A “water and carbon” near-zero emission WWTP system: Model development and techno-economic-environmental benefits assessment

Wastewater treatment plants (WWTPs) have been striving to recover energy and resources, targeting water and carbon near zero emissions. This study aims to develop a water-energy-tailored model for such a proposal. On one hand, this model will unveil the potential for resource and energy recovery by analyzing the energy flow and mass balance of the WWTP. On the other hand, it explores in-situ energy generation by calculating photovoltaic power generation at a specific location using high spatial-temporal resolution data. The model is employed in a typical town-level WWTP with a capacity of 4000 m³/d located in China. The potentials for carbon emission reduction and associated cost-benefit were analyzed under four different power supply paradigms from the perspective of life cycle assessment. Key findings are as follows: firstly, there is untapped chemical energy (1.65 kWh/m³) and thermal energy (2.32 kWh/m³ for heating) potential within wastewater. It is necessary to recover energy from it and enable water reuse to achieve near-zero wastewater discharge. Secondly, it is hard to balance operation energy consumption and in-situ solar energy recovery along with water-borne energy in the WWTP. The tipping point is identified at a scale of 10,000 m³/d, when constructing a photovoltaic and energy storage system within all available space on the plant premises, with a capacity potential of 95 kWh/(m²$\bullet$a). Thirdly, under this condition, the cost of the photovoltaic and energy storage system is at least 73% of the electricity cost from the grid over the assessed 25-year period. The economic viability of WWTPs throughout the entire lifecycle remains a challenge. Therefore, caution is warranted in claiming the feasibility of constructing near-zero carbon WWTPs. Policy implications are also carefully discussed, targeting to achieve a balance among technology, economy, and environment while making the model work in real.