant Regional Impact: In these basins, urban wastewater effluents contribute an average of 7.3% of total DIC and 9.9% of total DOC exports.
• North-South Divide: Northern basins, which are more water-scarce and have higher population densities (e.g., Haihe River Basin), show a much higher relative contribution of effluent-derived carbon (up to 36.2% of DOC). This is partly because treated wastewater is a crucial component of river flow, highlighting a complex trade-off between water security (SDG 6) and carbon pollution.

Factors Influencing Effluent Carbon and Implications for Sustainable Development

Key Drivers of Carbon Levels

The concentration and flux of DC from wastewater are influenced by a combination of technological, environmental, and socioeconomic factors relevant to sustainable infrastructure and climate adaptation.

• Wastewater Treatment Technology: Advanced treatment processes can lower DOC concentrations, demonstrating a pathway to mitigate this carbon source through investment in sustainable infrastructure (SDG 9).
• Socioeconomic Factors: Economic growth, urban population size, and the expansion of built-up areas show a strong positive correlation with DC flux. This underscores the need to integrate sustainable waste management into urban planning to achieve SDG 11.
• Climatic Factors: Temperature and precipitation influence both treatment efficiency and the dilution capacity of receiving rivers. Climate change (SDG 13) is likely to exacerbate these effects, particularly in water-scarce regions.

Conclusion and Recommendations for SDG-Aligned Policy

The findings of this report establish domestic wastewater as a globally significant and overlooked source of riverine dissolved carbon. This omission hinders accurate global carbon budgeting and masks a key pressure on aquatic ecosystems, thereby affecting the monitoring and achievement of several interconnected SDGs.

Policy Implications

1. Integrate Wastewater into Carbon Accounting: To support SDG 13, national and global carbon budgets must incorporate DC fluxes from both treated and untreated wastewater.
2. Enhance Monitoring for SDG 6: Achieving SDG 6.3 requires not only expanding wastewater treatment but also monitoring effluent quality for pollutants, including carbon. Implementing monitoring for effluent DC should be considered a key performance indicator for sustainable sanitation systems.
3. Promote Sustainable Urban Planning (SDG 11): As cities expand, strategies must be developed to reduce the carbon footprint of municipal services. This includes investing in advanced wastewater treatment technologies that minimize carbon discharge and promoting water reuse schemes that consider the downstream ecological impacts.

Addressing the challenge of wastewater-derived carbon is a critical cross-cutting issue that links sanitation, urban sustainability, climate action, and the protection of aquatic life. Acknowledging and managing this carbon source is essential for making meaningful progress on the 2030 Agenda for Sustainable Development.

1. Which SDGs are addressed or connected to the issues highlighted in the article?

1. SDG 6: Clean Water and Sanitation

   The article directly addresses the core themes of SDG 6 by focusing on the management of domestic wastewater. It quantifies the global volumes of treated and untreated wastewater, highlighting significant disparities in treatment capacity across different regions. The central issue is how wastewater discharge, whether treated or not, affects the water quality of riverine ecosystems by introducing dissolved carbon.

2. SDG 11: Sustainable Cities and Communities

   The research connects rapid urbanization and population density to increased wastewater generation and, consequently, higher dissolved carbon loads in rivers. The article states that contributions are “highest in densely populated regions” and that factors like “economic growth and urban development” influence effluent levels. This links the environmental impact of cities (a key concern of SDG 11) to water pollution and the global carbon cycle.

3. SDG 13: Climate Action

   The article establishes a clear link to climate action by quantifying a previously unquantified source of carbon in the global carbon cycle. It argues that “wastewater derived carbon” needs to be incorporated into “global carbon budget assessment” to improve the accuracy of climate dynamics prediction. By identifying and measuring this anthropogenic carbon source, the study provides crucial information for climate change mitigation strategies and policies.

4. SDG 14: Life Below Water

   The research examines the “land-to-ocean carbon transport” through rivers, which is a primary pathway for land-based pollution to reach marine environments. The article quantifies that domestic wastewater accounts for “3.13 ± 0.46% of the global riverine DC export” to the oceans. This discharge of dissolved carbon can alter the biogeochemistry of coastal waters, connecting directly to the goal of reducing marine pollution from land-based activities.

