Promoting coupling coordination of reclaimed water reuse based on carbon - emergy - economy through optimizing end uses

Reclaimed water reuse presents significant environmental and economic benefits, yet the optimal end-use strategy for maximizing its comprehensive performance remains underexplored. This study develops a coupling coordination degree (CCD) model integrating carbon emission, emergy, and economic analyses to evaluate the synergistic effects of different end-use scenarios. The proposed CCD model advances the existing frameworks by integrating a comprehensive multidimensional approach that incorporates resource structure and efficiency, as well as the environmental contributions to socioeconomic systems, besides considerations of greenhouse gas emissions and economic performance. A case study of a reclaimed water project in Chongqing, China, compares four scenarios (S1: park green space irrigation with pipeline transportation (PGP) + municipal miscellaneous water with truck transportation (MMT); S2: park green space irrigation with truck transportation (PGT) + MMT; S3: water source heat pump (WSHP) + PGP + MMT; S4: WSHP + PGT + MMT). Key results indicate: (1) Replacing PGP with PGT (S2) slightly increased carbon emissions, while integrating WSHP (S3) and combination of PGT and WHSP (S4) observably reduced emissions; (2) Emergy sustainability was moderately improved across S2, S3 and S4; (3) Economic returns varied, with S2 and S4 showing small gains but S3 declining slightly due to WSHP costs; (4) CCD values ranked S4 > S3 > S2 > S1, demonstrating that combined measures synergistically enhanced system coordination and co-benefits between climate mitigation and economic efficiency. The findings highlight the importance of organized combination of end-use ways for promoting coordinative development of reclaimed water systems.