Gravity-Driven Membrane as seawater desalination pretreatment: Understanding the role of membrane biofilm on water production and AOC removal

Gravity-Driven Membrane (GDM) is considered an emerging seawater reverse osmosis desalination pretreatment. The objective of this study was to assess the GDM process performance under different operating conditions and to evaluate the impact of membrane biofilm on the produced water quantity and quality. Three lab-scale submerged GDM systems (UF_12h, UF_24h and MF_24h) were tested under different conditions of Hydraulic Retention Time (HRT) (12 h/24 h) and membrane pore size (Microfiltration/Ultrafiltration). The best water quality in terms of Assimilable Organic Carbon (AOC) was achieved with the longer HRT (24 h), where significant AOC was removed by the biofilm developed on the membrane. The next generation sequencing indicated that a longer HRT increased the abundance of Proteobacteria and Planctomycetes in the biofilm, which due to their biodegradation capability yielded to the highest AOC removal (~50–55 %). Furthermore, the use of microfiltration led to the highest water production (9.1 L/m²/h) due to larger membrane pore size, and a less dense biofilm. These findings indicate that employing a microfiltration membrane and 24 h HRT provides the best performance in terms of water quantity and AOC-removal. This study demonstrates that controlling GDM design and operation enabled manipulation of the biofilm proprieties yielding to high water production and high AOC-removal.