2. What specific targets under those SDGs can be identified based on the article’s content?

1. Target 6.3: Improve water quality by reducing pollution and increasing wastewater treatment

   This target is central to the article. The study’s entire premise is built on differentiating between “treated and untreated domestic wastewater” and quantifying their respective impacts. It explicitly mentions that globally, 47.2% of wastewater is “released untreated directly into the environment.” It also highlights regional disparities in treatment, noting that in South and East Asia, “the relatively low rates of safely treated domestic wastewater” contribute significantly to riverine carbon loads. This directly relates to the target’s aim of halving the proportion of untreated wastewater and improving water quality.

2. Target 11.6: Reduce the adverse environmental impact of cities

   The article connects urbanization directly to environmental degradation. It notes that “anthropogenic activities exacerbate uncertainties in current global riverine carbon flux estimates, particularly in rapidly urbanizing watersheds.” The analysis shows that effluent DC flux has a “positive correlation with population density” and “urban population size.” This demonstrates how municipal waste management (specifically wastewater) in cities has a significant, measurable environmental impact, which is the focus of Target 11.6.

3. Target 13.2: Integrate climate change measures into national policies, strategies and planning

   The article’s conclusion that there is a “need to incorporate the wastewater derived carbon into global carbon budget assessment” is a direct call to action relevant to this target. Accurate carbon budgets are fundamental for effective climate policies. By quantifying the 21.4 Tg of dissolved carbon discharged annually from wastewater, the study provides data that should be integrated into national and global climate planning to better manage anthropogenic carbon sources.

4. Target 14.1: Prevent and reduce marine pollution from land-based activities

   The article quantifies a specific type of land-based pollution entering the marine environment. It calculates the “land-to-ocean carbon transport” from wastewater, showing it contributes significantly to the “global riverine DC exports to the oceans.” This analysis provides a metric for understanding how urban wastewater management on land directly results in the pollution of downstream aquatic and, ultimately, marine ecosystems.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

1. Indicator for Target 6.3 (Implied: 6.3.1 – Proportion of wastewater safely treated)

   The article heavily relies on and reports data directly related to this indicator. It provides global figures, stating that of the total wastewater volume, “41.4% treated in wastewater treatment plants (WWTPs) and discharged, and 47.2% released untreated.” It further breaks this down by region, comparing treatment rates in Asia (“24% in South Asia”) with those in “Europe and North America” (86.5%). These percentages are direct measures for Indicator 6.3.1.

2. Indicators for Target 11.6 (Implied)

   While not citing an official indicator, the article uses several metrics that measure the environmental impact of cities. These include:

   • Volume of treated and untreated wastewater generated: The article quantifies this at national and river basin scales (e.g., “Globally, ~267.55 billion m3 domestic wastewater generated each year”).
   • Concentration of pollutants in effluent: The study provides detailed measurements of “dissolved inorganic carbon (DIC)” and “dissolved organic carbon (DOC)” concentrations in wastewater effluent (e.g., “global DIC and DOC concentrations in WWTP effluents are ~4.2 and 1.8 times higher, respectively, than those observed in natural river waters”).

   These metrics can be used to track the progress of cities in managing their municipal waste.

3. Indicator for Target 13.2 (Implied)

   The primary indicator proposed by the article is the total flux of dissolved carbon (DC) from wastewater sources. The study’s main finding is the quantification of this flux: “globally, domestic wastewater discharges ~21.4 Tg DC annually.” The article advocates for the monitoring of this specific carbon flux so it can be included in carbon budgets, thereby serving as a metric to assess whether climate change measures are comprehensive.

4. Indicator for Target 14.1 (Implied)

   The article provides a direct indicator for measuring land-based pollution affecting marine ecosystems: the proportion of riverine dissolved carbon export originating from wastewater. It calculates that wastewater “collectively account[s] for 3.13 ± 0.46% of the global riverine DC export.” This percentage serves as a clear indicator of the contribution of a specific land-based activity (wastewater discharge) to the total pollution load transported to the oceans.

4. Create a table with three columns titled ‘SDGs, Targets and Indicators” to present the findings from analyzing the article.

Source: nature.com