Titanium xerogel coagulant outperforms PFS and PAC in water treatment: Better contaminant removal and less membrane fouling

Preloading the flocs on the membrane surface is beneficial to enhance the removal of contaminants and alleviate membrane fouling in the membrane-filtration process, but the appropriate flocs and its mechanism of action need to be further clarified. The present study first focused on the flocs generated by titanium xerogel coagulant (TXC) with PFS and PACl as comparation. Three flocs were separately pre-loaded on the ultrafiltration membrane to explore their performance of removing four toxic heavy metals (As(III), As(V), Sb(III) and Sb(V)) and alleviating membrane fouling. Results showed that the flocs produced by TXC and PFS could adsorb toxic oxygenates continuously, and the removal performance of TXC flocs is better than that of PFS, while the removal performance of PACl flocs is limited. After long-term operation, the membrane fouling of pre-loaded TXC floc was minimal, and the irreversible fouling was 1/3 and 1/6 of that of PFS and PACl, respectively. Meanwhile, the backwash time was 2.0 and 2.2 times longer than PFS and PACl, respectively. The mechanism of membrane fouling alleviation by floc preloading was clarified by floc characterization and model calculations. According to the n value in the model after fitting, an intermediate blocking mechanism dominated with the larger and more loose flocs in TXC system (n = 1.54), while a complete pore blocking mechanism dominated in the PFS (n = 1.98) and PACl (n = 2.11) systems. The results demonstrated that preloading the TXC flocs were superior to that of PFS and PACl in the membrane-filtration process.