Efficiency and mechanism of sludge thickening by a novel sandwich-like forward osmosis composite membrane constructed on microporous polyethylene separator

Sludge thickening is a critical step for deep sludge dewatering, but energy consumption and efficiency are still the bottlenecks. Forward osmosis (FO) provides a promising choice due to its flexible configurations, low energy consumption and high quality of recovered water. This study proposed a new method to construct a sandwich-like FO membrane on commercial polyethylene substrate for sludge thickening. The combined modification of FO membrane using Fe 3+ -mediated polydopamine (PDA) interlayer and hydroxylamine-O-sulfonic acid (HOSA) grafting polyamide (PA) layer obtained good performance for sludge thickening. Its pure water flux ( J w ) increased by 2.5 folds and the reverse salt flux ( J s ) decreased by 2.7 folds, respectively. Excellent antifouling property and higher water flux were also achieved even after running for several cycles (∼48 h) during sludge thickening. More importantly, more than 98 % of nutrients (e.g., COD, TN, TP and NH 4 + -N) were rejected in sludge, and thus good quality of the recovered water was obtained, greatly mitigating the burden of following wastewater treatment. Mechanism was revealed by comprehensively characterizing the membrane and foulants in the sludge using 3D fluorescence spectroscopy (F-EEM). Performance of FO method using different draw solutions was examined systematically, and the feasibility of utilizing synthetic seawater as draw solutions was assessed. This study proposed a new FO method for efficient sludge thickening, and highlighted the potential application of the method using seawater and industrial saline wastewater as draw solutions in decentralized wastewater treatment.