Critical flux-based fouling control method for forward osmosis during simultaneous thickening and digestion of waste activated sludge

The use of forward osmosis (FO) membranes for simultaneous thickening and digestion of waste activated sludge (WAS) (FO-MSTD) has recently garnered significant interest. However, a major challenge hindering the widespread adoption of the FO-MSTD process is severe membrane fouling, which results from increasing sludge concentrations and deteriorating sludge properties. In response, this study proposes a novel FO membrane fouling control strategy based on the critical flux concept, involving a variable draw solution (DS) operating mode. Two operating modes were examined based on the critical flux behavior of the FO membrane: a constant DS concentration mode (with a fixed DS concentration of 1.5 M, referred to as Constant DS1.5) and a variable DS concentration mode (with varying DS concentrations at different operational stages, referred to as Variable DS1.5-1.0 and Variable DS1.5-0.75). Over 21 days of operation, the MLSS concentrations of WAS in the Constant DS1.5, Variable DS1.5-1.0, and Variable DS1.5-0.75 reactors increased from initial values of 3.46, 3.88, and 4.20 g/L to 35.2, 31.5, and 29.2 g/L, respectively. Concurrently, the cumulative digestion rates of MLVSS in the three FO-MSTD processes reached 29.3 %, 29.0 %, and 36.3 %, respectively. After adjusting the DS concentration, the FO membrane flux decay rates for Constant DS1.5, Variable DS1.5-1.0, and Variable DS1.5-0.75 were 57.7 %, 49.4 %, and 13.8 %, respectively. In addition to experiencing a lower flux decline, the Variable DS1.5-0.75 mode achieved a significantly higher flux recovery rate of 98.5 % compared to the other two modes. Moreover, the biovolume of proteins and microorganisms in the membrane fouling layer decreased by 85.2 % and 72.7 %, respectively, in the Variable DS1.5-0.75 mode compared to the Constant DS1.5 mode. These findings indicate that operating with a DS below the critical concentration can significantly improve the reversibility of membrane fouling and reduce the presence of organic and biological foulants in the fouling layer, thereby mitigating membrane fouling in the FO-MSTD reactor.