A comparative study of ferrous, ferric and ferrate pretreatment for ceramic membrane fouling alleviation in reclaimed water treatment

Coagulation-membrane filtration hybrid process has been widely applied in water treatment to improve water quality and alleviate membrane fouling. Herein, we systematically investigated the comparable effect of ferrate, ferrous and ferric pretreatment on fouling alleviation of ceramic membrane in reclaimed water treatment. Our results showed that the pretreatment of ferric, ferrous and ferrate remarkably alleviated membrane fouling with the fouling alleviation performance following the order: ferrate > ferrous > ferric > control. The t(crit) (subcritical fouling time) after which the fouling rate (dTMP/dt) became much more severe was remarkably extended from 0 (control) to 432 (ferric), 576 (ferrous) and 888 h (ferrate), respectively. Meanwhile, hydraulically reversible fouling resistance had the greatest contribution (58.3%) to the overall fouling resistance in the control case, however, hydraulically irreversible membrane fouling caused by inorganics (Rir-citrate) became more severe in long-term operation with the contribution of Rir-citrate to the total fouling resistance of 40.7%, 46.3% and 50.3% in the cases with ferric, ferrous and ferrate pretreatment, respectively. Moreover, the redundancy analysis results showed that the alleviated membrane fouling was principally owing to the sharp reduction of organic matters and the significant increase of particle size and particle concentration in influent. Compared with ferric and ferrous, ferrate had the best fouling alleviation performance, which was mainly due to its dual functions of chemical oxidation (including (OH)-O-center dot, Fe(V) and Fe(IV)) and coagulation (ferric oxyhydroxides).