Insight into fouling potential analysis of a pilot-scale pressure-assisted forward osmosis plant for diluted seawater reverse osmosis desalination

[Display omitted] •A pilot-scale pressure-assisted forward osmosis (PAFO) desalination plant was installed and operated on-site.•The fouling potential was analyzed after 76 days of operation under the condition of 2bar pressurization.•The microbial ATP concentration and the MFI0.45 (one of the fouling index) showed a strong correlation.•Severe bacterial fouling occurred even under pressure at the level of 2bar on the FO membranes contacting with wastewater. In recent years, pressure-assisted forward osmosis (PAFO) process has been studied to overcome the low water productivity in FO-RO hybrid seawater desalination. Our research group demonstrated a pilot-scale PAFO-RO plant for seawater desalination operations using secondary treated wastewater and natural seawater. A pretreatment stage of mesh tube filter media (MTF) and cartridge filter (CF) was implemented for a pilot-scale PAFO. In this study, the water quality and fouling index (SDI15 and MFI0.45) of the influent and treated water at each process stage of the plant were measured. For the influent of both wastewater and seawater feature, it showed linear correlations between the concentration of microbial ATP (i.e., an indicator of microbial activity) and MFI0.45. After the PAFO was operated for 76 days, the fouling of the filtration media and the FO membrane was also analyzed. The observation of the fouled membranes by SEM showed that fouling was hardly observed on the side contacting with seawater, but severe bacterial biofilms were observed on the side contacting with wastewater. FT-IR results also confirmed protein-like substances on the membrane surface contacting with wastewater, indicating irreversible fouling. This study suggests that an advanced pretreatment process to remove bacteria and organic matter is essential for the protection of the FO membranes.