Micro-flocculation/sedimentation and ozonation for controlling ultrafiltration membrane fouling in recycling of activated carbon filter backwash water

Abstract

The reuse of activated carbon filter backwash water (ACFBW) was performed using an ultrafiltration (UF) system. The feed water was pretreated by the micro-flocculation/sedimentation, followed by ozonation, prior to its entry into the UF system. The dose of 2mg/L PACl and subsequent 10min sedimentation were involved in the pretreatment, which resulted in the removal of most particles and bacteria in ACFBW, causing a decrease of transmembrane pressure (TMP) and the mitigation of membrane fouling. In the parallel experiment with or without preozonation, the influence of dosing ozone at 0.15mg/L on membrane fouling was investigated. The results showed that the preozonation restrained bacteria breeding on the membrane surface and maintained the membrane permeability. The preozonation reduced the hydrophobic organic matters and changed the molecular weight distribution of organics in the influent ACFBW, reflecting macromolecular organic compounds to form small fractions. In addition, the preozonation narrowed the pore size of membrane, the virgin membrane varying from 2.1 to 26.3nm compared with the used membrane with preozonation from 1.5 to 23.6nm. The preozonation improved the hydrophobicity of membrane material in a long-running filtration, which could aggravate the membrane fouling. However, the preozonation enhanced UF performance due to the characteristics change of organic matters, which was dominant in alleviating membrane fouling compared with the adverse influence of the increased hydrophobicity of membrane material.