Exploring the pathway for the energy-environment synergistic WWTPs through comprehensive efficiency assessment

The wastewater treatment sector requires comprehensive efficiency evaluation to balance traditional pollutant removal metrics with greenhouse gas (GHG) reduction objectives. This study develops an extended slacks-based measure analysis framework integrating net energy intensity, surplus energy supply intensity, GHG emission, and effluent quality for 2359 Chinese wastewater treatment plants. Our assessment reveals that the comprehensive efficiency scores decrease to 0.261 under the extended evaluation framework, compared with 0.351 of the traditional evaluation frameworks, and the previously identified benchmark plants no longer meet standards when considering energy-environment synergies. The assessment of energy recovery pathways demonstrates that heat pump technology delivers superior standalone performance with 28.7 % efficiency gains, while integrated combined heat and power, heat pump, and photovoltaic systems enable 74.4 % of plants to achieve medium efficiency levels. However, a 52.7 % efficiency gap persists even under optimal technology deployment, indicating that energy recovery alone cannot achieve complete sustainability transformation. The analysis identifies three key constraints: non-linear scale effects where ultra-large plants experience regulatory-driven resource intensity requirements, regional disparities influenced by climate and management practices, and operational underutilization averaging 79.0 % loading rate. These findings support a sequential three-tiered strategic pathway: first establishing operational excellence and load-matching, followed by targeted energy recovery technology deployment, and ultimately requiring policy and infrastructure frameworks that economically justify stringent environmental standards and capture their comprehensive value rather than conventional compliance alone.