Systematic study of air-assisted AGMD modules: Conventional, coolant-side condensation, and bubble column dehumidification

This study presents a systematic experimental investigation of three air gap membrane distillation (AGMD) configurations: conventional AGMD (AGMD-C), air-injected with in-loop condensation (AGMD-CC), and air-injected with an external bubble column dehumidifier (AGMD-BC). In all cases, the air gap itself remains stagnant; the injected air is bubbled along the feed-side membrane surface, becomes humidified, and is subsequently dehumidified either within the coolant loop (AGMD-CC) or in the external bubble column (AGMD-BC). The novelty of this work lies in the direct comparison of two distinct vapor recovery strategies: external condensation in a bubble column and in-loop condensation through the coolant stream, under identical thermal and hydraulic conditions. This approach provides new insights into how module design influences vapor recovery, thermal efficiency, and long-term stability, thereby addressing a critical gap in the optimization of AGMD desalination systems. Performance was evaluated across a range of feed and coolant flowrates, feed and coolant temperatures, air injection rates, and air gap thicknesses. In addition, long term continuous operation with real seawater was conducted to assess fouling and scaling resistance and durability. Results showed that AGMD-BC consistently achieved the highest productivity and energy efficiency. At baseline conditions, AGMD-BC delivered roughly 17 % higher total distillate flux and almost 9 % higher gained output ratio (GOR) compared with AGMD-C, while AGMD-CC provided intermediate gains. Increasing air injection to 0.4 SCFM raised total distillate flux and GOR by nearly 87 % and about 26 %, respectively, relative to no-air operation. Reducing the air gap from 9 to 5 mm improved performance by up to 40 %. During long-term testing, AGMD-BC exhibited only around 9 % total distillate flux decline, compared with almost 21 % for AGMD-C, confirming superior fouling mitigation and stability. The findings establish AGMD-BC as a robust and high-performing configuration, demonstrating that external bubble column condensation coupled with air injection is a promising strategy for advancing AGMD desalination toward higher efficiency and long-term reliability.