Insight into organic fouling behavior in polyamide thin-film composite forward osmosis membrane: Critical flux and its impact on the economics of water reclamation

A strategy of using critical fluxes to control organic fouling of polyamide thin film composite (PA-TFC) forward osmosis (FO) membranes during wastewater reclamation was developed for FO mode. This work was a comprehensive investigation with various organic foulants covering complex mixtures as well as single foulants. The foulants were alginate (ALG), Humic acid (HA), and Bovine Serum Albumin (BSA) and the study covered different concentration (40; 80; 120; 160 mg/L). Our results indicated that there was a single value of critical flux, 35 LMH for 160 mg/L and single foulant. However the presence of mixed foulants i.e., ALG + BSA, ALG + HA, HA + BSA, at an overall foulant concentration of 160 mg/L gave rise to foulant-foulant-membrane interactions that caused a significant decrease in critical flux values to 25–30 LMH. Using these results as a guide, long-term tests in which there was no fouling or negligible fouling were successfully implemented. Operating below critical flux maintains a sustainable operation with the characteristic of full reversibility, which is vital if chemical cleaning is to be minimized. Characterization of fouling around critical values was made through physico-chemical analyses including SEM, EEM, aggregate size, zeta potential, and FTIR. It was found that FO fouling became irreversible when operated at a flux ≥35 LMH for single foulants and fluxes of 25 LMH and 30 LMH for ALG + BSA and HA + BSA foulants respectively, being a foulant concentration of 160 mg/L; such conditions are favorable for the formation of the cohesive and compact cake layer. Economic assessments based on specific energy consumption facilitated the production of guidelines for practical design and operation.