Long-term performance and initial fouling evaluation of an open-loop plate and frame forward osmosis element using wastewater treatment plant secondary effluent as a feed solution

In this study, the long-term performance of a plate and frame forward osmosis (PFFO) process was systematically assessed through an element-scale system using secondary effluent from a full-scale wastewater treatment plant. To the best of our knowledge, this represents the first time long-term operation of an element-scale PFFO process has been performed with relatively stable operation, water flux, and contaminant rejection rates without physical or chemical cleaning. Seasonal variations in water flux, inlet pressure, and contaminants in the element-scale PFFO process were monitored during the 10 months of operation. The results showed that at the end of the operating period, there was a 52 % reduction in water flux compared to the initial value due to the effects of initial membrane fouling, while the transmembrane inlet pressure increased by approximately 0.2 bar. High DOC removal of over 90.7 % was obtained with the presence of humic-like and protein-like substances in the diluted draw solution. In addition, the hydrophilicity of the diluted draw solution increased by 40.8 %. After the process, the hydrophobicity of the membrane surface (contact angle) increased by 16°, and a large amount of multivalent ions were detected on fouled membrane surface (i.e., Al, Ca, Fe, and Si). The results of this element-scale PFFO process demonstrate that sustainable long-term operation can be achieved compared with spiral wound type FO elements owing to the low water flux reduction and high rejection rate of contaminants.