Evaluation of the potential for operating carbon neutral WWTPs in China

- Organic energy could only offset 50% of the energy consumption of a WWTP in China.
- About 1/5 of the effluent could supply a net thermal energy of 0.26 kWh/(m3 °C) via WSHP.
- Photovoltaic technology is not economic albeit supplying 10% of the energy consumption.
- Organic plus thermal energy could realize the goal of WWTPs towards carbon neutrality.

Carbon neutrality is starting to become a hot topic for wastewater treatment plants (WWTPs) all over the world, and carbon neutral operations have emerged in some WWTPs. Although China is still struggling to control its water pollution, carbon neutrality will definitely become a top priority for WWTPs in the near future. In this review, the potential for operating carbon neutral WWTPs in China is technically evaluated. Based on the A2/O process of a typical municipal WWTP, an evaluation model is first configured, which couples the COD/nutrient removals (mass balance) with the energy consumption/recovery (energy balance). This model is then applied to evaluate the potential of the organic (COD) energy with regards to carbon neutrality. The model's calculations reveal that anaerobic digestion of excess sludge can only provide some 50% of the total amount of energy consumption. Water source heat pumps (WSHP) can effectively convert the thermal energy contained in wastewater to heat WWTPs and neighbourhood buildings, which can supply a net electrical equivalency of 0.26 kWh when 1 m3 of the effluent is cooled down by 1 °C. Photovoltaic (PV) technology can generate a limited amount of electricity, barely 10% of the total energy consumption. Moreover, the complexity of installing solar panels on top of tanks makes PV technology almost not worth the effort. Overall, therefore, organic and thermal energy sources can effectively supply enough electrical equivalency for China to approach to its target with regards to carbon neutral operations.

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