Draw solution contamination undermines FO-MD stability for transfer station leachate treatment

Transfer station leachate (TSL) is a high-strength, complex and high-risk liquid waste containing abundant emerging pollutants (EPs). Forward osmosis-membrane distillation (FO-MD) system holds great potential for on-site TSL treatment due to its high rejection and small footprint. However, the long-term stability of FO-MD (particularly the MD unit) in real TSL treatment as well as its impact on EPs control, has rarely been addressed. In this study, a long-term experiment with real TSL demonstrated the effectiveness of FO-MD for TSL volume reduction (over 90%) and EPs rejection (over 98% for both antibiotics and polyfluoroalkyl substances (PFAS)), meanwhile unveiled a previously overlooked problem: draw solution (DS) contamination and the consequences of MD membrane fouling and wetting. This led to a significant imbalance of the combined system and a sharp deterioration in EPs rejection (antibiotics and PFAS leakage increased by 3-9 times). Further investigation revealed that small amides/amino acids (the characteristic of fresh TSL) and scaling ions were accumulated in the DS and consequently induced fouling/wetting of the MD membrane. To this end, a coagulation pretreatment was applied to raw TSL, which selectively removed key foulants (amide-like compounds and CO₃^2-) in the DS, substantially alleviating MD membrane fouling/wetting, thereby sustaining stable water production and efficient EPs rejection over long-term operation. The findings of present study highlight the critical but often ignored role of DS contamination in FO-MD system failure and provide a practical mitigation strategy, paving the way for reliable application of FO-MD in high-strength and high-risk wastewater treatment.