Key Takeaways & Executive Findings
- •• • Indirect carbon emissions constitute 56.8% of total emissions, with electricity accounting for 33.4%, indicating that energy efficiency and chemical reduction are primary levers for decarbonization. • • Reclaimed water source heat pumps contribute 67.4% of total carbon reduction, achieving a balance ratio of 103.7%, making them the critical component for carbon neutrality. • • Coupling reclaimed water reuse with heat pumps requires carbon reduction shares of 34.82% and 65.18% respectively to reach a 100% balance ratio, offering a universally applicable strategy. • • The ratio of residual thermal energy to chemical energy carbon reduction is 8.76:1; extracting only 36% of residual heat alongside sludge anaerobic digestion-cogeneration achieves full carbon balance.
Abstract
In the context of carbon peaking and carbon neutrality, urban reclaimed water plants must adopt measures such as energy conservation, consumption reduction, and enhanced resource and energy utilization to achieve carbon neutrality. This study developed a carbon emission balance model and accounting method for such plants, incorporating strategies of carbon emission reduction, carbon substitution, and carbon sink. The optimal pathway towards carbon neutrality was evaluated based on the carbon emission balance ratio. Using a 1×10⁵ m³/d urban reclaimed water plant as a case study, the results showed total carbon emissions of 20,934 t CO2e. The carbon emission reduction from reclaimed water source heat pumps for heating and cooling was 21,701 t CO2e, yielding a carbon emission balance ratio of 103.7%. In contrast, other carbon reduction measures contributed 15,424 t CO2e, with a balance ratio of 73.7%, highlighting the pivotal role of reclaimed water source heat pumps. When the heat pump extracted 27% and 36% of residual thermal energy, coupled with reclaimed water reuse or sludge anaerobic digestion-cogeneration, respectively, both pathways achieved a 100% balance ratio. Assuming year-round extraction, the balance ratio reached 213%. The carbon reduction ratio between utilizing residual thermal energy and chemical energy was 8.76:1. This study demonstrates that urban reclaimed water plants can achieve carbon neutrality through multiple pathways, with residual thermal energy recovery exhibiting significant potential.
1. Introduction
Urban wastewater treatment in China, with a capacity of 2.58×10⁸ m³/d as of 2022, generates annual carbon emissions exceeding 1.97×10⁸ t CO2e, approximately 2% of national emissions. Projections indicate this will rise to 3.65×10⁸ t by 2030, underscoring the urgent need for decarbonization strategies. Conventional approaches focus on energy efficiency and process optimization, yet they often fail to fully exploit the embedded energy and resource potential of reclaimed water, particularly residual thermal energy.
This study addresses this bottleneck by constructing a comprehensive carbon emission balance model that integrates carbon reduction, substitution, and sink strategies. The model quantifies the contributions of various measures, with a specific focus on reclaimed water source heat pumps, which demonstrate a carbon reduction ratio of 8.76:1 compared to chemical energy recovery. By evaluating the carbon emission balance ratio, the study identifies optimal pathways for achieving carbon neutrality, offering a practical framework for the industry.
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PANG Hongtao, HOU Feng, ZHU Ke, ZHANG Lujing, LI Peng, JIANG Leyong, SUN Shihao (2026). Establishment of a Carbon Emission Balance Model and Analysis of Carbon Neutrality Pathways for Urban Reclaimed Water Plants. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202606005
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Frequently Asked Questions
What are the primary sources of indirect carbon emissions in urban reclaimed water plants, and how can they be mitigated?
Indirect emissions account for 56.8% of total emissions, with electricity contributing 33.4%. Mitigation should focus on energy-efficient equipment, renewable energy integration, and optimizing chemical dosing to reduce embedded carbon.
How does the carbon emission balance ratio of 103.7% for reclaimed water source heat pumps compare to other carbon reduction measures?
Reclaimed water source heat pumps achieve a balance ratio of 103.7%, surpassing other measures which collectively yield 73.7%. This indicates that heat pumps alone can offset all plant emissions, making them the most effective single technology.
What are the conditions for achieving a 100% carbon emission balance ratio using combined pathways?
Two pathways achieve 100% balance: (1) extracting 27% of residual thermal energy with reclaimed water reuse, and (2) extracting 36% with sludge anaerobic digestion-cogeneration. Both require specific integration of heat recovery with existing infrastructure.
What is the significance of the 8.76:1 ratio between residual thermal energy and chemical energy carbon reduction?
This ratio indicates that recovering residual thermal energy is nearly nine times more effective per unit of energy than chemical energy recovery from sludge digestion. Prioritizing heat recovery maximizes carbon reduction efficiency.
How scalable is the proposed model for different plant capacities?
The model was validated on a 1×10⁵ m³/d plant, but the framework is scalable. The carbon balance ratio depends on site-specific factors such as heat demand and sludge characteristics, but the methodology can be adapted to optimize pathways for any capacity.
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