Sidestream vs. mainstream AnMBR for low-carbon wastewater treatment: a comprehensive life-cycle techno-economic and environmental assessment under stochastic uncertainties.

Municipal wastewater treatment is transitioning towards energy and resource recovery. However, conventional activated sludge (CAS) and mainstream anaerobic membrane bioreactors (M-AnMBR), which directly treat the bulk flow of low-strength municipal wastewater for organic removal, are constrained by high carbon footprints or energy demands. Sidestream AnMBR (S-AnMBR) treats a small-volume, carbon-rich stream concentrated from the mainstream wastewater, physically decoupling carbon and water flows via chemically enhanced primary treatment (CEPT), offering a promising solution to these bottlenecks. This study developed an evaluation framework integrating techno-economic analysis, life cycle assessment, and data envelopment analysis coupled with Monte Carlo simulations to comprehensively compare CAS, M-AnMBR (including an aeration-optimized scenario), and S-AnMBR, given a range of uncertainties. Results show that S-AnMBR has a high probability of achieving net-negative energy consumption (80.4%), with a net energy consumption of -0.04 kWh/m3 (90% uncertainty interval: -0.11 to 0.03 kWh/m3), while also exhibiting the lowest carbon footprint (0.24 kg CO2e/m3, with a 90% uncertainty interval of 0.22 to 0.34 kg CO2e/m3). S-AnMBR achieves a cost efficiency of 0.85 (out of 1), comparable to the highest score of 0.88 achieved by CAS. Under the evaluated conditions, M-AnMBR remains uncompetitive despite aeration optimization. Sensitivity analysis reveals that the dominant performance bottlenecks shift markedly as the treatment architecture evolves from CAS to mainstream and sidestream AnMBR processes. While CAS is primarily influenced by external factors (e.g., discount rates and grid carbon emission factors), M-AnMBR relies typically on membrane flux and gas sparging. For S-AnMBR, the dominant constraints shift to the cost and performance of nitrogen-removal membranes and the carbon intensity of CEPT coagulants, highlighting membrane cost reduction as a key opportunity for further techno-economic improvement. By integrating front-end carbon capture, S-AnMBR emerges as a highly resilient and sustainable technological paradigm for next-generation wastewater treatment.